Negative electrode for lithium secondary battery, lithium secondary battery including the same, and method for manufacturing lithium secondary battery

Abstract

The present disclosure relates to a negative electrode for a lithium secondary battery, comprising: a negative electrode current collector; and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector and comprising a Si-based negative electrode active material, a conductive material, and a first binder polymer, wherein the Si-based negative electrode active material has cracks formed after activation, a second binder polymer is applied in the cracks, and the first binder polymer and the second binder polymer are of different species. The present disclosure also relates to a lithium secondary battery comprising the negative electrode and a method for manufacturing the lithium secondary battery. The lithium secondary battery exhibits improved lifespan characteristics.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2020-0132199 filed in Korea on October 13, 2020.

[0002] The present disclosure relates to a negative electrode for a lithium secondary battery, a lithium secondary battery including the negative electrode, and a method for manufacturing the lithium secondary battery. Background Art

[0003] Energy storage technology has recently received increasing attention. As the application of energy storage technology has expanded to include energy for mobile phones, video cameras, and notebook PCs, and even for electric vehicles, the demand for batteries with higher energy density that serve as power sources for these electronic devices is increasing. Lithium secondary batteries are among those that best meet this demand, and therefore, active research has been conducted on these lithium secondary batteries.

[0004] Generally, a lithium secondary battery includes a positive electrode containing lithium metal oxide, a negative electrode containing carbonaceous materials, etc., an electrolyte containing lithium salt and an organic solvent, and a separator interposed between the positive and negative electrodes so that the two electrodes can be electrically insulated from each other.

[0005] Carbonaceous materials have been frequently used as negative electrode active materials for forming the negative electrode of lithium secondary batteries. However, as the use of lithium secondary batteries has been expanded, the demand for high-capacity lithium secondary batteries has been increasing. Therefore, it is necessary to replace high-capacity negative electrode active materials with low-capacity carbonaceous materials. To meet this demand, attempts have been made to use Si as a negative electrode active material, which has a higher charge / discharge capacity than carbonaceous materials and can be electrochemically alloyed with lithium.

[0006] However, Si-based negative electrode active materials have a serious problem in that they undergo significant volume changes due to the insertion and extraction of lithium ions during charge / discharge. Si-based negative electrode active materials undergo volume expansion of 300% or more by charging, and the mechanical stress applied therein generates cracks inside and on the surface of the active material. In addition, when lithium ions are extracted by discharge, the Si-based negative electrode active material shrinks. Since these cracks are not restored again, repeated charge / discharge cycles lead to pulverization of the active material, and thus the negative electrode active material can be detached from the negative electrode current collector, or the negative electrode active material particles are detached from each other to form a dead volume that causes an electrical short circuit. Therefore, it is known that Si-based negative electrode active materials cause a rapid decrease in charge / discharge capacity as the charge / discharge cycle proceeds. In addition, while the interface exposed to the active material is increased by cracks, side reactions with the electrolyte occur, resulting in continuous consumption of lithium ions and electrolyte.

[0007] Under these circumstances, there is an urgent need for a technology capable of preventing problems caused by volume changes in Si-based negative electrode active materials. Summary of the Invention

[0008] Technical issues

[0009] The present disclosure is designed to solve the problems of the related art, and thus the present disclosure is directed to providing a negative electrode for a lithium secondary battery capable of preventing problems caused by volume change of a negative electrode active material, and a lithium secondary battery including the same.

[0010] The present disclosure also relates to providing a method for manufacturing the lithium secondary battery, and a lithium secondary battery obtained thereby.

[0011] Technical Solution

[0012] In one aspect of the present disclosure, there is provided a negative electrode for a lithium secondary battery according to any one of the following embodiments.

[0013] According to a first embodiment, there is provided a negative electrode for a lithium secondary battery, comprising:

[0014] a negative electrode current collector; and

[0015] a negative electrode active material layer disposed on at least one surface of the negative electrode current collector and comprising a Si-based negative electrode active material, a conductive material, and a first binder polymer,

[0016] wherein the Si-based negative electrode active material has cracks formed after activation of a lithium secondary battery including the negative electrode for a lithium secondary battery,

[0017] A second adhesive polymer is coated in the cracks, and

[0018] The first binder polymer and the second binder polymer are of different species.

[0019] According to a second embodiment, there is provided the negative electrode for a lithium secondary battery according to the first embodiment.

[0020] The second binder polymer may include a copolymer of a first monomer derived from vinylidene fluoride (VDF) and a second monomer derived from hexafluoropropylene (HFP), and the second monomer may be present in an amount of 20 wt % or more based on 100 wt % of the copolymer.

[0021] According to a third embodiment, the negative electrode for a lithium secondary battery according to the first embodiment or the second embodiment is provided.

[0022] The Si-based negative electrode active material may include Si, SiOx (1≤x≤2), Si / C, or two or more thereof.

[0023] According to a fourth embodiment, there is provided the negative electrode for a lithium secondary battery according to any one of the first to third embodiments.

[0024] The first binder polymer may include polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR), or two or more thereof.

[0025] In yet another aspect of the present disclosure, a lithium secondary battery according to the following embodiment is provided.

[0026] According to a fifth embodiment, there is provided a lithium secondary battery including:

[0027] positive electrode;

[0028] A negative electrode comprising a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer being provided on at least one surface of the negative electrode current collector and comprising a Si-based negative electrode active material having cracks formed after activation of the lithium secondary battery, a conductive material, and a first binder polymer, wherein a second binder polymer is coated in the cracks; and

[0029] A separator is interposed between the positive electrode and the negative electrode and includes a porous polymer substrate and a porous coating layer provided on at least one surface of the porous polymer substrate and including a plurality of inorganic particles and the second binder polymer.

[0030] In yet another aspect of the present disclosure, provided are a method of manufacturing a lithium secondary battery according to the following embodiments and a lithium secondary battery obtained thereby.

[0031] According to a sixth embodiment, there is provided a method for manufacturing a lithium secondary battery, comprising the following steps:

[0032] (S1) preparing a positive electrode and a primary negative electrode including a Si-based negative electrode active material and a first binder polymer;

[0033] (S2) applying a slurry for forming a porous coating layer comprising inorganic particles, a second binder polymer, and a solvent for the second binder polymer on at least one surface of a porous polymer substrate, followed by drying to obtain a separator;

[0034] (S3) interposing the separator obtained in step (S2) between the positive electrode prepared in step (S1) and the primary negative electrode, and laminating them to obtain an electrode assembly;

[0035] (S4) introducing the electrode assembly obtained in step (S3) into a battery case, and injecting an electrolyte therein to obtain a primary battery;

[0036] (S5) activating the primary battery of step (S4);

[0037] (S6) heating the primary battery of step (S5) so that the second binder polymer is dissolved in the electrolyte, and allowing the primary battery to stand; and

[0038] (S7) cooling the product obtained in step (S6),

[0039] The first binder polymer is not dissolved in the electrolyte at the heating temperature of step (S6).

[0040] According to a seventh embodiment, there is provided a method of manufacturing a lithium secondary battery as defined in the sixth embodiment,

[0041] The heating temperature in step (S6) may be 70°C to 90°C.

[0042] According to an eighth embodiment, there is provided a method of manufacturing a lithium secondary battery as defined in the sixth embodiment or the seventh embodiment,

[0043] The second binder polymer may include a copolymer of a first monomer derived from vinylidene fluoride (VDF) and a second monomer derived from hexafluoropropylene (HFP), and the second monomer may be present in an amount of 20 wt % or more based on 100 wt % of the copolymer.

[0044] According to a ninth embodiment, there is provided a method of manufacturing a lithium secondary battery as defined in any one of the sixth to eighth embodiments,

[0045] The first binder polymer may include polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR), or two or more thereof.

[0046] According to a tenth embodiment, there is provided a method of manufacturing a lithium secondary battery as defined in any one of the sixth to ninth embodiments,

[0047] The second binder polymer may be used in the slurry for forming the porous coating layer in an amount of 15 wt % to 25 wt % based on 100 wt % of the combined weight of the inorganic particles and the second binder polymer.

[0048] According to an eleventh embodiment, there is provided a lithium secondary battery obtained by the method as defined in any one of the sixth to tenth embodiments.

[0049] According to a twelfth embodiment, there is provided the lithium secondary battery as defined in the eleventh embodiment,

[0050] The separator included in the resulting product of step (S7) may include the second binder polymer in an amount of 1 wt % to 5 wt % based on 100 wt % of the combined weight of the inorganic particles and the second binder polymer.

[0051] Beneficial effects

[0052] The negative electrode for a lithium secondary battery according to an embodiment of the present disclosure includes a binder polymer present in cracks formed in a Si-based negative electrode active material after activation, and the binder polymer connects these cracks to provide improved durability, and thus the lithium secondary battery including the negative electrode can provide improved life characteristics.

[0053] A method for manufacturing a lithium secondary battery according to an embodiment of the present disclosure includes heating the battery to a temperature at which a second binder polymer dissolves in an electrolyte so that the second binder polymer can be present in cracks formed in a Si-based negative active material after activation, and thus can provide a lithium secondary battery with improved durability and lifespan characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the following disclosure, are used to provide a further understanding of the technical features of the present disclosure. Therefore, the present disclosure should not be construed as being limited to the drawings.

[0055] Figure 1 is a schematic diagram illustrating a negative electrode for a lithium secondary battery according to an embodiment of the present disclosure.

[0056] Figure 2 is an expanded view illustrating a Si-based negative electrode active material in a negative electrode for a lithium secondary battery according to an embodiment of the present disclosure.

[0057] Figure 3 is a schematic flowchart illustrating a method of manufacturing a lithium secondary battery according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0058] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terms used in the specification and the appended claims should not be interpreted as being limited to the common meanings and dictionary meanings, but should be interpreted based on the meanings and concepts of the technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to appropriately define the terms for the best interpretation.

[0059] Therefore, the descriptions presented herein are merely preferred examples for illustrative purposes and are not intended to limit the scope of the present disclosure, and it should be understood that other equivalents and modifications may be made thereto without departing from the scope of the present disclosure.

[0060] In one aspect of the present disclosure, there is provided a negative electrode for a lithium secondary battery, comprising:

[0061] a negative electrode current collector; and

[0062] a negative electrode active material layer disposed on at least one surface of the negative electrode current collector and comprising a Si-based negative electrode active material, a conductive material, and a first binder polymer,

[0063] wherein the Si-based negative electrode active material has cracks formed after activation, the second binder polymer is coated in the cracks, and

[0064] The first binder polymer and the second binder polymer are of different species.

[0065] Figure 1 is a schematic diagram illustrating a negative electrode for a lithium secondary battery according to an embodiment of the present disclosure.

[0066] Reference Figure 1 , a negative electrode 1 for a lithium secondary battery includes a negative electrode current collector 10 .

[0067] The negative electrode current collector 10 is not particularly limited as long as it includes a material conventionally used for negative electrode current collectors. According to an embodiment of the present disclosure, the negative electrode current collector 10 may include a foil made of copper, gold, nickel, a copper alloy, or two or more thereof.

[0068] According to an embodiment of the present disclosure, the thickness of the negative electrode current collector 10 is not particularly limited, but the negative electrode current collector may have a thickness of 3 μm to 500 μm.

[0069] Reference Figure 1 , the negative electrode 1 for a lithium secondary battery includes a negative electrode active material layer 20 on at least one surface of a negative electrode current collector 10. The negative electrode active material layer 20 includes a Si-based negative electrode active material 21, a conductive material 22, and a first binder polymer 23.

[0070] According to the present disclosure, the Si-based negative electrode active material 21 may be provided in the form of particles.

[0071] According to an embodiment of the present disclosure, the Si-based negative active material 21 may include Si, SiO x (1≤x≤2), Si / C, or two or more thereof.

[0072] According to an embodiment of the present disclosure, when the Si-based negative electrode active material 21 is provided in the form of particles, the negative electrode active material 21 may have an average particle diameter of 0.5 μm to 5 μm, 1 μm to 4 μm, or 2 μm to 3 μm. The average particle diameter of the Si-based negative electrode active material 21 means a particle diameter (D) corresponding to 50% of the cumulative value from smaller particles calculated based on the results of measuring the particle size distribution of the particles after classification using a general particle size distribution analyzer. 50 ). The average particle diameter of the Si-based negative electrode active material can generally be measured by using X-ray diffraction (XRD) or by using an electron microscope (SEM, TEM), or the like. When the Si-based negative electrode active material meets the above-defined range, the pulverization of the Si-based negative electrode active material caused by the continuous volume change of the Si-based negative electrode active material, especially the continuous expansion and contraction thereof, can be further reduced, and the specific surface area can be increased to improve the output characteristics.

[0073] The conductive material 22 is not particularly limited as long as it has conductivity and does not cause any chemical changes in the corresponding battery. According to an embodiment of the present disclosure, the conductive material 22 may include: carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; conductive fibers such as carbon fibers or metal fibers; carbon fluoride; metal powders such as aluminum or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, or two or more thereof.

[0074] According to the present disclosure, the first binder polymer 21 binds the Si-based negative electrode active material 21 and the conductive material 22 to each other, and binds the Si-based negative electrode active material 21 and / or the conductive material 22 to the negative electrode current collector 10 to each other. The first binder polymer is not particularly limited as long as it is generally used as a binder for a negative electrode and does not dissolve in an electrolyte at a heating temperature described below.

[0075] According to an embodiment of the present disclosure, the first binder polymer 23 may include polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR), or two or more thereof.

[0076] Reference Figure 1 , the first binder polymer 23 is shown to be in linear contact with the Si-based negative electrode active material 21, the conductive material 22, and the negative electrode current collector 10, but the scope of the present disclosure is not limited thereto. For example, the first binder polymer 23 may be in point contact with the Si-based negative electrode active material 21, the conductive material 22, and the negative electrode current collector 10.

[0077] Figure 2 is an expanded view illustrating a Si-based negative electrode active material in a negative electrode for a lithium secondary battery according to an embodiment of the present disclosure.

[0078] Reference Figure 2 , cracks 24 are formed in the Si-based negative electrode active material 21 after activation. The second binder polymer 25 is coated in the cracks 24.

[0079] According to the present disclosure, the expression "the second binder polymer is coated in the cracks" means that the second binder polymer is present in the vacant spaces of the cracks. For example, this covers the second binder polymer being coated in the cracks or inserted into the cracks, and also covers the second binder polymer completely filling the cracks.

[0080] The Si-based negative electrode active material 21 undergoes severe volume changes due to the insertion and extraction of lithium ions during charge / discharge. Due to this volume change, the pores of the Si-based negative electrode active material increase as the lithium secondary battery is repeatedly cycled, causing the formation of cracks 24. The negative electrode active material is pulverized (pulverization) due to the cracks 24, so that the negative electrode active material can be separated from the negative electrode collector and / or the negative electrode active material particles are separated from each other, resulting in a decrease in conductivity between the negative electrode active material and the current collector and / or between the negative electrode active material particles. As a result, this can cause the charge / discharge capacity of the lithium secondary battery to decrease, causing the life characteristics of the lithium secondary battery to decrease.

[0081] According to the present invention, the second binder polymer 25 is present in the cracks 24 to form connections within the cracks. While the first binder polymer 23 interconnects adjacent Si-based negative electrode active materials, the second binder polymer 25 is disposed within the cracks 24 formed in each Si-based negative electrode active material to form connections within the cracks. In the negative electrode according to an embodiment of the present disclosure, even when cracks form in the negative electrode active material, the second binder polymer 25 connects the cracks and thus can prevent the negative electrode active material from being crushed by the cracks, resulting in improved durability of the negative electrode. Consequently, the lifespan characteristics of a lithium secondary battery including this negative electrode can be improved.

[0082] According to the present disclosure, the first binder polymer 23 and the second binder polymer 25 are different polymers. When the first binder polymer 23 and the second binder polymer 25 are different polymers, the second binder polymer can be coated in cracks formed in the Si-based negative active material after activation, while preventing the negative active material from being separated from the negative current collector and causing an electrical short circuit.

[0083] According to an embodiment of the present disclosure, the second binder polymer 25 may include a copolymer of a first monomer derived from vinylidene fluoride (Vinylidene Fluoride; VDF) and a second monomer derived from hexafluoropropylene (Hexafluoropropylene; HFP). When polyvinylidene fluoride-co-hexafluoropropylene is used as the second binder polymer, it shows excellent adhesion so that the cracks 24 can be well connected. Here, the second monomer may be present in an amount of 20 wt % or more, or 30 wt % or more based on 100 wt % of the copolymer. When the second monomer is present within the range defined above, the copolymer of the first monomer derived from vinylidene fluoride (Vinylidene Fluoride; VDF) and the second monomer derived from hexafluoropropylene (Hexafluoropropylene; HFP) can be easily coated in the cracks formed in the Si-based negative active material after activation.

[0084] The negative electrode for a lithium secondary battery according to an embodiment of the present disclosure may be obtained by a method of manufacturing a lithium secondary battery as described below, but is not limited thereto.

[0085] The negative electrode for a lithium secondary battery according to an embodiment of the present disclosure may be used together with a positive electrode and a separator to manufacture a lithium secondary battery.

[0086] The positive electrode applied to the lithium secondary battery according to the embodiment of the present disclosure is not particularly limited, and may include a positive electrode active material layer formed on at least one surface of a positive electrode collector.

[0087] The positive electrode current collector is not particularly limited as long as it comprises a material that can be conventionally used for a positive electrode current collector. For example, the positive electrode current collector may comprise aluminum, nickel, or a combination thereof.

[0088] The positive electrode active material may include any positive electrode active material that can be conventionally used for the positive electrode of a lithium secondary battery, and specific examples thereof include lithium-containing transition metal oxides. For example, lithium-containing transition metal oxides include LiCoC2, LiNiO2, LiMnO2, LiMn2O4, Li(Ni a Co b Mn c )O2 (0<a<1, 0<b<1, 0<c<1, a+b+c=1), LiNi 1-y Co y O2、LiCo 1-y Mn y O2、LiNi 1-y Mn y O2(0≤y<1), Li(Ni a Co b Mn c )O4(0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn 2-z Ni z O4、LiMn 2-z Co z O4 (0 < z < 2), LiCoPO4, LiFePO4, or two or more thereof. In addition to these oxides, sulfides, selenides, and halides may also be used.

[0089] A separator used in a lithium secondary battery according to an embodiment of the present disclosure includes a porous polymer substrate and a porous coating layer provided on at least one surface of the porous polymer substrate and including a plurality of inorganic particles and a second binder polymer. The separator is interposed between a positive electrode and a negative electrode and serves to insulate the positive and negative electrodes from each other.

[0090] Any porous polymer substrate may be used as long as it is conventionally used in the art. For example, the porous polymer substrate may include a polyolefin-based porous polymer membrane or a nonwoven web, but is not limited thereto.

[0091] Non-limiting examples of polyolefin-based porous polymer membranes include membranes made of polyolefin polymers such as polyethylene including high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene, polypropylene, polybutylene, polypentene, or the like, or two or more thereof.

[0092] In addition to the polyolefin-based nonwoven web, the nonwoven web may also include a nonwoven web formed of polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, or two or more thereof. The nonwoven web may have a structure of a spunbond nonwoven web or a meltblown nonwoven web including long fibers.

[0093] Although there is no particular limitation on the thickness of the porous polymer substrate, the porous polymer substrate may have a thickness of 5 μm to 50 μm. Although there are no particular limitations on the pore size and porosity of the porous polymer substrate, the pore size and porosity may be 0.01 μm to 50 μm and 10% to 95%, respectively.

[0094] The porosity and pore size of the porous polymer substrate can be measured from scanning electron microscopy (SEM) images, by using a mercury porosimeter or a capillary flow porosimeter, or by the BET 6-point method based on nitrogen adsorption flow using a porosimetry analyzer (e.g., Porosimetry Analyzer; Belsorp-II mini, Bell Japan Inc.).

[0095] The porous coating layer includes inorganic particles and a second binder polymer and is disposed on at least one surface of the porous polymer substrate in order to improve the mechanical strength of the separator and the safety of the lithium secondary battery.

[0096] There are no particular limitations on the inorganic particles, as long as they are electrochemically stable. That is, there are no particular limitations on the inorganic particles that can be used herein, as long as they are within the operating voltage range of the applicable battery (e.g., based on Li / Li + According to an embodiment of the present disclosure, the inorganic particles may include high dielectric constant inorganic particles having a dielectric constant of 5 or greater, or 10 or greater, inorganic particles having lithium ion transport capability, or two or more thereof. Non-limiting examples of inorganic particles having a dielectric constant of 5 or greater may include particles selected from BaTiO3, BaSO4, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3 (PLZT, where 0<x<1, and 0<y<1), Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnium oxide (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, Y2O3, SiO2, Al2O3, γ-AlOOH, Al(OH)3, SiC, TiO2, or the like, or a mixture of two or more thereof. However, the scope of the present disclosure is not limited thereto.

[0097] According to an embodiment of the present disclosure, although there is no particular limitation on the particle size of the inorganic particles, from the perspective of forming a coating layer having a uniform thickness and appropriate porosity, the inorganic particles may have a particle size of about 0.01 μm to 10 μm, or about 0.05 μm to 1.0 μm. Here, the average particle diameter distribution of the inorganic particles means a particle diameter (D) corresponding to 50% of the cumulative value from smaller particles calculated based on the results of measuring the particle size distribution of the particles after classification using a general particle size distribution analyzer. 50 ). The particle size distribution can be determined by laser diffraction.

[0098] According to the present invention, the second binder polymer contained in the porous coating layer is the same type as the second binder polymer coated in the cracks formed in the negative electrode active material after activation.

[0099] The content of the inorganic particles and the content of the second binder polymer included in the porous coating layer of the separator may be determined in consideration of the thickness, pore size, and porosity of the finished porous coating layer.

[0100] A lithium secondary battery including the negative electrode for a lithium secondary battery according to an embodiment of the present disclosure may include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, a lithium ion polymer secondary battery, or the like.

[0101] Although there is no particular limitation on the outer shape of the lithium secondary battery, the lithium secondary battery may be provided in the form of a cylindrical battery using a can, a pouch-type battery, a coin-type battery, or the like.

[0102] A lithium secondary battery including the negative electrode for a lithium secondary battery according to an embodiment of the present disclosure shows improved durability and thus provides improved cycle life.

[0103] A lithium secondary battery including the negative electrode for a lithium secondary battery according to an embodiment of the present disclosure may be obtained by a method of manufacturing a lithium secondary battery as described below, but the scope of the present disclosure is not limited thereto.

[0104] In yet another aspect of the present disclosure, there is provided a method for manufacturing a lithium secondary battery, comprising the following steps:

[0105] (S1) preparing a positive electrode and a primary negative electrode including a Si-based negative electrode active material and a first binder polymer;

[0106] (S2) applying a slurry for forming a porous coating layer comprising inorganic particles, a second binder polymer, and a solvent for the second binder polymer on at least one surface of a porous polymer substrate, followed by drying to obtain a separator;

[0107] (S3) interposing the separator obtained in step (S2) between the positive electrode prepared in step (S1) and the primary negative electrode, and laminating them to obtain an electrode assembly;

[0108] (S4) introducing the electrode assembly obtained in step (S3) into a battery case, and injecting an electrolyte therein to obtain a primary battery;

[0109] (S5) activating the primary battery of step (S4);

[0110] (S6) heating the primary battery of step (S5) so that the second binder polymer is dissolved in the electrolyte, and allowing the primary battery to stand; and

[0111] (S7) cooling the product obtained in step (S6),

[0112] The first binder polymer is not dissolved in the electrolyte at the heating temperature of step (S6).

[0113] Figure 3 is a schematic flowchart illustrating a method of manufacturing a lithium secondary battery according to an embodiment of the present disclosure.

[0114] Hereinafter, a method of manufacturing a lithium secondary battery according to an embodiment of the present disclosure will be explained in more detail with reference to main parts thereof.

[0115] First, a positive electrode and a primary negative electrode (S1) comprising a Si-based negative electrode active material and a first binder polymer are prepared. The positive electrode can be obtained by any conventional method known to those skilled in the art. For example, a positive electrode slurry comprising the positive electrode active material can be prepared, and then the slurry can be directly coated on the positive electrode current collector. Reference will be made to the above description of the positive electrode current collector and the positive electrode active material.

[0116] The primary negative electrode can be obtained by mixing a Si-based negative electrode active material, a conductive material, a first binder polymer, and a solvent for the first binder polymer to prepare a negative electrode slurry according to any conventional method known to those skilled in the art, and coating the slurry on a negative electrode current collector, followed by pressing and drying. Reference will be made to the above description of the Si-based negative electrode active material and the conductive material.

[0117] According to the present disclosure, the first binder polymer is characterized in that it is not dissolved in the electrolyte at the heating temperature of step (S6) as described below. When the first binder polymer is dissolved in the electrolyte at the heating temperature of step (S6) as described below, the negative electrode active material may be separated from the negative electrode current collector, causing an internal short circuit. In general, the first binder polymer is not particularly limited as long as it can be used as a binder for the negative electrode and is not dissolved in the electrolyte at the heating temperature of step (S6) as described below.

[0118] According to an embodiment of the present disclosure, the first binder polymer may be an aqueous binder polymer. Specific examples of the first binder polymer include polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber (SBR), or two or more thereof.

[0119] According to the present disclosure, the solvent for the first binder polymer may serve as a solvent capable of dissolving the first binder polymer or as a dispersion medium that cannot dissolve the first binder polymer but can disperse it, depending on the specific type of the first binder polymer.

[0120] According to an embodiment of the present disclosure, the solvent for the first binder polymer may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), or water. The negative electrode slurry may have a solid content of 50 wt % to 95 wt %, or 70 wt % to 90 wt %.

[0121] According to an embodiment of the present disclosure, the Si-based negative active material may be used in an amount of 60 wt % to 80 wt %, or 70 wt % to 99 wt %, based on the total weight of the negative active material layer.

[0122] According to an embodiment of the present disclosure, the conductive material may be used in an amount of 0.1 wt % to 20 wt %, or 10 wt % to 15 wt %, based on the total weight of the negative active material layer.

[0123] According to an embodiment of the present disclosure, the first binder polymer may be used in an amount of 0.1 wt % to 20 wt %, or 5 wt % to 10 wt %, based on the total weight of the negative active material layer.

[0124] According to an embodiment of the present disclosure, the negative electrode slurry may further include an additive including a thickener such as carboxymethyl cellulose (CMC).

[0125] In addition, a slurry for forming a porous coating layer comprising inorganic particles, a second binder polymer, and a solvent for the second binder polymer is applied to at least one surface of the porous polymer substrate, followed by drying to obtain a separator (S2). The slurry for forming a porous coating layer can be prepared by any conventional method known to those skilled in the art and applied to the porous polymer substrate and dried. With reference to the above description of the porous polymer substrate and the inorganic particles.

[0126] The second binder polymer is characterized in that it dissolves in the electrolyte due to the heating of the primary battery in step (S6) as described below. That is, the second binder polymer does not dissolve in the electrolyte at room temperature, but dissolves in the electrolyte at the heating temperature of step (S6) as described below.

[0127] According to an embodiment of the present disclosure, the second binder polymer may include a copolymer of a first monomer derived from vinylidene fluoride (VDF) and a second monomer derived from hexafluoropropylene (HFP), wherein the second monomer may be present in an amount of 20 wt% or more based on 100 wt% of the copolymer. When the second monomer is present within the above-defined range, the copolymer of the first monomer derived from vinylidene fluoride (VDF) and the second monomer derived from hexafluoropropylene (HFP) can be easily dissolved in the electrolyte in the following steps.

[0128] According to an embodiment of the present disclosure, the solvent for the second binder polymer may include acetone, tetrahydrofuran, methylene chloride, chloroform, methylethylketone, dimethylacetamide, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, or two or more thereof.

[0129] According to an embodiment of the present disclosure, the slurry for forming the porous coating layer may include the second binder polymer in an amount of 15% to 25% by weight, or about 20% by weight, based on 100% by weight of the total content of the inorganic particles and the binder polymer. When the second binder polymer is included in the slurry for forming the porous coating layer within the above-defined range, the second binder polymer is more easily dissolved in the electrolyte in an amount such that the second binder polymer can connect cracks formed in the negative electrode active material. In addition, it is easier to prevent an excessive increase in the amount of the second binder polymer that cannot be discharged from the electrolyte when the battery is cooled.

[0130] Here, it is stated that the positive electrode and the primary negative electrode are prepared before the separator. However, the scope of the present disclosure is not limited to this. For example, the positive electrode and the primary negative electrode can be prepared after the separator is prepared. In addition, the positive electrode and the primary negative electrode can be prepared simultaneously with the separator.

[0131] Then, the separator prepared in step (S2) is inserted between the positive electrode prepared in step (S1) and the primary negative electrode, and laminated to obtain an electrode assembly (S3). The electrode assembly can be obtained by any conventional method known to those skilled in the art.

[0132] According to an embodiment of the present disclosure, lamination may be performed at a temperature of 25° C. to 150° C., at a pressure of 100 kgf / cm 2 Up to 400kgf / cm 2 or under both conditions.

[0133] After that, the electrode assembly obtained from step (S3) is housed in a battery case, and an electrolyte is injected therein to obtain a primary battery (S4). The primary battery can be obtained by using any conventional method known to those skilled in the art.

[0134] According to an embodiment of the present disclosure, the battery case may be a pouch-shaped case including a receiving portion receiving an electrode assembly and an electrolyte therein and a sealing portion configured to seal the battery case to prevent the electrode assembly and electrodes from being exposed to the outside, but is not limited thereto.

[0135] The electrolyte may be injected after the electrode assembly is housed in the battery case and before the battery case is sealed, or may be injected after the battery case is sealed. The electrolyte may be injected using any conventional method known to those skilled in the art.

[0136] According to the present disclosure, the electrolyte includes a lithium salt as an electrolyte and an organic solvent capable of dissolving the lithium salt. The lithium salt may be a lithium salt conventionally used for an electrolyte of a lithium secondary battery without particular limitation, and may be an organic solvent having an A + B - For example, A + Including Li + 、Na + , K + , or a mixture of two or more thereof, such as an alkali metal cation, B - Including PF6 - 、BF4 - 、F - 、Cl - Br - , I - 、ClO4 - 、AsF6 - 、CH3CO2 - CF3SO3 - 、N(CF3SO2)2 - 、C(CF2SO2)3 - , or a mixture of two or more thereof.

[0137] The organic solvent can be an organic solvent conventionally used for the electrolyte of lithium secondary batteries without particular limitation, and specific examples thereof include ethers, esters, linear carbonates, cyclic carbonates, or mixtures of two or more thereof. Typically, the organic solvent can include cyclic carbonates, linear carbonates, or carbonate compounds as mixtures thereof.

[0138] Specific examples of the cyclic carbonate compound include, but are not limited to, ethylene carbonate (EC), propylene carbonate (PC), 1,2-butenyl carbonate, 2,3-butenyl carbonate, 1,2-pentenyl carbonate, 2,3-pentenyl carbonate, vinylene carbonate, vinyl ethylene carbonate, or halides thereof, or a mixture of two or more thereof.

[0139] Specific examples of the halide include, but are not limited to, fluoroethylene carbonate (FEC), or the like.

[0140] Specific examples of the linear carbonate compound include, but are not limited to, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methylpropyl carbonate, ethylpropyl carbonate, or a mixture of two or more thereof.

[0141] In addition, specific examples of the ether of the organic solvent include, but are not limited to, dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, or a mixture of two or more thereof.

[0142] Specific examples of the ester of the organic solvent include, but are not limited to, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, ε-caprolactone, or a mixture of two or more thereof.

[0143] Then, the primary battery of step (S4) is activated (S5).

[0144] Step (S5) is a step of charging and discharging the primary battery injected with electrolyte so that the primary battery can be activated. As the primary battery is charged, lithium is inserted into the Si-based negative electrode active material, causing volume expansion. Due to the volume expansion, cracks are formed in the negative electrode active material.

[0145] According to an embodiment of the present disclosure, in step (S5), the primary battery may be charged to 30% to 100%, 50% to 100%, or 80% to 100% of the battery capacity, and may be discharged to 50% to 0%, or 30% to 0% of the battery capacity.

[0146] According to an embodiment of the present disclosure, charging / discharging may be based on a 0.1C rate, and the primary battery may be charged in a constant current (CC) / constant voltage (CV) mode and discharged in a CC mode. The charge / discharge cutoff voltage may be set between 2.3V and 4.5V, or between 3.2V and 4.2V. However, charge / discharge conditions such as the charge / discharge mode, cutoff voltage range, and C-rate are not limited to the above-defined ranges, and appropriate ranges or conditions may be determined in consideration of the electrode active material, battery type, and battery characteristics.

[0147] According to an embodiment of the present disclosure, the activation step may be performed under a pressurized condition by applying a predetermined pressure. For example, the pressure may vary from 1 kN to 20 kN.

[0148] After that, the primary battery of step ( S5 ) is heated so that the second binder polymer is dissolved in the electrolyte, and the primary battery including the electrolyte containing the second binder polymer dissolved therein is allowed to stand ( S6 ).

[0149] Step (S6) is a step in which the temperature of the primary battery is increased to a specific temperature at which the second binder polymer can dissolve in the electrolyte, or a higher temperature, so that the second binder polymer present in the separator, particularly the porous coating of the separator, can dissolve in the electrolyte, and the primary battery is stabilized in such a manner that the electrolyte containing the second binder polymer dissolved therein can be uniformly diffused into the electrode assembly. During this step, the electrolyte containing the second binder polymer dissolved therein penetrates into the cracks formed in the negative electrode active material after step (S5). Since the electrolyte containing the second binder polymer dissolved therein needs to penetrate into the cracks formed in the negative electrode active material after step (S5), step (S6) should be performed after step (S5).

[0150] As the size of the cracks formed in the negative electrode active material after step (S5) increases, the electrolyte containing the second binder polymer dissolved therein is more likely to be present in the cracks. Therefore, a larger amount of the second binder polymer discharged from the electrolyte after the cooling step as described below may be present in the cracks formed in the negative electrode active material after step (S5).

[0151] Even if the second binder polymer present in the separator dissolves due to heating the primary battery in step (S6), the positive electrode, the primary negative electrode, and the separator are laminated to prevent the inorganic particles contained in the separator from detaching from the porous polymer substrate. Since the inorganic particles contained in the separator need to be fixed to the porous polymer substrate even after the second binder polymer dissolves in the electrolyte, step (S6) should be performed after step (S3).

[0152] The heating temperature of the primary battery may vary depending on the size and shape of the battery. According to an embodiment of the present disclosure, the heating temperature of the primary battery in step (S6) may be 70°C to 90°C, 75°C to 85°C, or about 80°C. When the heating temperature of the primary battery falls within the range defined above, the second binder polymer can be fully dissolved in the electrolyte and can be dissolved in the electrolyte at a temperature lower than the boiling point of the organic solvent contained in the electrolyte. Therefore, the second binder polymer can be easily dissolved in the electrolyte in an amount sufficient to connect the cracks formed in the negative electrode active material after step (S5). In addition, although the second binder polymer is soluble in the electrolyte, it is easier to prevent the performance degradation of the lithium secondary battery caused by the temperature near the shutdown temperature of the pores blocking the separator and the reduction in ionic conductivity.

[0153] According to an embodiment of the present disclosure, when the heating temperature of the primary battery in step (S6) is 70° C. to 90° C., the first binder polymer may not be dissolved in the electrolyte at a temperature of 90° C. or lower. The first binder polymer may be dissolved in the electrolyte at a temperature higher than 90° C. Here, the second binder polymer is not dissolved in the electrolyte at room temperature, but may be dissolved in the electrolyte at a temperature of 70° C. or higher.

[0154] The primary battery can be left standing for a period of time that varies depending on the size and shape of the battery. According to an embodiment of the present disclosure, the primary battery can be left standing for 24 hours or longer, 48 hours or longer, or 72 hours or longer. When the primary battery is left standing for a time within the above-defined range, it is possible to ensure that the electrolyte containing the second binder polymer dissolved therein has sufficient time to uniformly penetrate into the electrode assembly.

[0155] According to an embodiment of the present disclosure, step (S6) may be performed under a pressurized condition by applying a predetermined pressure. For example, the pressure may vary from 1 kN to 20 kN.

[0156] According to an embodiment of the present disclosure, the content of the second binder polymer dissolved in the electrolyte may be 60% to 95% by weight based on the amount of the second binder polymer contained in the slurry used to form the porous coating layer during the manufacture of the separator. When the content of the second binder polymer dissolved in the electrolyte satisfies the above-defined range, it is easier to prevent the inorganic particles from detaching from the porous polymer substrate and to dissolve the second binder polymer in an amount sufficient to connect the cracks formed in the negative electrode active material.

[0157] According to an embodiment of the present disclosure, the method may further include a degassing step for removing the gas generated in steps (S5) and (S6).

[0158] After that, the resultant product of step (S6) is cooled to obtain a finished lithium secondary battery (S7).

[0159] In step (S7), since the solubility of the second binder polymer with respect to the electrolyte decreases due to a decrease in temperature, the second binder polymer dissolved in the electrolyte is discharged from the electrolyte inserted into the cracks formed in the negative electrode active material after step (S6). The discharged second binder polymer connects the cracks formed in the negative electrode active material after step (S5) to improve the durability of the negative electrode and the life characteristics of the lithium secondary battery including the negative electrode.

[0160] According to an embodiment of the present disclosure, in step ( S7 ), the primary battery may be cooled to 15° C. to 45° C., 20° C. to 30° C., or about 25° C. For example, the primary battery may be cooled to room temperature.

[0161] As described above, a negative electrode for a lithium secondary battery having improved durability may be obtained by the method of manufacturing a lithium secondary battery according to an embodiment of the present disclosure.

[0162] In addition, a lithium secondary battery having improved lifespan characteristics may be obtained by using the negative electrode for a lithium secondary battery having improved durability by the method of manufacturing a lithium secondary battery according to an embodiment of the present disclosure.

[0163] In the lithium secondary battery obtained by the above method, the second binder polymer contained in the separator is dissolved and thus can remain in the separator in a significantly reduced amount compared to the amount used in the step of preparing the separator. Even if the content of the second binder polymer remaining in the separator is reduced, the separator, the positive electrode, and the primary negative electrode are laminated to prevent the inorganic particles contained in the separator from detaching from the porous polymer substrate.

[0164] The second binder polymer is dissolved in the electrolyte in step (S6) and can remain in the separator of the finished lithium secondary battery in an amount significantly reduced compared to the initial content added to the slurry for forming the porous coating layer during the manufacture of the separator. According to an embodiment of the present disclosure, the second binder polymer can be present in an amount of 1% to 5% by weight of the total content of the inorganic particles and the second binder polymer in the separator contained in the resulting product of step (S7) based on 100% by weight.

[0165] Hereinafter, the present disclosure will be explained in detail with reference to the examples. However, the following examples may be embodied in a variety of different forms and should not be construed as being limited to the exemplary embodiments set forth herein. On the contrary, these exemplary embodiments are provided so that the present disclosure will be thorough and complete and will fully convey the scope of the present disclosure to those skilled in the art.

[0166] Example 1

[0167] Cathode manufacturing

[0168] First, 97.5 wt% of Li(Ni 0.333 Co 0.334 Mn 0.333 )O2 (Sigma-Aldrich), 1.0 wt% of carbon black (Timcal) as a conductive material, and 1.5 wt% of PVDF (Solvey) as a binder polymer were added to N-methylpyrrolidone at 25° C. to prepare a positive electrode slurry. Then, 600 mg / 25 m 2 The positive electrode slurry was coated on the top surface of an aluminum foil having a thickness of 15 μm by slot-die coating to obtain a positive electrode having a thickness of 162 μm.

[0169] Fabrication of primary negative electrode

[0170] First, 80 wt% of Si particles (Sigma-Aldrich, average particle diameter (D 50 ):2.3μm), 10 wt% of carbon black (Timcal) as a conductive material, and 10 wt% of carboxymethyl cellulose (Diacell) as a first binder polymer were added to water at 25°C to prepare a negative electrode slurry. Then, 100 mg / 25m 2 The negative electrode slurry was coated on the top surface of a copper foil having a thickness of 8 μm by slot die coating to obtain a primary negative electrode having a thickness of 58 μm.

[0171] Manufacturing of partitions

[0172] First, 80 wt% of Al2O3 (Sigma-Aldrich, average particle diameter: 500 nm) as inorganic particles and 20 wt% of PVDF-HFP (Sigma-Aldrich, 20 wt% of HFP-derived monomer based on 100 wt% of PVDF-HFP) as a second binder polymer were added to N-methylpyrrolidone to prepare a slurry for forming a porous coating. The slurry for forming a porous coating was coated on the top surface of a polyethylene film (Toray) having a thickness of 9 μm by dip coating. Then, phase separation was performed at a relative humidity of 50% to obtain a separator. The finished separator had a thickness of 17.5 μm.

[0173] Manufacturing of lithium secondary batteries

[0174] The separator obtained as described above was interposed between the positive electrode obtained as described above and the primary negative electrode, and heated at 25° C. under 250 kgf / cm 2 The electrode assembly was housed in a battery case, and an electrolyte comprising 1.0 M LiPF6 lithium salt dissolved in an organic solvent containing a mixture of EC:DEC at a volume ratio of 3:7 was injected thereinto to obtain a primary battery.

[0175] The primary battery was first cycled at 0.15 C / 0.33 C and then cycled once at 0.33 C / 0.33 C for 2 cycles of activation. The primary battery was charged in constant current (CC) / constant voltage (CV) mode and discharged in CC mode with a CV cutoff current of 0.05 C.

[0176] The activated primary battery was allowed to stand at 75° C. for 24 hours to allow the second binder polymer to dissolve in the electrolyte. The primary battery was then cooled to 25° C. to complete a lithium secondary battery.

[0177] Example 2

[0178] A lithium secondary battery was obtained in the same manner as in Example 1, except that the activated primary battery was allowed to stand at 80°C.

[0179] Example 3

[0180] A lithium secondary battery was obtained in the same manner as in Example 1, except that the activated primary battery was allowed to stand at 85°C.

[0181] Example 4

[0182] Manufacturing of positive electrode, primary negative electrode, and separator

[0183] A positive electrode, a primary negative electrode, and a separator were obtained in the same manner as in Example 1, except that 75 wt % of the inorganic particles and 25 wt % of the second binder polymer were used.

[0184] Manufacturing of lithium secondary batteries

[0185] In the same manner as in Example 2, a lithium secondary battery was obtained.

[0186] Example 5

[0187] Manufacturing of positive electrode, primary negative electrode, and separator

[0188] A positive electrode, a primary negative electrode, and a separator were obtained in the same manner as in Example 1, except that 85 wt % of the inorganic particles and 15 wt % of the second binder polymer were used.

[0189] Manufacturing of lithium secondary batteries

[0190] In the same manner as in Example 2, a lithium secondary battery was obtained.

[0191] Comparative Example 1

[0192] A lithium secondary battery was obtained in the same manner as in Example 1, except that the steps of heating the activated primary battery to dissolve the second binder polymer in the electrolyte and cooling the primary battery after the primary battery was activated were not performed.

[0193] Test Example 1: Analysis of separator thickness differences before assembly of the electrode assembly and separator of a finished lithium secondary battery

[0194] The separator obtained from Example 1 before assembly of the electrode assembly, i.e., the separator obtained from step (S2), was prepared. In addition, the separator contained in the finished battery according to Example 1, i.e., the separator contained in the resulting product of step (S7), was prepared. The separator contained in the finished lithium secondary battery according to Example 1 was obtained by decomposing it from the finished lithium secondary battery according to Example 1.

[0195] The separator before assembly of the electrode assembly and the separator included in the finished lithium secondary battery according to Example 1 were observed in terms of thickness difference.

[0196] It can be seen that the separator obtained from Example 1 before assembly of the electrode assembly had a thickness of 17.5 μm.

[0197] In comparison, it can be seen that the separator included in the finished lithium secondary battery according to Example 1 had a thickness of 14.89 μm.

[0198] Therefore, it can be seen that the second binder polymer contained in the separator is dissolved in the electrolyte by the heating step.

[0199] Test Example 2: Determination of the content of the second binder polymer remaining in the separator included in the lithium secondary battery

[0200] The content of the second binder polymer remaining in the separator included in each of the lithium secondary batteries according to Examples 1 to 5 and Comparative Example 1, ie, the separator included in the resultant product of step ( S7 ), was measured. The results are shown in Table 1 below.

[0201] The content of the second binder polymer remaining in the separator contained in the finished lithium secondary battery was measured as follows.

[0202] A separator obtained by decomposing each of the finished lithium secondary batteries according to Examples 1 to 5 and Comparative Example 1 was prepared.

[0203] Thermogravimetric analysis (TGA) was used to measure the change in separator weight when repeatedly heated and cooled from 25°C to 700°C at a rate of 10°C / min, and then the content of the second binder polymer remaining in the separator contained in the finished lithium secondary battery was calculated from the change. The content of the second binder polymer is expressed based on the total content of the inorganic particles and the second binder polymer remaining in the separator.

[0204] [Table 1]

[0205]

[0206] As can be seen from Table 1, the content of the second binder polymer remaining in the separator contained in each of the finished batteries according to Examples 1 to 5 was significantly reduced compared to the content of the second binder polymer contained in the separator before assembly of the electrode assembly, that is, the separator that was not subjected to the heating step. From this result, it can be inferred that the second binder polymer contained in the separator in each of the finished batteries according to Examples 1 to 5 was dissolved in the electrolyte by the heating step, and the electrolyte containing the second binder polymer dissolved therein permeated the entire battery, so that the second binder polymer was coated in the cracks of the negative electrode active material formed after the activation step.

[0207] In contrast, the content of the second binder polymer remaining in the separator contained in the finished battery according to Comparative Example 1 showed little difference compared to the content of the second binder polymer contained in the separator before assembly of the electrode assembly, that is, the separator not subjected to the heating step. From this result, it can be inferred that the second binder polymer contained in the separator in the finished battery according to Comparative Example 1 was not dissolved in the electrolyte, and therefore the second binder polymer could not be coated in the cracks of the negative electrode active material formed after the activation step.

[0208] Test Example 3: Evaluation of the cycle life of lithium secondary batteries

[0209] Each of the lithium secondary batteries according to Examples 1 to 5 and Comparative Example 1 was subjected to a charge / discharge cycle in which the lithium secondary battery was continuously charged to 4.2 V at 1.0 C in a constant current mode and then discharged at 0.5 C. The capacity retention rate after 100 charge / discharge cycles was then measured. The results are shown in Table 2 below.

[0210] [Table 2]

[0211] Capacity retention after 100 cycles (%) Example 1 81.9 Example 2 82.5 Example 3 83.2 Example 4 84.6 Example 5 80.1 Comparative Example 1 68.2

[0212] As can be seen from Table 2, each of the lithium secondary batteries according to Examples 1 to 5 showed a capacity retention rate of 80% or higher even after 100 cycles.

[0213] Specifically, in the case of Examples 1 to 3, it can be seen that the capacity retention rate after 100 cycles increases with increasing the heating temperature of the primary battery. This is believed to be because the solubility of the second binder polymer in the electrolyte increases as the heating temperature of the primary battery increases, and thus a larger amount of the second binder polymer can be applied to the cracks in the negative electrode active material.

[0214] Referring to Examples 2, 4, and 5, it can be seen that the capacity retention rate after 100 cycles increases as the content of the second binder polymer contained in the slurry for forming the porous coating layer increases. This is believed to be because the content of the second binder dissolved in the electrolyte increases as the content of the second binder polymer contained in the slurry for forming the porous coating layer increases.

[0215] In contrast, it can be seen that the secondary battery according to Comparative Example 1 showed a capacity retention rate after 100 cycles that decreased to less than 80%. This is believed to be because the lithium secondary battery according to Comparative Example 1 did not undergo the step of heating the primary battery, and therefore the second binder polymer was not dissolved in the electrolyte and was not coated in the cracks formed in the negative electrode active material, so that the negative electrode active material may be partially separated from the negative electrode current collector and / or negative electrode active material particles, resulting in a decrease in the capacity of the lithium secondary battery.

Claims

1. A negative electrode for a lithium secondary battery, comprising: negative electrode current collector; and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector and comprising a Si-based negative electrode active material, a conductive material, and a first binder polymer, Wherein, the Si-based negative electrode active material is in the form of particles, The Si-based negative electrode active material has cracks formed after activation of a lithium secondary battery including the negative electrode for a lithium secondary battery, A second adhesive polymer is coated in the cracks, and The first binder polymer and the second binder polymer are of different species.

2. The negative electrode for a lithium secondary battery according to claim 1, wherein the second binder polymer comprises a copolymer of a first monomer derived from vinylidene fluoride and a second monomer derived from hexafluoropropylene, and The second monomer is present in an amount of 20 wt% or more based on 100 wt% of the copolymer.

3. The negative electrode for a lithium secondary battery according to claim 1, wherein the Si-based negative electrode active material comprises Si, SiO x , Si / C, or two or more thereof. 4 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the first binder polymer comprises polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber, or two or more thereof.

5. A lithium secondary battery comprising: positive electrode; A negative electrode comprising a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer being provided on at least one surface of the negative electrode current collector and comprising a Si-based negative electrode active material having cracks formed after activation of the lithium secondary battery, a conductive material, and a first binder polymer, wherein a second binder polymer is coated in the cracks; and a separator interposed between the positive electrode and the negative electrode and comprising a porous polymer substrate and a porous coating layer provided on at least one surface of the porous polymer substrate and comprising a plurality of inorganic particles and the second binder polymer, Wherein, the Si-based negative electrode active material is in the form of particles.

6. A method for manufacturing a lithium secondary battery, comprising the following steps: (S1) preparing a positive electrode and a primary negative electrode including a Si-based negative electrode active material and a first binder polymer; (S2) applying a slurry for forming a porous coating layer comprising inorganic particles, a second binder polymer, and a solvent for the second binder polymer on at least one surface of a porous polymer substrate, followed by drying to obtain a separator; (S3) interposing the separator obtained in step (S2) between the positive electrode prepared in step (S1) and the primary negative electrode, and laminating them to obtain an electrode assembly; (S4) introducing the electrode assembly obtained in step (S3) into a battery case, and injecting an electrolyte therein to obtain a primary battery; (S5) activating the primary battery of step (S4); (S6) heating the primary battery of step (S5) so that the second binder polymer in the separator is dissolved in the electrolyte, and allowing the primary battery to stand; and (S7) cooling the product obtained in step (S6), The first binder polymer is not dissolved in the electrolyte at the heating temperature of step (S6). 7 . The method for manufacturing a lithium secondary battery according to claim 6 , wherein the heating temperature of step ( S6 ) is 70° C. to 90° C.

8. The method for manufacturing a lithium secondary battery according to claim 6, wherein the second binder polymer comprises a copolymer of a first monomer derived from vinylidene fluoride and a second monomer derived from hexafluoropropylene, and The second monomer is present in an amount of 20 wt% or more based on 100 wt% of the copolymer. 9 . The method of manufacturing a lithium secondary battery according to claim 6 , wherein the first binder polymer comprises polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, styrene-butadiene rubber, or two or more thereof.

10. The method of manufacturing a lithium secondary battery according to claim 6, wherein the second binder polymer is used in the slurry for forming a porous coating layer in an amount of 15 wt% to 25 wt% based on 100 wt% of the combined weight of the inorganic particles and the second binder polymer.

11. A lithium secondary battery obtained by the method defined in any one of claims 6 to 10. 12 . The lithium secondary battery according to claim 11 , wherein the separator included in the resultant product of step ( S7 ) includes the second binder polymer in an amount of 1 wt % to 5 wt % based on 100 wt % of the combined weight of the inorganic particles and the second binder polymer.

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