Anode for lithium secondary battery and lithium secondary battery including the same

Abstract

One embodiment of the present invention provides a negative electrode for a lithium secondary battery and a lithium secondary battery including the same, which substantially suppresses volume expansion of a silicon-based active material included in the negative electrode, thereby providing excellent lifespan characteristics, and can substantially alleviate process problems such as electrode detachment during notching, and problems such as cracking and electrode detachment during the charging / discharging process after battery fabrication. The negative electrode for the lithium secondary battery comprises a current collector; a first negative composite layer formed on at least one surface of the current collector; and a second negative composite layer formed on the first negative composite layer, wherein the first negative composite layer and the second negative composite layer each comprise a carbon-based active material, the first negative composite layer comprises a first binder, a first silicon-based active material, and a first conductive material, and the second negative composite layer comprises a second binder, a second silicon-based active material, and a second conductive material, wherein the content of the first conductive material and the second conductive material differs from each other based on the total weight of the first negative composite layer and the second negative composite layer, and the types of the first silicon-based active material and the second silicon-based active material differ from each other.

Description

Technology Field

[0001] The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the same, and more specifically, to a negative electrode for a high-capacity lithium secondary battery having a multilayer structure and excellent lifespan characteristics, and a lithium secondary battery including the same. Background Technology

[0002] With the recent growing interest in environmental issues, extensive research is being conducted on electric vehicles (EVs) and hybrid electric vehicles (HEVs) that can replace fossil fuel-using vehicles, such as gasoline and diesel cars, which are one of the major causes of air pollution.

[0003] Lithium-ion batteries with high discharge voltage and output stability are primarily used as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs). Furthermore, as the need for high-energy secondary batteries with high energy density increases, the development and research of high-capacity anodes for this purpose are also actively underway.

[0004] Recently, silicon-based active materials with a higher discharge capacity than graphite are being applied to negative electrodes for secondary batteries in order to realize high-capacity and high-energy-density secondary batteries. However, these silicon-based active materials have a large volume expansion rate compared to graphite and cause relatively large shrinkage / expansion during repeated charging and discharging processes of the battery, which can lead to delamination of the active material layer, increased internal resistance of the electrode, adverse reactions with the electrolyte, and deterioration of the electrode's life characteristics. The problem to be solved

[0005] One objective of the present invention is to provide a negative electrode for a high-capacity lithium secondary battery in which the volume expansion of a silicon-based active material is substantially suppressed, and a lithium secondary battery including the same.

[0006] One objective of the present invention is to provide a negative electrode for a lithium secondary battery that has excellent adhesion between a current collector and an active material and can improve the resistance characteristics and lifespan characteristics of the battery, and a lithium secondary battery including the same. means of solving the problem

[0007] A negative electrode for a lithium secondary battery according to one embodiment of the present invention comprises: a current collector; a first negative composite layer formed on at least one surface of the current collector; and a second negative composite layer formed on the first negative composite layer, wherein the first negative composite layer and the second negative composite layer each comprise a carbon-based active material, the first negative composite layer comprises a first binder, a first silicon-based active material, and a first conductive material, and the second negative composite layer comprises a second binder, a second silicon-based active material, and a second conductive material, wherein the content of the first conductive material and the second conductive material differs from each other based on the total weight of the first negative composite layer and the second negative composite layer, and the types of the first silicon-based active material and the second silicon-based active material differ from each other.

[0008] The first silicon-based active material includes a silicon oxide-based active material, and the second silicon-based active material may include a Si-C composite.

[0009] The types of the first binder and the second binder mentioned above may differ from each other.

[0010] The second binder above may include at least one of a polyacrylic acid (PAA)-based binder, a polyvinyl alcohol (PVA)-based binder, and a polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA copolymer)-based binder.

[0011] The content of polyacrylic acid monomer in the polyacrylic acid (PAA)-based binder; the content of polyvinyl alcohol monomer in the polyvinyl alcohol (PVA)-based binder; or the content of polyvinyl alcohol-polyacrylic acid copolymer monomer in the polyvinyl alcohol-polyacrylic acid (PVA-PAA-Copolymer) copolymer-based binder may be 10 to 90 mol% based on the total content of each monomer.

[0012] The content of the first conductive material may be greater than the content of the second conductive material based on the total weight of the first cathode composite layer and the second cathode composite layer.

[0013] The content of the second silicon-based active material may be greater than the content of the first silicon-based active material based on the total weight of the first cathode composite layer and the second cathode composite layer.

[0014] The content ratio of the first silicon-based active material and the second silicon-based active material may be 1:9 to 4:6 based on the total weight of the first cathode composite layer and the second cathode composite layer.

[0015] The first binder above may include styrene-butadiene rubber (SBR).

[0016] The content of butadiene monomer in the above styrene-butadiene rubber (SBR) may be 50 to 95 mol% based on the total content of the monomer.

[0017] The types of the first and second conductive materials mentioned above may differ from each other.

[0018] The Raman R value is expressed by the following Equation 1, and the negative electrode for the lithium secondary battery may have a Raman R value of the first conductive material greater than or equal to the Raman R value of the second conductive material.

[0019] [Equation 1]

[0020] Raman R = I D / I G

[0021] In the above Equation 1, the I Dis 1350 to 1380 cm -1 It is the peak intensity value of the absorption region, and the above I G is 1580 to 1600 cm -1 This is the peak intensity value of the absorption region.

[0022] The Raman R value of the first conductive material may be 0.1 to 1.8, and the Raman R value of the second conductive material may be 0.01 to 0.1.

[0023] The loading weight (LW) ratio of the first cathode composite layer and the second cathode composite layer may be 2:8 to 5:5.

[0024] The adhesion strength between the above current collector and the first cathode composite layer may be 0.45 N / 18 mm or more.

[0025] A lithium secondary battery according to one embodiment of the present invention may include the negative electrode for a lithium secondary battery described above. Effects of the invention

[0026] According to one embodiment of the present invention, the volume expansion of a silicon-based active material included in a negative electrode for a lithium secondary battery is substantially suppressed, so that the capacity characteristics and lifespan characteristics of the lithium secondary battery can be excellent.

[0027] According to one embodiment of the present invention, a negative electrode for a lithium secondary battery and a lithium secondary battery including the same can be provided, in which process problems such as electrode detachment during notching and problems such as cracking and electrode detachment during the charging / discharging process after battery fabrication are substantially alleviated. Brief explanation of the drawing

[0028] FIG. 1 is a schematic cross-sectional view showing the structure of a negative electrode for a lithium secondary battery according to one embodiment of the present invention. Specific details for implementing the invention

[0029] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0031] When silicon-based active materials are included to achieve high capacity characteristics in a negative electrode for a lithium secondary battery, problems such as electrode detachment and reduced lifespan characteristics due to a large volume expansion rate exist. Accordingly, in the case of a negative electrode containing silicon-based active materials, it is difficult to secure high capacity characteristics while simultaneously satisfying adhesion between the current collector and the active material, resistance characteristics, and lifespan characteristics.

[0032] Accordingly, the inventors of the present invention have confirmed that the above-mentioned problems can be substantially resolved by appropriately controlling the composition of each layer in a negative electrode for a lithium secondary battery with a multilayer structure, in particular, the type of silicon-based active material included in each layer, the content of the conductive material, and the type of binder. With reference to FIG. 1, the present invention will be described in more detail below through embodiments.

[0034] FIG. 1 is a schematic cross-sectional view showing the structure of a negative electrode for a lithium secondary battery according to one embodiment of the present invention.

[0036] Negative electrode (100) for lithium secondary battery

[0037] A negative electrode (100) for a lithium secondary battery according to one embodiment of the present invention comprises: a current collector (10); a first negative electrode composite layer (21) formed on at least one surface of the current collector; and a second negative electrode composite layer (22) formed on the first negative electrode composite layer, wherein the first negative electrode composite layer and the second negative electrode composite layer each comprise a carbon-based active material, the first negative electrode composite layer comprises a first binder, a first silicon-based active material, and a first conductive material, and the second negative electrode composite layer comprises a second binder, a second silicon-based active material, and a second conductive material, wherein the content of the first conductive material and the second conductive material differs from each other based on the total weight of the first negative electrode composite layer and the second negative electrode composite layer, and the types of the first silicon-based active material and the second silicon-based active material differ from each other.

[0038] The above-mentioned negative electrode (100) for a lithium secondary battery has a multilayer structure and includes a negative electrode composite layer (20) on a current collector (10), and the negative electrode composite layer includes a first negative electrode composite layer (21) and a second negative electrode composite layer (22).

[0039] The first cathode composite layer (lower layer) is an active material layer on one side adjacent to the current collector, and the second cathode composite layer (upper layer) is an active material layer formed on the first cathode composite layer and relatively spaced apart from the current collector.

[0040] A negative electrode (100) for a lithium secondary battery according to one embodiment of the present invention includes a first silicon-based active material and a second silicon-based active material, which are different types of silicon-based active materials, in the first negative electrode composite layer (lower layer) and the second negative electrode composite layer (upper layer), respectively, and accordingly, the content of the first conductive material and the second conductive material, which are conductive materials included in each layer, may also be different.

[0042] The types of the first silicon-based active material and the second silicon-based active material may differ from each other. Specifically, the first silicon-based active material may include a silicon oxide-based active material, and the second silicon-based active material may include a Si-C composite.

[0043] The above silicon oxide-based active material is SiO x(0 <x<2)의 화학식으로 표현되는 화합물이며, Si-C 복합체는 SiC의 화학식으로 표현되는 화합물일 수 있다.

[0044] Generally, while Si-C composites have high capacity and low resistance characteristics compared to conventional silicon oxide-based active materials, the breakdown of the active material's structure during the charging and discharging process can cause a short circuit (isolation) between the Si-C composite and the carbon-based active material, which may degrade the battery's lifespan characteristics.

[0045] Therefore, when a silicon oxide-based active material with relatively less structural collapse during the charging and discharging process is applied to the first cathode composite layer (lower layer) formed on one side adjacent to the current collector, unlike when a Si-C composite is applied, problems such as short circuit (isolation) between the carbon-based active material and the silicon oxide-based active material within the electrode can be mitigated, and at the same time, a Si-C composite having relatively high capacity and low resistance characteristics can be applied to the second cathode composite layer (upper layer) to simultaneously secure high energy density.

[0047] The content of the second silicon-based active material may be greater than the content of the first silicon-based active material based on the total weight of the first cathode composite layer and the second cathode composite layer.

[0048] The content ratio of the first silicon-based active material and the second silicon-based active material may be 1:9 to 4:6 based on the total weight of the first cathode composite layer and the second cathode composite layer.

[0049] The content of the first silicon-based active material may be 0.1 to 10 weight%, 0.5 to 5 weight%, or 1 to 3 weight% based on the total weight of the first cathode composite layer, and the content of the second silicon-based active material may be 0.5 to 20 weight%, 1 to 10 weight%, or 4 to 10 weight% based on the total weight of the second cathode composite layer.

[0050] When the content and content ratio of the first silicon-based active material and the second silicon-based active material are as described above, the silicon-based active material content of the first negative composite layer (lower layer) formed on one surface adjacent to the current collector can be adjusted to a relatively low level to mitigate problems such as electrode detachment due to volume expansion, and the low resistance characteristics and high lifespan characteristics of the electrode can be secured by adjusting the content of the silicon-based active material, which has a lower lithium ion diffusion rate compared to the carbon-based active material, to a relatively high level in the second negative composite layer (upper layer).

[0052] The first binder may include a rubber-based binder. For example, the rubber-based binder may be at least one selected from the group consisting of styrene-butadiene rubber (SBR), fluororubber, ethylene propylene rubber, butyl acrylate rubber, butadiene rubber, isoprene rubber, acrylonitrile rubber, acrylic rubber, and silane rubber.

[0053] Specifically, the first binder may include styrene-butadiene rubber (SBR). In this case, the adhesion between the first cathodic composite layer (lower layer) formed on one side adjacent to the current collector and the current collector may be further improved.

[0054] In the present invention, the styrene-butadiene rubber (SBR) included in the first binder refers to a polymer comprising repeating units of a styrene-derived structure and repeating units of a butadiene-derived structure. In the styrene-butadiene rubber, the repeating units of a butadiene-derived structure may be repeating units of a structure derived from 1,3-butadiene or derivatives thereof, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, or 2-ethyl-1,3-butadiene. In addition, the repeating unit of the styrene-derived structure may be a repeating unit of a structure derived from styrene or a derivative thereof, which is an aromatic vinyl compound such as styrene, α-methylstyrene, p-methylstyrene, 3-methylstyrene, 4-methylstyrene, 4-propylstyrene, 1-vinylnaphthalene, 4-cyclohexylstyrene, 4-(p-methylphenyl)styrene, 1-vinyl-5-hexylnaphthalene, etc.

[0055] When the first binder is as described above, the multilayer structure electrode can have excellent flexibility and adhesion, thereby substantially mitigating problems such as electrode detachment during the process or cracking during the charging / discharging process.

[0056] The content of butadiene monomer in the above styrene-butadiene rubber (SBR) may be 50 to 95 mol% based on the total content of the monomer.

[0057] If the butadiene monomer content in the styrene-butadiene rubber (SBR) is less than 50 mol% based on the total monomer content, the adhesion between the current collector and the active material layer is low, so delamination of the outermost die-cut surface may occur when immersed in the electrolyte, and if it exceeds 90 mol%, the electrical resistance is high and the affinity with the electrolyte is low, which may cause problems such as increased battery resistance.

[0058] The first cathode composite layer may further include a thickener. When the thickener is included in the cathode composite layer together with a binder, viscosity is further imparted, which can further improve electrode adhesion.

[0059] The above-mentioned thickener may include a cellulose-based compound, for example, one or more of carboxymethylcellulose (CMC), hydroxypropylmethylcellulose, methylcellulose, or alkali metal salts thereof may be used in a mixture. Na, K, or Li may be used as the alkali metal. Specifically, the above-mentioned thickener may include carboxymethylcellulose (CMC) or an alkali metal salt thereof.

[0061] The second binder above may include rubber-based binders such as styrene-butadiene rubber (SBR), fluoropolymer, ethylene propylene rubber, butyl acrylate rubber, butadiene rubber, isoprene rubber, acrylonitrile rubber, acrylic rubber, and silane rubber; water-soluble polymer-based binders such as polyacrylic acid (PAA)-based binders, polyvinyl alcohol (PVA)-based binders, and polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA copolymer)-based binders, or combinations thereof. A detailed description of the rubber-based binder above is omitted as it overlaps with the description given above.

[0062] The second cathode composite layer may further include a thickening agent. The thickening agent may include a cellulose-based compound. A detailed description of the thickening agent is omitted as it overlaps with the description above.

[0064] The types of the first binder and the second binder mentioned above may differ from each other.

[0065] Specifically, the first cathode composite layer (lower layer) may include a binder capable of improving the flexibility and adhesion of the electrode, and the second cathode composite layer (upper layer) may include a binder capable of mitigating volume expansion of the silicon-based active material.

[0066] More specifically, the second binder may include at least one of a polyacrylic acid (PAA)-based binder, a polyvinyl alcohol (PVA)-based binder, and a polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA copolymer)-based binder. In this case, the second binder may not include a rubber-based binder and a thickener.

[0067] The above-mentioned water-soluble polymer binders have brittle characteristics with substantially little volume change, and when included together with silicon-based active materials, they can substantially alleviate the volume expansion of silicon-based active materials.

[0068] Accordingly, if the second binder includes a binder of the same type as the first binder, unlike the first binder, the adhesion between the first negative composite layer, which is an active material layer on one side adjacent to the current collector, and the current collector can be improved and the flexibility of the multilayer structure electrode can be secured, and even if a silicon-based active material with a higher content is applied to the second negative composite layer on the first negative composite layer, a negative electrode for a high-capacity lithium secondary battery can be provided in which volume expansion, etc., is substantially mitigated.

[0069] The content of polyacrylic acid monomer in the polyacrylic acid (PAA)-based binder; the content of polyvinyl alcohol monomer in the polyvinyl alcohol (PVA)-based binder; or the content of polyvinyl alcohol-polyacrylic acid copolymer monomer in the polyvinyl alcohol-polyacrylic acid (PVA-PAA-Copolymer) copolymer-based binder may be 10 to 90 mol% based on the total content of each monomer.

[0070] When the monomer content in the binder is less than 10 mol%, the content of the monomer having brittle characteristics is low, so there is a limit to substantially suppressing the volume expansion of the silicon-based active material. On the other hand, when the monomer content in the binder exceeds 90 mol%, the brittle characteristics of the binder become excessively large, so there is a limit to substantially suppressing the occurrence of cracks in the electrode.

[0072] The total content of the first binder and thickener in the first cathode composite layer may be 0.2 to 7 weight%, 1.0 to 5.0 weight%, and 3.0 to 4.0 weight% based on the total weight of the first cathode composite layer.

[0073] Specifically, the content of the first binder may be 0.1 to 5 weight%, 0.7 to 3.3 weight%, or 2.0 to 2.5 weight% based on the total weight of the first cathode composite layer. If the content of the first binder is excessively low, the adhesion strength is low, which may cause delamination during the notching process, and if it is excessively high, the electrical resistance is high, which may degrade battery performance.

[0074] Specifically, the content of the thickener may be 0.1 to 2.0 weight%, 0.3 to 1.7 weight%, or 1.0 to 1.5 weight% based on the total weight of the first cathode composite layer. If the content of the thickener is too low, it is difficult to secure interlayer adhesion of the active material, which may result in scrap generation and partial detachment during the notching process, and if it is too high, the electrical resistance may increase.

[0075] The content ratio of the first binder to the thickener in the first cathode composite layer may be 1 to 5, and specifically, 1.5 to 3.

[0076] The total content of the second binder and thickener in the second cathode composite layer may be 0.1 to 5 weight%, 0.5 to 4.0 weight%, and 1.0 to 3.0 weight% based on the total weight of the second cathode composite layer.

[0077] Specifically, the content of the second binder may be 0.07 to 3.5 weight%, 0.3 to 3.2 weight%, or 1.0 to 1.5 weight% based on the total weight of the second cathode composite layer, and the content of the thickener may be 0.03 to 1.7 weight%, 0.15 to 1.6 weight%, or 0.5 to 0.7 weight% based on the total weight of the second cathode composite layer.

[0078] The content ratio of the second binder to the thickener in the second cathode composite layer may be 0.1 to 1, and specifically, 0.3 to 0.7.

[0079] As previously described, if the types of the first binder and the second binder are different, the thickener may not be present in the second cathode composite layer.

[0080] When the specific content of the first binder and the second binder is as described above, the multilayer structure electrode can have excellent flexibility and adhesion, thereby substantially mitigating problems such as electrode detachment during the process or cracking during the charging / discharging process. Due to the brittle characteristics that substantially do not change in volume, the volume expansion of the silicon-based active material can be substantially mitigated.

[0082] The contents of the first conductive material and the second conductive material may differ from each other based on the total weight of the first cathode composite layer and the second cathode composite layer. Specifically, the content of the first conductive material may be greater than the content of the second conductive material based on the total weight of the first cathode composite layer and the second cathode composite layer.

[0083] The first conductive material may be 0.05 to 5 weight%, 0.1 to 3 weight%, or 0.3 to 1 weight% based on the total weight of the first cathode composite layer, and the second conductive material may be 0.01 to 1 weight%, 0.05 to 0.5 weight%, or 0.07 to 0.3 weight% based on the total weight of the second cathode composite layer.

[0084] When the content of the first and second conductive materials is as described above, the content of the conductive material in the first cathode composite layer (lower layer) formed on one surface adjacent to the current collector is controlled to be relatively high, thereby increasing the contact points between the current collector and the first cathode composite layer. Increasing these contact points can reduce the resistance between the electrode and the current collector, and the contact points between the current collector and the electrode can be maintained even if the electrode expands during use. Furthermore, by controlling the content of the conductive material in the second cathode composite layer (upper layer) to be relatively low, the total content of the conductive material included in the electrode can be reduced to ensure economic efficiency. Additionally, by increasing the content of the active material within the composite layer, an electrode with the same energy density can be realized with a relatively lower loading weight. Moreover, if an excessive amount of conductive material is included in the upper layer, it can mitigate the problem of reduced interfacial resistance and lifespan characteristics of the electrode caused by blocking pores.

[0085] The first and second conductive materials mentioned above are used to impart conductivity to the electrode and to maintain the structure of the electrode, and may be used as they possess conductivity without causing adverse reactions with other elements of the secondary battery. Examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used.

[0086] Specifically, the first conductive material and the second conductive material may each include carbon nanotubes (CNT).

[0087] Carbon nanotubes (CNTs) have higher electron mobility compared to conventional conductive materials such as carbon black, enabling high energy density to be achieved with a small amount, and possess high strength due to their stable structure, and can substantially alleviate the volume expansion of silicon-based active materials.

[0088] Therefore, if the first conductive material and the second conductive material include carbon nanotubes (CNT), the energy density, lifespan characteristics, and resistance characteristics of the electrode may be even better.

[0089] The types of the first and second conductive materials mentioned above may differ from each other.

[0090] The Raman R value of the first conductive material may be greater than or equal to the Raman R value of the second conductive material.

[0091] The Raman R value is expressed by the following Equation 1.

[0092] [Equation 1]

[0093] Raman R = I D / I G

[0094] In the above Equation 1, the I D is 1350 to 1380 cm -1 It is the peak intensity value of the absorption region, and the above I G is 1580 to 1600 cm -1 This is the peak intensity value of the absorption region.

[0095] The above Raman R value is a parameter indicating the relative degree of crystallinity of the material, and the above I D The value represents the peak intensity of the region associated with the amorphous state, and the above I G The value represents the peak intensity of the region associated with the crystalline state. Therefore, an increase in the Raman R value indicates a lower relative degree of crystallinity of the material.

[0096] The Raman R value of the first conductive material may be 0.1 to 1.8. Specifically, the Raman R value of the first conductive material may be 0.3 to 1.3.

[0097] The Raman R value of the second conductive material may be 0.01 to 0.1. Specifically, the Raman R value of the second conductive material may be 0.01 to 0.05.

[0098] Specifically, the first conductive material may include multi-walled carbon nanotubes (MWCNT), and the second conductive material may include single-walled carbon nanotubes (SWCNT).

[0099] When the Raman R values, etc., of the first conductive material and the second conductive material are the same, the conductive material with relatively superior performance such as conductivity is included in the second cathode composite layer (upper layer) to maintain excellent performance as a whole electrode, while the conductive material with relatively low crystallinity is included in the first cathode composite layer (lower layer) to substantially alleviate problems caused by volume expansion, etc.

[0101] The first cathode composite layer and the second cathode composite layer may each include a carbon-based active material.

[0102] The above carbon-based active material may include, for example, one or more carbon-based materials selected from crystalline artificial graphite, crystalline natural graphite, amorphous hard carbon, low-crystalline soft carbon, carbon black, acetylene black, Ketjen black, Super P, graphene, and fibrous carbon.

[0103] The above carbon-based active material may be 85 to 100 weight%, 90 to 97 weight%, or 93 to 95 weight% based on the total weight of the first cathode composite layer.

[0104] The above carbon-based active material may be 80 to 100 weight%, 85 to 95 weight%, or 90 to 93 weight% based on the total weight of the first cathode composite layer.

[0106] The loading weight (LW) ratio of the first cathode composite layer and the second cathode composite layer may be 2:8 to 5:5.

[0107] The above loading weight (LW) refers to the amount of a cathode composite layer formed on a current collector, that is, a layer including an active material, a binder, a conductive material, etc., formed on the current collector, expressed in units of weight per area. At this time, the area is based on the area of ​​the current collector, and the weight is based on the total weight of the formed cathode composite layer.

[0108] The loading weight of the first cathode composite layer is 10 to 20 mg / cm² 2 It could be.

[0109] The loading weight of the second cathode composite layer is 15 to 30 mg / cm² 2 It could be.

[0110] When the loading weight (LW) ratio of the first cathode composite layer and the second cathode composite layer is as described above, by controlling the amount of the second cathode composite layer to be relatively high compared to the first cathode composite layer, a cathode with a multilayer structure having excellent capacitance characteristics, lifespan characteristics, and electrode adhesion can be provided.

[0112] The negative electrode for the lithium secondary battery may have an adhesion strength between the current collector and the first negative electrode composite layer of 0.45 N / 18 mm or more. Specifically, the adhesion strength between the current collector and the first negative electrode composite layer may be 0.5 to 1 N / 18 mm and 0.55 to 0.7 N / 18 mm.

[0113] When the adhesion between the above current collector and the first cathode composite layer is as described above, the excellent adhesion can substantially suppress problems such as detachment after multiple charges and discharges, as well as during the process.

[0115] The method for manufacturing a negative electrode for a secondary battery according to the present invention is not specifically limited and can be performed by known methods. For example, a first negative electrode slurry comprising a first solvent, a carbon-based active material, a first silicon-based active material, a first binder, and a first conductive material is applied and dried on a current collector by a method such as bar coating, casting, or spraying to form a first negative electrode composite layer, and then a second negative electrode slurry comprising a second solvent, a carbon-based active material, a second silicon-based active material, a second binder, and a second conductive material is applied and dried on the first negative electrode composite layer by a method such as bar coating, casting, or spraying.

[0116] The above solvent may be, for example, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and the amount of the above solvent used is sufficient to dissolve or disperse the active material, conductive material, and binder in consideration of the coating thickness and manufacturing yield of the composition for forming the cathode composite layer, and to have a viscosity that can exhibit excellent thickness uniformity when applied for forming the cathode composite layer.

[0118] lithium secondary battery

[0119] A lithium secondary battery according to one embodiment of the present invention may include the negative electrode for a lithium secondary battery described above.

[0120] The above lithium secondary battery may include a cathode comprising a lithium-transition metal composite oxide. Specifically, the lithium-transition metal composite oxide is Li x Ni a Co b Mn c O y (0 <x≤1.1, 2≤y≤2.02, 0<a<1, 0<b<1, 0<c<1, 0<a+b+c≤1)의 화학식으로 표시되는 NCM계 양극 활물질일 수 있다. 또한, 상기 리튬-전이금속 복합 산화물은 LiFePO4의 화학식으로 표시되는 리튬인산철(LFP)계 양극 활물질일 수도 있다.

[0121] The lithium secondary battery described above has excellent high capacity, lifespan, and resistance characteristics, making it highly suitable for use as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0123] The following examples are provided to aid in understanding the present invention; however, these examples are merely illustrative of the invention and do not limit the appended claims. It is obvious to those skilled in the art that various changes and modifications to the examples are possible within the scope and spirit of the invention, and such changes and modifications are also included within the appended claims.

[0125] Examples and Comparative Examples

[0126] 1) Preparation of the cathode

[0127] A first cathode slurry was applied and dried on a copper foil to form a first cathode composite layer, and a second cathode slurry was applied and dried on the first cathode composite layer to form a second cathode composite layer, thereby manufacturing a cathode for a lithium secondary battery of the example and comparative example. At this time, the loading weight (LW) ratio of the first cathode composite layer and the second cathode composite layer was applied as 5:5.

[0128] In addition, the types and contents of the first silicon-based active material, second silicon-based active material, first conductive material, second conductive material, first binder, and second binder applied in the examples and comparative examples are shown in Table 1 below, and artificial graphite was applied as the carbon-based active material in the remainder of the first cathode composite layer and the second cathode composite layer. In addition, a polyacrylic acid (PAA)-based binder having a polyacrylic acid (PAA) monomer content of 40 mol% in the binder was applied as the second binder in Example 1.

[0130] 2) Manufacturing of secondary batteries

[0131] A positive electrode was prepared by coating and drying a slurry containing an NCM-based active material, which is a Li-transition metal composite oxide, onto an aluminum foil. A polyolefin separator was interposed between the positive electrode and the negative electrode prepared above, and a lithium secondary battery was prepared by injecting an electrolyte in which 1M LiPF6 was dissolved in a solvent mixed with ethylene carbonate (EC) and diethyl carbonate (DEC). The prepared lithium secondary battery was applied as a secondary battery sample for the examples and comparative examples. .

[0133] Second cathode composite layer (upper layer) First cathode composite layer (lower layer) Secondary silicon-based active material Second Challenge Second binder / thickener First silicon-based active material First Challenge First binder / thickener Example 1 SiC 7% SWCNT 0.1% PAA 1.7% SiOx 2%(0 <x<2) MWCNT 0.5% SBR 2.4% CMC 1.2% Example 2 SiC 7% SWCNT 0.1% SBR 0.6% CMC 1.2% SiOx 2%(0 <x<2) MWCNT 0.5% SBR 2.4% CMC 1.2% Comparative Example 1 SiC 7% SWCNT 0.1% SBR 0.6% CMC 1.2% SiOx 2%(0 <x<2) MWCNT 0.1% SBR 2.4% CMC 1.2% Comparative Example 2 SiC 7% MWCNT 0.5% PAA 1.7% SiOx 2%(0 <x<2) MWCNT 0.5% SBR 2.4% CMC 1.2% Comparative Example 3 SiOx 14%(0 <x<2) SWCNT 0.1% SBR 0.6% CMC 1.2% SiOx 2%(0 <x<2) MWCNT 0.5% SBR 2.4% CMC 1.2% Comparative Example 4 SiOx 14%(0 <x<2) SWCNT 0.1% SBR 0.6% CMC 1.2% SiOx 2%(0 <x<2) MWCNT 0.1% SBR 2.4% CMC 1.2%

[0135] 3) Adhesion evaluation

[0136] The cathode prepared above was cut to a size of 18 mm in width and 150 mm in length. A tape with a width of 18 mm was attached to the current collector, and a roller with a load of 2 kg was used to ensure sufficient adhesion. Subsequently, the cathode composite layer was attached to one side of a tensile tester (IMADA, DS2-50N) using double-sided tape, and the tape attached to the current collector was fastened to the opposite side of the tensile tester to measure the adhesion strength. The results are shown in Table 2.

[0138] 4) Evaluation of resistance characteristics

[0139] For the above secondary battery sample, resistance characteristics were measured by adjusting the SOC to 50% at 25℃, allowing for a resting time of 1 hour, and then discharging at a current of 1C for 10 seconds, and the results are shown in Table 2. Specifically, the resistance value of the secondary battery sample was measured according to the following Equation 2, and the results are shown in Table 2.

[0140] [Equation 2]

[0141] R = (V0- V1) / I

[0142] In Equation 2 above, R is the resistance value of the secondary battery, V0 is the voltage of the secondary battery measured after a rest period of 1 hour after reaching SOC 50% at 25℃, V1 is the voltage of the secondary battery measured after discharging with a current of 1C for 10 seconds, and I is the current value of 1C.

[0144] 5) Evaluation of life characteristics

[0145] For the above secondary battery samples, the cycle of charging at 1 / 3C and discharging at 0.5C in the range of SOC 4-98% at 25℃ was repeated 300 times, and the discharge capacity retention rate relative to the initial discharge capacity was measured as %, and the results are shown in Table 2.

[0147] Electrode adhesion (N / 18mm) Resistance characteristics (mΩ) Lifespan characteristics (%) Example 1 0.57 0.85 98.5 Example 2 0.56 0.88 97.1 Comparative Example 1 0.41 1.05 94.6 Comparative Example 2 0.56 0.95 88.0 Comparative Example 3 0.42 0.89 96.2 Comparative Example 4 0.40 1.10 88.8

[0149] When comparing Examples 1 and 2 with Comparative Examples 1 to 4, it was found that the cathodes of Examples 1 and 2, which have different content of the first conductive material and the second conductive material and different types of the first silicon-based active material and the second silicon-based active material, have superior electrode adhesion compared to the cathodes of Comparative Examples 1 to 4, and also have superior resistance characteristics and lifespan characteristics.

[0150] Among them, the cathode of Example 1, which has different characteristics for the types of the first binder and the second binder, was found to have the best adhesion, resistance, and lifespan characteristics.

[0151] Considering the above results, it is determined that when the conductive material content of the upper layer and the type of silicon-based active material are applied differently to the upper and lower layers, respectively, in a multilayer cathode containing a silicon-based active material as in Examples 1 and 2, it is possible to manufacture an electrode suitable for the characteristics required for each layer and provide a cathode with excellent capacity and lifespan characteristics, as well as a secondary battery containing the same.

[0152] In addition, it is determined that if the type of binder for each layer is applied differently to suit the characteristics required for each layer, as in Example 1, a negative electrode with superior performance and a lithium secondary battery including the same can be provided. Explanation of the symbols

[0154] 100: Negative electrode for lithium secondary batteries 10: The whole house 20: Cathode composite layer 21: First cathode composite layer 22: Second cathode composite layer

Claims

Claim 1 Current collector; a first cathode composite layer formed on at least one surface of the current collector; and includes a second cathode composite layer formed on the first cathode composite layer, wherein the first cathode composite layer and the second cathode composite layer each include a carbon-based active material, the first cathode composite layer includes a first binder, a first silicon-based active material, and a first conductive material, and the second cathode composite layer includes a second binder, a second silicon-based active material, and a second conductive material, wherein the first silicon-based active material includes a silicon oxide-based active material, and the second silicon-based active material includes a Si-C composite, wherein the content of the second silicon-based active material is greater than the content of the first silicon-based active material based on the total weight of the first cathode composite layer and the second cathode composite layer, and the content of the first conductive material is greater than the content of the second conductive material based on the total weight of the first cathode composite layer and the second cathode composite layer, and the Raman R value is expressed by the following Equation 1, and The Raman R value of the first conductive material is greater than the Raman R value of the second conductive material, [Equation 1] Raman R = I D / I G (In the above Equation 1, the above I D is 1350 to 1380 cm -1 It is the peak intensity value of the absorption region, and the above I G is 1580 to 1600 cm -1 It is the peak intensity value of the absorption region.) Anode for lithium secondary battery. Claim 2 delete Claim 3 A negative electrode for a lithium secondary battery according to claim 1, wherein the types of the first binder and the second binder are different from each other. Claim 4 A negative electrode for a lithium secondary battery according to claim 3, wherein the second binder comprises at least one of a polyacrylic acid (PAA)-based binder, a polyvinyl alcohol (PVA)-based binder, and a polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA copolymer)-based binder. Claim 5 A negative electrode for a lithium secondary battery according to claim 4, wherein the content of a polyacrylic acid monomer in the polyacrylic acid (PAA)-based binder; the content of a polyvinyl alcohol monomer in the polyvinyl alcohol (PVA)-based binder; or the content of a polyvinyl alcohol-polyacrylic acid copolymer monomer in the polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA-Copolymer)-based binder is 10 to 90 mol% based on the total content of each monomer. Claim 6 delete Claim 7 delete Claim 8 A negative electrode for a lithium secondary battery according to claim 1, wherein the content ratio of the first silicon-based active material and the second silicon-based active material is 1:9 to 4:6 based on the total weight of the first negative electrode composite layer and the second negative electrode composite layer. Claim 9 A negative electrode for a lithium secondary battery according to claim 1, wherein the first binder comprises styrene-butadiene rubber (SBR). Claim 10 A negative electrode for a lithium secondary battery according to claim 9, wherein the content of butadiene monomer in the styrene-butadiene rubber (SBR) is 50 to 95 mol% based on the total content of the monomer. Claim 11 A negative electrode for a lithium secondary battery according to claim 1, wherein the types of the first conductive material and the second conductive material are different from each other. Claim 12 delete Claim 13 A negative electrode for a lithium secondary battery according to claim 1, wherein the Raman R value of the first conductive material is 0.1 to 1.8 and the Raman R value of the second conductive material is 0.01 to 0.

1. Claim 14 A negative electrode for a lithium secondary battery according to claim 1, wherein the loading weight (LW) ratio of the first negative electrode composite layer and the second negative electrode composite layer is 2:8 to 5:

5. Claim 15 A negative electrode for a lithium secondary battery according to claim 1, wherein the adhesion between the current collector and the first negative electrode composite layer is 0.45 N / 18 mm or more. Claim 16 A lithium secondary battery comprising a negative electrode for a lithium secondary battery according to any one of claims 1, 3 to 5, 8 to 11 and 13 to 15.

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