SILICON-CARBON COMPOSITE, PREPARATION METHOD THEREFOR, AND ANODE ACTIVE MATERIAL AND LITHIUM SECONDARY BATTERY COMPRISING SAME

Abstract

The present invention relates to a silicon-carbon composite, a preparation method therefor, and an anode active material comprising same. The silicon-carbon composite includes silicon particles, silicon oxides, magnesium compounds, and carbon. As a molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite satisfies 0.01 to 0.60, when the silicon-carbon composite is applied to an anode active material, the discharge capacity, initial efficiency, and capacity retention ratio after cycles of a lithium secondary battery may be simultaneously improved.

Description

TECHNICAL FIELD

The present invention relates to a silicon-carbon composite, to a method for preparing the same, and to a negative electrode active material and a lithium secondary battery comprising the same.

BACKGROUND ART

In recent years, as electronic devices become smaller, lighter, thinner, and more portable in tandem with the development of the information and communication industry, the demand for a high energy density of batteries used as power sources for these electronic devices is increasing. A lithium secondary battery is a battery that can best meet this demand, and research on small batteries using the same, as well as the application thereof to large electronic devices such as automobiles and power storage systems, is being actively conducted.

Carbon materials are widely used as a negative electrode active material for such a lithium secondary battery. Silicon-based negative electrode active materials are being studied in order to further enhance the capacity of batteries. Since the theoretical capacity of silicon (4,199 mAh/g) is greater than that of graphite (372 mAh/g) by 10 times or more, a significant enhancement in the battery capacity is expected.

The reaction scheme when lithium is intercalated into silicon is, for example, as follows:

22Li+5Si═Li₂₂Si₅  [Reaction Scheme 1]

In a silicon-based negative electrode active material according to the above reaction scheme, an alloy containing up to 4.4 lithium atoms per silicon atom with a high capacity is formed. However, in most silicon-based negative electrode active materials, volume expansion of up to 300% is induced by the intercalation of lithium, which destroys the negative electrode, making it difficult to exhibit high cycle characteristics.

In addition, this volume change may cause cracks on the surface of the negative electrode active material, and an ionic material may be formed inside the negative electrode active material, thereby causing the negative electrode active material to be electrically detached from the current collector. This electrical detachment phenomenon may significantly reduce the capacity retention rate of a battery.

In order to solve this problem, Japanese Patent No. 4393610 discloses a negative electrode active material in which silicon and carbon are mechanically processed to form a composite, and the surfaces of the silicon particles are coated with a carbon layer using a chemical vapor deposition (CVD) method.

In addition, Japanese Laid-open Patent Publication No. 2016-502253 discloses a negative electrode active material comprising porous silicon-based particles and carbon particles, wherein the carbon particles comprise fine carbon particles and coarse-grained carbon particles having different average particle diameters.

However, although these prior art documents relate to a negative electrode active material comprising silicon and carbon, there is a limit to suppressing the volume expansion and contraction during charging and discharging. Thus, there is still a demand for research to solve these problems.

PRIOR ART DOCUMENTS

Patent Documents

• • (Patent Document 1) Japanese Patent No. 4393610
  • (Patent Document 2) Japanese Laid-open Patent Publication No. 2016-502253
  • (Patent Document 3) Korean Laid-open Patent Publication No. 2018-0106485

DISCLOSURE OF INVENTION

Technical Problem

The present invention is devised to solve the above problems of the prior art. An object thereof is to provide a silicon-carbon composite comprising silicon particles, a magnesium compound, and carbon and having a molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite that satisfies a specific range, thereby having excellent capacity retention rate, along with significantly improved discharge capacity and initial efficiency, when it is applied to a negative electrode active material.

In addition, another object of the present invention is to provide silicon-carbon composites of various structures each having a core-shell structure.

In addition, another object of the present invention is to provide a method for preparing the silicon-carbon composite.

In addition, another object of the present invention is to provide a negative electrode active material comprising the silicon-carbon composite and a lithium secondary battery comprising the same.

Solution to Problem

To accomplish the above objects, the present invention provides a silicon-carbon composite, which comprises silicon particles, a magnesium compound, and carbon, wherein the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite is 0.01 to 0.60.

In addition, the present invention provides a method for preparing the silicon-carbon composite, which comprises a first step of obtaining a silicon composite oxide using a silicon-based raw material and a magnesium-based raw material; a second step of etching the silicon composite oxide using an etching solution containing a fluorine (F) atom-containing compound to obtain a silicon composite; and a third step of coating carbon inside the silicon composite, or coating carbon inside the silicon composite and forming a carbon layer on the surface thereof, using a chemical thermal decomposition deposition method to obtain a silicon-carbon composite.

In addition, the present invention provides a negative electrode active material comprising the silicon-carbon composite.

Further, the present invention provides a lithium secondary battery comprising the negative electrode active material.

Advantageous Effects of Invention

According to an embodiment, the silicon-carbon composite comprises silicon particles, a magnesium compound, and carbon and has a molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite, which satisfies a specific range. Thus, when it is used as a negative electrode active material of a lithium secondary battery, along with a binder and a conductive material, to prepare a negative electrode active material composition, it is readily dispersed, mechanical properties such as strength are excellent, and the performance of a lithium secondary battery can be enhanced. Further, when the porosity of the silicon-carbon composite is controlled, the discharge capacity, initial efficiency, and capacity retention rate of a lithium secondary battery can be further enhanced.

According to another embodiment, the silicon-carbon composite has a core-shell structure, and the core comprises silicon particles, a magnesium compound, and carbon, and, in particular, the shell comprises at least one or two or more carbon layers, whereby the strength of the shell layer is further increased, and the bonding (adhesion) between the silicon-carbon composite and a binder is increased, which can reduce the electrode expansion ratio. As a result, it is possible to enhance the discharge capacity, initial efficiency, and capacity retention rate of a lithium secondary battery at the same time.

In addition, the preparation method according to an embodiment has an advantage in that mass production is possible through a continuous process with minimized steps.

BRIEF DESCRIPTION OF DRAWINGS

The following drawings attached to the present specification illustrate preferred embodiments of the present invention and serve to further understand the technical idea of the present invention together with the description of the present invention. Accordingly, the present invention should not be construed as being limited only to those depicted in the drawings.

FIG. 1 is a schematic diagram illustrating the preparation process of a silicon-carbon composite according to an embodiment.

FIG. 2 is a photograph observing a cross-section of the silicon-carbon composite prepared in Example 1 at a magnification of 30,000 using a focused ion beam scanning electron microscope (FIB-SEM).

FIG. 3 is a photograph showing the spot locations of the outer part (A), center part (C), and outer part (B) of a cross-section of the silicon-carbon composite prepared in Example 1 through spot scanning using a focused ion beam scanning electron microscope (FIB-SEM).

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is not limited to what is disclosed below. Rather, it may be modified in various forms as long as the gist of the invention is not altered.

In the drawings attached to the present specification, some components are exaggerated, omitted, or schematically depicted. The size of each component does not entirely reflect the actual size.

The same reference numerals refer to the same elements throughout the present specification.

The terms used in the present specification are for the purpose of describing particular embodiments only and are not intended to limit the present invention. Singular expressions cover plural expressions unless the context clearly indicates otherwise.

In this specification, when one component is described as being formed on or under another component, it means that one component is formed directly on or under the other component, or indirectly through another component.

In this specification, when a part is referred to as “comprising” an element, it is to be understood that the part may comprise other elements as well, unless otherwise indicated.

In addition, all numbers and expressions related to the quantities of components, reaction conditions, and the like used herein are to be understood as being modified by the term “about,” unless otherwise indicated.

[Silicon-Carbon Composite]

The silicon-carbon composite according to an embodiment comprises silicon particles, a magnesium compound, and carbon, wherein the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite is 0.01 to 0.60.

As the silicon-carbon composite comprises silicon particles, a magnesium compound, and carbon together, it is possible to further enhance the performance of a lithium secondary battery. Specifically, since the silicon particles in the silicon-carbon composite charge lithium, they can enhance the capacity of a lithium secondary battery; and since the magnesium compound hardly reacts with lithium ions, it can reduce the degree of expansion and contraction of the electrode when lithium ions are intercalated, and it can produce the effect of suppressing the volume expansion of silicon particles; thus, it can be effective in enhancing the capacity retention rate of a lithium secondary battery. In addition, the carbon can impart excellent electrical conductivity and suppress side reactions of the electrolyte, thereby further enhancing the performance of a lithium secondary battery.

In addition, the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite satisfies the above specific range, whereby it is possible to significantly enhance discharge capacity and initial efficiency while an excellent capacity retention rate is maintained.

Further, according to an embodiment, it is possible to remove most of the silicon dioxide contained in the silicon-carbon composite by etching. In such a case, the surface of the silicon particle may comprise silicon (Si) atoms in a very high fraction relative to oxygen (O) atoms. That is, the molar ratio O/Si may be significantly reduced. As the silicon-carbon composite is used as a negative electrode active material, a lithium secondary battery having excellent discharge capacity can be preferably obtained while the initial efficiency of the lithium secondary battery can be enhanced.

The molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite may be 0.01 to 0.60, preferably, 0.01 to 0.45, more preferably, 0.05 to 0.45, even more preferably, 0.06 to 0.40, most preferably, 0.06 to 0.30.

When the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite is equal to, or greater than, the lower limit within the above range, expansion and contraction due to charging and discharging can be suppressed. Thus, when the silicon-carbon composite is used as a negative electrode active material, it is possible to prevent the negative electrode active material from being delaminated from the negative electrode current collector. In addition, if the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms is equal to, or lower than, the upper limit within the above range, the sufficient discharge capacity of a lithium secondary battery can be secured, so that it is possible to maintain high charge and discharge characteristics.

Hereinafter, the structure of the silicon-carbon composite according to an embodiment will be described in detail.

Structure of the Silicon-Carbon Composite

The silicon-carbon composite according to an embodiment comprises silicon particles, a magnesium compound, and carbon.

FIG. 1 is a schematic diagram illustrating the preparation process of a silicon-carbon composite according to an embodiment. FIGS. 1(C) and 1(D) each show a schematic diagram that simplifies the cross-section of the structure of silicon-carbon composites according to various embodiments of the present invention. FIG. 1(B) and its preparation process are described in detail in the preparation method of a silicon-carbon composite described below.

First, referring to FIGS. 1(C) and 1(D), the silicon-carbon composite comprises silicon particles (20), a magnesium compound (60), and carbon (45).

In the silicon-carbon composite according to an embodiment, silicon particles as an active material, a magnesium compound, and carbon are uniformly dispersed and distributed to be firmly combined together. As a result, it is possible to prevent the silicon-carbon composite from being pulverized due to a change in volume thereof during the charging and discharging of a lithium secondary battery.

In addition, the silicon-carbon composite may comprise silicon particles (20), specifically, a silicon aggregate in which the silicon particles (20) are combined with (or interconnected to) each other. A carbon layer (40) may be formed on the surfaces of the silicon particles (20) and/or the silicon aggregate, or on the surface of the magnesium compound (60), or both. In addition, carbon (45) may be formed as it is uniformly dispersed and/or distributed between the silicon particle (20) and the magnesium compound (60).

In the present specification, the term “combined with (or interconnected to) each other” refers to a state in which silicon particles in the silicon-carbon composite are each combined (fused) with at least another silicon particle and connected to each other, or a structure (silicon aggregate) is formed in which the silicon particles are continuously connected.

The inside of the silicon-carbon composite may be more firmly combined by the carbon (45) and the carbon layer (40). As a result, it is possible to minimize the pulverization of the silicon-carbon composite that may be caused by a change in volume during the charging and discharging of a lithium secondary battery.

The silicon-carbon composite is a composite in which silicon aggregates, in which a plurality of silicon particles (20) are combined with each other, are uniformly distributed in a composite whose structure is in the form of a single mass, for example, a polyhedral, spherical, or similar shape. It may be in a single composite in which carbon (45) and a carbon layer (40) comprising carbon surround a part or all of the surfaces of one or more silicon particles or the surfaces of secondary silicon particles (silicon aggregates) formed by the aggregation of two or more silicon particles.

That is, in the silicon-carbon composite, a large amount of carbon (45) is formed inside thereof and a carbon layer (40) comprising carbon is uniformly formed in a state to surround the silicon particles (20) and/or the magnesium compound (60); thus, there are advantages in that it is possible to ensure high conductivity and that the addition of a carbon-based negative electrode active material can be reduced when a negative electrode is manufactured. In addition, the specific surface area of the silicon-carbon composite is appropriately adjusted such that the performance of a lithium secondary battery can be further enhanced.

In addition, the silicon particles (20) may be present between the magnesium compound (60) particles and may be surrounded by the magnesium compound (60).

Since the magnesium compound hardly reacts with lithium ions during the charging and discharging of a lithium secondary battery, it is possible to reduce the expansion and contraction of the electrode when lithium ions are occluded in the electrode, thereby enhancing the cycle characteristics of the lithium secondary battery. In addition, the strength of the matrix, which is a continuous phase surrounding the silicon particles, can be fortified by the magnesium compound.

In addition, according to an embodiment, the silicon-carbon composite may have a core-shell structure, wherein the core may comprise silicon particles, a magnesium compound, and carbon, and the shell may comprise a carbon layer.

For example, in the silicon-carbon composite, the core comprises silicon particles comprising carbon (e.g., amorphous carbon and/or an amorphous carbon layer) and a magnesium compound therein, and a carbon layer is formed on the surface of the core, whereby a high degree of fusion is achieved at the interface between them, and the interfacial resistance can be reduced. As a result, the intercalation and detachment of lithium can be smoothly achieved, thereby simultaneously securing the high-capacity and high-output characteristics of a lithium secondary battery.

In addition, the silicon-carbon composite may comprise pores (10).

When the silicon-carbon composite comprises pores (10), the volume expansion of silicon particles (20) that may occur during the charging and discharging of a lithium secondary battery can be accommodated through the pores, thereby effectively alleviating and suppressing the problems caused by the volume expansion.

In addition, the silicon-carbon composite, which comprises a core-shell structure, may comprise a shell comprising a carbon layer (carbon film) on the surface of the core.

In the silicon-carbon composite, the carbon layer that constitutes the shell may be formed or laminated in various ways as long as the desired effect is not impaired. The shell may comprise up to four layers, specifically, one to four layers, preferably, one to three layers, for example, one or two layers of carbon.

For example, according to an embodiment, there may be provided a silicon-carbon composite (hereinafter referred to as “silicon-carbon composite (1)”) having a core-shell structure wherein the shell has a single-layer structure composed of a first carbon layer.

Specifically, referring back to FIG. 1(C), the silicon-carbon composite (1) may have a core-shell structure, wherein the core may comprise silicon particles (20), a magnesium compound (60), and carbon (45), and the shell may comprise a first carbon layer (40). In addition, the silicon-carbon composite (1) may comprise pores (10) therein.

In addition, the silicon-carbon composite may comprise a silicon composite and a carbon layer on its surface.

The silicon composite may be present in the core.

Specifically, the silicon composite may be present in the core and may comprise silicon particles (20), a magnesium compound (60), and carbon (45). In addition, a shell comprising a first carbon layer (40) may be formed on the surface of the silicon composite.

In addition, the magnesium compound (60) may comprise a fluorine-containing magnesium compound (61), magnesium silicate (62), or a mixture thereof.

For example, the magnesium compound (60) may comprise a fluorine-containing magnesium compound (61).

In addition, the magnesium compound (60) may further comprise magnesium silicate (62).

For example, the magnesium compound (60) may comprise a fluorine-containing magnesium compound (61) and magnesium silicate (62).

Since the magnesium compound hardly reacts with lithium ions during the charging and discharging of a lithium secondary battery, it is possible to reduce the expansion and contraction of the electrode when lithium ions are occluded in the electrode, thereby enhancing the cycle characteristics of the lithium secondary battery.

In addition, the strength of the matrix, which is a continuous phase surrounding the silicon particles, can be further fortified by the magnesium compound.

In addition, the fluorine-containing magnesium compound (61) may be contained more in the outer part (O) than in the center part (C) of the silicon-carbon composite.

In the present specification, referring to FIG. 1(C), the “center part” refers to the region within 50% of the distance from the center of the silicon-carbon composite toward its surface, and the “outer part” refers to the region between the center part (C) and the carbon layer (40).

In addition, the fluorine-containing magnesium compound (61) does not release lithium ions during the charging of a lithium secondary battery. For example, it may be an inactive material that does not occlude or release lithium ions during the charging of a lithium secondary battery.

Lithium ions are released from the silicon particles, whereas lithium ions, which have been steeply increased during charging, are not released from the fluorine-containing magnesium compound. Thus, a matrix comprising a fluorine-containing magnesium compound does not participate in the chemical reaction of a lithium secondary battery, but it may function as a body that suppresses the volume expansion of silicon particles during the charging of the lithium secondary battery.

As the silicon-carbon composite (1) comprises a carbon layer having a single-layer structure on the surface of the core, the strength may be fortified, excellent electrical conductivity may be secured even after the electrode expands during charging and discharging, and side reactions with the electrolyte are suppressed, so that it is possible to enhance the charge and discharge capacity, initial charge and discharge efficiency, and capacity retention rate of a lithium secondary battery.

According to another embodiment, there may be provided a silicon-carbon composite (hereinafter referred to as “silicon-carbon composite (2)”) having a core-shell structure wherein the shell has a multilayer structure comprising at least two carbon layers comprising a first carbon layer and a second carbon layer.

That is, the shell may have a multilayer structure comprising at least two carbon layers comprising a first carbon layer and a second carbon layer.

Specifically, referring back to FIG. 1(D), the silicon-carbon composite (2) may have a core-shell structure, wherein the core may comprise silicon particles (20), a magnesium compound (60), and carbon (45), and the shell may comprise a first carbon layer (40) and a second carbon layer (50) formed on the first carbon layer (40). In addition, the silicon-carbon composite (2) may comprise pores (10) therein.

According to an embodiment, the inside of the silicon-carbon composite may have a structure similar to that of a three-dimensional pomegranate (hereinafter, referred to as a pomegranate structure). In the pomegranate structure, a wall may be formed inside a core in a state in which nano-sized silicon particles and a magnesium compound, for example, magnesium silicate, a fluorine-containing magnesium compound, or a mixture thereof, are combined with each other. In addition, internal pores may be formed as surrounded by these walls. The internal pores may comprise inner pores and open pores, which will be described later.

In addition, the silicon-carbon composite may further comprise silicon oxide (SiO_x, 0.4<x≤2). The silicon oxide (SiO_x, 0.4<x≤2) may be present on the surfaces of the silicon particles.

In addition, the carbon is present on the surface of at least one selected from the group consisting of the silicon particles, silicon oxide (SiO_x, 0.4<x≤2), and magnesium compound as contained inside the core (inside the silicon composite). The carbon may serve as a matrix, and the silicon particles, silicon oxide (SiO_x, 0.4<x≤2), and magnesium compound may be present as dispersed in the carbon matrix. In addition, the carbon may be present on the surface of the core (the surface of the silicon composite) while it forms a carbon layer.

Meanwhile, when the shell is composed of at least two carbon layers, even if cracks are formed on the surface of one of the carbon layers, it is possible to maintain the state in which the carbon layers are electrically connected until the other carbon layer without cracks is completely detached.

The first carbon layer may be present on the surface of the core.

In addition, the first carbon layer may be present on the surface of the silicon particles, silicon aggregates, and/or magnesium compound.

In addition, the shell has a structure that surrounds the core, wherein at least one carbon layer adopted therein, that is, the first carbon layer, or at least two carbon layers comprising the first carbon layer and the second carbon layer, may produce an effect as a buffer layer (cushioning layer).

The buffer layer is effective as a mechanical buffer support surrounding the core. More specifically, the buffer layer may suppress the deterioration, cracking, and volume expansion of the core caused by the mechanical expansion of silicon during charging and discharging.

The specific characteristics of the silicon-carbon composites (1) and (2) are described in more detail in the various examples of the silicon-carbon composite described below.

Physical Properties of the Silicon-Carbon Composite

The silicon-carbon composite may have an average particle diameter (D₅₀) of 2 μm to 15 μm.

In addition, the average particle diameter (D₅₀) of the silicon-carbon composite is a value measured as a diameter-averaged particle diameter (D₅₀), i.e., a particle size or median diameter when the cumulative volume is 50% in particle size distribution measurement according to a laser beam diffraction method. If the average particle diameter (D₅₀) of the silicon-carbon composite is less than the above range, there may be a concern that the dispersibility may be deteriorated due to the aggregation of the silicon-carbon composite particles during the preparation of a negative electrode slurry (i.e., a negative electrode active material composition) using the same. On the other hand, if the average particle diameter (D₅₀) of the silicon-carbon composite exceeds the above range, the expansion of the silicon-carbon composite particles due to the charging of lithium ions becomes severe, and the binding capability between the silicon-carbon composite particles and the binding capability between the particles and the current collector are deteriorated as charging and discharging are repeated, so that the lifespan characteristics may be significantly reduced. In addition, there is a concern that the activity may be deteriorated due to a decrease in the specific surface area.

Specifically, the average particle diameter (D₅₀) of the silicon-carbon composite may be preferably 3 μm to 10 μm, more preferably 3 μm to 8 μm. More specifically, in view of the remarkable improvement effect of the optimization of the average particle diameter (D₅₀) of the silicon-carbon composite, the average particle diameter (D₅₀) of the silicon-carbon composite particles may be 3 μm to 6 μm.

The silicon-carbon composite may have a core-shell structure, and the core may have an average particle diameter of 2 μm to 10 μm, preferably, 2 μm to 8 μm, more preferably, 2 μm to 6 μm.

The silicon-carbon composite may have a specific surface area (Brunauer-Emmett-Teller method; BET) of 2 m²/g to 60 m²/g, preferably, 2 m²/g to 50 m²/g, more preferably, 2 m²/g to 40 m²/g, even more preferably, 3 m²/g to 40 m²/g. If the specific surface area of the silicon-carbon composite is less than the above range, the charge and discharge characteristics of a lithium secondary battery may be deteriorated. If it exceeds the above range, it may be difficult to prepare a negative electrode active slurry suitable for the application to a negative electrode current collector of a lithium secondary battery, the contact area with an electrolyte increases, and the decomposition reaction of the electrolyte may be accelerated, or a side reaction of a lithium secondary battery may be caused.

In addition, the silicon-carbon composite may have a density of 1.8 g/cm³ to 2.6 g/cm³, preferably, 1.8 g/cm³ to 2.55 g/cm³, more preferably, 1.9 g/cm³ to 2.55 g/cm³, even more preferably, 1.9 g/cm³ to 2.5 g/cm³. The density may vary depending on the amount of coverage of the carbon layer, and the strength of the matrix of a negative electrode active material may be enhanced, thereby enhancing the initial efficiency or cycle life characteristics. In such an event, the density may refer to true density.

When the density of the silicon-carbon composite is equal to, or greater than, the lower limit of the above range, cracks in the negative electrode active material powder due to the volume expansion and contraction of the negative electrode active material powder during charging and discharging may be minimized, whereby the cycle deterioration may be suppressed. When the density is equal to, or less than, the upper limit of the above range, the impregnability of an electrolyte is enhanced, which enhances initial charge and discharge capacity.

When the silicon-carbon composite is used as a negative electrode active material in an electrode (negative electrode), the electrode expansion rate may be 55% to 150%, preferably, 55% to 100%, more preferably, 60% to 80%, based on the thickness of the electrode after one cycle of charge and discharge.

The electrode expansion rate may be a value converted into a percentage of the ratio of the thickness (N1) of a press-rolled negative electrode before electrode manufacture and the thickness (N2) of the negative electrode when the cycle lifespan is measured during the lithium secondary battery evaluation using a lithium secondary battery manufactured using the negative electrode and the measurement is terminated at 80% of the cycle life.

$\begin{array}{cc}\mathrm{Electrode}\mathrm{expansion}\mathrm{rate}\left(\%\right)=\mathrm{thickness}\left(N2\right)\mathrm{of}a\mathrm{negative}\mathrm{electrode}\mathrm{when}\mathrm{the}\mathrm{measurement}\mathrm{is}\mathrm{terminated}\mathrm{at}80\%\mathrm{of}\mathrm{the}\mathrm{cycle}\mathrm{life}/\mathrm{thickness}\mathrm{of}a\mathrm{press}-\mathrm{rolled}\mathrm{negative}\mathrm{electrode}\mathrm{before}\mathrm{electrode}\mathrm{manufacture}\left(N1\right)\times 100& \left[\mathrm{Equation}A\right]\end{array}$

In the silicon-carbon composite, the silicon-carbon composite (1) having a single-layer structure in which the shell is composed of a first carbon layer, and the silicon-carbon composite (2) having a multilayer structure in which the shell comprises at least two carbon layers comprising a first carbon layer and a second carbon layer, may also have the same or similar properties.

Hereinafter, each component of the silicon-carbon composite will be described in detail.

Silicon Particles

The silicon-carbon composite according to an embodiment comprises silicon particles.

In addition, the silicon-carbon composite may comprise a silicon aggregate in which silicon particles are combined with each other. Specifically, the silicon-carbon composite may comprise a silicon aggregate having a three-dimensional (3D) structure in which two or more of the silicon particles are combined with each other.

Since the silicon particles charge lithium, the capacity of a lithium secondary battery may decrease if silicon particles are not employed. The silicon particles may be crystalline or amorphous and specifically may be amorphous or in a similar phase thereto.

If the silicon particles are crystalline, a dense composite may be obtained as the size of the crystallites is small, which fortifies the strength of the matrix to prevent cracks. Thus, the initial efficiency or cycle lifespan characteristics of a lithium secondary battery can be further enhanced. In addition, if the silicon particles are amorphous or in a similar phase thereto, the expansion or contraction during the charging and discharging of a lithium secondary battery is small, and battery performance such as capacity characteristics can be further enhanced.

Although the silicon particles have high initial efficiency and battery capacity, there is a very complex crystal change by electrochemically absorbing, storing, and releasing lithium atoms.

The silicon particles may be uniformly distributed inside the silicon-carbon composite. In such a case, excellent mechanical properties such as strength may be achieved.

In addition, the silicon-carbon composite may have a structure in which silicon particles, a magnesium compound, and carbon are uniformly dispersed.

In addition, it is preferable that silicon particles and/or silicon aggregates in which silicon particles are combined with each other in the silicon-carbon composite are uniformly distributed inside the silicon-carbon composite.

In addition, the silicon aggregates as an active material may be uniformly distributed inside the silicon-carbon composite, and silicon particles that are not combined with each other may be contained. The silicon particles and/or silicon aggregates may be uniformly distributed inside the silicon-carbon composite. In such a case, excellent electrochemical properties such as charge and discharge may be achieved.

The silicon particles contained in the silicon-carbon composite according to an embodiment may be in an amorphous form, a crystalline form having a crystallite size of 2 nm to 10 nm, or a mixture thereof.

Specifically, when the silicon-carbon composite is subjected to an X-ray diffraction (Cu-Kα) analysis using copper as a cathode target and calculated by the Scherrer equation based on a full width at half maximum (FWHM) of the diffraction peak of Si (220) around 2θ=47.5°, the silicon particles may have a crystallite size of 2 nm to 10 nm, preferably, 2 nm to 8 nm, more preferably, 2 nm to 5 nm.

If the silicon particles comprise a crystalline form, and if the crystallite size of the silicon particles is less than 2 nm, it is not easy to prepare them, and the yield after etching may be low. In addition, if the crystallite size of the silicon particles exceeds the above range, the micropores cannot adequately suppress the volume expansion of the silicon particles that may occur during charging and discharging, a region that does not contribute to discharging is present, and a reduction in the Coulombic efficiency that stands for the ratio of charge capacity to discharge capacity cannot be suppressed.

The content of silicon (Si) in the silicon-carbon composite may be 30% by weight to 80% by weight, preferably, 40% by weight to 80% by weight, more preferably, 50% by weight to 80% by weight, even more preferably, 50% by weight to 70% by weight, based on the total weight of the silicon-carbon composite.

If the content of silicon (Si) is less than the above range, the amount of an active material for the occlusion and release of lithium is small, which may reduce the charge and discharge capacity of a lithium secondary battery. On the other hand, if it exceeds the above range, the charging and discharge capacity of a lithium secondary battery may be increased, whereas the expansion and contraction of the electrode during charging and discharging may be excessively increased, and the negative electrode active material powder may be further pulverized, which may deteriorate the cycle characteristics.

Silicon Oxide Compound

According to an embodiment, the silicon-carbon composite may further comprise a silicon oxide compound.

In the present invention, the silicon oxide compound is a generic term for an amorphous silicon compound obtained by cooling and precipitation of silicon monoxide formed by the oxidation of metal silicon, the reduction of silicon dioxide, or heating a mixture of silicon dioxide and metal silicon. It may be represented by the formula SiO_x. In the above formula, x may be 0.4≤x≤2, preferably, 0.4≤x<1.5, more preferably, 0.4≤x<1.2.

In addition, when a lower silicon oxide powder, that is, powder of SiO_x where x is 0.4≤x≤1.2 is used, lithium silicate can be uniformly formed in the silicon-carbon composite, thereby alleviating volume expansion. As a result, it is possible to enhance the cycle characteristics of a lithium secondary battery.

In the formula SiO_x, if the value of x is less than the above range, expansion and contraction may be increased and lifespan characteristics may be deteriorated during the charging and discharging of a lithium secondary battery. In addition, if x exceeds the above range, there may be a problem in that the initial efficiency of a lithium secondary battery is decreased as the amount of inactive oxides increases.

In addition, the silicon-carbon composite may further comprise a silicon oxide (SiO_x, 0.4≤x≤2) formed on the surface of the silicon particles and/or silicon aggregates.

Meanwhile, the content of oxygen (O) in the silicon-carbon composite may be 0.5% by weight to 10% by weight, preferably, 0.5% by weight to 8% by weight, more preferably, 0.5% by weight to 6% by weight, based on the total weight of the silicon-carbon composite. If the content of oxygen (O) in the silicon-carbon composite is less than the above range, the degree of expansion may be increased during the charging of a lithium secondary battery. If the content of oxygen (O) in the silicon-carbon composite exceeds the above range, an irreversible reaction with lithium increases, which may deteriorate the initial charge and discharge efficiency of a lithium secondary battery, the negative electrode active material may be readily delaminated from the negative electrode current collector, and the charge and discharge cycle may be deteriorated.

Meanwhile, in general, as the ratio of oxygen decreases in a negative electrode active material that comprises silicon, the ratio of silicon particles as an active material increases, thereby increasing discharge capacity and discharge efficiency, whereas there may arise a problem in that cracks may be formed in the silicon-carbon composite by the volume expansion caused by the charging and discharging of silicon particles. As a result, the cycle characteristics of a lithium secondary battery may be deteriorated. In order to solve this problem, silicon dioxide as an inert material or an inert phase may be contained in the silicon-carbon composite.

Specifically, since the silicon dioxide is an inert material or an inert phase, it is effective in mitigating the volume expansion caused by the charging and discharging of silicon particles. The silicon dioxide is a component for forming a matrix in the silicon-carbon composite, which increases the mechanical strength of the matrix of the silicon-carbon composite to make it more robust and is effective in suppressing the formation of cracks even during volume expansion.

Further, as the content of the silicon dioxide is optimized, it is possible to suppress a decrease in the initial capacity and initial efficiency of a lithium secondary battery and to enhance the cycle characteristics by increasing the strength of the matrix.

Specifically, according to an embodiment, the silicon-carbon composite may comprise the silicon dioxide in an amount of 0.3% to 6% by weight, preferably, 0.3% by weight to 5% by weight, more preferably, 0.5% by weight to 5% by weight, most preferably, 1% by weight to 4% by weight, based on the total weight of the silicon-carbon composite. If the content of the silicon dioxide is less than the above range, the effect of enhancing the cycle characteristics of a lithium secondary battery may be insignificant. If it exceeds the above range, it may be difficult to achieve the desired molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite, and the cycle characteristics or discharge capacity of a lithium secondary battery may be deteriorated.

Magnesium Compound

According to an embodiment, the silicon-carbon composite comprises a magnesium compound.

Since the magnesium compound hardly reacts with lithium ions during the charging and discharging of a lithium secondary battery, it is possible to reduce the expansion and contraction of the electrode when lithium ions are occluded in the electrode, thereby enhancing the cycle characteristics of the lithium secondary battery. In addition, the strength of the matrix, which is a continuous phase surrounding the silicon, can be fortified by the magnesium compound.

The magnesium compound may comprise magnesium silicate, a fluorine-containing magnesium compound, or a mixture thereof.

Specifically, the magnesium compound may comprise a fluorine-containing magnesium compound.

When the silicon-carbon composite comprises a fluorine-containing magnesium compound, the change in crystal lattice constant during the occlusion and release of lithium ions can be reduced.

In general, silicon particles may occlude lithium ions during the charging of a lithium secondary battery to form an alloy, which may increase the lattice constant to thereby expand the volume thereof. During the discharging of the lithium secondary battery, lithium ions are released to return to the original metal nanoparticles, thereby reducing the lattice constant. The fluorine-containing magnesium compound may be a zero-strain material that does not accompany such a change.

The silicon particles may be present between the fluorine-containing magnesium compound particles and may be surrounded by the fluorine-containing magnesium compound particles.

In addition, referring back to FIG. 1, the fluorine-containing magnesium compound may be contained more in the outer part (O) than in the center part (C) of the silicon-carbon composite.

In addition, the fluorine-containing magnesium compound does not release lithium ions during the charging of a lithium secondary battery. For example, it may be an inactive material that does not occlude or release lithium ions during the charging of a lithium secondary battery.

Lithium ions are released from the silicon particles, whereas lithium ions, which have been steeply increased during charging, are not released from the fluorine-containing magnesium compound. Thus, a matrix comprising a fluorine-containing magnesium compound does not participate in the chemical reaction of a lithium secondary battery, but it may function as a body that suppresses the volume expansion of silicon particles during the charging of the lithium secondary battery.

The fluorine-containing magnesium compound may comprise magnesium fluoride (MgF₂), magnesium fluoride silicate (MgSiF₆), or a mixture thereof.

In addition, when the magnesium fluoride is subjected to an X-ray diffraction (Cu-Kα) analysis using copper as a cathode target and calculated by the Scherrer equation based on a full width at half maximum (FWHM) of the diffraction peak of MgF₂ (111) around 2θ=40°, MgF₂ may have a crystallite size of 2 nm to 50 nm, 2 nm to 45 nm, or 10 nm to 38 nm. When the crystallite size of MgF₂ is within the above range, it may function as a body for suppressing the volume expansion of silicon particles during the charging and discharging of a lithium secondary battery.

According to an embodiment, in an X-ray diffraction (Cu-Kα) analysis using copper corresponding to an Si (220) crystal plane of the silicon particles as a cathode target, IB/IA as a ratio of the diffraction peak intensity (IB) corresponding to an MgF₂ (111) crystal plane around 2θ=40.4° to the diffraction peak intensity (IA) of Si (220) around 2θ=47.3° may be greater than 0 to 1. If IB/IA exceeds 1, there may be a problem in that the capacity of a lithium secondary battery is deteriorated.

The magnesium compound may comprise magnesium silicate.

In addition, referring back to FIG. 1, the magnesium silicate may be contained more in the center part (C) than in the outer part (O) of the silicon-carbon composite.

The magnesium silicate may comprise MgSiO₃, Mg₂SiO₄, or a mixture thereof.

In particular, as the silicon-carbon composite comprises MgSiO₃, the Coulombic efficiency or capacity retention rate may be increased.

The content of the magnesium silicate may be 0% by weight to 30% by weight, 0.5% by weight to 25% by weight, or 0.5% by weight to 20% by weight, based on the total weight of the silicon-carbon composite.

In the silicon-carbon composite according to an embodiment of the present invention, magnesium silicate may be converted to a fluorine-containing magnesium compound by etching.

For example, some, most, or all of the magnesium silicate may be converted to a fluorine-containing magnesium compound depending on the etching method or etching degree. More specifically, most of the magnesium silicate may be converted to a fluorine-containing magnesium compound.

Meanwhile, the content of magnesium (Mg) in the silicon-carbon composite may be 0.2% by weight to 20% by weight, preferably, 0.2% by weight to 10% by weight, more preferably, 1% by weight to 9% by weight, based on the total weight of the silicon-carbon composite. When the content of magnesium (Mg) in the silicon-carbon composite satisfies the above range, the initial efficiency of a lithium secondary battery may be enhanced, and it may be advantageous in terms of the effect of enhancing charge and discharge capacity and cycle characteristics, along with handling stability.

As the silicon-carbon composite according to an embodiment comprises the magnesium compound, along with the silicon particles, it is possible to further enhance the performance of a lithium secondary battery.

In addition, the matrix, which is a continuous phase surrounding the silicon particles, is fortified in strength by the magnesium compound to suppress the volume expansion of the silicon particles, so that it is possible to reduce the volume change during occlusion and release of lithium ions and to minimize the occurrence of cracks in the electrode active material even upon repeated charging and discharging. Further, the magnesium compound may be present adjacent to the silicon particles. In such a case, the contact of the silicon particles with the electrolyte solvent is minimized, which reduces the reaction between the silicon particles and the electrolyte solvent, whereby it is possible to prevent a decrease in the initial charge and discharge efficiency and to suppress the expansion of silicon particles, thereby enhancing the capacity retention rate.

Carbon

The silicon-carbon composite according to an embodiment comprises carbon.

According to an embodiment, as the silicon-carbon composite comprises carbon, it is possible to secure adequate electrical conductivity of the silicon-carbon composite and to adjust the specific surface area appropriately. Thus, when it is used as a negative electrode active material of a lithium secondary battery, the lifespan characteristics and capacity of the lithium secondary battery can be enhanced.

In general, the electrical conductivity of a negative electrode active material is an important factor for facilitating electron transfer during an electrochemical reaction. If the composite as a negative electrode active material does not comprise carbon, the electrical conductivity may not reach an appropriate level.

In the silicon-carbon composite, the carbon may be present on the surface of the silicon particles contained in the silicon-carbon composite. In addition, the carbon may be present on the surface, or inside, of the silicon aggregates contained in the silicon-carbon composite.

In addition, the carbon may serve as a matrix, and the silicon particles and the pores may be dispersed in the composite matrix.

Specifically, the silicon-carbon composite may have a sea-island structure in which the silicon particles or closed pores (v) form islands and carbon forms a sea. The pores may be open pores and closed pores. The closed pores may comprise pores whose inside is not coated with carbon.

The state in which the silicon particles or carbon are uniformly dispersed is confirmed through image observation of a dark field image or a bright field image by a transmission electron microscope (TEM).

According to an embodiment, the silicon-carbon composite comprises a silicon composite and a carbon layer on its surface. Specifically, the silicon-carbon composite comprises a silicon composite comprising silicon particles, a magnesium compound, and carbon, and it may comprise a carbon layer comprising carbon on the surface thereof.

In particular, as the silicon-carbon composite comprises a carbon layer, it is possible to enhance the electrical contact between particles and to provide excellent electrical conductivity even after the electrode has been expanded during charging and discharging, so that the performance of a lithium secondary battery can be further enhanced.

In addition, according to an embodiment, the thickness of the carbon layer or the amount of carbon may be controlled, so that it is possible to achieve appropriate electrical conductivity, as well as to prevent a deterioration of the lifespan characteristics, to thereby achieve a negative electrode active material with high capacity.

The content of carbon (C) may be 15% by weight to 60% by weight based on the total weight of the silicon-carbon composite. Specifically, the content of carbon (C) may preferably be 20% by weight to 55% by weight, 20% by weight to 50% by weight, or 25% by weight to 45% by weight, based on the total weight of the silicon-carbon composite.

Specifically, when the silicon-carbon composite has a core-shell structure, the content of the carbon (C) may be the sum of the content of carbon (C) in the core and the amount of carbon (C) in the carbon layer contained in the shell.

If the content of carbon (C) is less than the above range, a sufficient effect of enhancing conductivity cannot be expected, and there is a concern that the electrode lifespan of a lithium secondary battery may be deteriorated. In addition, if the content of carbon (C) exceeds the above range, the discharge capacity of a lithium secondary battery may decrease, and the bulk density may decrease, so that the charge and discharge capacity per unit volume may be deteriorated.

In addition, the silicon-carbon composite comprises carbon inside the core, and the content of carbon (C) inside the core of the silicon-carbon composite may be 16% by weight to 45% by weight, preferably, 18% by weight to 40% by weight, more preferably, 20% by weight to 40% by weight, based on the total weight of the silicon-carbon composite.

When the content of carbon (C) inside the core of the silicon-carbon composite satisfies the above range, sufficient electrical conductivity can be obtained, thereby further enhancing conductivity. As a result, a lithium secondary battery with excellent charge and discharge capacity and cycle characteristics can be obtained.

In addition, the amount of carbon (C) in the carbon layer contained in the shell may be 2% by weight to 10% by weight, preferably, 2% by weight to 8% by weight, more preferably, 3% by weight to 6% by weight, based on the total weight of the silicon-carbon composite.

According to an embodiment, the carbon in the core may comprise at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, reduced graphene oxide, and carbon nanotubes, for example, amorphous carbon. In such a case, the interfacial resistance between the silicon particles and the carbon layer on the surface of the core is reduced, and the intercalation and detachment of lithium are smoothly achieved, thereby securing the high output characteristics of a lithium secondary battery.

In addition, the amorphous carbon inside the core can serve as a support for fixing silicon particles, suppress volume changes of the silicon particles during charge and discharge, and effectively block the contact between the silicon particles and the electrolyte.

In addition, the shell may comprise a carbon layer (first carbon layer) comprising at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, reduced graphene oxide, and carbon nanotubes.

Meanwhile, the thickness of the carbon layer may be 2 nm to 100 nm, preferably, 2 nm to 80 nm, more preferably, 3 nm to 50 nm. When the thickness of the carbon layer is equal to, or greater than, the lower limit of the above range, an enhancement in conductivity may be achieved. When it is equal to, or less than, the upper limit of the above range, a decrease in the capacity of a lithium secondary battery may be suppressed.

The average thickness of the carbon layer may be measured, for example, by the following procedure.

First, a negative electrode active material is observed at an arbitrary magnification by a transmission electron microscope (TEM).

The magnification is preferably, for example, a degree that can be confirmed with the naked eye. Subsequently, the thickness of the carbon layer is measured at arbitrary 15 points. In such an event, it is preferable to select the measurement positions at random widely as much as possible, without concentrating on a specific region. Finally, the average value of the thicknesses of the carbon layer at the 15 points is calculated.

Pores

The silicon-carbon composite may comprise pores on its surface, inside, or both.

When the silicon-carbon composite comprises pores, the volume expansion of silicon particles that may occur during the charging and discharging of a lithium secondary battery can be accommodated through the pores, thereby effectively alleviating and suppressing a problem caused by the volume expansion.

In the present specification, pores may be used interchangeably with voids. In addition, the pores may comprise inner pores (also referred to as closed pores) and/or internal pores.

The inner pore refers to an independent pore that is not connected to other pores because the entire wall of the pore is closed to form a closed structure.

The internal pores comprise inner pores and open pores. At least a part of the wall of a pore may be open to form an open structure, so that it may be connected to other pores, or it may not be so. In addition, they may refer to pores exposed to the outside as they are disposed on the surface of the silicon-carbon composite.

Meanwhile, the silicon-carbon composite comprises pores inside thereof, and the porosity of the silicon-carbon composite may be 12% or less. In such an event, the porosity may refer to internal porosity. Specifically, the porosity may be 12% or less, specifically, 0.01% to 12%, preferably, 0.01% to 10%, more preferably, 0.1% to 8%, most preferably, 0.5% to 5%.

If the porosity of the silicon-carbon composite exceeds the above range, the mechanical strength is reduced due to a large number of pores present in the negative electrode active material, and there is a concern that the negative electrode active material may be collapsed in the process of manufacturing a lithium secondary battery, for example, during the mixing of the negative electrode active material slurry and the press-rolling step after coating. Meanwhile, if the porosity of the silicon-carbon composite is less than the above range, the manufacture of the silicon-carbon composite may be difficult.

When the porosity of the silicon-carbon composite satisfies the above range, it is possible to obtain a buffering effect of volume expansion while maintaining sufficient mechanical strength. Thus, it is possible to minimize the problem of volume expansion due to the use of silicon particles and to enhance the achievement of high capacity and lifespan characteristics of a lithium secondary battery.

In addition, the volume expansion that takes place during the charging and discharging of a lithium secondary battery can be concentrated on the pores rather than the outer part of the negative electrode active material, thereby effectively controlling the volume expansion and enhancing the lifespan characteristics of a lithium secondary battery. In addition, an electrolyte can easily penetrate into the inside of the silicon-carbon composite to enhance the charge and discharge characteristics, so that the performance of a lithium secondary battery can be further enhanced.

The internal pores comprise open pores on the surface of the silicon-carbon composite and inner pores.

The porosity of the silicon-carbon composite may be defined as in the following Relationship 1. The porosity may refer to internal porosity.

$\begin{array}{cc}\mathrm{Porosity}\left(\%\right)=\mathrm{pore}\mathrm{volume}\mathrm{per}\mathrm{unit}\mathrm{mass}/\left(\mathrm{specific}\mathrm{volume}+\mathrm{pore}\mathrm{volume}\mathrm{per}\mathrm{unit}\mathrm{mass}\right)& \left[\mathrm{Relationship}1\right]\end{array}$

The measurement of the porosity is not particularly limited. Based on an embodiment, for example, it may be measured with BELSORP (BET equipment) of BEL JAPAN using an adsorption gas such as nitrogen or by a mercury permeation method (Hg porosimeter).

However, it may be difficult in the present invention to measure an accurate value using a nitrogen adsorption method or a mercury permeation method. This may be a case where the surface of the composite is closed by a carbon coating, so that nitrogen gas does not reach the inner pores. In such a case, the porosity, that is, internal porosity, may be obtained by the sum of a value obtained by adsorption gas (BET) such as nitrogen or a mercury permeation method and the inner porosity of the following Relationship 2.

$\begin{array}{cc}\mathrm{Inner}\mathrm{porosity}\left(\%\right)=\left\{1-\left(\mathrm{measured}\mathrm{density}/\mathrm{calculated}\mathrm{density}\right)\right\}\times 100& \left[\mathrm{Relationship}2\right]\end{array}$

In Relationship 2, the measured density may be measured using helium.

In the silicon-carbon composite, when the inner pores are closed and nitrogen gas or mercury does not reach the inner pores, the porosity measured by the nitrogen adsorption method may be considered to be close to open porosity.

According to an embodiment, most of the pores formed after etching may be present in small amounts or almost absent after the formation of a carbon layer (after carbon coating).

Specifically, in the method for preparing a silicon-carbon composite described below, when a carbon layer is formed, the pores present inside the composite, or inside and on the surface thereof, can be largely filled with carbon. As a result, the silicon-carbon composite may comprise only a small amount of pores.

In addition, the amount of pores, for example, porosity, of the silicon-carbon composite can be controlled by adjusting the etching conditions, such as the etching solution, its concentration, and etching time, in the method for preparing the silicon-carbon composite described below, and by changing the type of gas and the coating temperature during carbon coating to control the coating amount of carbon. The silicon-carbon composite may comprise only a small amount of pores or almost no pores inside thereof by the above adjustment.

Meanwhile, the silicon-carbon composite may have an inner porosity of 0.01% to 8%, preferably, 0.1% to 6%, more preferably, 0.5% to 5%. The inner porosity may be calculated by the above Relationship 2.

When the internal porosity and the inner porosity of the silicon-carbon composite each satisfy the above range, it is possible to obtain a buffering effect of volume expansion while sufficient mechanical strength is maintained when it is applied to a negative electrode active material of a lithium secondary battery. Thus, it is possible to minimize the problem of volume expansion attributed to the use of silicon particles and to optimally enhance the achievement of high capacity and lifespan characteristics.

The silicon-carbon composite may have various structures.

Specifically, the silicon-carbon composite has a core-shell structure, while it comprises a silicon-carbon composite (1) having a single-layer structure in which the shell is composed of a first carbon layer, a silicon-carbon composite (2) having a multilayer structure in which the shell comprises at least two carbon layers comprising a first carbon layer and a second carbon layer, and a combination thereof.

Hereinafter, the characteristics of the silicon-carbon composite (1) and the silicon-carbon composite (2) are specifically described.

Silicon-Carbon Composite (1)

The silicon-carbon composite (1) according to an embodiment may be a silicon-carbon composite having a core-shell structure wherein the shell has a single-layer structure composed of a first carbon layer.

The structure of the core of the silicon-carbon composite (1), each component, and its characteristics are as described above.

The first carbon layer may comprise at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, reduced graphene oxide, and carbon nanotubes.

Specifically, the first carbon layer may use the same components as the type and properties of the first carbon layer of the silicon-carbon composite (2) described below.

The first carbon layer may be present on the surface of the core. In addition, the first carbon layer may be present on the surfaces of the silicon particles, silicon aggregates, and/or magnesium compound.

In addition, the shell is a structure surrounding the core, and the first carbon layer contained therein can suppress the expansion of the core and the growth of silicon particles (silicon crystals) in the core and can further enhance conductivity.

The content of carbon (C) in the silicon-carbon composite (1) may be 15% by weight to 60% by weight, 20% by weight to 55% by weight, 20% by weight to 50% by weight, or 25% by weight to 45% by weight, based on the total weight of the silicon-carbon composite (1). If the content of carbon (C) is less than the above range, a sufficient effect of enhancing conductivity cannot be expected, and there is a concern that the electrode lifespan of a lithium secondary battery may be deteriorated. In addition, if the content of carbon (C) exceeds the above range, the discharge capacity of a lithium secondary battery may decrease, and the bulk density may decrease, so that the charge and discharge capacity per unit volume may be deteriorated.

In addition, the amount of carbon (C) in the first carbon layer in the shell may be 2% by weight to 10% by weight, preferably, 2% by weight to 8% by weight, more preferably, 3% by weight to 6% by weight, based on the total weight of the silicon-carbon composite (1).

When the amount of carbon (C) in the first carbon layer satisfies the above range, the desired effect can be effectively achieved.

The thickness of the first carbon layer may be 2 nm to 80 nm, preferably, 3 nm to 50 nm, more preferably, 3 nm to 30 nm.

If the thickness of the first carbon layer is less than the above range, the electrical conductivity of the first carbon layer is too low; thus, when it is applied to a negative electrode active material, there may be a problem in that the reactivity with an electrolyte is increased, and the initial efficiency of a lithium secondary battery is deteriorated. Further, the effect of suppressing the volume expansion of the silicon particles and the capacity enhancement may be insignificant, and a uniform coating may not be obtained. In addition, if the thickness of the first carbon layer exceeds the above range, the mobility of lithium ions may be hindered, which increases resistance, and the adhesive strength to the core may be deteriorated.

Silicon-Carbon Composite (2)

The silicon-carbon composite (2) according to an embodiment may be a silicon-carbon composite having a core-shell structure wherein the shell has a multilayer structure comprising at least two carbon layers comprising a first carbon layer and a second carbon layer.

The structure of the core of the silicon-carbon composite (2), each component, and its characteristics are as described above.

The shell of the silicon-carbon composite (2) may comprise at least two carbon layers comprising a first carbon layer and a second carbon layer formed on the first carbon layer.

The first carbon layer may be present on the surface of the core. In addition, the first carbon layer may be present on the surfaces of the silicon particles, silicon aggregates, and/or magnesium compound.

The first carbon layer may comprise at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, reduced graphene oxide, and carbon nanotubes. Here, the chemical vapor graphene refers to graphene formed by chemical thermal decomposition deposition (chemical vapor deposition, CVD).

Specifically, the first carbon layer may comprise at least one selected from the group consisting of amorphous carbon, crystalline carbon, and chemical vapor graphene. For example, when the amorphous carbon or crystalline carbon is disposed on the core to form a first carbon layer, the strength of the first carbon layer is appropriately maintained, and the expansion of the core and the growth of silicon crystals in the core can be suppressed. In addition, when the crystalline carbon is disposed on the core to form a first carbon layer, the conductivity of a negative electrode active material can be further enhanced.

For example, when the carbon inside the core and the first carbon layer on the surface of the core comprise amorphous carbon, it is preferable that the thickness of the first carbon layer is 2 nm or more or 3 nm or more.

If the thickness of the first carbon layer is less than the above range, the effect of suppressing volume change may be insufficient, and the volume change due to charging and discharging may increase, thereby deteriorating the cycle performance of a lithium secondary battery.

In particular, according to an embodiment, when the carbon inside the core and the first carbon layer on the surface of the core comprise amorphous carbon, the buffering property against volume expansion is further enhanced due to low crystallinity, and the conductivity is further increased, so that the output characteristics and cycle characteristics of a lithium secondary battery can be further enhanced.

The amorphous carbon may comprise a substance selected from the group consisting of soft carbon (low-temperature calcined carbon), hard carbon, pitch carbide, mesophase pitch carbide, calcined coke, and a combination thereof.

In addition, in a Raman spectrum obtained by Raman spectroscopy of the silicon-carbon composite (2) in which the first carbon layer on the surface of the core comprises amorphous carbon, when the intensity of the D band peak appearing in the range of 1,280 cm⁻¹ to 1,400 cm⁻¹ is ID, and the intensity of the G band peak appearing in the range of 1,500 cm⁻¹ to 1,660 cm⁻¹ is IG, the following Relationship 3 is satisfied.

$\begin{array}{cc}\mathrm{ID}/\mathrm{IG}>3& \left[\mathrm{Relationship}3\right]\end{array}$

ID/IG may preferably be 3 to 5, more preferably 3 to 4.

The G band peak is a peak having a Raman shift of, for example, about 1,550 cm⁻¹ to about 1,610 cm⁻¹ (e.g., around 1,580 cm⁻¹), and it may be a peak attributed to graphite. In addition, the D band peak is a peak having a Raman shift of, for example, about 1,290 cm⁻¹ to about 1,350 cm⁻¹ (e.g., around 1,320 cm⁻¹), and it may be a peak attributed to amorphous carbon.

That is, the value of ID/IG may stand for the ratio of amorphous carbon to graphite in the carbon layer. In addition, the value of ID/IG indicates the degree of crystallinity of the carbon layer. A higher value of ID/IG may indicate a higher proportion of amorphous carbon.

If ID/IG is less than the above range, the cycle characteristics of a lithium secondary battery may deteriorate.

The Raman spectrum is measured at a laser wavelength of 780 nm using a microscopic Raman spectrometer (Nicolet Almega Ga XR manufactured by Thermo Fisher Scientific).

In the Raman spectrum of the first carbon layer, the intensities of the D band peak and the G band peak may be obtained as follows.

The Raman spectrum, which is composed of the sum of the peak of the D band and the peak of the G band, is divided into two peaks as fitted in a Gaussian function or a Gaussian/Lorenz mixed function using the nonlinear least squares method. The integrated intensities of the obtained D band and G band are obtained, and the peak intensities of the two divided D band peak and the G band peak are calculated, respectively.

Meanwhile, the crystalline carbon may comprise natural graphite, artificial graphite, expandable graphite, graphene, carbon black, fullerene soot, or a combination thereof, but it is not limited thereto.

Natural graphite may be naturally produced graphite and may include flake graphite, high crystalline graphite, microcrystalline or crypto crystalline (amorphous) graphite, or the like.

Artificial graphite is artificially synthesized graphite and is made by heating amorphous carbon at high temperatures. It may be primary graphite, electro graphite, secondary graphite, graphite fiber, or the like. Expandable graphite is made by intercalating chemicals such as acid or alkali between the layers and heating them to solidify the vertical layers of the molecular structure. Graphene refers to a single layer of graphite.

The second carbon layer may be present on the first carbon layer.

The second carbon layer may comprise at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, reduced graphene oxide, and carbon nanotubes. In such an event, the first carbon layer and the second carbon layer comprise different types.

According to an embodiment, in the silicon-carbon composite (2), the first carbon layer may comprise at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, and carbon nanotubes, and the second carbon layer may comprise reduced graphene oxide (RGO).

The reduced graphene oxide may be formed by partially or substantially completely reducing graphene oxide.

When the second carbon layer comprises reduced graphene oxide, it can function as a barrier layer or a buffer layer, and the buffer layer is effective as a mechanical buffer support surrounding the core and can enhance adhesion with a binder. More specifically, the buffer layer may suppress the deterioration, cracking, and volume expansion of the core caused by the mechanical expansion of silicon during charging and discharging.

When the silicon-carbon composite (2) is used as a negative electrode active material, the contact resistance between the negative electrode active material layer and the current collector and between the negative electrode active material particles can be lowered, thereby enhancing the intercalation and detachment characteristics of lithium.

In addition, the second carbon layer may further comprise graphite.

According to an embodiment, in the silicon-carbon composite, a first carbon layer and a second carbon layer may be sequentially disposed on the core to form a shell.

The first carbon layer may be formed using chemical thermal decomposition deposition (chemical vapor deposition, CVD). The second carbon layer may be formed using one or more methods selected from a liquid coating method and a spray drying method.

Specifically, in the silicon-carbon composite (2), a first carbon layer may be formed on the core by chemical thermal decomposition deposition, and a second carbon layer comprising reduced graphene oxide may be subsequently formed on the first carbon layer using one or more methods selected from a liquid coating method and a spray drying method.

When the first carbon layer is formed on the core by chemical thermal decomposition deposition, carbon (carbon layer) may be mainly formed in the pores in the core. As carbon is uniformly formed in the inner pores, it is possible to minimize the volume expansion of silicon during charging and discharging and the formation of large silicon particles caused by the sintering of nanosilicon by the volume expansion.

Meanwhile, when the second carbon layer comprising reduced graphene oxide is formed on the first carbon layer using at least one method selected from a liquid coating method and a spray drying method, the carbon content can be readily controlled according to the purpose, and low specific surface area, high conductivity, high capacity, and long lifespan can be achieved.

In the silicon-carbon composite (2), the carbon layers that form the shell may be laminated in various ways as long as the desired effect is not impaired. It may be up to four layers, specifically, two to four layers, preferably, two to three layers, for example, two layers.

When the silicon-carbon composite (2) in which at least two carbon layers are sequentially formed on all, most, or part of the surface of the core is used as a negative electrode active material, it is possible to suppress the release of lithium in the core and to suppress the volume change of silicon particles that occurs during the intercalation and detachment of lithium, thereby maintaining high conductivity and conduction path between the negative electrode active material particles. As a result, it is possible to provide a negative electrode active material and a lithium secondary battery having high charge and discharge capacity and excellent cycle lifespan characteristics.

The content of carbon (C) contained in the silicon-carbon composite (2) may be 18% by weight to 60% by weight, 20% by weight to 60% by weight, 20% by weight to 55% by weight, or 30% by weight to 55% by weight, based on the total weight of the silicon-carbon composite (2).

If the content of carbon (C) in the silicon-carbon composite (2) is less than the above range, the effect of suppressing the volume expansion of a negative electrode active material and enhancing the conductivity thereof may be insignificant. If it exceeds the above range, there may be a problem in that the capacity of a lithium secondary battery may decrease and that lithium ions may hardly be detached.

The content of carbon (C) contained in the core of the silicon-carbon composite (2) may be the content of carbon (C) contained in the core of the silicon-carbon composite.

In addition, in the shell of the silicon-carbon composite (2), the amount of carbon (C) in the first carbon layer may be 2% by weight to 10% by weight, preferably, 2% by weight to 8% by weight, more preferably, 3% by weight to 6% by weight, based on the total weight of the silicon-carbon composite (2).

In addition, in the shell of the silicon-carbon composite (2), the amount of carbon (C) in the second carbon layer may be 2% by weight to 10% by weight, preferably, 2% by weight to 8% by weight, more preferably, 2% by weight to 6% by weight, even more preferably, 2% by weight to 5% by weight, based on the total weight of the silicon-carbon composite (2).

If the content of carbon (C) in the second carbon layer is less than the above range, there may be a problem in that the effect of suppressing the volume expansion of a negative electrode active material and enhancing the conductivity thereof may be insignificant. If it exceeds the above range, there may be a problem in that the capacity of a lithium secondary battery may decrease and that lithium ions may hardly be detached.

Specifically, the shell allows lithium to pass through, but prevents oxygen from passing through. It can prevent oxygen from reacting with silicon and/or a silicon oxide compound in the core. In addition, if the shell is uniformly coated to be formed on the core, there is an effect of suppressing cracking caused by stress due to the rapid volume expansion of silicon. Since cracks are formed irregularly and discontinuously, there is a possibility that the electrolyte does not come into contact with the inside of the negative electrode active material or that a portion that is electrically blocked may be present, which causes a defect in the lithium secondary battery. In this regard, the role of the reduced graphene oxide of the second carbon layer is of significance. Since the reduced graphene oxide has high electrical conductivity, it is possible to maintain a state in which the carbon layers are electrically connected.

Meanwhile, the total thickness of the first carbon layer and the second carbon layer may be 5 nm to 100 nm, preferably, 5 nm to 80 nm, more preferably, 5 nm to 50 nm. When the total thickness of the first carbon layer and the second carbon layer satisfies the above range, even if the volume of the core is changed due to the intercalation and detachment of lithium, it is possible to effectively prevent or alleviate the pulverization of the core and to effectively prevent or alleviate side reactions between silicon and electrolyte, thereby enhancing the performance of a lithium secondary battery.

Specifically, the thickness of the first carbon layer may be 2 nm to 50 nm, preferably, 2 nm to 40 nm, more preferably, 3 nm to 30 nm. If the thickness of the first carbon layer is less than the above range, the electrical conductivity of the first carbon layer is too low; thus, when it is applied to a negative electrode active material, there may be a problem in that the reactivity with an electrolyte is increased, and the initial efficiency of a lithium secondary battery is deteriorated. Further, the effect of suppressing the volume expansion of the silicon particles and the capacity enhancement may be insignificant, and a uniform coating may not be obtained. In addition, if the thickness of the first carbon layer exceeds the above range, the mobility of lithium ions may be hindered, which increases resistance, and the adhesive strength to the core may be deteriorated.

The thickness of the second carbon layer may be 3 nm to 50 nm, preferably, 3 nm to 30 nm, more preferably, 5 nm to 20 nm. If the thickness of the second carbon layer is less than the above range, the electrical conductivity of the second carbon layer is too low; thus, when it is applied to a negative electrode active material, there may be a problem that the reactivity with an electrolyte is increased, and the initial efficiency is deteriorated. In addition, the effect of suppressing the volume expansion of the silicon particles and the capacity enhancement may be insignificant, and a uniform coating may not be obtained. If the thickness of the second carbon layer exceeds the above range, the thickness of the second carbon layer is too thick, the mobility of lithium ions may be hindered, which increases resistance, and the adhesive strength to the core may be deteriorated.

In the silicon-carbon composite (2), it is preferable that the carbon layer comprising at least two carbon layers is formed such that each carbon layer is formed thinly and uniformly along the entire outer shape of the shell while the outer shape of the shell is maintained. However, it may be in the form in which some or most thereof covers the entire surface of the core. When each of the first carbon layer and the second carbon layer is formed thinly and uniformly on the surface of the core, the electrical contact between the particles contained in the negative electrode active material can be enhanced.

The silicon-carbon composite (2) may have a specific surface area (Brunauer-Emmett-Teller; BET) of 2 m²/g to 60 m²/g, 2 m²/g to 30 m²/g, or 2 m2/g to 10 m2/g.

In addition, in a Raman spectrum obtained by Raman spectroscopy of the silicon-carbon composite (2), when the intensity of the 2D band peak appearing in the range of 2,600 cm⁻¹ to 2,760 cm⁻¹ is I2D, the intensity of the G band peak appearing in the range of 1,500 cm⁻¹ to 1,660 cm⁻¹ is IG, and the intensity of the D band peak appearing in the range of 1,280 cm⁻¹ to 1,400 cm⁻¹ is ID, the following Relationship 4 is satisfied.

$\begin{array}{cc}0.5<\left(I2D+\mathrm{IG}\right)/\mathrm{ID}\le 2.& \left[\mathrm{Relationship}4\right]\end{array}$

In addition, the content of oxygen (O) in the reduced graphene oxide in the second carbon layer may be 0.5% by weight to 10% by weight, preferably, 1% by weight to 10% by weight, more preferably, 3% by weight to 10% by weight, based on the total weight of the reduced graphene oxide. When the content of oxygen (O) in the reduced graphene oxide in the second carbon layer satisfies the above range, the degree of reduction of the graphene oxide increases, whereby reduced graphene oxide with high electrical conductivity can be achieved; thus, the effect of a lithium secondary battery can be further enhanced.

The reduced graphene oxide in the second carbon layer may comprise at least one selected from the group consisting of lithium (Li), sodium (Na), and potassium (K) in an amount of 0.1% by weight to 5% by weight, preferably, 0.1% by weight to 4% by weight, more preferably, 0.1% by weight to 3% by weight, based on the total weight of carbon (C) in the second carbon layer. When the reduced graphene oxide in the second carbon layer comprises at least one selected from the group consisting of lithium (Li), sodium (Na), and potassium (K) in the above content, it may be even more advantageous for achieving the desired effects.

The silicon-carbon composite (2) may have an electrode expansion rate of 50% to 150%, preferably, 50% to 100%, more preferably, 55% to 80%, based on the thickness of the electrode after one cycle of charge and discharge.

The silicon-carbon composite (2) comprises at least two carbon layers comprising a first carbon layer and a second carbon layer, thereby further increasing the strength of the shell layer, increasing the bonding (adhesion) between the silicon-carbon composite (2) and the binder, resulting in a reduced electrode expansion rate, and enhancing the capacity retention rate after cycles of a lithium secondary battery.

[Method for Preparing a Silicon-Carbon Composite]

The method for preparing a silicon-carbon composite according to an embodiment comprises a first step of obtaining a silicon composite oxide using a silicon-based raw material and a magnesium-based raw material; a second step of etching the silicon composite oxide using an etching solution containing a fluorine (F) atom-containing compound to obtain a silicon composite; and a third step of coating carbon inside the silicon composite, or coating carbon inside the silicon composite and forming a carbon layer on the surface thereof, using a chemical thermal decomposition deposition method to obtain a silicon-carbon composite.

The preparation method according to an embodiment has an advantage in that mass production is possible through a continuous process with minimized steps.

Specifically, in the method for preparing a silicon-carbon composite, the first step may comprise obtaining a silicon composite oxide using a silicon-based raw material and a magnesium-based raw material. The first step may be carried out by, for example, using the method described in Korean Laid-open Patent Publication No. 2018-0106485.

The silicon-based raw material powder may comprise a powder comprising silicon capable of reacting with lithium, for example, a powder comprising two or more of silicon, silicon oxide, and silicon dioxide.

According to an embodiment, the silicon composite oxide may comprise magnesium silicate.

The content of magnesium (Mg) in the silicon composite oxide obtained in the first step may be 0.2% by weight to 15% by weight, 0.2% by weight to 10% by weight, or 0.2% by weight to 8% by weight, based on the total weight of the silicon composite oxide. When the content of magnesium (Mg) in the silicon composite oxide satisfies the above range, the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite may satisfy the desired range. In such an event, it is possible to further enhance the performance of a lithium secondary battery such as the discharge capacity, initial charge and discharge efficiency, and capacity retention rate of the lithium secondary battery.

The silicon composite oxide may be represented by, for example, the general formula Mg_xSiO_y (0<x≤2, 0.8<y<4.1).

According to an embodiment, the preparation method may further comprise forming a carbon layer on the surface of the silicon composite oxide by using a chemical thermal decomposition deposition method.

Specifically, once a carbon layer has been formed on the surface of the silicon composite oxide, the etching process of the second step may be carried out. In such a case, uniform etching is possible, and the yield of a silicon composite upon etching can be further enhanced.

The step of forming a carbon layer may be carried out by a process similar or identical to the process of coating carbon or forming a carbon layer on the surface in the third step of the method for preparing a silicon-carbon composite to be described below.

In the method for preparing a silicon-carbon composite, the second step may comprise etching the silicon composite oxide using an etching solution containing a fluorine (F) atom-containing compound to obtain a silicon composite.

The etching step may comprise dry etching and wet etching.

If dry etching is used, selective etching may be possible.

Silicon dioxide of the silicon composite oxide is dissolved and eluted by the etching step to thereby form pores. That is, the silicon composite oxide is etched using an etching solution comprising a fluorine (F) atom-containing compound in the etching step to thereby form pores.

In addition, a part, or almost, of the magnesium silicate is converted to a fluorine-containing magnesium compound by the etching step, so that a silicon composite comprising silicon particles and a magnesium compound (e.g., magnesium silicate and a fluorine-containing magnesium compound) may be prepared.

When the silicon composite oxide is etched using the fluorine (F) atom-containing compound (e.g., HF), a part of magnesium silicate is converted to a fluorine-containing magnesium compound, and pores are formed at the same time in the portion from which silicon oxide has been eluted and removed. As a result, a silicon composite comprising silicon particles and a magnesium compound (e.g., magnesium silicate and a fluorine-containing magnesium compound) may be prepared.

According to an embodiment, since silicon dioxide is removed by the etching, whereby the oxygen concentration is significantly reduced, the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms can be significantly reduced.

Meanwhile, in the method for preparing a silicon-carbon composite, pores may be formed in the portion where the silicon dioxide is removed by etching the silicon composite oxide; however, the pores may be filled with carbon by the carbon coating process of the third step described above, so that a small amount of, or almost no, pores may be present.

When a small amount of, or almost no, pores are present, the strength and density of the silicon-carbon composite increase, which preferably increases the discharge capacity per volume of a lithium secondary battery. If pores remain after carbon coating, it is preferable that the porosity of the silicon-carbon composite is 12% or less.

For example, in the etching step in which HF is used, when dry etching is carried out, it may be represented by the following Reaction Schemes G1 and G2, and when wet etching is carried out, it may be represented by the following Reaction Schemes L1a to L2:

MgSiO₃+6HF(gas)→SiF₄(g)+MgF₂+3H₂O  (G1)

Mg₂SiO₄+8HF(gas)→SiF₄(g)+2MgF₂+4H₂O  (G2)

MgSiO₃+6HF(aq.solution)→MgSiF₆+3H₂O  (L1a)

MgSiF₆+2HF(aq.solution)→MgF₂+H₂SiF₆  (L1b)

MgSiO₃+2HF→SiO₂+MgF₂+H₂O  (L1c)

SiO₂+6HF(l)→H₂SiF₆+2H₂O  (L1d)

MgSiO₃+8HF(aq.solution)→MgF₂+H₂SiF₆+3H₂O  (L1)

Mg₂SiO₄+8HF(aq.solution)→MgSiF₆+MgF₂+4H₂O  (L2a)

MgSiF₆+2HF(aq.solution)→MgF₂+H2SiF₆  (L2b)

Mg₂SiO₄+4HF(aq.solution)→SiO₂+2MgF₂+2H₂O  (L2c)

SiO₂+6HF(aq.solution)→H₂SiF₆+2H₂O  (L2d)

Mg₂SiO₄+10HF(aq.solution)→2MgF₂+H₂SiF₆+4H₂O  (L2)

In addition, pores may be considered to be formed by the following Reaction Schemes (2) and (3).

SiO₂+4HF(gas)→SiF₄+2H₂O  (2)

SiO₂+6HF(aq.solution)→H₂SiF₆+2H₂O  (3)

Pores (voids) may be formed where silicon dioxide is dissolved and removed in the form of SiF₄ and H₂SiF₆ by the reaction mechanism as in the above reaction schemes.

In addition, silicon dioxide contained in the silicon composite may be removed depending on the degree of etching, and pores may be formed therein.

In addition, the ratio of O/Si and the specific surface area of the silicon composite before and after etching may significantly vary, respectively.

In addition, the specific surface area and the density of the silicon composite may significantly vary before and after the coating of carbon.

According to an embodiment, the silicon composite upon etching may comprise silicon particles and a magnesium compound such as a fluorine-containing magnesium compound, magnesium silicate, or a combination thereof. In addition, the silicon composite upon etching may further comprise silicon oxide.

In addition, the silicon composite upon etching may hardly show a pattern of magnesium silicate in an X-ray diffraction analysis, and it may comprise silicon particles, silicon oxide, and a magnesium compound such as a fluorine-containing magnesium compound.

Here, etching refers to a process in which the silicon composite oxide is treated with an acidic aqueous solution containing an acid, for example, an etching solution containing a fluorine (F) atom-containing compound in the solution as an acidic aqueous solution.

A commonly used etching solution containing a fluorine atom may be used as the etching solution containing a fluorine (F) atom-containing compound without limitation within a range that does not impair the effects.

Specifically, the fluorine (F) atom-containing compound may comprise at least one selected from the group consisting of HF, NH₄F, and HF₂. As the fluorine (F) atom-containing compound is used, the silicon composite may comprise magnesium fluoride, and the etching step may be carried out more quickly.

The etching solution may further comprise one or more acids selected from the group consisting of organic acids, sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chromic acid.

The stirring temperature (treatment temperature) as the etching conditions is not particularly limited. For example, it may be 10° C. to 90° C., preferably, 10° C. to 80° C., more preferably, 20° C. to 70° C.

The silicon composite may have some remaining silicon oxide and some remaining silicon dioxide.

Specifically, the content of silicon dioxide in the silicon composite may be 0.1 to 5% by weight, preferably, 0.2 to 5% by weight, more preferably, 0.5 to 4% by weight. A negative electrode active material comprising a composite containing silicon dioxide in the above range is preferable because its charge and discharge cycle characteristics are good, and it can provide a lithium secondary battery having a high battery capacity.

In addition, the number of oxygen present on the surface of the silicon composite may be lowered by the etching. That is, it is possible to significantly lower the oxygen fraction on the surface of the silicon composite and to reduce the surface resistance through the etching. As a result, the electrochemical characteristics, particularly, the lifespan characteristics of a lithium secondary battery may be significantly enhanced.

In addition, according to an embodiment, as the selective etching removes silicon dioxide, the silicon composite may comprise silicon (Si) at a very high fraction relative to oxygen (O). That is, the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O/Si) present in the composite may be reduced.

In such a case, a lithium secondary battery having a high capacity and excellent cycle characteristics as well as an improved first charge and discharge efficiency can be obtained.

The silicon composite after etching and before carbon coating may be porous.

In addition, it is characterized in that the average particle diameter of the silicon composite hardly changes by the etching.

The specific surface area of the silicon composite may be increased relative to the specific surface area of the silicon composite oxide before etching.

The molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms present in the silicon composite may be 0.01 to 0.60, preferably, 0.01 to 0.45, more preferably, 0.05 to 0.45, even more preferably, 0.06 to 0.40, most preferably, 0.06 to 0.30. If the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms is outside the above range, it acts as a resistance during the intercalation reaction of lithium, so that the electrochemical characteristics of a lithium secondary battery may be deteriorated. As a result, the electrochemical characteristics, particularly, the lifespan characteristics of the lithium secondary battery may be deteriorated.

The content of oxygen (O) in the silicon composite may be 0.1% by weight to 20% by weight, preferably, 3% by weight to 20% by weight, more preferably, 4.5% by weight to 20% by weight, based on the total weight of the silicon composite. If the content of oxygen (O) in the silicon composite is less than the above range, the degree of expansion is increased during the charging of a lithium secondary battery, which is not preferable because the cycle characteristics are deteriorated. If the content of oxygen (O) in the silicon composite exceeds the above range, there may be a concern that an irreversible reaction with lithium increases, which deteriorates the initial charge and discharge efficiency, it may be readily delaminated from the negative electrode current collector, and the charge and discharge cycle is deteriorated.

Meanwhile, the content of silicon (Si) in the silicon composite may be 30% by weight to 90% by weight, 40% by weight to 90% by weight, or 50% by weight to 85% by weight, based on the total weight of the silicon composite. If the content of silicon (Si) is less than the above range, the amount of an active material for the occlusion and release of lithium is small, which may reduce the charge and discharge capacity of a lithium secondary battery. On the other hand, if it exceeds the above range, the charge and discharge capacity of a lithium secondary battery may be increased, whereas the expansion and contraction of the electrode during charging and discharging may be excessively increased, and the negative electrode active material powder may be further pulverized, which may deteriorate the cycle characteristics of the lithium secondary battery.

The content of magnesium (Mg) in the silicon composite may be 0.2% by weight to 20% by weight, 0.5% by weight to 15% by weight, or 1% by weight to 8% by weight, based on the total weight of the silicon composite.

If the content of magnesium (Mg) is less than the above range, there may be a problem in that the cycle characteristics of a lithium secondary battery are deteriorated. If it exceeds the above range, there may be a problem in that the charge capacity of a lithium secondary battery is reduced.

According to an embodiment, physical properties such as element content and specific surface area may vary before and after the etching step. That is, physical properties such as element content and specific surface area in the silicon composite oxide before the etching step and the silicon composite after the etching step may vary.

The silicon composite may have a specific surface area (Brunauer-Emmett-Teller method; BET) of 100 m²/g to 600 m²/g, 103 m²/g to 550 m²/g, or 200 m²/g to 550 m²/g. If the specific surface area of the silicon composite is less than the above range, the lifespan characteristics of a lithium secondary battery may be deteriorated. If it exceeds the above range, it is difficult to control the heat generated during the etching process in the second step, and the yield of the composite upon etching may be reduced.

The silicon composite may have a density of 1.5 g/cm³ to 2.5 g/cm³, preferably, 1.6 g/cm³ to 2.4 g/cm³, more preferably, 1.8 g/cm³ to 2.4 g/cm³. The measurement of density is as described above. As the density of the silicon composite satisfies the above range, it is possible to suppress the deterioration of cycle characteristics of a lithium secondary battery and to enhance the impregnability of an electrolyte solution, which increases the utilization rate of the negative electrode active material, thereby enhancing the initial charge and discharge capacity.

Meanwhile, the silicon composite may be formed from a silicon composite oxide (Mg_xSiO_y, 0<x≤2, 0.8<y<4.1). It is a composite in which a plurality of silicon particles are uniformly distributed in a composite whose structure is in the form of a single mass, for example, a polyhedral, spherical, or similar shape. It may comprise secondary silicon particles formed by the combination of two or more silicon particles (primary silicon particles) with each other. The magnesium compound may be present on the surface of the silicon particles, present between the silicon particles, or present inside the silicon particles as intercalated thereinto. In addition, the magnesium compound may be present as uniformly dispersed with the silicon particles.

The method for preparing a silicon-carbon composite may further comprise filtering and drying the composite obtained by the etching.

The filtration and drying step may be carried out by a commonly used method.

In the method for preparing a silicon-carbon composite, the third step may comprise coating carbon inside the silicon composite, or coating carbon inside the silicon composite and forming a carbon layer on the surface thereof, using a chemical thermal decomposition deposition method to obtain a silicon-carbon composite.

The electrical contact between the particles of the silicon-carbon composite may be enhanced by the step of formation of a carbon layer. In addition, as the charging and discharging are carried out, excellent electrical conductivity may be imparted even after the electrode is expanded, so that the performance of a lithium secondary battery can be further enhanced. Specifically, the carbon layer may increase the conductivity of the negative electrode active material to enhance the output characteristics and cycle characteristics of a battery and may increase the stress relaxation effect when the volume of the active material is changed.

The carbon layer may comprise at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, reduced graphene oxide, and carbon nanotubes.

The carbon coating in the third step may be carried out by injecting at least one carbon source selected from compounds represented by the following Formulae 1 to 3 and carrying out a reaction in a gaseous state at 400° C. to 1,200° C.:

C_NH_(2N+2−A)[OH]_A  [Formula 1]

• • in Formula 1, N is an integer of 1 to 20, and A is 0 or 1,

C_NH_(2N−B)  [Formula 2]

• • in Formula 2, N is an integer of 2 to 6, and B is an integer of 0 to 2,

C_xH_yO_z  [Formula 3]

• • in Formula 3, x is an integer of 1 to 20, y is an integer of 0 to 25, and z is an integer of 0 to 5.

Specifically, the third step may comprise forming carbon inside the silicon composite and a first carbon layer on its surface by injecting at least one selected from compounds represented by the above Formulae 1 to 3 and carrying out a reaction in a gaseous state at 400° C. to 1,200° C.

In addition, the third step may comprise forming carbon inside the core and a first carbon layer of the shell by injecting at least one selected from a compound represented by the above Formulae 1 to 3 and carrying out a reaction in a gaseous state at the above temperature.

That is, when the third step is performed, a silicon-carbon composite (1) having a core-shell structure can be obtained in which the core comprises silicon particles, a magnesium compound, and carbon, and the shell comprises a carbon layer (first carbon layer)

The compound represented by Formula 1 may be at least one selected from the group consisting of methane, ethane, propane, butane, methanol, ethanol, propanol, propanediol, and butanediol. The compound represented by Formula 2 may be at least one selected from the group consisting of ethylene, acetylene, propylene, butylene, butadiene, and cyclopentene. The compound represented by Formula 3 may be at least one selected from the group consisting of benzene, toluene, xylene, ethylbenzene, naphthalene, anthracene, and dibutyl hydroxytoluene (BHT).

In addition, a carbon coating may be uniformly formed on the surface of the pores inside the silicon-carbon composite. This is preferable since the cycle lifespan is further enhanced.

The carbon source gas may further comprise at least one inert gas selected from hydrogen, nitrogen, helium, and argon. In addition, the reaction may be carried out at, for example, 400° C. to 1,100° C., specifically, 500° C. to 1,100° C., more specifically, 600° C. to 1,000° C.

The reaction time (or thermal treatment time) may be appropriately adjusted depending on the desired amount of carbon coating. For example, the reaction time may be 10 minutes to 100 hours, specifically, 30 minutes to 90 hours, more specifically, 50 minutes to 40 hours, but it is not limited thereto.

In the method for preparing a silicon-carbon composite according to an embodiment, it is possible to form a thin and uniform carbon layer comprising amorphous carbon, crystalline carbon, a carbon nanofiber, chemical vapor graphene, reduced graphene oxide, or a carbon nanotube as a main component on the surface of the silicon composite even at a relatively low temperature through a gas-phase reaction of the carbon source gas. In addition, the detachment reaction in the carbon layer thus formed does not substantially take place.

In addition, since carbon is uniformly formed inside the silicon composite and over the entire surface thereof through the gas-phase reaction, a carbon film (carbon layer) having high crystallinity can be formed.

Thus, when the silicon-carbon composite is used as a negative electrode active material, the electrical conductivity of the negative electrode active material can be enhanced without changing the structure.

In addition, according to an embodiment, when a reactive gas containing the carbon source gas and an inert gas is supplied to the silicon composite, the reactive gas penetrates into the open pores of the silicon composite, and a conductive carbon material such as one or more graphene-containing materials selected from graphene, reduced graphene oxide, and graphene oxide, a carbon nanotube, and a carbon nanofiber is grown on the surface of the silicon composite.

For example, as the reaction time elapses, the conductive carbon material deposited on the surface of silicon in the silicon composite is gradually grown to obtain the silicon-carbon composite.

Meanwhile, according to an embodiment, the specific surface area of the silicon-carbon composite may decrease depending on the amount of carbon coating.

The structure of the graphene-containing material may be a layer, a nanosheet type, or a structure in which several flakes are mixed.

According to an embodiment, the step of forming a carbon layer allows most of the open and closed pores present inside and on the surface of the silicon composite to be filled with carbon. As a result, the porosity of the silicon-carbon composite can be significantly reduced. That is, the silicon-carbon composite may be devoid of pores or may have a small number of pores. For example, since the silicon-carbon composite has almost no pores, the strength and density of the silicon-carbon composite are increased, which can increase the discharge capacity per volume of a lithium secondary battery.

If pores are present in the silicon-carbon composite after the carbon coating, the porosity is as described above.

In addition, according to an embodiment, the silicon-carbon composite may have a three-dimensional (3D) structure in which the silicon-carbon composite particles are combined with each other.

In addition, if a carbon layer comprising a graphene-containing material is uniformly formed over the entire surface of the silicon composite, it is possible to suppress the volume expansion as a graphene-containing material that has enhanced conductivity and is flexible for volume expansion is directly grown on the surface of silicon aggregates, silicon particles, and/or magnesium fluoride. In addition, the formation (coating) of a carbon layer may prevent the direct contact of silicon with the electrolyte, thereby reducing the formation of a solid electrolyte interphase (SEI) layer.

In addition, according to an embodiment, it may further comprise, after the third step (after the formation of a carbon layer in the third step), pulverizing and classifying the silicon-carbon composite such that the average particle diameter of the silicon-carbon composite is 2 μm to 15 μm.

The classification may be carried out to adjust the particle size distribution of the silicon-carbon composite, for which dry classification, wet classification, or classification using a sieve may be used. In the dry classification, the steps of dispersion, separation, collection (separation of solids and gases), and discharge are carried out sequentially or simultaneously using an air stream, in which pretreatment (adjustment of moisture, dispersibility, humidity, and the like) may be carried out prior to the classification so as not to decrease the classification efficiency caused by interference between particles, particle shape, airflow disturbance, velocity distribution, and influence of static electricity, and the like, to thereby adjust the moisture or oxygen concentration in the air stream used. In addition, a desired level of particle size distribution may be obtained by carrying out pulverization and classification of the silicon-carbon composite at one time. After the pulverization, it is effective to divide the coarse powder part and the granular part with a classifier or sieve.

A silicon-carbon composite having an average particle diameter of 2 μm to 15 μm, 3 μm to 10 μm, or 3 μm to 8 μm may be obtained through the pulverization and classification treatment. The silicon-carbon composite may have a Dmin of 0.3 μm or less and a Dmax of 8 μm to 30 μm. Within the above ranges, the specific surface area of the silicon-carbon composite may be reduced, and the initial efficiency and cycle characteristics may be enhanced by about 10% to 20% as compared with before classification.

The silicon-carbon composite upon the pulverization and classification comprises an amorphous grain boundary and a crystal grain boundary together, so that particle collapse upon charging and discharging may be reduced by virtue of the stress relaxation effect of the amorphous grain boundary and the crystal grain boundary.

Meanwhile, the method for preparing a silicon-carbon composite according to an embodiment may further comprise a fourth step of forming a second carbon layer on the first carbon layer using a liquid coating method after forming the first carbon layer.

In the method for preparing a silicon-carbon composite, the silicon-carbon composite (1) comprising the first carbon layer can be obtained as it comprises the first to third steps, and the silicon-carbon composite (2) comprising the first carbon layer and the second carbon layer can be obtained as it comprises the first to fourth steps.

Specifically, referring back to FIG. 1, a silicon composite of FIG. 1(B) can be obtained by the second step comprising the etching process.

For example, as shown in FIG. 1(B), the silicon composite may comprise silicon particles (20) and a plurality of pores (10).

Silicon dioxide contained in the silicon composite may be removed depending on the degree of etching, and pores may be formed therein. Specifically, the silicon composite after the etching and before the carbon coating may be porous.

For example, the porosity (internal porosity) of the silicon composite after the etching and before the carbon coating may preferably be 10% to 60%, more preferably, 15% % to 60%, even more preferably, 20% to 50%.

When the porosity of the silicon composite after the etching and before the carbon coating satisfies the above range, it may be advantageous for achieving the desired effects.

In addition, the number of oxygen present on the surface of the silicon composite may be lowered by the etching. That is, it is possible to significantly lower the oxygen fraction on the surface of the silicon composite and to reduce the surface resistance through the etching. As a result, when the silicon-carbon composite is applied to a negative electrode active material, the electrochemical properties, particularly, the lifespan characteristics of a lithium secondary battery can be remarkably improved.

Meanwhile, a silicon-carbon composite (1) of FIG. 1(C) can be obtained by the third step of using a chemical thermal decomposition deposition method ((1) in FIG. 1). In such an event, in the silicon-carbon composite (1), carbon may be coated inside the silicon composite, or carbon may be coated inside the silicon composite and a carbon layer (first carbon layer) may be formed on the surface thereof.

Specifically, the silicon-carbon composite (1) prepared by the third step may comprise silicon particles (20), specifically, a silicon aggregate in which the silicon particles (20) are combined with (or interconnected to) each other, and a magnesium compound (60), wherein a carbon layer (first carbon layer) (40) may be formed on the surface of the silicon particles (20), and carbon (45) may be formed as it is uniformly dispersed and/or distributed between the silicon particles (20), between the magnesium compounds (60), and between both of them. In addition, the silicon-carbon composite (1) may comprise pores (10) therein.

When a chemical thermal decomposition deposition method is used to form a carbon layer (first carbon layer) in the third step, most of the pores inside the silicon composite are filled with carbon, or, for example, an amorphous carbon layer is uniformly coated, which has the advantage of suppressing volume expansion of silicon and/or sintering of silicon during discharge.

The electrical contact between the particles of the silicon-carbon composite (1) may be enhanced by the step of forming a carbon layer (first carbon layer) in the third step. In addition, as the charge and discharge are carried out, excellent electrical conductivity may be retained even after the electrode is expanded, so that the performance of a lithium secondary battery can be further enhanced.

In addition, a silicon-carbon composite (2) of FIG. 1(D) can be obtained in which a second carbon layer is formed on the first carbon layer by the fourth step of using the liquid coating method ((2) in FIG. 1).

Specifically, the silicon-carbon composite (2) prepared by the fourth step may have a core-shell structure, wherein the core may comprise silicon particles (20), a magnesium compound, and carbon (45), and the shell may comprise a first carbon layer (40) and a second carbon layer (50) on the first carbon layer (40). In addition, the silicon-carbon composite (2) may comprise pores (10) therein. In such an event, the magnesium compound (60) may comprise a fluorine-containing magnesium compound (61), magnesium silicate (62), or a mixture thereof.

The second carbon layer may be formed using reduced graphene oxide. The reduced graphene oxide may be formed by partially or substantially completely reducing graphene oxide.

The formation of the second carbon layer may be carried out using a spray drying method as a liquid coating method. Specifically, a second carbon layer comprising reduced graphene oxide may be formed on the surface of the first carbon layer through a spray drying method (spray dryer), which may form a partial π-π bond or a physical bond between carbon-carbon layers.

When a spray drying method is used to form a second carbon layer, it is possible to control the carbon content by adjusting the concentration of reduced graphene oxide and to carry out the coating at a low temperature, unlike a chemical thermal decomposition deposition method, resulting in the advantages that an increase in the crystal size of silicon caused during carbon coating at a high temperature would not take place.

In the formation of a second carbon layer, graphene oxide or reduced graphene oxide is dispersed in a solvent to prepare a dispersion, in which the concentration of the dispersion may be 0.01 to 50 mg/ml.

In addition, the temperature in the spray drying may be 100° C. to 500° C., specifically, 150° C. to 350° C. In addition, when spray drying is carried out at the above temperature using graphene oxide, the graphene oxide can be reduced to form reduced graphene oxide.

That is, in the preparation of the silicon-carbon composite (2), the first carbon layer may be formed using a chemical thermal decomposition deposition method, and the second carbon layer may be formed using a spray drying method.

The second carbon layer formed using the spray drying method may be thermally treated at 500° C. to 800° C. under inert gas to adjust the oxygen content of the second carbon layer.

According to another embodiment, the formation of the second carbon layer may be carried out using a dry coating method.

In addition, the formation of the second carbon layer may be carried out using both of the dry coating method and the liquid coating method.

Meanwhile, according to an embodiment, once the first carbon layer or the second carbon layer has been formed in any one or more of the third step and the fourth step, a step of carrying out at least one of pulverization and classification of the silicon-carbon composite may be further carried out to have the desired average particle size.

For example, one or more of the silicon composite powders may be combined with each other to form aggregates after the first carbon layer is formed in the third step, and the method may further comprise pulverizing and classifying the silicon-carbon composite such that the average particle diameter thereof is 2 μm to 15 μm. The classification may be carried out to adjust the particle size distribution of the silicon-carbon composite, for which dry classification, wet classification, or classification using a sieve may be used.

In the dry classification, the steps of dispersion, separation, collection (separation of solids and gases), and discharge are carried out sequentially or simultaneously using an air stream, in which pretreatment (adjustment of moisture, dispersibility, humidity, and the like) may be carried out prior to the classification so as not to decrease the classification efficiency caused by interference between particles, particle shape, airflow disturbance, velocity distribution, and influence of static electricity, and the like, to thereby adjust the moisture or oxygen concentration in the air stream used.

In addition, a desired particle size distribution may be obtained by carrying out pulverization and classification at one time. After the pulverization, it is effective to divide the coarse powder part and the granular part with a classifier or sieve.

Specifically, once the first carbon layer and/or second carbon layer has been formed, the pulverization process may be carried out.

Alternatively, a step of performing at least one step of pulverization and classification of the silicon-carbon composite having a core-shell structure obtained in the fourth step may be further carried out (fifth step).

According to an embodiment, as the silicon-carbon composite having a core-shell structure is subjected to pulverization and/or classification, a silicon-carbon composite having the desired average particle size can be obtained.

Specifically, in the fifth step, the silicon-carbon composite (2) obtained in the fourth step may be classified. Alternatively, the silicon-carbon composite (2) may be pulverized. Alternatively, the silicon-carbon composite (2) may be pulverized and classified. Specifically, the silicon-carbon composite (2) obtained in the fourth step may be classified and, if necessary, pulverized. Alternatively, the silicon-carbon composite (2) obtained in the fourth step may be pulverized.

According to the above preparation method, a silicon-carbon composite comprising silicon particles, a magnesium compound, and carbon and having a molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms within a specific range can be provided. When the silicon-carbon composite is used in a negative electrode active material to manufacture a lithium secondary battery, it is possible to significantly enhance the discharge capacity and initial efficiency, along with an excellent capacity retention rate.

In addition, as the silicon-carbon composite comprises at least one carbon layer, it is possible to secure adequate electrical conductivity and to adjust the specific surface area appropriately. Thus, the performance of a lithium secondary battery can be enhanced. Further, the characteristics of a lithium secondary battery can be controlled by varying the structure of the shell of the silicon-carbon composite.

In particular, the silicon-carbon composite comprises at least two carbon layers comprising a first carbon layer and a second carbon layer, thereby further increasing strength, increasing the bonding (adhesion) between the silicon-carbon composite (2) and the binder, resulting in a reduced electrode expansion rate, and further enhancing the capacity retention rate after cycles of a lithium secondary battery.

In addition, the preparation method according to an embodiment has an advantage in that mass production is possible through a continuous process with minimized steps.

The silicon-carbon composite prepared by the above preparation method can be advantageously used in a negative electrode active material and a lithium secondary battery, and it can efficiently achieve the desired effect.

[Negative Electrode Active Material]

The negative electrode active material according to an embodiment may comprise the silicon-carbon composite.

In addition, the negative electrode active material may further comprise a carbon-based negative electrode material, specifically, a graphite-based negative electrode material.

The negative electrode active material may be used as a mixture of the silicon-carbon composite and the carbon-based negative electrode material, for example, a graphite-based negative electrode material. In such an event, the electrical resistance of the negative electrode active material can be reduced, while the expansion stress involved in charging can be relieved at the same time.

The carbon-based negative electrode material may comprise, for example, at least one selected from the group consisting of natural graphite, synthetic graphite, soft carbon, hard carbon, mesocarbon, a carbon fiber, a carbon nanotube, pyrolytic carbon, coke, a glass carbon fiber, a sintered organic high molecular compound, and carbon black.

The content of the carbon-based negative electrode material may be 10% by weight to 95% by weight, preferably, 30% by weight to 90% by weight, more preferably, 30% by weight to 80% by weight, based on the total weight of the negative electrode active material.

In addition, if a silicon-carbon composite containing silicon particles having a crystallite size of 2 nm to 10 nm is used as mixed with graphite-based materials generally having low volume expansion, only the silicon particles do not cause large volume expansion; thus, a lithium secondary battery with excellent cycling characteristics can be obtained.

In addition, the negative electrode active material may further comprise silicon powder having carbon coated on the surface and a composite composed of silicon powder and graphite.

[Lithium Secondary Battery]

According to an embodiment, there is provided a negative electrode comprising the negative electrode active material and a lithium secondary battery comprising the same.

The lithium secondary battery may comprise a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous liquid electrolyte in which a lithium salt is dissolved, wherein the negative electrode may comprise a negative electrode active material comprising the silicon-carbon composite.

The negative electrode may be composed of a negative electrode mixture only or may be composed of a negative electrode current collector and a negative electrode mixture layer (negative electrode active material layer) supported thereon. Similarly, the positive electrode may be composed of a positive electrode mixture only or may be composed of a positive electrode current collector and a positive electrode mixture layer (positive electrode active material layer) supported thereon. In addition, the negative electrode mixture and the positive electrode mixture may each further comprise a conductive agent and a binder.

Materials known in the art may be used as a material constituting the negative electrode current collector and a material constituting the positive electrode current collector. Materials known in the art may be used as a binder and a conductive material added to the negative electrode and the positive electrode.

If the negative electrode is composed of a current collector and an active material layer supported thereon, the negative electrode may be prepared by coating the negative electrode active material composition comprising the silicon-carbon composite on the surface of the current collector and drying it.

In addition, the lithium secondary battery comprises a non-aqueous liquid electrolyte in which the non-aqueous liquid electrolyte may comprise a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent.

A solvent commonly used in the field may be used as a non-aqueous solvent. Specifically, an aprotic organic solvent may be used.

Examples of the aprotic organic solvent include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, cyclic carboxylic acid esters such as furanone, chain carbonates such as diethyl carbonate, ethylmethyl carbonate, and dimethyl carbonate, chain ethers such as 1,2-methoxyethane, 1,2-ethoxyethane, and ethoxymethoxyethane, and cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran. They may be used alone or in combination of two or more.

The lithium secondary battery may comprise a non-aqueous lithium secondary battery.

The negative electrode active material and the lithium secondary battery using the silicon-carbon composite according to an embodiment can enhance charge and discharge capacity, as well as initial charge and discharge efficiency and capacity retention rate.

MODE FOR THE INVENTION

Hereinafter, the present invention will be described in detail with reference to examples. The following examples are only illustrative of the present invention, and the scope of the present invention is not limited thereto.

EXAMPLE

Example 1

Preparation of a Silicon-Carbon Composite

• • (1) Step 1: A silicon composite oxide powder having the element content and physical properties shown in Table 1 below was prepared using a silicon powder, a silicon dioxide powder, and metallic magnesium by the method described in Example 1 of Korean Laid-open Patent Publication No. 2018-0106485.
  • (2) Step 2:50 g of the silicon composite oxide powder was dispersed in water. While it was stirred at a speed of 300 rpm, 650 ml of an aqueous solution of 30% by weight of HF was added as an etching solution to etch the silicon composite oxide powder for 1 hour at room temperature.

The product obtained by the above etching was filtered and dried at 150° C. for 2 hours. Then, in order to control the particle size of the composite, it was pulverized using a mortar to have an average particle diameter of 5.8 μm, to thereby prepare a silicon composite.

• • (4) Step 3: 10 g of the silicon composite was placed inside a tubular electric furnace, and methane gas was injected at a rate of 1 liter/minute. It was maintained at 850° C. for 3 hours and then cooled to room temperature, whereby the inside of the silicon composite was coated with carbon and a carbon layer was formed on its surface, to thereby prepare a silicon-carbon composite having the content of each component and physical properties shown in Table 3 below.
  • (4) Step 4: In order to control the particle size of the silicon-carbon composite, it was pulverized and classified to have an average particle diameter of 6.1 μm by a mechanical method, thereby preparing a silicon-carbon composite (1).

Fabrication of a Lithium Secondary Battery

A negative electrode and a battery (coin cell) comprising the silicon-carbon composite (1) as a negative electrode active material were each prepared.

The negative electrode active material, Super-P as a conductive material, and polyacrylic acid were mixed at a weight ratio of 80:10:10 with water to prepare a negative electrode active material composition having a solids content of 45%.

The negative electrode active material composition was applied to a copper foil having a thickness of 18 μm and dried to prepare an electrode having a thickness of 70 μm. The copper foil coated with the electrode was punched in a circular shape having a diameter of 14 mm to prepare a negative electrode plate for a coin cell.

Meanwhile, a metallic lithium foil having a thickness of 0.3 mm was used as a positive electrode plate.

A porous polyethylene sheet having a thickness of 25 μm was used as a separator. A liquid electrolyte in which LiPF₆ had been dissolved at a concentration of 1 M in a mixed solvent of ethylene carbonate (EC) and diethylene carbonate (DEC) at a volume ratio of 1:1 was used as an electrolyte. The above components were employed to fabricate a coin cell (battery) having a thickness of 3.2 mm and a diameter of 20 mm.

Examples 2 to 9

The same method as in Example 1 was carried out to prepare a silicon-carbon composite (1), and a negative electrode active material and a lithium secondary battery were manufactured using the same, except that a silicon composite oxide and a silicon composite having an element content and physical properties as shown in Tables 1 and 2 below were used, the type and amount of carbon source gas were changed as shown in Table 3, and the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite was adjusted.

Example 10

A silicon-carbon composite (2) having a second carbon layer formed using a spray dryer on the surface of the first carbon layer of the silicon-carbon composite (1) prepared in Example 7 was obtained

Formation of the second carbon layer by a spray dryer is as follows:

• • (1) to (3) Steps 1 to 3: The same procedures as in Example 7 were carried out.
  • (4) Step 4: The silicon-carbon composite with a first carbon layer obtained in Example 7 was added to graphene oxide of 0.5% by weight at the same weight ratio to prepare a graphene oxide dispersion. While the dispersion was injected at 100 ml/minute using a metering pump, spray drying was carried out at a concentration of the dispersion of 1.0 mg/ml and a spray temperature of 250° C., thereby obtaining a silicon-carbon composite comprising a first carbon layer and a second carbon layer.
  • (5) Step 5: In order to control the particle size of the silicon-carbon composite, it was pulverized and classified to have an average particle diameter (D₅₀) as shown in Table 4 by a mechanical method, thereby preparing a final silicon-carbon composite (2).

The silicon-carbon composite (2) was used to manufacture a negative electrode active material and a lithium secondary battery in the same method as in Example 1.

Example 11

The same method as in Example 10 was carried out to prepare a silicon-carbon composite (2), and a negative electrode active material and a lithium secondary battery were manufactured using the same, except that a spray dryer was used on the first carbon layer of the silicon-carbon composite (1) prepared in Example 9, and it was adjusted to have the components and properties as shown in Table 4.

Example 12

The same method as in Example 10 was carried out to prepare a silicon-carbon composite (2), and a negative electrode active material and a lithium secondary battery were manufactured using the same, except that a spray dryer was used on the first carbon layer of the silicon-carbon composite (1) prepared in Example 5, and it was adjusted to have the components and properties as shown in Table 4.

Comparative Example 1

A negative electrode active material and a lithium secondary battery using the same were prepared in the same manner as in Example 1, except that the etching process of step (2) using an etching solution in Example 1 was not carried out, and that the content of each component and the physical properties of the composite were adjusted as shown in Tables 1 to 3 below.

Comparative Example 2

A negative electrode active material and a lithium secondary battery using the same were prepared in the same manner as in Example 1, except that the silicon composite oxide of Example 5 was used, that nitrohydrochloric acid instead of HF was used as an etching solution and etching was carried out for 12 hours at 70° C., and that the amount of carbon source gas was changed to adjust the content of each component and the physical properties of the composite as shown in Tables 1 to 3 below.

Comparative Example 3

The same method as in Example 1 was carried out to prepare a negative electrode active material and a lithium secondary battery were prepared using the same, except that a silicon composite oxide and a silicon composite having an element content and physical properties as shown in Tables 1 and 2 below were used, and the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite and the content of each element were changed as shown in Table 3.

Comparative Example 4

A negative electrode active material and a lithium secondary battery using the same were prepared in the same manner as in Example 8, except that the silicon composite of Example 8 was used, and step 3 using a chemical thermal decomposition deposition method was not carried out.

Test Example

<Test Example 1> Electron Microscope Analysis

The surface of the silicon-carbon composite prepared in Example 1 was observed using a focused ion beam scanning electron microscope (FIB-SEM) photograph (QUANTA 3D FEG) at a magnification of 30,000. The results are shown in FIGS. 2 and 3 and Table A.

Specifically, FIG. 2 is a photograph observing a cross-section of the silicon-carbon composite prepared in Example 1 at a magnification of 30,000 using an FIB-SEM. FIG. 3 is a photograph showing the left outer part A, center part C, and right outer part B of a cross-section of the silicon-carbon composite prepared in Example 1 through spot scanning. The composition of the elements contained in those parts was analyzed. The results are shown in Table A below.

As can be seen from FIG. 3 and Table A, carbon (C), oxygen (O), magnesium (Mg), fluorine (F), and silicon (Si) components were observed in the silicon-carbon composite prepared in Example 1.

	TABLE A
		% by	% by			% by	% by			% by	% by
	Element	weight	element		Element	weight	element		Element	weight	element
Part	C	38.2	57.4	Part	C	37.2	56.3	Part	C	0.2	0.4
A	O	2.3	2.6	B	O	3.1	3.5	C	O	34.8	48.1
	Mg	2.4	1.8		Mg	2.5	1.9		Mg	3.3	3.0
	F	3.5	3.6		F	3.9	3.7		F	0	0
	Si	53.6	34.4		Si	53.3	34.5		Si	61.7	48.5

<Test Example 2> Analysis of the Content of the Component Elements in the Silicon-Carbon Composite

The content of each component element of magnesium (Mg), oxygen (O), and carbon (C) in each of the composites prepared in the Examples and Comparative Examples were analyzed.

The contents of silicon and magnesium (Mg) were each analyzed by inductively coupled plasma (ICP) emission spectroscopy. The contents of oxygen (O) and carbon (C) were each measured by an elemental analyzer.

Meanwhile, the content of MgF₂ was calculated using the contents of oxygen (O), silicon (Si), and magnesium (Mg).

<Test Example 3> Analysis of the Crystallite Size of Silicon Particles

In the silicon-carbon composite, silicon particles were subjected to an X-ray diffraction (Cu-Kα) analysis using copper as a cathode target to calculate the crystallite size of the silicon particles by the Scherrer equation based on a full width at half maximum (FWHM) of the diffraction peak of Si (220) around 2θ=47.5°.

<Test Example 4> Analysis of Density of the Silicon-Carbon Composite

For the density (true density) of each silicon-carbon composite, Acupick II1340 manufactured by Shimadzu Corporation was used, and the measurement was carried out after 200 times of purging with helium gas in a sample holder set at a temperature of 23° C.

<Test Example 5> Measurement of the Average Particle Diameter of the Silicon Composite or Silicon-Carbon Composite

The average particle diameter (D₅₀) of the composite particles prepared in the Examples and Comparative Examples was measured as an average diameter D₅₀, i.e., a particle diameter or median diameter when the cumulative volume is 50% in particle size distribution measurement according to a laser beam diffraction method.

<Test Example 6> Raman Spectroscopic Analysis

The silicon-carbon composites prepared in the Examples were each subjected to a Raman spectroscopic analysis. The Raman analysis was carried out using a micro-Raman analyzer (Renishaw, RM1000-In Via) at 2.41 eV (514 nm).

The G band (1,580 cm⁻¹) and the D band (1,350 cm⁻¹) were observed in the Raman spectra obtained by Raman spectroscopy, from which it was confirmed that the silicon-carbon composites prepared in the Examples comprised carbon having conductivity.

Accordingly, the silicon-carbon composites prepared in the Examples were excellent in conductivity, and the performance of a lithium secondary battery could be remarkably enhanced.

<Test Example 7> Measurement of Electrode Expansion Rate

The thickness (N1) of a press-rolled negative electrode using the negative electrode active material prepared in the Examples before electrode manufacture was measured.

The cycle lifespan of the lithium secondary battery (coin cell) manufactured using the negative electrode was measured, and the measurement was terminated at 80% of the cycle lifespan. Thereafter, the coin cell was disassembled, the negative electrode was separated, and it was immersed in a dimethyl carbonate (DMC) solvent and then taken out. Next, after the solvent was completely dried, the thickness (N2) of the side surface of the separated negative electrode was measured using SEM.

The ratio of the thickness (N1) of the press-rolled negative electrode before the electrode manufacture and the thickness (N2) of the negative electrode whose measurement was terminated at 80% of the cycle lifespan was converted into a percentage and calculated as follows:

$\begin{array}{cc}\mathrm{Electrode}\mathrm{expansion}\mathrm{rate}\left(\%\right)=\mathrm{thickness}\left(N2\right)\mathrm{of}a\mathrm{negative}\mathrm{electrode}\mathrm{when}\mathrm{the}\mathrm{measurement}\mathrm{is}\mathrm{terminated}\mathrm{at}80\%\mathrm{of}\mathrm{the}\mathrm{cycle}\mathrm{life}/\mathrm{thickness}\mathrm{of}a\mathrm{press}-\mathrm{rolled}\mathrm{negative}\mathrm{electrode}\mathrm{before}\mathrm{electrode}\mathrm{manufacture}\left(N1\right)\times 100& \left[\mathrm{Equation}A\right]\end{array}$

<Test Example 8> Measurement of Capacity, Initial Efficiency, and Capacity Retention Rate of the Lithium Secondary Battery

The coin cells (lithium secondary batteries) prepared in the Examples and Comparative Examples were each charged at a constant current of 0.1 C until the voltage reached 0.005 V and discharged at a constant current of 0.1 C until the voltage reached 2.0 V to measure the charge capacity (mAh/g), discharge capacity (mAh/g), and initial efficiency (%). The results are shown in Table 4 below.

$\begin{array}{cc}I\mathrm{nitial}\mathrm{efficiency}\left(\%\right)=\mathrm{discharge}\mathrm{capacity}/\mathrm{charge}\mathrm{capacity}\times 100& \left[\mathrm{Relationship}5\right]\end{array}$

In addition, the coin cells prepared in the Examples and Comparative Examples were each charged and discharged once in the same manner as above and, from the second cycle, charged at a constant current of 0.5 C until the voltage reached 0.005 V and discharged at a constant current of 0.5 C until the voltage reached 2.0 V to measure the cycle characteristics (capacity retention rate for 50 cycles and 100 cycles, %). The results are shown in Tables 4 and 5 below.

$\begin{array}{cc}\mathrm{Capacity}\mathrm{retention}\mathrm{rate}\mathrm{upon}50\mathrm{cycles}\left(\%\right)={51}^{\mathrm{st}}\mathrm{discharge}\mathrm{capacity}/2^{\mathrm{nd}}\mathrm{discharge}\mathrm{capacity}\times 100& \left[\mathrm{Relationship}6\right]\end{array}$ $\begin{array}{cc}\mathrm{Capacity}\mathrm{retention}\mathrm{rate}\mathrm{upon}100\mathrm{cycles}\left(\%\right)={101}^{\mathrm{st}}\mathrm{discharge}\mathrm{capacity}/2^{\mathrm{nd}}\mathrm{discharge}\mathrm{capacity}\times 100& \left[\mathrm{Relationship}7\right]\end{array}$

The characteristics of the secondary batteries using the composites prepared in the Examples and Comparative Examples are summarized in Tables 3 and 4 below.

	TABLE 1
	Example	Comparative Example
	1	2	3	4	5	6	7	8	9	1	2	3	4
Silicon	Mg content (% by weight)	3.3	1.2	12	6.7	3.3	6.7	3.3	12	6.7	3.3
composite	O content (% by weight)	35.1	35.7	29.8	32.6	35.1	32.6	35.1	29.8	32.6	35.1
oxide	D50 (μm)	4.8	5.2	5.4	5.8	4.8	5.8	4.8	5.4	5.8	4.8
	Density (g/cm3)	2.4	2.36	2.51	2.47	2.4	2.47	2.4	2.51	2.47	2.4
	BET (m2/g)	3.1	2.3	7.7	8.7	3.1	8.7	3.1	7.7	8.7	3.1

	TABLE 2
	Example	Comparative Example
	1	2	3	4	5	6	7	8	9	1	2	3	4
Silicon	Oxygen reduction	51.3	47.0	75.9	67.2	56.4	64.7	85.0	93	75	—	—	32	93
composite	rate (%)
	Mg content (% by	4	5.4	4.1	3.4	14.8	7.9	5.7	7.4	8.5		11.6	7.6	7.4
	weight)
	O content (% by	17.1	18.6	8.6	11.7	13	11.5	4.9	2.3	8.3		31.8	21.8	2.3
	weight)
	Si content (% by	75.7	72	82.4	81.3	59.1	72.6	81.8	79.5	73.3		56.6	58.7	79.5
	weight)
	MgF2/Si molar ratio	0.03	0.04	0.04	0.03	0.16	0.08	0.07	0.1	0.1		—	0.05	0.1
	O/Si molar ratio	0.40	0.44	0.18	0.25	0.38	0.28	0.10	0.05	0.20		0.98	0.65	0.05
	Crystallite size of	4.7	4.9	5.3	4.1	5.7	4.8	5.0	4.8	4.9		5.9	4.9	4.8
	silicon particles
	(nm)
	MgF2 (111)/	0.06	0.07	0.07	0.15	0.48	0.07	0.25	0.20	0.13		—	0.22	0.20
	Si (220)
	Density (g/cm3)	2.2	2.25	1.92	1.95	2.34	2.27	2.02	2.27	2.29		2.41	2.26	2.27
	Pore volume	0.44	0.38	0.53	0.47	0.23	0.59	0.60	0.39	0.60		0.02	0.43	0.39
	(cm3/g)
	BET (m2/g)	223	209	503	385	103	204	492	473	263		12.4	136	473

	TABLE 3
	Example
		1	2	3	4	5	6	7
Silicon-	Carbon source gas	CH4	CH4	CH4	CH4	CH4	CH4	CH4
carbon	C content (% by weight)	33.5	33.6	28.2	30.5	45.3	39.5	37.4
composite	Si content (% by weight)	50.3	48.6	59.2	56.5	32.3	43.9	51.1
(1)	Mg content (% by weight)	2.7	3.6	2.9	2.4	8.1	4.8	3.6
	O/Si molar ratio	0.41	0.45	0.19	0.27	0.41	0.29	0.11
	Crystallite size of	5.6	5.5	6.0	4.6	6.7	6.2	5.8
	silicon particles (nm)
	MgF2 crystallite size (nm)	37.1	32.8	34	32.5	39.3	36.8	38.1
	Density (g/cm3)	2.35	2.38	2.19	2.24	2.51	2.42	2.43
	D50 (μm)	6.1	5.1	7.5	6.4	9.2	5.9	8.3
	BET (m2/g)	11.2	12.1	15.8	13.1	21.3	16.7	13.6
	Porosity (%)	1.3	1.3	1.2	1.4	1.2	1.8	1.7
Characteristics	Discharge capacity (mAh/g)	1691	1630	2066	1554	1480	1553	1734
of the	Initial efficiency (%)	86.5	81.7	92.3	86.4	85.6	87.5	86.5
secondary	Capacity retention rate	86.9	83.5	85.1	85.9	84.2	85.1	85.6
battery	upon 50 cycles (%)
	Example	Comparative Example
			8	9	1	2	3	4
	Silicon-	Carbon source gas	CH4	C2H4	CH4	CH4	CH4	—
	carbon	C content (% by weight)	36.2	17	5.8	5.2	35	0
	composite	Si content (% by weight)	50.7	61.3	58	53.7	38.1	79.5
	(1)	Mg content (% by weight)	4.7	7.1	3.1	11	4.8	7.4
		O/Si molar ratio	0.06	0.21	1.0	0.98	0.65	0.05
		Crystallite size of	5.6	5.8	6.0	6.3	5.7	4.8
		silicon particles (nm)
		MgF2 crystallite size (nm)	38.5	37.0	—	—	36.5	38.4
		Density (g/cm3)	2.40	2.49	2.42	2.46	2.38	2.27
		D50 (μm)	5.4	6.2	5.3	6.6	6.5	5.2
		BET (m2/g)	10.4	30	2.1	9.2	12.5	473
		Porosity (%)	1.2	1.4	0.9	1.8	1.3	47
	Characteristics	Discharge capacity (mAh/g)	1683	1940	1582	1291	1530	264
	of the	Initial efficiency (%)	88.9	85.9	77.6	85.1	80.1	52.2
	secondary	Capacity retention rate	85.5	83.1	86.8	83.7	83.8	30.5
	battery	upon 50 cycles (%)

	TABLE 4
	Example
	10	11	12
Shell	First carbon layer	Example 7	Example 9	Example 5
	Second	Type	Reduced	Reduced	Reduced
	carbon		graphene	graphene	graphene
	layer		oxide	oxide	oxide
		Method of formation	Spray drying	Spray drying	Spray drying
		Total weight of Li,	3	4	2
		Na, and K
		(% by weight)
		Carbon content	3.7	5	2
		(% by weight)
		Thickness (nm)	9	12	6
Silicon-carbon	O/Si molar ratio	0.11	0.21	0.41
composite (2)	C content (% by weight)	41	22	47.3
	Mg content (% by weight)	3.2	6.5	7.5
	Silicon particles crystallite size	5.8	5.8	6.7
	(nm)
	Porosity (%)	1	0.8	1
	D50 (μm)	8.4	6.3	9.2
	Density (g/cm3)	2.44	2.51	2.52
	BET (m2/g)	10	15	19
Characteristics	Initial charge and discharge	1630	1850	1450
of the	capacity (mAh/g)
secondary	Electrode expansion rate (%)	60	62	55
battery	Initial efficiency (%)	84.8	84.5	83.9
	Capacity retention rate upon	87.2	83.4	85.8
	100 cycles (%)

As can be seen from Tables 3 and 4 above, the silicon-carbon composites of Examples 1 to 12 each comprised silicon particles, a magnesium compound, and carbon, wherein the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms satisfied a specific range. Thus, the lithium secondary batteries manufactured using the same were significantly enhanced in performance in terms of discharge capacity, initial efficiency, and capacity retention rate as compared with the lithium secondary batteries of Comparative Examples 1 to 4.

Specifically, the lithium secondary batteries of Examples 1 to 9 using the silicon-carbon composite comprising silicon particles, a magnesium compound, and carbon, wherein the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms was 0.06 to 0.45, had a discharge capacity of 1,480 mAh/g to 2,066 mAh/g, an initial efficiency of 81.7% to 92.3%, and a capacity retention rate of 85% or more in most cases, indicating that the performance of the lithium secondary batteries was overall excellent.

Meanwhile, when the lithium secondary battery of Comparative Example 1 in which only etching was not carried out in the process of Example 1 is compared, the secondary battery of Example 1 had a discharge capacity of 1,691 mAh/g, an initial efficiency of 86.5%, and a capacity retention rate of 86.9%, whereas the lithium secondary battery of Comparative Example 1 had a discharge capacity of 1,582 mAh/g and an initial efficiency of 77.6%, indicating that the latter was significantly reduced in the discharge capacity and initial efficiency as compared with the lithium secondary battery of Example 1. This is understood as a result of the fact that in the lithium secondary battery of Comparative Example 1, etching was not carried out in the preparation of the silicon-carbon composite, so that an excessive amount of SiO₂, an irreversible substance, remained.

In addition, in the silicon-carbon composite of Comparative Example 2, in which the etching was carried out using nitrohydrochloric acid instead of HF in the process of Example 5, the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms was about 0.98, which was significantly increased as compared with the silicon-carbon composite of Example 5. The lithium secondary battery of Comparative Example 2 prepared using the same had an initial efficiency similar to that of the silicon-carbon composite of Example 5, whereas it had a discharge capacity of 1,291 mAh/g and a capacity retention rate of 83.7%, which were significantly decreased as compared with the silicon-carbon composite of Example 5 in terms of discharge capacity and capacity retention rate.

Further, in the lithium secondary battery of Comparative Example 3, in which the etching process in the method of Example 6 was changed such that the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms was adjusted to 0.65, the discharge capacity and initial efficiency were significantly decreased as compared with the lithium secondary battery of Example 6.

In addition, in the lithium secondary battery of Comparative Example 4, in which a silicon-carbon composite comprising no carbon was used, all of the discharge capacity, initial efficiency, and capacity retention rate were significantly decreased as compared with the lithium secondary batteries of Examples 1 to 9.

In addition, when the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms was less than 0.01, the preparation of a silicon-carbon composite was difficult, making the measurements impossible.

Meanwhile, in Table 4 above, the silicon-carbon composites (2) of Examples 10 to 12, each comprising two carbon layers comprising a first carbon layer and a second carbon layer, were composites in which a second carbon layer was formed by spray drying on the first carbon layer of the silicon-carbon composites of Examples 7, 9 and 5, respectively. By virtue of the double carbon layers, the strength of the shell layer was further increased, and the combining force (adhesion) between the silicon-carbon composite and the binder was increased, thereby reducing the electrode expansion rate. As a result, reliability was enhanced by maintaining an excellent capacity retention rate even after 100 cycles of the lithium secondary batteries.

Accordingly, the performance of a lithium secondary battery can be uniformly enhanced by controlling the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite. Further, the characteristics of the lithium secondary battery can be controlled by changing the shell structure of the silicon-carbon composite, whereby the desired characteristics of a lithium secondary battery can be achieved, thereby further increasing reliability.

REFERENCE NUMERALS OF THE DRAWINGS

• • 10: pore (void)
  • 20: silicon particle
  • 40: carbon layer (first carbon layer)
  • 45: carbon
  • 50: carbon layer (second carbon layer)
  • 60: magnesium compound
  • 61: fluorine-containing magnesium compound
  • 62: magnesium silicate
  • O: outer part
  • C: center part
  • (1) Chemical thermal decomposition deposition method
  • (2) Liquid coating method

Claims

1. A silicon-carbon composite, which comprises silicon particles, a magnesium compound, and carbon, wherein the molar ratio (O/Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon composite is 0.01 to 0.60.

2. The silicon-carbon composite of claim 1, wherein the silicon-carbon composite has a core-shell structure, the core comprises the silicon particles, magnesium compound, and carbon, and the shell comprises a carbon layer.

3. The silicon-carbon composite of claim 1, wherein the silicon-carbon composite comprises pores inside thereof, and the porosity of the silicon-carbon composite is 12% or less.

4. The silicon-carbon composite of claim 1, wherein the magnesium compound comprises a fluorine-containing magnesium compound, and the fluorine-containing magnesium compound comprises magnesium fluoride (MgF2), magnesium fluoride silicate (MgSiF6), or a mixture thereof.

5. The silicon-carbon composite of claim 4, wherein the magnesium compound may further comprise magnesium silicate, and the magnesium silicate comprises MgSiO3, Mg2SiO4, or a mixture thereof.

6. The silicon-carbon composite of claim 5, wherein the fluorine-containing magnesium compound is contained more in the outer part than in the center part of the silicon-carbon composite, and the magnesium silicate is contained more in the center part than in the outer part of the silicon-carbon composite.

7. The silicon-carbon composite of claim 2, wherein the shell comprises a first carbon layer comprising at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, reduced graphene oxide, and carbon nanotubes.

8. The silicon-carbon composite of claim 2, wherein the shell comprises two or more carbon layers comprising a first carbon layer and a second carbon layer, the first carbon layer comprises at least one selected from the group consisting of amorphous carbon, crystalline carbon, carbon nanofibers, chemical vapor graphene, and carbon nanotubes, andthe second carbon layer comprises reduced graphene oxide.

9. The silicon-carbon composite of claim 1, wherein, based on the total weight of the silicon-carbon composite, the content of magnesium (Mg) is 0.2% by weight to 20% by weight, the content of oxygen (O) is 0.5% by weight to 10% by weight, the content of silicon (Si) is 30% by weight to 80% by weight, and the content of carbon (C) is 15% by weight to 60% by weight, in the silicon-carbon composite.

10. The silicon-carbon composite of claim 1, wherein the silicon-carbon composite further comprises silicon oxide (SiOx, 0.4<x≤2).

11. A method for preparing the silicon-carbon composite of claim 1, which comprises: a first step of obtaining a silicon composite oxide using a silicon-based raw material and a magnesium-based raw material;a second step of etching the silicon composite oxide using an etching solution containing a fluorine (F) atom-containing compound to obtain a silicon composite; anda third step of coating carbon inside the silicon composite, or coating carbon inside the silicon composite and forming a carbon layer on the surface thereof, using a chemical thermal decomposition deposition method to obtain a silicon-carbon composite.

12. The method for preparing the silicon-carbon composite according to claim 11, wherein the third step comprises forming carbon inside the silicon composite and a first carbon layer on its surface by injecting at least one selected from compounds represented by the following Formulae 1 to 3 and carrying out a reaction in a gaseous state at 400° C. to 1,200° C.: CNH(2N+2−A)[OH]A  [Formula 1]in Formula 1, N is an integer of 1 to 20, and A is 0 or 1, CNH(2N−B)  [Formula 2]in Formula 2, N is an integer of 2 to 6, and B is an integer of 0 to 2, CxHyOz  [Formula 3]in Formula 3, x is an integer of 1 to 20, y is an integer of 0 to 25, and z is an integer of 0 to 5.

13. The method for preparing the silicon-carbon composite according to claim 12, which further comprises a fourth step of forming a second carbon layer on the first carbon layer using a liquid coating method after forming the first carbon layer.

14. A negative electrode active material, which comprises the silicon-carbon composite of claim 1.

15. A lithium secondary battery, which comprises the negative electrode active material of claim 14.