Negative electrode for lithium secondary battery, method for pre-lithiation of the negative electrode, and lithium secondary battery including the negative electrode

Abstract

The present invention relates to a negative electrode for a lithium secondary battery, wherein the negative electrode is pre-lithiated, a method for manufacturing the negative electrode, and a lithium secondary battery comprising the negative electrode, and the pre-lithiated negative electrode of the present invention can increase the capacity of the lithium secondary battery and improve the electrochemical performance of the lithium secondary battery by ensuring the initial reversibility of the negative electrode.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to and the benefit of Korean Patent Application No. 10-2018-0078681, filed on Jul. 6, 2018, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0004] The present invention relates to a negative electrode for a lithium secondary battery, a method for pre-lithiation of the negative electrode, a lithium secondary battery manufactured using the negative electrode, and a method for manufacturing a lithium secondary battery. Specifically, the present invention relates to a negative electrode for a lithium secondary battery that can increase the capacity of the battery and improve the electrochemical performance by ensuring the initial reversibility of the negative electrode through pre-lithiation, and can allow more lithium ions to diffuse into the negative electrode active material layer during pre-lithiation, a method for pre-lithiation of the negative electrode, and a lithium secondary battery manufactured using the negative electrode. Background Art

[0005] With the technological development and increase in demand for mobile devices, the demand for secondary batteries as energy sources has rapidly increased, and among these secondary batteries, lithium secondary batteries with high energy density, high operating voltage, long cycle life and low self-discharge rate are being commercialized and widely used.

[0006] During this period, metal oxides such as LiCoO2, LiMnO2, LiMn2O4 or LiCrO2 are used as positive electrode active materials constituting the positive electrode of lithium secondary batteries, while metal lithium, carbon-based materials such as graphite or activated carbon, or silicon oxide (SiO x ) is used as a negative electrode active material constituting the negative electrode. Among the materials used as negative electrode active materials, metallic lithium was mainly used initially, but as the charge and discharge cycles proceed, lithium atoms grow on the surface of metallic lithium, thereby damaging the separator and damaging the battery, and therefore, recently, carbon-based materials are generally used. However, since the disadvantage of carbon-based materials is that the theoretical capacity is only about 400 mAh / g, the capacity is small, and various studies have been conducted to replace carbon-based materials with silicon (Si)-based materials having a high theoretical capacity (4,200 mAh / g) as negative electrode active materials.

[0007] The lithium secondary battery is charged and discharged while lithium ions of the positive electrode active material of the positive electrode are intercalated into and deintercalated from the negative electrode active material of the negative electrode.

[0008] Theoretically, the reaction of lithium intercalation into the negative electrode active material and the reaction of lithium deintercalation from the negative electrode active material are completely reversible, but in reality, more lithium is consumed than the theoretical capacity of the negative electrode active material, and only a portion of the lithium is recovered during discharge. Therefore, starting from the second cycle, a smaller amount of lithium ions are intercalated during charging, and most of the intercalated lithium ions are deintercalated during discharge. As described above, the capacity difference that occurs in the reaction during the first charge and discharge is called irreversible capacity loss, and since commercial lithium secondary batteries are manufactured in a state where lithium ions are supplied only from the positive electrode and lithium is not present in the negative electrode, it is important to minimize the irreversible capacity loss during initial charge and discharge.

[0009] It is known that this initial irreversible capacity loss is mainly caused by the electrolyte decomposition reaction on the surface of the negative electrode active material, and a solid electrolyte interface (SEI) film is formed on the surface of the negative electrode active material through an electrochemical reaction via electrolyte decomposition. Since the formation of such SEI films consumes a large amount of lithium ions, the formation of SEI films leads to the problem of irreversible capacity loss, but the SEI film formed at the beginning of charging prevents lithium ions from reacting with the negative electrode or other materials during charging and discharging, and acts as an ion tunnel that only allows lithium ions to pass, thereby further suppressing the electrolyte decomposition reaction and helping to improve the cycle characteristics of lithium secondary batteries.

[0010] Therefore, a method for improving the initial irreversibility caused by the formation of SEI film, etc. is needed, and one method is to perform pre-lithiation before manufacturing a lithium secondary battery so as to experience the side reactions that occur during the first charge in advance. As described above, in the case of performing pre-lithiation, when the actually manufactured secondary battery is charged and discharged, the advantage is that since the first cycle is performed in a state where the irreversibility is reduced, the initial irreversibility can be reduced.

[0011] Conventional pre-lithiation methods may include, for example, a method of depositing lithium on the negative electrode and a method of bringing lithium into direct contact with the negative electrode. For example, a lithiated material layer for pre-lithiation may be formed on the negative electrode active material layer, however, the lithiated material is easily oxidized and is easily oxidized when exposed to moisture or oxygen.

[0012] Therefore, there is a need to develop a new negative electrode for lithium secondary batteries that can achieve more efficient pre-lithiation.

[0013] [Prior art literature]

[0014] [Patent Document]

[0015] KR2008-0025002 A Summary of the invention

[0016] [Technical issues]

[0017] The present invention aims to provide a negative electrode for a lithium secondary battery, a method for pre-lithiation of the negative electrode, and a lithium secondary battery manufactured using the negative electrode, in which the initial reversibility of the negative electrode can be ensured and at the same time, lithium ions can be fully diffused into the negative electrode active material during pre-lithiation.

[0018] [Technical solution]

[0019] One aspect of the present invention provides a negative electrode for a lithium secondary battery, the negative electrode for the lithium secondary battery comprising: a first negative electrode active material layer, the first negative electrode active material layer is formed on a negative electrode collector and comprises a first negative electrode active material; a lithium metal layer, the lithium metal layer is formed on the first negative electrode active material layer and comprises lithium metal; and a second negative electrode active material layer, the second negative electrode active material layer is formed on the lithium metal layer and comprises a second negative electrode active material.

[0020] Another aspect of the present invention provides a negative electrode for a lithium secondary battery,

[0021] The negative electrode for a lithium secondary battery is manufactured by pre-lithiating the negative electrode and includes: a first negative electrode active material layer formed on a negative electrode current collector and including a first negative electrode active material; and

[0022] A second negative electrode active material layer is formed on the first negative electrode active material layer and includes a second negative electrode active material, and the first negative electrode active material layer and the second negative electrode active material layer each include lithium therein.

[0023] Another aspect of the present invention provides a method for pre-lithiation of a negative electrode for a lithium secondary battery, the method comprising:

[0024] forming a first negative electrode active material layer on the negative electrode current collector;

[0025] forming a lithium metal layer including lithium metal on the first negative electrode active material layer; forming a second negative electrode active material layer on the lithium metal layer; and impregnating the resultant with a pre-lithiation solution to achieve pre-lithiation.

[0026] Another aspect of the present invention provides a lithium secondary battery manufactured using the negative electrode for a lithium secondary battery of the present invention.

[0027] Another aspect of the present invention provides a method for manufacturing a lithium secondary battery, the method comprising: forming a first negative electrode active material layer on a negative electrode current collector;

[0028] forming a lithium metal layer including lithium metal on the first negative electrode active material layer;

[0029] forming a second negative electrode active material layer on the lithium metal layer to manufacture a negative electrode for a lithium secondary battery;

[0030] manufacturing an electrode assembly including a negative electrode, a positive electrode, and a separator and housing the electrode assembly in a battery case; and,

[0031] The electrolyte solution is injected into the battery case, and the electrode assembly is placed at a temperature of 10° C. to 200° C. for 2 hours to 48 hours so that the electrode assembly is impregnated with the electrolyte solution.

[0032] Another aspect of the present invention provides a method for manufacturing a lithium secondary battery, the method comprising: forming a first negative electrode active material layer on a negative electrode current collector;

[0033] forming a lithium metal layer including lithium metal on the first negative electrode active material layer;

[0034] forming a second negative electrode active material layer on the lithium metal layer;

[0035] impregnating the resultant with a pre-lithiation solution, thereby achieving pre-lithiation to manufacture a pre-lithiated negative electrode for a lithium secondary battery;

[0036] manufacturing an electrode assembly including a negative electrode, a positive electrode, and a separator and housing the electrode assembly in a battery case; and,

[0037] An electrolyte solution is injected into the battery case.

[0038] [Beneficial Effects]

[0039] The negative electrode for a lithium secondary battery of the present invention has a structure in which the negative electrode active material exists above and below the lithium metal layer, thereby consuming more lithium metal in pre-lithiation and reducing the residual unreacted lithium metal, thereby improving the pre-lithiation effect.

[0040] The pre-lithiated negative electrode of the present invention can ensure the initial reversibility of the negative electrode, thereby improving the electrochemical performance of the lithium secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a cross-sectional view showing a stacked structure of a negative electrode according to an embodiment of the present invention.

[0042] Figure 2 is a cross-sectional view showing a stacked structure of a negative electrode according to Comparative Example 1 of the present invention.

[0043] Figure 3 is a cross-sectional view showing a stacked structure of a negative electrode according to Comparative Example 2 of the present invention.

[0044] Figure 4 is a cross-sectional view showing a stacked structure of a negative electrode according to Comparative Example 3 of the present invention.

[0045] Figure 5 is a cross-sectional view showing a stacked structure of a negative electrode according to Comparative Example 4 of the present invention.

[0046] Figure 6 is a cross-sectional view showing a stacked structure of a negative electrode according to Comparative Example 5 of the present invention. DETAILED DESCRIPTION

[0047] Hereinafter, the present invention will be described in further detail.

[0048] The terms or words used in this specification and claims should not be construed as limited to the ordinary meaning or dictionary meaning, but should be construed as meanings and concepts consistent with the spirit of the present invention based on the principle that the inventor can appropriately define concepts and terms in order to interpret the inventor's invention in the best manner.

[0049] Negative electrode and method for manufacturing the same

[0050] The negative electrode and the pre-lithiation method thereof of the present invention will be described with reference to the accompanying drawings of the present invention.

[0051] The negative electrode of the present invention may have the following components before pre-lithiation (see Figure 1 ):

[0052] A first negative electrode active material layer 112, the first negative electrode active material layer 112 is formed on the negative electrode current collector 130 and includes a first negative electrode active material;

[0053] a lithium metal layer 120 formed on the first negative active material layer 112 and including lithium metal; and

[0054] The second negative electrode active material layer 114 is formed on the lithium metal layer 120 and includes a second negative electrode active material.

[0055] Each of the first negative electrode active material and the second negative electrode active material includes silicon (Si), a silicon-based alloy, or silicon oxide (SiO x ,0 < x ≤ 2) as the silicon-based negative electrode active material.

[0056] Specifically, in addition to the silicon-based negative electrode active material, the negative electrode active material may further include a carbon-based negative electrode active material such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon. In this case, each of the first negative electrode active material layer and the second negative electrode active material layer may independently include a silicon-based negative electrode active material and a carbon-based negative electrode active material in a weight ratio of 1:99 to 50:50, preferably 5:95 to 20:80.

[0057] When the proportion of the silicon-based negative electrode active material is lower than the above range, it is difficult to improve the energy density of the battery, so it is difficult to achieve a high capacity of the battery. When the proportion of the silicon-based negative electrode active material exceeds the above range, the volume expansion degree of the negative electrode may increase.

[0058] The lithium metal layer may be composed of lithium metal powder or lithium metal foil.

[0059] In addition, the negative electrode of the present invention is a prelithiated negative electrode manufactured by prelithiation of the negative electrode, and includes:

[0060] A first negative electrode active material layer formed on the negative electrode current collector and including a first negative electrode active material; and

[0061] A second negative electrode active material layer formed on the first negative electrode active material layer and including a second negative electrode active material,

[0062] and each of the first negative electrode active material layer and the second negative electrode active material layer includes lithium therein.

[0063] In the negative electrode, lithium in the lithium metal layer moves to the first negative electrode active material layer and the second negative electrode active material layer formed above and below the lithium metal layer through prelithiation, and exists in the form of lithium ions diffusing or in the form of combining with the first negative electrode active material in the first negative electrode active material layer and the second negative electrode active material in the second negative electrode active material layer.

[0064] The prelithiation method for manufacturing a prelithiated negative electrode for a lithium secondary battery according to the present invention may include the following operations:

[0065] Forming a first negative electrode active material layer on the negative electrode current collector;

[0066] forming a lithium metal layer including lithium metal on the first negative electrode active material layer;

[0067] forming a second negative electrode active material layer on the lithium metal layer; and

[0068] The resultant is impregnated with a pre-lithiation solution to achieve pre-lithiation.

[0069] The lithium metal layer can be formed by adding and dispersing lithium metal powder and a binder in an organic solvent and then uniformly applying the solution to the formed first negative electrode active material layer, or by placing a lithium metal foil on the first negative electrode active material layer and pressing it.

[0070] The pre-lithiation solution is a solution including an ionizable lithium salt and an organic solvent, and corresponds to a general electrolyte solution.

[0071] Ionizable lithium salts include Li + as cation, and the anion can be selected from F - , Cl - Br - ,I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - 、AlO4 - 、AlCl4 - PF6 - 、SbF6 - 、AsF6 - , B 10 Cl 10 - 、BF2C2O4 - , BC4O8 - PF4C2O4 - PF2C4O8 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - 、C4F9SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH- 、CH3SO3 - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - The group composed of.

[0072] The organic solvent may be one or more selected from the following: a cyclic carbonate-based organic solvent selected from the group consisting of ethylene carbonate, 1,2-butenyl carbonate, 2,3-butenyl carbonate, 1,2-pentenyl carbonate, 2,3-pentenyl carbonate, vinylene carbonate and fluoroethylene carbonate (FEC); a linear carbonate-based organic solvent selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methylpropyl carbonate and ethylpropyl carbonate; and a linear ester-based organic solvent selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0073] The impregnation operation is a pre-lithiation operation, and can be performed by impregnating the high temperature heat-treated negative electrode active material layer with a pre-lithiation solution at a temperature of 10°C to 200°C for 2 to 48 hours, and preferably at a temperature of 20°C to 70°C for 2 to 36 hours.

[0074] When the pre-lithiation temperature and time are less than 10°C and less than 2 hours, respectively, pre-lithiation may not be fully achieved, when the pre-lithiation temperature exceeds 200°C, the lithium metal may melt and its shape may not be maintained, and since pre-lithiation can be fully achieved within 48 hours, there is no need to further impregnate the negative electrode.

[0075] Since the negative electrode of the present invention includes a negative electrode active material layer located above and below the lithium metal layer, more lithium metal is consumed in pre-lithiation, thereby fully realizing pre-lithiation, and the residual unreacted lithium metal can also be reduced to improve the pre-lithiation effect. Therefore, the electrochemical performance of the lithium secondary battery can be improved by ensuring the initial reversibility of the negative electrode.

[0076] In contrast, for example, when the negative electrode active material is present only in the layer below the lithium metal layer (see Figure 2 ), since the contact area between the negative electrode active material and the lithium metal is small, the lithium metal is not fully used for pre-lithiation and remains as unreacted lithium metal, and thus pre-lithiation may not be fully performed.

[0077] In addition, in addition to the case where the negative electrode active material layer surrounds the lithium metal layer above and below as in the present invention, for example, in the case where the negative electrode active material exists only in the layer above the lithium metal layer (see Figure 4 ), the case where the lithium metal layer exists above and below the negative electrode active material (see Figure 5 ), and the case where the lithium metal layer exists in the negative electrode active material layer (see Figure 6 ) cannot be fully pre-lithiated.

[0078] That is, the structure of the lithium secondary battery of the present invention can be the optimal structure of the negative electrode in pre-lithiation.

[0079] Lithium secondary battery and method for manufacturing the same

[0080] The negative electrode of the present invention can be effectively used to manufacture a lithium secondary battery.

[0081] Specifically, the lithium secondary battery according to the present invention includes a negative electrode, a positive electrode disposed opposite to the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte, and here, the negative electrode is the above-mentioned negative electrode of the present invention.

[0082] In addition, the secondary battery may selectively further include: a battery case (eg, a pouch) accommodating an electrode assembly consisting of a positive electrode, a negative electrode, and a separator; and a sealing member sealing the battery case.

[0083] The lithium secondary battery may be manufactured according to a conventional method of manufacturing a secondary battery except that the negative electrode of the present invention is used.

[0084] In the secondary battery, the positive electrode includes a positive electrode collector and a positive electrode active material layer on at least one surface of the positive electrode collector.

[0085] The positive electrode can be manufactured according to the conventional method for manufacturing the positive electrode known in the art. For example, the positive electrode can be manufactured by dissolving or dispersing the components constituting the positive electrode active material layer (i.e., positive electrode active material, conductive material and / or binder) in a solvent to prepare a positive electrode mixture, applying the positive electrode mixture to at least one surface of the positive electrode collector, and then drying and pressing the resulting product, or the positive electrode can be manufactured by casting the positive electrode mixture on a separate support, and then laminating the film stripped from the support on the positive electrode collector.

[0086] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and may be, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver or the like. In addition, the positive electrode current collector may generally have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase adhesion to the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, and the like.

[0087] Examples of positive electrode active materials may include: layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; lithium manganese oxides such as those of the formula Li 1+y Mn 2- y Compounds represented by LiMnO4 (wherein y is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2 and the like; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7 and the like; compounds represented by the formula LiNi 1-y M y Ni-type lithium nickel oxide represented by O2 (wherein M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and y = 0.01 to 0.3); represented by the formula LiMn 2-y M y O2 (wherein M = Co, Ni, Fe, Cr, Zn or Ta, and y = 0.01 to 0.1) or a lithium manganese composite oxide represented by the formula Li2Mn3MO8 (wherein M = Fe, Co, Ni, Cu or Zn); LiMn2O4, in which a portion of Li is replaced by alkaline earth metal ions; disulfide compounds; Fe2(MoO4)3; and the like, but the present invention is not limited thereto.

[0088] Furthermore, the binder and the conductive material may be the same as those described above for the negative electrode.

[0089] In addition, in the secondary battery, the separator is not particularly limited, as long as it is commonly used in the secondary battery to separate the negative electrode from the positive electrode and provide a path for the movement of lithium ions, and in particular, the separator is preferably low resistance to the ion movement of the electrolyte, and has excellent ability to be impregnated with an electrolyte solution. Specifically, a porous polymer film can be used, for example, a porous polymer film formed by a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a stacked structure having two or more layers thereof. In addition, conventional porous non-woven fabrics can be used, for example, non-woven fabrics formed by high melting point glass fibers or polyethylene terephthalate fibers. In addition, a coated separator including a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and it can be selectively used in a monolayer or multilayer structure.

[0090] In addition, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte that can be used to manufacture a secondary battery may be used as the electrolyte, but the present invention is not limited thereto.

[0091] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0092] The organic solvent is not particularly limited as long as it is used as a medium that enables the ions involved in the electrochemical reaction of the battery to move. Specifically, the organic solvent may be: an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, and the like; an ether-based solvent such as dibutyl ether, tetrahydrofuran, or the like; a ketone-based solvent such as cyclohexanone; an aromatic hydrocarbon-based solvent such as benzene, fluorobenzene, and the like; a carbonate-based solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (EPC), and the like. carbonate, PC) and the like; alcohol-based solvents such as ethanol, isopropanol and the like; nitriles such as Ra-CN (wherein Ra is a linear, branched or cyclic C2 to C20 hydrocarbon group, and may include a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolane; and the like. Among them, carbonate-based solvents are preferred, and a mixture of a cyclic carbonate (such as ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant and a linear carbonate-based compound (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate or the like) having low viscosity, which can improve the charge and discharge performance of the battery, is more preferred. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the electrolyte solution can exhibit excellent performance.

[0093] Lithium salts can be used without particular limitation, as long as they are compounds that can provide lithium ions used in lithium secondary batteries. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2 or the like can be used as lithium salts. The concentration of the lithium salt can be in the range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity, so that excellent electrolyte performance can be exhibited, and lithium ions can be effectively moved.

[0094] In addition to the components of the electrolyte, in order to improve the life characteristics of the battery, suppress the reduction of the battery capacity, and improve the discharge capacity of the battery, the electrolyte may further include one or more types of additives: for example, halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, ethylene glycol dimethyl ether (glyme), hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidinones, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum chloride. Here, the additives may be included in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0095] In addition, in the present invention, a lithium secondary battery can be manufactured using the pre-lithiation negative electrode of the present invention, but a lithium secondary battery can also be manufactured by performing pre-lithiation after assembling the battery, that is, by housing an electrode assembly including the negative electrode before pre-lithiation in a battery casing, and then injecting an electrolyte solution into the battery casing to impregnate the electrode assembly with the electrolyte solution to manufacture a lithium secondary battery.

[0096] The method of manufacturing a lithium secondary battery by first pre-lithiating the negative electrode comprises the following operations:

[0097] forming a first negative electrode active material layer on the negative electrode current collector;

[0098] forming a lithium metal layer including lithium metal on the first negative electrode active material layer;

[0099] forming a second negative electrode active material layer on the lithium metal layer;

[0100] impregnating the resultant with a pre-lithiation solution, thereby achieving pre-lithiation to manufacture a pre-lithiated negative electrode for a lithium secondary battery;

[0101] manufacturing an electrode assembly including a negative electrode, a positive electrode, and a separator, and housing the electrode assembly in a battery case; and

[0102] An electrolyte solution is injected into the battery case.

[0103] The method of manufacturing a lithium secondary battery by performing pre-lithiation after assembling the battery includes the following operations:

[0104] forming a first negative electrode active material layer on the negative electrode current collector;

[0105] forming a lithium metal layer including lithium metal on the first negative electrode active material layer;

[0106] forming a second negative electrode active material layer on the lithium metal layer to manufacture a negative electrode for a lithium secondary battery;

[0107] manufacturing an electrode assembly including a negative electrode, a positive electrode, and a separator, and housing the electrode assembly in a battery case; and

[0108] The electrolyte solution is injected into the battery case, and the electrode assembly is placed at a temperature of 10° C. to 200° C. for 2 hours to 48 hours so that the electrode assembly is impregnated with the electrolyte solution.

[0109] Example

[0110] Hereinafter, the present invention will be described in detail with reference to Examples. However, the following Examples are intended to illustrate the present invention and are not intended to limit the present invention.

[0111] Example 1.

[0112] Formation of the first negative electrode active material layer

[0113] A negative electrode active material slurry was prepared by adding 92 wt % of a negative electrode active material (graphite:SiO=7:3), 3 wt % of a conductive agent (Denkablack), 3.5 wt % of a binder (styrene butadiene rubber (SBR)), and 1.5 wt % of a thickener (carboxymethyl cellulose (CMC)) to water. One surface of a copper current collector 130 was coated with the prepared negative electrode active material slurry, dried, and rolled to form a first negative electrode active material layer 112, wherein the loading amount of the negative electrode active material was 2.6 mg / cm 2 .

[0114] Formation of lithium metal layer

[0115] A solution in which lithium metal powder and polyvinylidene fluoride (PVdF) binder were added to tetrahydrofuran (THF) at a weight ratio of 95:5 and dispersed was uniformly applied on the formed first negative electrode active material layer 112 and dried to form a lithium metal layer 120, wherein the loading amount of lithium was 0.25 mg / cm 2 .

[0116] Formation of the Second Negative Electrode Active Material Layer

[0117] A negative electrode active material slurry was prepared by adding 92 wt % of a negative electrode active material (graphite:SiO=7:3), 3 wt % of a conductive agent (Denkablack) and 5 wt % of a binder (PVdF) to THF. The slurry was uniformly applied on the formed lithium metal layer, and then dried and rolled to form a second negative electrode active material layer 114, wherein the loading amount of the negative electrode active material was 2.6 mg / cm 2 (See Figure 1 ).

[0118] Pre-lithiation

[0119] A pre-lithiation solution was prepared by dissolving 1M LiPF6 in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 50:50, and the above-mentioned fabricated structure (a structure in which a first negative electrode active material layer, a lithium metal layer, and a second negative electrode active material layer were sequentially formed on a current collector) was impregnated with the solution and then allowed to stand at a temperature of 25°C.

[0120] Here, lithium ions from the lithium metal layer 120 diffuse into the first negative active material layer 112 and the second negative active material layer 114 for pre-lithiation. After 3 hours, the electrode was taken out of the pre-lithiation solution, washed with DMC, and dried to produce a pre-lithiation negative electrode.

[0121] Manufacturing of lithium secondary batteries

[0122] After the pre-lithiated anode fabricated above was punched into a button cell size, a polyolefin separator was interposed between the anode and a lithium metal foil as a counter electrode, and then an electrolyte solution in which 1 M LiPF6 was dissolved in a solvent in which EC and DEC were mixed in a volume ratio of 50:50 was injected to fabricate a coin-type half-cell.

[0123] Example 2.

[0124] A pre-lithiated negative electrode and a lithium secondary battery using the pre-lithiated negative electrode were manufactured in the same manner as in Example 1, except that when forming the lithium metal layer, a lithium metal foil was placed on the first negative electrode active material layer and pressed instead of applying lithium metal powder on the first negative electrode active material layer. The loading amount of the negative electrode active material and the loading amount of the lithium metal were equal to those in Example 1.

[0125] Comparative Example 1.

[0126] A pre-lithiated negative electrode and a lithium secondary battery using the pre-lithiated negative electrode were prepared in the same manner as in Example 1, except that a first negative electrode active material layer 12 and a second negative electrode active material layer 14 were sequentially formed on one surface of a copper current collector 30, and a lithium metal layer 20 was formed on the second negative electrode active material layer (see Figure 2 ). The loading amount of the negative electrode active material and the loading amount of the lithium metal are equal to those in Example 1.

[0127] Comparative Example 2.

[0128] A lithium secondary battery was manufactured in the same manner as in Example 1, except that a second negative electrode active material layer 14 was sequentially formed on the first negative electrode active material layer 12 without forming a lithium metal layer (see Figure 3 ), so the pre-lithiation process was not performed. The loading amount of the negative electrode active material was equal to that of Example 1.

[0129] Comparative Example 3.

[0130] Fabrication of lithium metal layer

[0131] A solution in which lithium metal powder and PVdF binder were added to THF and dispersed at a weight ratio of 95:5 was uniformly applied on one surface of the copper current collector 30 and dried to form a lithium metal layer 20 on the copper current collector 30. The loading amount of lithium metal was equal to that of Example 1.

[0132] Formation of negative electrode active material layer

[0133] A negative electrode active material slurry was prepared by adding 92 wt % of a negative electrode active material (graphite:SiO=7:3), 3 wt % of a conductive agent (Denkablack) and 5 wt % of a binder (PVdF) to THF. The slurry was uniformly applied on the formed lithium metal layer 20, and then dried and rolled to form a negative electrode active material layer 10 (see Figure 4 ). The loading amount of the negative electrode active material is equal to the sum of the loading amounts of the negative electrode active material used in the first negative electrode active material layer and the second negative electrode active material layer of Example 1.

[0134] Pre-lithiation and manufacturing of lithium secondary batteries

[0135] A lithium secondary battery was manufactured in the same manner as in Example 1 using a pre-lithiated negative electrode (a two-layer structure in which a current collector, a lithium metal layer and a negative electrode active material layer were sequentially stacked) manufactured by pre-lithiating a negative electrode having a negative electrode active material layer formed thereon in the same manner as in Example 1.

[0136] Comparative Example 4.

[0137] A pre-lithiated negative electrode (a three-layer structure of a current collector / lithium metal layer / negative electrode active material layer / lithium metal layer stacked sequentially, see Figure 5 ) and a lithium secondary battery, except that: a lithium metal layer 22 and a negative electrode active material layer 10 are sequentially formed on a copper current collector 30 in the same manner as in Comparative Example 3, and then a lithium metal layer 24 is formed again on the negative electrode active material layer 10 (by the same method as the method of forming a lithium metal layer on the current collector). The loading amount of the negative electrode active material and the loading amount of the lithium metal are equal to those in Example 1. Specifically, the loading amount of the negative electrode active material is equal to the sum of the loading amounts of the negative electrode active materials used in the first negative electrode active material layer and the second negative electrode active material layer, and the amount of lithium metal is such that the sum of the lithium metal loading amounts of the two lithium metal layers 22 and 24 is equal to the lithium loading amount of the lithium metal layer of Example 1.

[0138] Comparative Example 5.

[0139] Formation of lithium metal layer

[0140] A portion of one surface of the copper current collector 30 was masked, and a lithium layer 20 was formed on a portion of the copper current collector 30 by vacuum deposition using lithium metal as a target. The loading amount of lithium metal was equal to that of Example 1.

[0141] Formation of negative electrode active material layer

[0142] A negative electrode active material slurry was prepared by adding 92 wt % of a negative electrode active material (graphite:SiO=7:3), 3 wt % of a conductive agent (Denkablack) and 5 wt % of a binder (PVdF) to THF. The slurry was uniformly applied on the formed lithium metal layer, and then dried and rolled to form a negative electrode active material layer 10 (see Figure 6 ). The loading amount of the negative electrode active material is equal to that in Example 1.

[0143] The structure formed as described above is a structure in which the lithium metal layer is included in the negative electrode active material layer because the lithium metal layer is partially formed on the current collector.

[0144] Pre-lithiation and manufacturing of lithium secondary batteries

[0145] A lithium secondary battery was manufactured in the same manner as in Example 1 using a pre-lithiated negative electrode manufactured by pre-lithiating a negative electrode having a negative electrode active material layer formed thereon in the same manner as in Example 1.

[0146] Test Example 1. Initial reversibility test

[0147] The coin-type half-cells manufactured in the examples and comparative examples were subjected to a charge-discharge reversibility test using an electrochemical charge-discharge device. The cells were charged by applying a current at a current density of 0.1C until the voltage reached 0.005 V (vs. Li / Li + ), and discharged at the same current density until the voltage reached 1.5 V. Here, the initial reversibility was determined by the ratio of the charge capacity to the discharge capacity, and the results are shown in Table 1 below.

[0148] [Table 1]

[0149]

[0150] In the case of Examples 1 and 2, since lithium metal exists between the negative electrode active material layers to widen the contact area with the negative electrode active material, the lithium metal is fully ionized and diffused into the negative electrode active material during pre-lithiation, so that all the lithium metal is used for pre-lithiation and very little lithium metal remains, thereby improving the initial efficiency of the negative electrode.

[0151] On the other hand, in the case of Comparative Example 1, since the negative electrode active material exists only in the layer below the lithium metal layer, the contact area between the negative electrode active material and the lithium metal is small, and therefore, the lithium metal is not all used for pre-lithiation, and the lithium metal remains as unreacted lithium metal, and therefore, pre-lithiation may not be fully achieved, and thus the initial efficiency is not significantly improved.

[0152] In the case of Comparative Example 2, since no lithium metal layer was included, pre-lithiation could not be performed, and thus the initial efficiency was very low.

[0153] In the case of Comparative Examples 3 to 5, since the negative electrode active material layer was not formed to surround the lithium metal layer above and below, pre-lithiation was not sufficiently achieved compared with the case of Examples 1 and 2, and thus the initial efficiency was very low.

[0154] [Explanation of Reference Numerals]

[0155] 10: Negative electrode active material layer

[0156] 12 and 112: first negative electrode active material layer

[0157] 14 and 114: second negative electrode active material layer

[0158] 20, 22, 24 and 120: Lithium metal layer

[0159] 30 and 130: negative electrode current collector

Claims

1. A method for pre-lithiation of a negative electrode for a lithium secondary battery, the method comprising: forming a first negative electrode active material layer on the negative electrode current collector; forming a lithium metal layer including lithium metal on the first negative electrode active material layer; wherein the lithium metal layer is made of lithium metal powder or lithium metal foil; forming a second negative electrode active material layer on the lithium metal layer; and impregnating the resultant with a pre-lithiation solution at a temperature of 10° C. to 200° C. for 2 to 48 hours to perform pre-lithiation; wherein the first negative electrode active material layer and the second negative electrode active material layer each independently include a silicon-based negative electrode active material and a carbon-based negative electrode active material in a weight ratio of 1:99 to 50:50; The carbon-based negative electrode active material is one or two selected from the group consisting of artificial graphite and natural graphite.

2. The method of claim 1, wherein the pre-lithiation solution comprises an ionizable lithium salt and an organic solvent. 3 . The method according to claim 1 , wherein the impregnation is performed at a temperature of 20° C. to 70° C. for 2 hours to 36 hours.

4. A negative electrode for a lithium secondary battery prepared by the method according to any one of claims 1 to 3, the negative electrode comprising: a first negative electrode active material layer formed on the negative electrode current collector and including a first negative electrode active material; a lithium metal layer formed on the first negative electrode active material layer and including lithium metal; and a second negative electrode active material layer formed on the lithium metal layer and including a second negative electrode active material; wherein the first negative electrode active material layer and the second negative electrode active material layer each independently include a silicon-based negative electrode active material and a carbon-based negative electrode active material in a weight ratio of 1:99 to 50:50; The carbon-based negative electrode active material is one or two selected from the group consisting of artificial graphite and natural graphite.

5. The negative electrode for a lithium secondary battery according to claim 4, wherein the lithium metal layer is formed by adding and dispersing lithium metal powder and a binder in an organic solvent and then uniformly applying the solution to the first negative electrode active material layer, or by placing a lithium metal foil on the first negative electrode active material layer and pressing. 6 . The negative electrode for a lithium secondary battery according to claim 4 , wherein the first negative electrode active material layer and the second negative electrode active material layer each independently include a silicon-based negative electrode active material and a carbon-based negative electrode active material in a weight ratio of 5:95 to 20:

80.

7. A method for manufacturing a lithium secondary battery, the method comprising: forming a first negative electrode active material layer on the negative electrode current collector; forming a lithium metal layer including lithium metal on the first negative electrode active material layer; wherein the lithium metal layer is made of lithium metal powder or lithium metal foil; forming a second negative electrode active material layer on the lithium metal layer; impregnating the resultant with a pre-lithiation solution at a temperature of 10° C. to 200° C. for 2 to 48 hours, thereby achieving pre-lithiation to manufacture a pre-lithiated negative electrode for a lithium secondary battery; manufacturing an electrode assembly including the negative electrode, the positive electrode, and a separator and housing the electrode assembly in a battery case; and injecting an electrolyte solution into the battery housing; wherein the first negative electrode active material layer and the second negative electrode active material layer each independently include a silicon-based negative electrode active material and a carbon-based negative electrode active material in a weight ratio of 1:99 to 50:50; The carbon-based negative electrode active material is one or two selected from the group consisting of artificial graphite and natural graphite.

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