Patent Application
20230387413
The present invention relates to a lithium secondary battery containing a positive electrode additive capable of supplementing lithium ions lost in an irreversible reaction and a method of manufacturing the same.
As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries that have a high energy density, a high operating potential, a long cycle life and a low self-discharging rate have been widely studied, also commercialized and used in various fields.
Recently, as lithium secondary batteries are used as a power source for medium-to-large devices such as electric vehicles, the high capacity, high energy density and low cost of a lithium secondary battery are further required.
Accordingly, research on an irreversible positive electrode additive that can improve capacity during subsequent charging/discharging by supplementing lithium ions lost by an irreversible reaction occurring at a negative electrode in initial charging has been actively conducted, and irreversible additives such as Li2NiO2 and Li6CoO4 have been developed.
However, such a conventional irreversible additive is generally prepared by reacting an improve of lithium oxide and a precursor such as cobalt oxide or nickel oxide. The irreversible additive prepared as described above is structurally unstable and generates a large amount of oxygen gas (O2) as charging progresses, and the generated oxygen gas may cause volume expansion of an electrode assembly to be served as one of the main causes triggering the decrease in battery performance.
In addition, such an irreversible additive may be transformed into a thermally unstable structure when stored at a high temperature of 60° C. or more after initial charging/discharging, and thus oxygen gas may be additionally released, and there is a limit to self-discharging of a battery.
Therefore, it is necessary to develop a lithium secondary battery that uses an irreversible additive in a positive electrode not only to realize a high operating voltage, but also to enhance the safety of the battery and improve self-discharging due to reduced oxygen gas generation in activation and subsequent high-temperature storage.
Therefore, the present invention is directed to providing a lithium secondary battery, which uses an irreversible additive in a positive electrode not only to realize a high operating voltage, but also to enhance the safety of the battery and improve self-discharging due to reduced oxygen gas generation in activation and subsequent high-temperature storage.
To solve the above problems,
LipCo(1-q)M1qO4 [Formula 1]
M1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and
Specifically, the ratio (CC/DC) of initial charge capacity (CC) to initial discharge capacity (DC) during initial charging/discharging may be 60 to 80.
In addition, the positive electrode mixture layer may have a weight change ratio before and after initial charging/discharging of 0.01 to 2.00%.
Moreover, the positive electrode additive contained in the positive electrode mixture layer may have a tetragonal structure with a space group of P42/nmc, and the content of the positive electrode additive may be 0.01 to 5 parts by weight with respect to a total of 100 parts by weight of the positive electrode mixture layer.
In addition, the positive electrode active material contained in the positive electrode mixture layer may be a lithium metal composite oxide represented by Formula 2 below:
Lix[NiyCozMnwM2v]Ou [Formula 2]
In addition, the positive electrode mixture layer may include 0.1 to 5 parts by weight of a conductive material with respect to the total weight, and as a usable conductive material, one or more selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber may be included.
In addition, the negative electrode mixture layer includes a carbon material and a silicon material, and the content of the silicon material may be 1 to 20 parts by weight with respect to 100 parts by weight of the negative electrode mixture layer.
Here, the carbon material may include one or more selected from the group consisting of natural graphite, artificial graphite, graphene, carbon nanotubes, carbon black, acetylene black, Ketjen black and carbon fiber.
In addition, the silicon material may include one or more of a silicon (Si) particle and a silicon oxide (SiOx, 1<x≤2) particle.
Further, another aspect of the present invention provides a method of manufacturing a lithium secondary battery, which includes
LipCo(1-q)M1qO4 [Formula 1]
In Formula 1,
M1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and
Here, the initial charging step may be performed by first activation for applying a current of 0.05C to 0.2C to a lithium secondary battery to charge to an SOC of 30% or less; second activation for applying a current of 0.3C to 0.5C to the lithium secondary battery that has undergone the first activation to charge to an SOC of more than 30% and less than 70%; and third activation for applying a current of 0.6C to 0.9C to the lithium secondary battery that has undergone the second activation to charge to an SOC of 70% or more.
In addition, the initial charging may be performed at a temperature condition of 20 to 70° C., and under a pressure condition of 5 to 900 kgf/cm2 for the lithium secondary battery.
In a lithium secondary battery according to the present invention, a positive electrode additive represented by Formula 1 as an irreversible additive is included in a positive electrode mixture layer, and a ratio (CC/DC) of an initial charge capacity (CC) to an initial discharge capacity (DC) is adjusted within a specific range, thereby reducing the amount of oxygen gas generated in the charging/discharging of the lithium secondary battery, and at the same time, preventing self-discharging and improving an operating voltage by inhibiting a decrease in open circuit voltage (OCV) of the battery in initial charging/discharging and/or subsequent high-temperature storage. Therefore, the lithium secondary battery including the same can be effectively used as a power source for mid-to-large devices such as electric vehicles.
The present invention may have various modifications and various examples, and thus specific examples are illustrated in the drawings and described in detail in the detailed description.
However, it should be understood that the present invention is not limited to specific embodiments, and includes all modifications, equivalents or alternatives within the spirit and technical scope of the present invention.
The terms “comprise,” “include” and “have” used herein designate the presence of characteristics, numbers, steps, actions, components or members described in the specification or a combination thereof, but it should be understood that these terms do not preclude the possibility of the presence or addition of one or more other characteristics, numbers, steps, actions, components, members or a combination thereof.
In addition, when a part of a layer, film, region or plate is disposed “on” another part, this includes not only a case in which one part is disposed “directly on” another part, but also a case in which still another part is interposed therebetween.
In contrast, when a part of a layer, film, region or plate is disposed “under” another part, this includes not only a case in which one part is disposed “directly under” another part, but also a case in which still another part is interposed therebetween. In addition, in this application, “on” may include not only a case of disposed on an upper part but also a case of disposed on a lower part.
Moreover, the “main component” used herein may be a component contained at 50 wt % or more, 60 wt % or more, 70 wt % or more, 80 wt % or more, 90 wt % or more, 95 wt % or more, or 97.5 wt % or more with respect to the total weight of a composition or specific component, and in some cases, when the main component constitutes the entire composition or specific component, it may be contained at 100 wt %.
In addition, the “Ah” used herein refers to a capacity unit of a lithium secondary battery, and is also called “ampere hour,” meaning a current amount per hour. For example, when the battery capacity is “3000 μmAh,” it means that a battery can be discharged with a current of 3000 μmA for 1 hour.
Hereinafter, the present invention will be described in further detail.
Lithium Secondary Battery
In one embodiment of the present invention, a lithium secondary battery which includes a positive electrode including a positive electrode current collector; and a positive electrode mixture layer disposed on the positive electrode current collector and containing a positive electrode active material and a positive electrode additive represented by Formula 1 below, and a negative electrode including a negative electrode current collector; and a negative electrode mixture layer disposed on the negative electrode current collector, and wherein the positive electrode additive has a ratio (CC/DC) of an initial charge capacity (CC) to an initial discharge capacity (DC) of 50 to 100:
LipCo(1-q)M1qO4 [Formula 1]
In Formula 1,
M1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and
p and q are 5≤p≤7 and 0≤q≤0.4, respectively.
The lithium secondary battery according to the present invention includes a positive electrode and a negative electrode, and a separator may be interposed between the positive electrode and the negative electrode or excluded. In addition, the positive electrode and the negative electrode has a structure impregnated with an electrolyte for the migration of lithium ions (Li+) between them.
Here, the positive electrode may include a positive electrode mixture layer formed by coating, drying and pressing a positive electrode slurry including a positive electrode active material and a positive electrode additive on the positive electrode current collector, and the positive electrode mixture layer may selectively further include a conductive material, a binder, and other additives as needed.
The positive electrode active material may include a lithium metal composite oxide including three or more types of elements selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn) and aluminum (Al), and in some cases, the lithium metal composite oxide may have a form in which another transition metal (M1) is doped. For example, the positive electrode active material may be a lithium metal composite oxide represented by Formula 2 below enabling reversible intercalation/deintercalation:
Lix[NiyCozMnwM2v]Ou [Formula 2]
In Formula 2,
M2 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and
x, y, z, w, v and u are 1.0<x≤1.30, 0.1≤y≤0.95, 0.01≤z≤0.5, 0≤w≤0.5, 0≤v≤0.2, and 1.5≤u≤4.5, respectively.
The lithium metal composite oxide represented by Formula 2 may be a composite metal oxide including lithium and nickel, and may include one or more compounds selected from the group consisting of LiNi1/3Co1/3Mn1/3O2, LiNi0.8Co0.1Mn0.1, LiNi0.6Co0.2Mn0.2O2, LiNi0.9Co0.05Mn0.05O2, LiNi0.6Co0.2Mn0.1Al0.1O2, LiNi0.06Co0.2Mn0.15Al0.05O2, and LiNi0.7Co0.1Mn0.1Al0.1O2.
In addition, the content of the positive electrode active material may be 85 to 95 parts by weight, specifically, 88 to 95 parts by weight, 90 to 95 parts by weight, 86 to 90 parts by weight, or 92 to 95 parts by weight with respect to 100 parts by weight of the positive electrode mixture layer.
Moreover, the positive electrode mixture layer may include a positive electrode additive imparting an irreversible capacity along with a positive electrode active material exhibiting electrical activity, wherein the positive electrode additive may include a lithium metal oxide represented by Formula 1 below:
LipCo(1-q)M1qO4 [Formula 1]
M1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and
p and q are5≤p≤7 and 0≤q≤0.4, respectively.
The positive electrode additive may contain lithium in excess to provide lithium for lithium consumption caused by an irreversible, chemical and physical reaction at a negative electrode upon initial charging, i.e., activation, thereby increasing charge capacity, reducing irreversible capacity, and improving lifetime characteristics.
Among positive electrode additives, the positive electrode additive represented by Formula 1 may have a higher content of lithium ions than a nickel-containing oxide that is commonly used in the art, and thus can replenish lithium ions lost in an irreversible reaction during the initial activation of the battery, so the charge/discharge capacity of the battery can be significantly improved. In addition, compared to the iron and/or manganese-containing oxide(s) commonly used in the art, there is no side reaction caused by the elution of a transition metal during the charging/discharging of the battery, so excellent stability of the battery is exhibited. Examples of the lithium metal oxides represented by Formula 1 may include Li6CoO4, Li6Co0.5Zn0.5O4, and Li6Co0.07Zn0.3O4.
In addition, the lithium cobalt oxide represented by Formula 1 may have a tetragonal crystalline structure, and among the tetragonal crystal structures, may be included in a space group of P42/nmc having a twisted tetrahedral structure consisting of a cobalt element and an oxygen element. Since the positive electrode additive has a twisted tetrahedral structure consisting of a cobalt element and an oxygen element and thus is structurally unstable, a large amount of oxygen gas may be generated in initial charging/discharging (that is, activation) and/or subsequent charging/discharging, thereby reducing battery performance and safety due to the volume expansion of an electrode assembly; and when stored at high temperature of 60° C. or more after initial charging/discharging, the positive electrode additive is transformed into a thermally unstable structure, so oxygen gas is additionally released and there is a limit to the self-discharge of the battery.
However, the lithium secondary battery according to one aspect of the present invention includes a positive electrode additive represented by Formula 1, and the charge/discharge capacity ratio (CC/DC) of the positive electrode additive during the initial charging/discharging is adjusted within a specific range, thereby reducing the amount of oxygen gas generated in the charging/discharging of the lithium secondary battery, and also inhibiting self-discharging and improving an operating voltage by improving the open circuit voltage of the battery in initial activation and subsequent high-temperature storage.
Specifically, the lithium secondary battery may have a ratio (CC/DC) of the charge capacity (CC) to the discharge capacity (DC) of the positive electrode additive in initial charging/discharging of 50 to 100. For example, the lithium secondary battery may have a ratio (CC/DC) of the charge capacity (CC) to the discharge capacity (DC) of the positive electrode additive in initial charging/discharging of 50 to 90; 50 to 75; 50 to 70; 60 to 90; 60 to 80; 60 to 70; 75 to 100; 65 to 75; or 70 to 78. The ratio (CC/DC) of the charge capacity (CC) to the discharge capacity (DC) of the positive electrode additive may be adjusted according to the type or content of the positive electrode additive contained in the positive electrode mixture layer, and in some cases, may be controlled by initial charging/discharging conditions. In one embodiment of the present invention, by adjusting the initial charge/discharge capacity ratio (CC/DC) of the positive electrode additive to satisfy the above range, the amount of oxygen gas generated in the charging/discharging of the lithium secondary battery may be reduced, and the open circuit voltage of the battery may be improved in initial charging/discharging and subsequent high-temperature storage, thereby inhibiting self-discharging and improving an operating voltage. Specifically, when the ratio (CC/DC) exceeds 100, a considerable decrease in open circuit voltage of the battery in high-temperature storage may be prevented, and when the ratio (CC/DC) is significantly low, i.e., less than 50, the generation of a large amount of gas during charging/discharging after battery activation may be improved.
In addition, the lithium secondary battery may have a weight change rate of the positive electrode mixture layer before and after the initial charging/discharging of 0.01 to 2.00%, and specifically, 0.01 to 1.80%; 0.01 to 1.60%; 0.01 to 1.50%; 0.01 to 1.40%; 0.1 to 1.20%; 0.01 to 1.00%; 0.01 to 0.08%; 0.01 to 0.06%; 0.01 to 0.05%; 0.01 to 0.04%; 0.04 to 0.4%; 0.04 to 0.5%; 0.04 to 1.0%; 0.04 to 1.5%; 0.05 to 1.0%; 0.08 to 0.5%; 1.0 to 2.0%; 1.5 to 2.0%; 0.05 to 0.9%; 0.2 to 1.7%; 1.0 to 1.7%; or 0.1 to 0.8%.
By adjusting the weight change rate of the positive electrode mixture layer before and after the initial charging/discharging of the lithium secondary battery within the above range, the amount of oxygen gas generated in the charging/discharging of the lithium secondary battery may be reduced, and the open circuit voltage of the battery may also be improved in initial charging/discharging and subsequent high-temperature storage, thereby inhibiting self-discharging and improving an operating voltage.
In addition, the positive electrode additive may be included at 0.01 to 5 parts by weight with respect to 100 parts by weight of the positive electrode mixture layer, and specifically, 0.01 to 4 parts by weight; 0.01 to 3 parts by weight; 0.01 to 2 parts by weight; 0.01 to 1 part by weight; 0.5 to 2 parts by weight; 0.1 to 0.9 parts by weight; 0.2 to 1.5 parts by weight; or 1 to 2 parts by weight with respect to 100 parts by weight of the positive electrode mixture layer. By adjusting the content of the positive electrode additive within the above range, a decrease in charge/discharge capacity caused by insufficient supplementation of lithium ions lost in an irreversible reaction due to a low content of the positive electrode additive may be prevented, and the generation of a large amount of oxygen gas during the charging/discharging of the battery due to an excess positive electrode additive may be prevented.
Further, the positive electrode mixture layer may further include a conductive material, a binder, or an additive in addition to the positive electrode active material and the positive electrode additive.
Here, the conductive material may be used to improve the performance of the positive electrode, such as electric conductivity, and may include one or more selected from the group consisting of natural graphite, artificial graphite, graphene, carbon nanotubes, carbon black, acetylene black, Ketjen black, and carbon fiber. For example, the conductive material may include acetylene black.
In addition, the conductive material may be included at 0.1 to 5 parts by weight, and specifically, 0.5 to 4 parts by weight; 1 to 3.5 parts by weight; or 0.5 to 1.5 parts by weight with respect to 100 parts by weight of the positive electrode mixture layer.
Moreover, the binder may include one or more resins selected from the group consisting of a polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethylmethacrylate, and a copolymer thereof. In one example, the binder may include polyvinylidene fluoride.
In addition, the binder may be included at 1 to 10 parts by weight, and specifically, 2 to 8 parts by weight; or 1 to 5 parts by weight with respect to a total of 100 parts by weight of the positive electrode mixture layer.
Moreover, the average thickness of the positive electrode mixture layer is not particularly limited, but specifically, may be 50 to 300 μm, and more specifically, 100 to 200 μm; 80 to 150 μm; 120 to 170 μm; 150 to 300 μm; 200 to 300 μm; or 150 to 190 μm.
In addition, in the positive electrode, a material that has high conductivity without causing a chemical change in the battery may be used as a positive electrode current collector. For example, as the positive electrode collector, stainless steel, aluminum, nickel, titanium, or calcined carbon may be used, and in the case of aluminum or stainless steel, one that is surface treated with carbon, nickel, titanium or silver may also be used. In addition, the positive electrode current collector may have fine irregularities formed on a surface thereof to increase the adhesion of the positive electrode active material, and may be formed in various shapes such as a film, a sheet, a foil, a net, a porous body, a foam body, and a non-woven fabric body.
Moreover, the average thickness of the current collector may be appropriately applied within 3 to 500 μm in consideration of the conductivity and total thickness of the positive electrode to be manufactured.
Meanwhile, the negative electrode may include a negative electrode mixture layer prepared by coating, drying and pressing a negative electrode active material on a negative electrode current collector, and if needed, a conductive material, a binder, and other additives may selectively be further included optionally in the negative electrode mixture layer, like the positive electrode.
In addition, the negative electrode active material may include, for example, a carbon material and a silicon material. The carbon material refers to a carbon material including a carbon atom as a main component, and examples of the carbon material may include graphite having a completely layered crystalline structure such as natural graphite, soft carbon having a low crystalline layered crystalline structure (graphene structure; a structure in which hexagonal honeycomb planes of carbon are arranged in layers) and hard carbon in which the above-described structures are mixed with amorphous parts, artificial graphite, expanded graphite, carbon nanofibers, non-graphitizing carbon, carbon black, acetylene black, Ketjen black, carbon nanotubes, fullerenes, activated carbon, and graphene, and preferably, one or more selected from the group consisting of natural graphite, artificial graphite, graphene and carbon nanotubes. More preferably, the carbon material includes natural graphite and/or artificial graphite, and may include any one or more of graphene and carbon nanotubes in addition to the natural graphite and/or artificial graphite. In this case, the carbon material may include 50 to 95 parts by weight, and more specifically, 60 to 90 parts by weight or 70 to 80 parts by weight of graphene and/or carbon nanotubes with respect to a total of 100 parts by weight of the carbon material.
In addition, the silicon material is a particle including silicon (Si), which is a metal component, as a main component, and may include one or more of silicon (Si) particles and silicon oxide (SiOx, 1≤X≤2) particles. In one example, the silicon material may include silicon (Si) particles, silicon monoxide (SiO) particles, silicon dioxide (SiO2) particles, or a mixture thereof.
Moreover, the silicon material may have a form in which crystalline particles and amorphous particles are mixed, and the proportion of the amorphous particles may be 50 to 100 parts by weight, and specifically, 50 to 90 parts by weight; 60 to 80 parts by weight, or 85 to 100 parts by weight based on a total of 100 parts by weight of the silicon material. In one embodiment of the present invention, thermal stability and flexibility may be improved without degrading the electrical properties of an electrode by controlling the proportion of the amorphous particles included in the silicon material to the above range.
In addition, the negative electrode active material contains a carbon material and a silicon material, and the silicon material may be included at 1 to 20 parts by weight, and particularly, 5 to 20 parts by weight; 3 to 10 parts by weight; 8 to 15 parts by weight; 13 to 18 parts by weight; or 2 to 7 parts by weight based on 100 parts by weight of the negative electrode mixture layer.
In one aspect of the present invention, an amount of lithium consumption and an irreversible capacity loss during the initial charging/discharging of the battery may be reduced and charge capacity per unit mass may also be improved by adjusting the contents of the carbon material and the silicon material included in the negative electrode active material to the above range.
In one example, the negative electrode active material may include 95±2 parts by weight of graphite; and 5±2 parts by weight of a mixture in which silicon monoxide (SiO) particles and silicon dioxide (SiO2) particles are uniformly mixed with respect to 100 parts by weight of the negative electrode active material. An amount of lithium consumption and an irreversible capacity loss during the initial charging/discharging of the battery may be reduced and charge capacity per unit mass may also be improved by adjusting the contents of the carbon material and the silicon material included in the negative electrode active material to the above range.
In addition, the negative electrode mixture layer may have an average thickness of 100 to 200 μm, and specifically, 100 to 180 μm, 100 to 150 μm, 120 to 200 μm, 140 to 200 μm, or 140 to 160 jm.
Moreover, the negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has high conductivity, and for example, copper, stainless steel, nickel, titanium, or calcined carbon may be used, and in the case of copper or stainless steel, one whose surface is treated with carbon, nickel, titanium or silver may be used. In addition, the negative electrode current collector, like the positive electrode current collector, has fine irregularities on a surface thereof to reinforce the adhesion of the positive electrode active material and may be formed in various shapes such as a film, a sheet, a foil, a net, a porous body, a foam body, and a non-woven fabric body. In addition, the average thickness of the negative electrode current collector may be suitably applied within 3 to 500 μm in consideration of the conductivity and total thickness of the negative electrode to be manufactured.
Further, in the lithium secondary battery according to the present invention, a separator may be interposed between the positive electrode and the negative electrode. Here, as the separator, an insulating thin film, which is interposed between a positive electrode and a negative electrode and has high ion permeability and mechanical strength, is used. The separator is not particularly limited as long as it is conventionally used in the art, and specifically, a sheet or non-woven fabric made of chemically-resistant and hydrophobic polypropylene, glass fiber, or polyethylene may be used. In some cases, a composite separator in which a porous polymer base material such as a sheet or non-woven fabric is coated with inorganic/organic particles by an organic binder polymer may be used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separator. Moreover, the separator may have a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm on average.
Meanwhile, the positive electrode and the negative electrode may be wound in a jelly roll shape and accommodated in a cylindrical, prismatic or pouch-type battery, or accommodated in a pouch-type battery in a folding or stack-and-folding form, but the present invention is not limited thereto.
In addition, a lithium salt-containing electrolyte according to the present invention may consist of an electrolyte and a lithium salt, and as the electrolyte, a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte may be used.
As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidinone, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethyoxy ethane, tetrahydroxyfuran, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, a dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, a tetrahydrofuran derivative, ether, methyl propionate, and ethyl propionate may be used.
As the organic solid electrolyte, for example, polymers such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphoric acid ester polymer, poly alginate lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers including an ionic dissociation group may be used.
As the inorganic solid electrolyte, for example, a nitride, halide or sulfate of lithium such as Li3N, LiI, Li5Ni2, Li3N—LiI—LiOH, LiSiO4, LiSiO4—LiI—LiOH, Li2SiS3, Li4SiO4, Li4SiO4—LiI—LiOH, or Li3PO4—Li2S—SiS2 may be used.
The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, and may be, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiBioClio, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate, or lithium imide.
In addition, to improve charging/discharging characteristics and flame retardancy, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexamathylphosphoric acid triamine, a nitrobenzene derivative, sulfur, a quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, an ammonium salt, pyrrole, 2-methoxy ethanol, or aluminum trichloride may be added to the electrolyte. In some cases, to impart incombustibility, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further included, and to enhance high-temperature storage properties, carbon dioxide gas may be further included, and fluoro-ethylene carbonate (FEC) or propene sultone (PRS) may be also included.
Method of Manufacturing Lithium Secondary Battery
Further, in one embodiment, the present invention provides a method of manufacturing a lithium secondary battery, which includes:
LipCo(1-q)M1qO4
In Formula 1,
M1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and
p and q are 5≤p≤7 and 0≤q≤0.4, respectively.
The method of manufacturing a lithium secondary battery according to embodiment of the present invention may include applying a current to the above-described lithium secondary battery, which includes a positive electrode including a positive electrode current collector, and a positive electrode mixture layer disposed on the positive electrode current collector and containing a positive electrode active material and a positive electrode additive represented by Formula 1 below; and a negative electrode including a negative electrode current collector, and a negative electrode mixture layer disposed on the negative electrode current collector to perform initial charging. Here, the current applied to the lithium secondary battery is 1C or less, so the ratio (CC/DC) of initial charge capacity (CC) to initial discharge capacity (DC) during initial charging/discharging can be adjusted to be in a range of 50 to 100.
Specifically, the initial charging is implemented by continuously performing a three-step charging process, that is, one to three steps of activation.
More specifically, the initial charging step may be performed by first activation for applying a current of 0.05C to 0.2C to a lithium secondary battery to charge to an SOC of 30% or less; second activation for applying a current of 0.3C to 0.5C to the lithium secondary battery that has undergone the first activation to charge to an SOC of more than 30% and less than 70%; and third activation for applying a current of 0.6C to 0.9C to the lithium secondary battery that has undergone the second activation to charge to an SOC of 70% or more.
In one example, the initial charging may be performed by first activation for applying a current of 0.08C to 0.15C to a lithium secondary battery to charge to an SOC of 30% or less in initial charging; second activation for applying a current of 0.35C to 0.45C to the lithium secondary battery that has undergone the first activation to charge to an SOC of more than 30% and less than 70%; and a third activation for applying a current of 0.65C to 0.8C to the lithium secondary battery that has undergone the second activation to charge to an SOC of 70% or more.
In one embodiment of the present invention, by controlling the current value so that the current applied to the lithium secondary battery is 1 C or less during initial charging, but is gradually increased as SOC increases, a SEI film may be uniformly formed on the negative electrode mixture layer, and it is possible to prevent the dissociation of lithium ions by the positive electrode active material before the positive electrode additive at a considerably low current or the deterioration of electricity due to the induction of overlithiation of the positive electrode additive caused by a current exceeding 1C.
In addition, the initial charging may be performed at a temperature condition higher than room temperature in order to lower the resistance of an electrode. Specifically, each stage of activation may be performed under a temperature condition of 20 to 70° C., and more specifically, 20 to 60° C.; 20 to 40° C.; 20 to 25° C.; 40 to 60° C.; 45 to 60° C.; 50 to 65° C.; 50 to 60° C.; 52 to 58° C.; 40 to 50° C.; or 42 to 48° C. When the initial charging is carried out under the above temperature condition, the secondary battery can be charged under a lower resistance condition of the positive electrode, so charging efficiency may increase and the uniformity of the SEI film formed on the negative electrode mixture layer may be improved.
Meanwhile, in the initial charging, the outer surface of the lithium secondary battery may be pressurized concurrently with charging of the lithium secondary battery or sequentially after the charging using a using a pressurizing device such as a jig, and the pressurizing may be performed at 5 to 900 kgf/cm2 for 4 to 6 seconds.
Moreover, discharging of the lithium secondary battery after initial charging may be further included, and in this case, the discharging may be performed under a current condition of 0.1 to 1 C, and specifically, 0.1 to 0.9 C; 0.1 to 0.8 C; 0.1 to 0.6 C; 0.1 to 0.5 C; 0.5 to 1 C; 0.2 to 0.8 C; or 0.4 to 0.6 C.
Further, the method of manufacturing a lithium secondary battery according to the present invention may further include aging the charged lithium secondary battery after the initial charging. The aging may allow the SEI film to be more stabilized and reformed to a uniform thickness by thermal energy and electrochemical energy. To this end, the aging may be performed at 20 to 80° C., and specifically, 30 to 70° C. or 45 to 65° C. for 0.5 to 30 hours.
According to the method of manufacturing a lithium secondary battery according to an aspect of the present invention, the initial charge/discharge capacity ratio (CC/DC) of the positive electrode additive represented by Formula 1, contained in the positive electrode mixture layer, and/or the weight ratio of the positive electrode mixture layer before and after the initial charging/discharging may be easily controlled in the above-described ranges, and thus, not only the amount of oxygen gas generated during the charging/discharging of the manufactured lithium secondary battery may be reduced, but also the open-circuit voltage of the battery may be improved in initial activation and/or subsequent high-temperature storage, thereby inhibiting self-discharging and improving an operating voltage. Therefore, the lithium secondary battery of the present invention can be effectively used as a power source of a mid-to-large devices such as electric vehicles.
Hereinafter, the present invention will be described in further detail with reference to examples and an experimental example.
However, the following examples and experimental example merely illustrate the present invention, and the content of the present invention is not limited to the following examples and experimental examples.
A positive electrode slurry for a lithium secondary battery was prepared by injecting N-methyl pyrrolidone into a homo mixer, weighing and inputting 97.8 parts by weight of a positive electrode active material LiNi0.6Co0.2Mn0.2O2 and a positive electrode additive Li6CoO4 or Li6Co0.7Zn0.3O4; 0.7 parts by weight of a conductive material, which is a mixture of carbon nanotubes (average size: 60±10 nm) and Denka black (average size: 2±0.5 μm) (75:25 wt./wt.); and 1.5 parts by weight of a binder PVdF with respect to 100 parts by weight of the solid content of the positive electrode slurry, and mixing the resultant at 2,000 rpm for 60 minutes. Here, a positive electrode was manufactured by applying the prepared positive electrode slurry to one surface of an aluminum current collector, drying the slurry at 100° C., and rolling the resultant. Here, the total thickness of the positive electrode mixture layer was 130 μm, and the total thickness of the manufactured positive electrode was approximately 200 μm.
With respect to 100 parts by weight of the solid content of a negative electrode slurry, 84 parts by weight of a negative electrode active material natural graphite and 14 parts by weight of silicon oxide (SiOx, 1<x≤2) particles; and 2 parts by weight of a binder styrene butadiene rubber (SBR) were prepared, and a negative electrode slurry was prepared in the same manner as the positive electrode slurry. Here, the graphite used in the formation of the negative electrode mixture layer was natural graphite (average particle diameter: 0.01 to 0.5 μm), and the silicon oxide (SiOx) particle had an average particle size of 0.9 to 1.1 μm. A negative electrode was manufactured by applying the prepared negative electrode slurry to one surface of a copper current collector, drying the slurry at 100° C. and rolling the resultant. Here, the total thickness of the negative electrode mixture layer was 150 μm, and the total thickness of the manufactured negative electrode was approximately 250 am.
A full-cell type lithium secondary battery was manufactured by stacking a separator (thickness: approximately 16 jm) consisting of a porous polyethylene (PE) film to be interposed between the prepared positive electrode and negative electrode and injecting E2DVC as an electrolyte.
Here, the “E2DVC” refers to a type of carbonate-based electrolyte, which is a mixed solution in which lithium hexafluorophosphate (LiPF6, 1.OM) and vinyl carbonate (VC, 2 wt %) are added to a mixture of ethylene carbonate (EC): dimethyl carbonate (DMC): diethyl carbonate (DEC)=1:1:1 (volume ratio).
By performing initial charging of the manufactured full cell at 22±2° C. under conditions shown in Table 1 below, the initial charge capacity (CC) and the initial discharge capacity (DC) of the positive electrode additive were measured, and their ratio (CC/DC) was calculated. Here, the potential range of the battery was 2.5 to 4.25V. In addition, the positive electrode was dissembled from the full cell, the positive electrode mixture layer was delaminated from the positive electrode current collector, and the weight of the positive electrode mixture layer was measured after the initial charging/discharging. The weight change rate before and after the initial charging/discharging was deduced from the weights measured before and after the initial charging/discharging.
To confirm the difference in effect according to the ratio (CC/DC) of the initial charge capacity (CC) and the initial discharge capacity (DC) of the positive electrode additive, the following experiment was performed.
a) Manufacture of Lithium Secondary Battery
To confirm the effect according to the initial charge/discharge capacity ratio of the positive electrode additive contained in the lithium secondary battery, lithium secondary batteries according to Examples 2a and 4a and Comparative Examples 1a to 4a were manufactured in the same manner as described in the corresponding Examples 2 and 4 and Comparative Examples 1 to 4, except that a positive electrode active material was not included and 97.8 parts by weight of a positive electrode additive such as Li6CoO4 or Li6Co0.7Zn0.3O4 was used with respect to 100 parts by weight of the solid content of a positive electrode slurry.
By performing initial charging of the manufactured full cell at 22±2° C. under conditions shown in Table 2 below, the initial charge capacity (CC) and the initial discharge capacity (DC) of the positive electrode additive were measured, and their ratio (CC/DC) was calculated. Here, the potential range of the battery was 2.5 to 4.25V.
b) Evaluation of Amount of Gas Generation
After performing initial charging/discharging on the lithium secondary batteries manufactured in Examples 2a and 4a and Comparative Examples 1a to 4a, for each secondary battery, {circle around (1)} the amounts of gas generated during 50 cycles of repeated charging/discharging at 45° C. under a condition of 0.3C/0.3C and {circle around (2)} the amounts of gas generated in 4-week storage at 60° C. were measured. The results are shown in Table 3 below, and
c) Evaluation of High-Temperature Storage Characteristic
The lithium secondary batteries manufactured in Examples 2a, 4a and Comparative Examples 1a to 4a were stored at 60° C. for 4 weeks, open circuit voltages (OCV) were measured, and the changed open circuit voltages were calculated based on those before storage. The results are shown in Table 3 below and
As shown in Table 3 and
Specifically, in the case of the lithium secondary batteries manufactured in Examples 2a and 4a that contain the positive electrode additive represented by Formula 1, satisfy a weight change rate of the positive electrode mixture layer in a predetermined range by adjusting the ratio (CC/DC) of the charge capacity (CC) and the discharge capacity (DC) of the positive electrode additive during initial charging/discharging within a specific range, it was confirmed that the amount of cumulative gas generation in charging/discharging after the initial charging/discharging and 4-week high-temperature storage increased according to an increase in content of the positive electrode additive, but decreased in comparison with the same amount. In addition, the lithium secondary batteries of the examples showed a decrease in open circuit voltage variation (AOCV) after high-temperature storage.
From the above results, it can be seen that, in the lithium secondary battery according to the present invention, the positive electrode additive represented by Formula 1 as an irreversible additive is included in the positive electrode mixture layer, the ratio (CC/DC) of the charge capacity (CC) and the discharge capacity (DC) of the positive electrode additive in the initial charging/discharging is adjusted within a specific range, thereby reducing the amount of oxygen gas generated in the charging/discharging of the lithium secondary battery, and at the same time, inhibiting self-discharging and improving an operating voltage by improving the open circuit voltage of the battery in initial activation and/or subsequent high-temperature storage.
In the above, the present invention has been described with reference to exemplary embodiments, but it should be understood by those skilled in the art or those of ordinary skill in the art that the present invention can be variously modified and changed without departing the spirit and technical scope of the present invention described in the accompanying claims.
Accordingly, the technical scope of the present invention is not limited to the content described in the detailed description of the specification, but should be defined by the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2021-0071872 | Jun 2021 | KR | national |
The present application is a national phase entry under 35 U.S.C. § 371 of International Application No. PCT/KR2022/007846, filed on Jun. 2, 2022, which claims priority from Korean Patent Application No. 10-2021-0071872, filed on Jun. 3, 2021, the contents of which are incorporated by reference herein.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2022/007846 | 6/2/2022 | WO |
