Patent Application
20190190020
The present invention relates to a secondary battery technology, and more particularly, to a silicon-based cathode active material and a preparation method thereof.
A secondary battery is a lithium secondary battery that can be charged and discharged using an electrode material having excellent reversibility, and one of most popular commercialized examples is a lithium secondary battery. The lithium secondary battery is expected to be applied as a medium/large-sized power source mounted on a transportation means (e.g. automobiles) or used for power storage of a power supply network (e.g. smart grid), as well as a compact power source for small IT devices (e.g. smart phones, portable computers, and electronic paper).
When a lithium metal is used as a negative electrode (cathode) of a lithium secondary, there is a risk that short-circuit or explosion may occur due to formation of a dendrite. Therefore, instead of the lithium metal, crystalline carbon (e.g. graphite and artificial graphite) soft carbon, hard carbon, and carbon-based active materials to which lithium can be intercalated and deintercalated, are widely used. However, with the expansion of the application of the secondary battery, there is an increasing demand for higher capacity and higher output of a secondary battery. Accordingly, non-carbon negative electrode materials having a capacity of 500 mAh/g or greater (e.g. silicon (Si), tin (Sn), or aluminum (Al)), which can replace carbon-based negative electrode materials having a theoretical capacity of 372 mAh/g and can be alloyed with lithium, are spotlighted.
Among these non-carbon negative electrode materials, silicon has the largest theoretical capacity of about 4,200 mAh/g, and thus the commercialization of silicon is very important in terms of capacity. However, the volume of silicon increases by about four times during charging, and thus, the electrical connection between active materials and the electrical connection between a current collector and the active materials may be destroyed due to the volume change during charging and discharging, and the progress of an irreversible reaction (e.g. formation of a solid electrolyte interface (SEI) layer (e.g. Li2O)) due to corrosion of the active material by the electrolyte, and subsequent deterioration in lifetime is the major obstacle for commercialization.
While many methods have been proposed to achieve a relatively high capacity battery with minimized volume expansion and shrinkage of an active material to improve the lifetime, the most feasible method for the commercialization of a battery is to make silicon particles nano particles. However, even in the cases of nano-sized silicon particles, these nano-sized silicon particles were not sufficient for commercialization because of the undifferentiation of particles due to repeated volume expansion/shrinkage, even at a small degree, and subsequent rapid deterioration of lifetime of the active material. Therefore, for commercialization of silicon materials, it is required to design particles capable of maximizing the capacity of a battery while suppressing volume change during charging/discharging, and a manufacturing technique thereof.
Accordingly, a technical object to be solved in the present invention is to provide a method for preparing silicon-based active material particles having a long lifetime by improving energy density by enhancing irreversible capacity and reducing changes in volume according to charging/discharging, by using a silicon-based material.
Additionally, another technical object to be solved in the present invention is to provide silicon-based active material particles having the advantages described above.
To achieve the above objects, the method for preparing silicon-based active material particles according to an embodiment of the present invention may include a step of providing silicon powder; a step of providing a pre-pulverization mixture, in which the silicon powder is dispersed in a solvent for dispersion containing an antioxidant; a step of applying mechanical compression and shear stress to the silicon powder of the pre-pulverization mixture to refine the silicon powder, thereby forming silicon particles having an oxygen content controlled by the antioxidant; and a step of drying the resulting material containing the silicon particles to obtain silicon-based active material particles.
In an embodiment, the antioxidant ay be selected from the group consisting of alpha lipoic acid, octene, propyl gallate, tocopherol, glutathione, butylated hydroxyl anisole, butylated hydroxy toluene, tertiary butyl hydroquinone, nordihydroguaiaretic acid, sesamol, vitamin E, vitamin C, and quercetin. In another embodiment, the antioxidant may include alpha lipoic acid or octene.
In an embodiment, the antioxidant may he used in an amount of 1 wt % to 10 wt % relative to the weight of the silicon powder. The solvent for dispersion may further include water, deionized water, an alcoholic solvent, or a mixture of two or more thereof.
In an embodiment, the alcoholic solvent may include any one selected from the group consisting of ethyl alcohol, methyl alcohol, glycerol, propylene glycol, isopropyl alcohol, isobutyl alcohol, polyvinyl alcohol, cyclohexanol, octyl alcohol, decanol, hexatecanol, ethylene glycol, 1,2-octanediol, 1,2-dodecanediol, 1,2-and hexadecanediol, or a mixture thereof.
In an embodiment, the step of applying mechanical compression and shear stress may be performed by a mill pulverization process using a composition of abrasive particles along with the solvent for dispersion.
To achieve the above objects, the silicon-based active material particle according to an embodiment may be a silicon-based active material particle which includes a core of the silicon particle and a chemical oxide layer formed on the core of the silicon particle, and the oxygen content relative to the total weight of the silicon-based active material particles may be limited to the range of 6 wt % to 10 wt %.
In an embodiment, the silicon-based active material particle may be formed by a pre-pulverization mixture, which is dispersed in a solvent for dispersion that includes silicon powder and an antioxidant, and the oxygen content may be controlled by the weight of the antioxidant relative to the weight of the silicon powder.
According to an embodiment of the present invention, in the preparation of a silicon-based active material particle, a method for preparing a silicon-based active material particle capable of contributing to the improvement of lifetime of an active material particle may be provided, in which excessive expansion of the core of the silicon particle during charging/discharging and subsequent undifferentiation are prevented and stable formation of solid electrolyte interface (SEI) is induced, by controlling the oxygen content within the silicon particle by providing a pre-pulverization mixture, which is dispersed in a solvent for dispersion that includes silicon powder and an antioxidant.
Additionally, the formation of an oxide film may be controlled by a solvent for dispersion used in the process of refining the silicon powder. As a result, a method for preparing a silicon-based active material particle with a long lifetime may be provided, in which the oxygen content in the active material is reduced and thereby the initial capacity of the battery and the battery efficiency are improved.
Hereinafter, preferred embodiments will be described in detail with reference to accompanying drawings.
Examples of the present invention are provided to more fully describe the present invention to those skilled in the art. The following Examples may be modified in various ways, but the scope of the present invention is not limited to these Examples described hereinbelow. Rather, these Examples are provided so that the present disclosure will be more faithful and complete and fully convey the spirit of the present invention to those skilled in the art.
Additionally, in the drawings, thickness and size of each layer are exaggerated for convenience and clarity of explanation, and like reference numerals refer to like elements in the drawings. As used herein, the term “and/or” includes any and all combinations of any of listed items.
The terminology used herein is used for the purpose of describing particular embodiments and is not intended to be limiting the invention. As used herein, the singular forms may include the plural forms as well, unless the context explicitly indicates otherwise. Additionally, it is apparent that the terms “comprise” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, members, components, and/or groups thereof, but do not exclude the presence or addition of one or more other features, integers, steps, operations, members, components, and/or groups thereof.
The refined silicon particles (SC) have a potato shape as shown in
The silicon particles (SC) micronized by mechanical compression and shear stress may be in the form of irregular particles. Additionally, by the above stress, the micronized silicon particles formed together with the chemical oxide may have low crystallinity or be easily amorphous particles having no crystal domain of several tens nanometers or more. When the size of mechanical compression and shear stress are too small, micronized silicon particles will have large crystal domains of tens of nanometers or more. Additionally, when the mechanical compression and shear stress are too large, the silicon particles are undifferentiated and sufficient capacity as the active material cannot be obtained, and thus stress control in the pulverizing method is required.
The grain type particles shown in
The average particle diameter of the silicon-based active material particles (100P) may be in the range of 30 nm to 300 nm, and preferably in the range of 30 nm to 200 nm. When the average particle diameter of the silicon-based active material particles (100P) is less than 30 nm, the relative proportion of the conductive layer or the conductive material in particle form in the active material slurry becomes large and thus the battery capacity deteriorates, whereas when the average particle diameter of the particle exceeds 300 nm, there is a problem in that it is difficult to amorphize so as to minimize the surface energy.
The silicon-based active material particles (100P) according to an embodiment of the present invention, as shown in
In the silicon-based active material particles (100P), the oxygen content relative to the total weight of the cores of silicon particles (SC) and the total weight of the chemical oxide layer (CO) may be in the range of 5 wt % to 15 wt %, and preferably 6 wt % to 10 wt %. Since the initial charging rate and the capacity retention characteristics are both maintained at 75% or greater within the oxygen content range, silicon-based active material particles suitable for commercialization may be provided. When the oxygen content exceeds 10 wt %, there is a problem in that the initial charging rate is reduced to 75% or less, whereas when the oxygen content is less than 6 wt %, a chemical oxide layer may not be formed to have an appropriate thickness, thereby deteriorating the charging/discharging efficiency of the silicon particles.
In an embodiment, the oxygen content may be measured using an infrared detection technique using a commercial element analyzer (ELTRA ONH-2000). Specifically, the oxygen contained in a sample is converted to carbon dioxide using the sample (2 mg to 10 mg), a calorie of 8 kW, and helium (purity of 99.995%) as a carrier gas, and the amount of oxygen can be measured by measuring the amount of the carbon dioxide generated. The method of measuring the oxygen content using the element analyzer (ELTRA ONH-2000) is only one example, and the measurement method is not limited thereto.
The chemical oxide layer (CO) has an advantage over the silicon oxide layer formed by a conventional thermal oxidation process or thin-film deposition process (e.g. a chemical vapor deposition process) in that the chemical oxide layer (CO) can form a stable phase with minimal influence to crystal structures of silicon particles. Generally, in cases of thermal oxidation and chemical vapor deposition, a high processing temperature hinders amorphization of silicon particles. However, since the chemical oxide layer (CO) can be formed at a low temperature and may be formed under high compression and shear stress, a silicon oxide layer may be formed while pursuing refinement of the silicon particles (SC) and, selectively, amorphization of the silicon particles (SC). As a result, the charging/discharging efficiency of the silicon particles may be improved and lifetime thereof may be increased. The degree of irregularity of the silicon-based active material particles (100P) may be determined by the shape of the cores inside the silicon particles (SC) and may depend on the compressive stress and the shear stress of the refinement process.
In an embodiment, since the chemical oxide layer (CO) of the silicon-based active material particles (100P) is electrically insulative, a conductive layer (CS) may be further formed on the outermost portions of the silicon-based active material particles (100P) so as to reduce contact resistance between the silicon-based active material particles (100P) and to secure an electron moving path. The conductive layer (CS) may include a carbon-based conductive layer (e.g. a graphite layer, a soft carbon layer, or a graphene layer). Additionally, the conductive layer (CS) may be nanoscale particles of conductive metal oxides (e.g. antimony zinc oxide or antimony tin oxide) or layers including the same.
Referring to
In an embodiment, the silicon-based active material complex (SEP) may be comprised of secondary particles, including at least one silicon-based active material particles (200P). The average particle diameter of the silicon-based active material complex (SEP) may be in the range of 1 μm to 10 μm.
The carbon matrix (CM) is for the electrical connection between the active material particles (200P) that are in contact with each other and can reduce the internal resistance to the current collector (not shown). In an embodiment, the carbon matrix (CM) may be crystalline or at least partially amorphous carbon film.
In an embodiment, the carbon matrix (CM) may include graphite, soft carbon, or grapheme. When the carbon matrix (CM) has high crystallinity as in graphite, the carbon matrix (CM) may cause a reaction with an electrolyte, and thus, low crystalline or amorphous carbon film having chemical resistance may be used. Additionally, the carbon matrix (CM) may have a mixture of a conductive graphite structure SP2 and an insulating diamond structure SP3. For the carbon matrix (CM) to have conductivity, the graphite structure SP2 may have a larger mole fraction than the diamond structure SP3, which may be controlled via a heat treatment process.
The silicon-based active material particles (200P) may be comprised of primary particles. In an embodiment, the average particle diameter of the silicon-based active material particles (200P) may be in the range of 30 nm to 300 nm, and preferably, in the range of 30 nm to 200 nm. When the average particle diameter of the silicon-based active material particles (200P) is less than 30 nm, the relative proportion of the conductive layer or the conductive material in particle form in the active material slurry becomes large and thus the battery capacity deteriorates, whereas when the average particle diameter of the particle exceeds 300 nm, there is a problem in that it is difficult to amorphize so as to minimize the surface energy.
In an embodiment, the silicon particles (SC) of the silicon-based active material particles (200P) may correspond to the silicon particles (SC) shown in
Referring to
A pre-pulverization mixture in which the silicon powder is dispersed in a solvent for dispersion in a liquid state containing an antioxidant is provided (S20). In an embodiment, as the solvent for dispersion, a solvent containing an antioxidant may be used so as to control the oxygen content. In an embodiment, the antioxidant may include alpha lipoic acid, octene, propyl gallate, tocopherol, glutathione, butylated hydroxyl anisole, butylated hydroxy toluene, tertiary butyl hydroquinone, nordihydroguaiaretic acid, sesamol, vitamin E, vitamin C, and quercetin.
In an embodiment, the antioxidant may be used in an amount of 1 wt % to 10 wt % relative to the weight of the silicon powder. For example, when the antioxidant is used less than 1 wt % relative to the weight of the silicon powder, the amount of oxygen contained in the silicon-based active material particles may be excessive and thereby the irreversible capacity may be increased. When the antioxidant is used greater than 10 wt % relative to the weight of the silicon powder, a chemical oxide layer (CO) is hardly formed and thus it is difficult to suppress the volume expansion of the silicon-based active material particles thereby resulting in a drastic deterioration of the lifetime of the battery. Accordingly, it is preferred that the antioxidant be used in an amount of 1 wt % to 10 wt % relative to the weight of the silicon powder.
The solvent for dispersion may further include water, deionized water, an alcoholic solvent, or a mixture of two or more thereof. The alcoholic solvent may be at least one selected from the group consisting of ethyl alcohol, methyl alcohol, glycerol, propylene glycol, isopropyl alcohol, isobutyl alcohol, polyvinyl alcohol, cyclohexanol, octyl alcohol, decanol, hexatecanol, ethylene glycol, 1,2-octanediol, 1,2-dodecanediol, and 1,2-hexadecanediol, and a mixture thereof. Preferably, the alcoholic solvent is ethyl alcohol.
Mechanical compression and shear stress are applied to the silicon powder of the pre-pulverization mixture to refine the silicon powder and thereby the silicon particles that will be the cores of the silicon-based active material particles are formed (S30). Such a refinement process is efficient in preparing amorphous silicon particles.
In an embodiment, the application of mechanical compression and shear stress to the silicon powder may be achieved by grinding, which performs pressing and abrasion while simultaneously supplying the pre-pulverization mixture between a spinning abrasive plate and a fixed plate. In another embodiment, the application of mechanical compression and shear stress to the silicon powder may be performed by a milling method, in which the pre-pulverization mixture and abrasive particles are charged into a cylindrical or conical container rotating about a central axis. The abrasive particles may be beads including ceramic particles, metal particles, or a mixture thereof, but the present invention is not limited thereto. The abrasive particles may apply mechanical compression and shear stress to the silicon powder of the dispersion mixture by having an appropriate average size relative to the size of the silicon powder.
In an embodiment, the silicon-based active material particles may be formed from the refined silicon particles while simultaneously applying the mechanical compression and shear stress. In an embodiment, a chemical oxide layer may be further formed on the refined silicon particles. As described above, the chemical oxide layer may include a silicon oxide. The thickness of the chemical oxidation layer may be in the range of 2 nm to 30 nm, and preferably in the range of 3 nm to 15 nm.
In the silicon-based active material particles (see 100P of
Then, silicon complex particles may be obtained by drying the resulting product including the silicon-based active material particles (S40). Preferably, a subsequent step such as a classification step for disintegrating the obtained silicon complex particles may be further performed.
The silicon-based secondary active material particles are formed using at least one of the silicon-based active material particles (S50). The silicon-based active material particles including the silicon particles and a liquid organic compound including a carbon precursor are mixed to form a dispersion solution.
Then, the silicon particles dispersed in the dispersion solution are obtained again and dried to form solid secondary particles. The solid secondary particles are heat treated, and thereby the carbon matrix including the carbon precursor is carbonized to form silicon-based secondary active material particles. These preparation methods are illustrative, and the present invention is not limited thereto.
In an embodiment, as the carbon precursor, any one selected from the group consisting of hydrocarbon-based compounds, alcohol-based compounds, ether-based compounds, and ester-based compounds, or a mixed solution of two or more thereof may be used. In an embodiment, the hydrocarbon may be hexene, nonene, dodecene, pentatetene, toluene, xylene, chlorobenzoic acid, benzene, hexadecyne, tetradecyne, or octadecyne. However, these compounds are illustrative, and other liquid hydrocarbons of linear or branched type having a carbon number in the range of 6 to 20 may be applied.
Additionally, the alcohol may be any one selected from ethyl alcohol, methyl alcohol, glycerol, propylene glycol, isopropyl alcohol, isobutyl alcohol, polyvinyl alcohol, cyclohexanol, octyl alcohol, decanol, hexatecanol, ethylene glycol, 1,2-octanediol, 1,2-dodecanediol, and 1,2-hexadecanediol, or a mixture thereof. As the alcohol-based organic solution, other primary alcohols, secondary alcohols, and tertiary alcohols may be used.
The ether may be cyclic ethers (e.g. octyl ether, butyl ether, hexyl ether, benzyl ether, phenyl ether, decyl ether, ethyl methyl ether, dimethyl ether, diethyl ether, diphenyl ether, tetrahydrofuran, 1,4-dioxane,) and polyethers (e.g. polyethylene glycol (PEG), polypropylene glycol (PPG), polytetramethylene glycol (PTMG), polyoxymethylene (POM), polytetrahydrofuran). The above-described polyethers are exemplary, and other aliphatic or aromatic polyethers may be used as the ether-based organic solvent. The ester may be a cyclic ester (e.g. polyethylene terephthalate, acrylate esters and cellulose acetate, isobutyl acetate, isopropyl acetate, allyl hexanoate, benzyl acetate, bornyl acetate, butyl acetate, and lactone).
In another embodiment, as an additional carbon precursor for increasing the carbon-containing concentration, may further include any one among the carbon-containing natural and synthetic polymer materials that can be dissolved using the liquid organic compound as a solvent, or a mixture thereof. In still another embodiment, any one among additional carbon-containing natural and synthetic polymer materials may be dissolved in a solvent other than the liquid organic compound such as water (e.g. distilled water or deionized).
The carbon-containing polymer materials may be chitosan, glucose, sucrose, maltose, lactose, starch, glycogen, polystyrene (PS), polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyacrylonitrile (PAN), polyethylene (PE), polyacrylic acid (PAA), polyvinylpyrrolidone (PVP), or a mixture thereof. For example, solid secondary particles may be obtained by dissolving the polymer materials (e.g. PAN, PAA, PVP, etc.) in an appropriate solvent, dissolving the silicon-based active material particles in the solvent, obtaining intermediate particles wetted with the polymer material, followed by drying.
Hereinafter, the characteristics and advantages of the present invention will be described with specific Experimental Examples selected from the above-mentioned materials and the composition ratios disclosed thereto in Examples described above. The following disclosure is for illustrative purposes only and the present invention is not to be construed as limiting the invention thereto:
A commercially available polysilicon powder having an average diameter of about 5 μm was prepared and dispersed in a solvent for dispersion, which contained 95 wt % of ethanol and 5 wt % each of alpha lipoic acid octene, propyl gallate,
tocopherol, glutathione, butylated hydroxyl anisole, and phenol, and thereby a pre-pulverization mixture was prepared. A time-controlled mill pulverization process, which can apply compressive and shear stress to the pre-pulverization mixture, was repeated at least once and thereby the silicon-based active material particles were prepared. The amounts of the alpha lipoic acid and the ethanol used are values relative to 100 parts by weight (wt %) relative to silicon powder.
A commercially available polysilicon powder having an average diameter of about 5 μm was prepared and the polysilicon powder was dissolved in a solvent for dispersion, which contained 100 wt % of ethanol, and thereby a pre-pulverization mixture was prepared. A time-controlled mill pulverization process, which can apply compressive and shear stress to the pre-pulverization mixture, was repeated at least once and thereby the silicon-based active material particles were prepared.
Battery characteristics were evaluated for samples containing silicon cathode active materials according to
Referring to Table 1 and
According to an embodiment of the present invention, in the preparation of a silicon-based active material particle, a method for preparing a silicon-based active material particle capable of contributing to the improvement of lifetime of an active material particle may be provided, in which excessive expansion of the core of the silicon particle during charging/discharging and subsequent undifferentiation are prevented and stable formation of solid electrolyte interface (SEI) is induced, by controlling the oxygen content within the silicon particle by providing a pre-pulverization mixture, which is dispersed in a solvent for dispersion that includes silicon powder and an antioxidant.
Additionally, the formation of an oxide film may be controlled by a solvent for dispersion used in the process of refining the silicon powder. As a result, a method for preparing a silicon-based active material particle with a long lifetime may be provided, in which the oxygen content in the active material is reduced and thereby the initial capacity of the battery and the battery efficiency are improved.
The present invention described above is not limited to exemplary embodiments and accompanying drawings thereof, and it will be apparent to those skilled in the art that various substitutions, modifications, and changes can be made hereto without departing from the technical concept of the present invention defined in the following claims.
100P, 200P: silicon-based active material particles
SC: silicon particles
PS: plane surface
CS: conductive layer
CO: chemical oxide layer
SEP: silicon-based active material complex
CM: carbon matrix
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2016-0106853 | Aug 2016 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2017/009223 | 8/23/2017 | WO | 00 |
