Negative electrode for secondary battery and method for manufacturing negative electrode for secondary battery

Abstract

A negative electrode for a secondary battery according to an embodiment of the present invention may comprise: a negative electrode current collector; a first negative electrode active material layer formed on at least one surface of the negative electrode current collector and including a first silicon-based negative electrode active material; and a second negative electrode active material layer which is formed on the first negative electrode active material layer and contains a second silicon-based negative electrode active material, the thickness of the second negative electrode active material layer being greater than the thickness of the first negative electrode active material layer, the content of the first silicon-based negative electrode active material in the first negative electrode active material layer may be 5 wt% or less. The weight percentage of the second silicon-based negative electrode active material in the second negative electrode active material layer based on the total weight of the second negative electrode active material layer may be greater than the weight percentage of the first silicon-based negative electrode active material in the first negative electrode active material layer based on the total weight of the first negative electrode active material layer.

Description

Technical Field

[0001] Embodiments of the present invention relate to a negative electrode for a secondary battery and a method of manufacturing the negative electrode for a secondary battery. Background Art

[0002] With the development of electronics, communications and aerospace industries, the demand for lithium secondary batteries as energy sources is increasing dramatically. In particular, as the importance of global environmental protection policies is emphasized, the electric vehicle market is growing rapidly, and research and development on lithium secondary batteries are being actively carried out at home and abroad.

[0003] A lithium secondary battery includes a positive electrode (cathode), a negative electrode (anode), and a separator between the positive electrode and the negative electrode. The positive electrode and the negative electrode each contain an active material that can insert and extract lithium ions.

[0004] When lithium secondary batteries are repeatedly charged and discharged, the capacity may decrease sharply as the volume of the negative electrode active material repeatedly expands and contracts, and the negative electrode may fall off. In particular, when silicon-based negative electrode active materials are used, the volume change may be more serious. Therefore, a new negative electrode that can overcome this problem is needed. Summary of the invention

[0005] Technical issues to be solved

[0006] According to one aspect of the present invention, a negative electrode for a secondary battery having a large capacity and a long cycle life and a method for manufacturing the negative electrode for a secondary battery can be provided.

[0007] In addition, the negative electrode for a secondary battery and the method for manufacturing the negative electrode for a secondary battery of the present invention can be widely used in electric vehicles (Electric Vehicle), battery charging stations (Battery Charging Station), energy storage devices (Energy Storage System (ESS)) and other green technology fields such as solar power generation (Photovoltaics) and wind power generation (Wind Power) using batteries.

[0008] In addition, the secondary battery negative electrode and the method for manufacturing the secondary battery negative electrode of the present invention can be used in eco-friendly mobile devices including electric vehicles and hybrid vehicles to prevent climate change by suppressing air pollution and greenhouse gas emissions.

[0009] Technical Solution

[0010] According to an embodiment of the present invention, a negative electrode for a secondary battery includes: a negative electrode collector; a first negative electrode active material layer, the first negative electrode active material layer is formed on at least one side of the negative electrode collector and includes a first silicon-based negative electrode active material; and a second negative electrode active material layer, the second negative electrode active material layer is formed on the first negative electrode active material layer and includes a second silicon-based negative electrode active material, the thickness of the second negative electrode active material layer is thicker than the thickness of the first negative electrode active material layer, wherein, based on the total weight of the first negative electrode active material layer, the content of the first silicon-based negative electrode active material in the first negative electrode active material layer is 5 weight % or less, and the weight percentage of the second silicon-based negative electrode active material in the second negative electrode active material layer based on the total weight of the second negative electrode active material layer is greater than the weight percentage of the first silicon-based negative electrode active material in the first negative electrode active material layer based on the total weight of the first negative electrode active material layer.

[0011] In one embodiment, the first silicon-based negative electrode active material and the second silicon-based negative electrode active material may include a material selected from SiO x (0≤x<2), one or more of Si / C composite and Si alloy.

[0012] In one embodiment, the first negative electrode active material layer and the second negative electrode active material layer may further include a carbon-based negative electrode active material.

[0013] In one embodiment, the carbon-based negative active material may include one or more selected from artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, Super P and graphene.

[0014] In one embodiment, based on the total weight of the first negative electrode active material layer and the second negative electrode active material layer, the sum of the contents of the first silicon-based negative electrode active material and the second silicon-based negative electrode active material contained in the entire first negative electrode active material layer and the second negative electrode active material layer may be 5-20 weight %.

[0015] In one embodiment, based on the total weight of the first negative electrode active material layer and the second negative electrode active material layer, the content of the carbon-based negative electrode active material contained in the entire first negative electrode active material layer and the second negative electrode active material layer may be 75-95% by weight.

[0016] In one embodiment, a ratio of a thickness of the first negative electrode active material layer to a thickness of the second negative electrode active material layer may be 4:6 to 1:9.

[0017] In one embodiment, based on the total weight of the second negative electrode active material layer, the content of the second silicon-based negative electrode active material in the second negative electrode active material layer may be 6-35 wt %.

[0018] In one embodiment, the first negative electrode active material layer and the second negative electrode active material layer may further include a conductive material.

[0019] In one embodiment, the first negative active material layer and the second negative active material layer may further include a binder.

[0020] In one embodiment, the weight percentage of the binder in the first negative electrode active material layer based on the total weight of the first negative electrode active material layer may be greater than the weight percentage of the binder in the second negative electrode active material layer based on the total weight of the second negative electrode active material layer.

[0021] In one embodiment, the weight percentage of the carbon-based negative electrode active material in the first negative electrode active material layer based on the total weight of the first negative electrode active material layer may be greater than the weight percentage of the carbon-based negative electrode active material in the second negative electrode active material layer based on the total weight of the second negative electrode active material layer.

[0022] In one embodiment, based on the total weight of the first negative electrode active material layer, the sum of the contents of the first silicon-based negative electrode active material and the carbon-based negative electrode active material in the first negative electrode active material layer may be 85-98 wt %.

[0023] In one embodiment, based on the total weight of the second negative electrode active material layer, the sum of the contents of the second silicon-based negative electrode active material and the carbon-based negative electrode active material in the second negative electrode active material layer may be 85-98 wt %.

[0024] According to an embodiment of the present invention, a method for manufacturing a negative electrode for a secondary battery includes the following steps: coating a first composition containing a first silicon-based negative electrode active material on a negative electrode collector to form a first layer; and coating a second composition containing a second silicon-based negative electrode active material on the first layer to form a second layer thicker than the first layer, wherein, based on the total weight of solids contained in the first composition, the content of the first silicon-based negative electrode active material in the first composition is less than 5 weight%, and the weight percentage of the second silicon-based negative electrode active material in the second composition based on the total weight of the solids contained in the second composition is greater than the weight percentage of the first silicon-based negative electrode active material in the first composition based on the total weight of the solids contained in the first composition.

[0025] In one embodiment, the first composition and the second composition may further include a carbon-based negative active material.

[0026] In one embodiment, the weight percentage of the carbon-based negative electrode active material in the first composition based on the total weight of the solids contained in the first composition may be greater than the weight percentage of the carbon-based negative electrode active material in the second composition based on the total weight of the solids contained in the second composition.

[0027] In one embodiment, the first composition and the second composition may further include a binder.

[0028] In one embodiment, the weight percentage of the binder in the first composition based on the total weight of solids contained in the first composition may be greater than the weight percentage of the binder in the second composition based on the total weight of solids contained in the second composition.

[0029] In one embodiment, a ratio of the thickness of the first layer to the thickness of the second layer may be 4:6 to 1:9.

[0030] Beneficial Effects

[0031] According to the present invention, a negative electrode for a secondary battery having a large capacity and a long cycle life and a method for producing the negative electrode for a secondary battery can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a diagram for explaining a negative electrode for a secondary battery according to an example of the present invention.

[0033] Figure 2 1 is a flowchart for explaining a method for manufacturing a negative electrode for a secondary battery according to an embodiment of the present invention.

[0034] Figure 3 It is a diagram for explaining a negative electrode for a secondary battery according to a comparative example of the present invention.

[0035] Figure 4 It is a diagram for explaining a negative electrode for a secondary battery according to another comparative example of the present invention.

[0036] Figure 5 It is a diagram for explaining a negative electrode for a secondary battery according to another comparative example of the present invention.

[0037] Figure 6 1 is a simulation result showing changes in porosity of a negative electrode for a secondary battery according to an embodiment of the present invention.

[0038] Figure 7 1 is a simulation result showing the change in porosity of a negative electrode for a secondary battery according to a comparative example of the present invention.

[0039] Figure 8 1 is a simulation result showing the change in porosity of a negative electrode for a secondary battery according to another comparative example of the present invention.

[0040] Fig. 9 1 is a simulation result showing the change in porosity of a negative electrode for a secondary battery according to another comparative example of the present invention.

[0041] Fig.10 : is a graph showing the capacity retention ratio of an example of the present invention and a comparative example according to an experimental example of the present invention.

[0042] Fig.11 : is a graph showing the capacity retention ratio of an example of the present invention and a comparative example according to an experimental example of the present invention.

[0043] Description of Reference Numerals

[0044] 10: Negative electrode current collector

[0045] 20: Negative electrode active material layer

[0046] 21: First negative electrode active material layer

[0047] 22: Second negative electrode active material layer

[0048] 100: Negative electrode DETAILED DESCRIPTION

[0049] The description of the structure or function of the implementation scheme disclosed in this specification or this application is merely exemplified for the purpose of illustrating the implementation scheme according to the technical idea of ​​the present invention. The implementation scheme according to the technical idea of ​​the present invention can be implemented in various ways in addition to the implementation scheme disclosed in this specification or this application, and it is not to be construed that the technical idea of ​​the present invention is limited to the implementation scheme described in this specification or this application.

[0050] Negative electrode for secondary battery

[0051] In one aspect of the present invention, a negative electrode for a secondary battery is provided, comprising: a negative electrode collector; a first negative electrode active material layer, the first negative electrode active material layer is formed on at least one side of the negative electrode collector and comprises a first silicon-based negative electrode active material; and a second negative electrode active material layer, the second negative electrode active material layer is formed on the first negative electrode active material layer and comprises a second silicon-based negative electrode active material, the thickness of the second negative electrode active material layer is thicker than the thickness of the first negative electrode active material layer, wherein, based on the total weight of the first negative electrode active material layer, the content of the first silicon-based negative electrode active material in the first negative electrode active material layer is less than 5 weight %, and the weight percentage of the second silicon-based negative electrode active material in the second negative electrode active material layer based on the total weight of the second negative electrode active material layer is greater than the weight percentage of the first silicon-based negative electrode active material in the first negative electrode active material layer based on the total weight of the first negative electrode active material layer.

[0052] Figure 1 1 is a diagram for explaining a negative electrode for a secondary battery according to an embodiment of the present invention. Figure 1 , a negative electrode for a secondary battery according to an embodiment of the present invention is described.

[0053] refer to Figure 1 The negative electrode 100 for a secondary battery includes a negative electrode current collector 10 and a negative electrode active material layer 20 .

[0054] The negative electrode current collector 10 is not particularly limited as long as it is a material that does not cause chemical changes in the secondary battery and has conductivity, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, a material in which the surface of copper or stainless steel is treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, fine concavoconvexity can be formed on the surface to enhance the binding force of the negative electrode active material, and it can be used in various forms such as film, sheet, foil, net, porous body, foam, non-woven fabric, etc.

[0055] The negative electrode active material layer 20 may be formed on at least one side of the negative electrode collector 10 . That is, the negative electrode active material layer 20 may be formed on one side of the negative electrode collector 10 , or may be formed on both sides of the negative electrode collector 10 .

[0056] The negative electrode active material layer 20 includes a first negative electrode active material layer 21 and a second negative electrode active material layer 22 .

[0057] The first negative electrode active material layer 21 is formed on at least one side of the negative electrode current collector 10. The first negative electrode active material layer 21 includes a first silicon-based negative electrode active material.

[0058] In an embodiment, the first silicon-based negative electrode active material may include a material selected from SiO x(0≤x<2), Si / C composite and Si alloy. x corresponds to SiO x In one embodiment, SiO x The average particle size (D50) of (0≤x<2) can be 4-9 μm or 5-7 μm. When the above range is met, the life of the secondary battery can be prevented from being reduced due to the volume expansion of the silicon-based material, and the side reaction on the surface of the silicon-based material can be suppressed. In one embodiment, SiO x The specific surface area (0≤x<2) can be 4-9m 2 / g or 5-8m 2 / g. When the above range is met, the conductive network with the conductive material can be improved. The Si / C composite can be obtained by combining carbon with SiO x (0≤x<2) The particles are heat treated (fired) in a bonded state so that the carbon material is coated on the SiO x (0≤x<2) in the form of particle surface or carbon dispersed in SiO x (0≤x<2) The form inside the particle. Si alloy (Si-alloy) can be a form of Si alloyed with one or more metals selected from Zn, Al, Mn, Ti, Fe and Sn.

[0059] Compared with carbon-based materials, silicon-based negative electrode active materials show higher capacity and can therefore achieve high energy density per unit volume.

[0060] In the embodiment, the content of the first silicon-based negative electrode active material in the first negative electrode active material layer 21 may be 5 wt % or less, based on the total weight of the first negative electrode active material layer 21. Specifically, based on the total weight of the first negative electrode active material layer 21, the content of the first silicon-based negative electrode active material in the first negative electrode active material layer 21 may be 0.2-4.5 wt %, more specifically 0.5-4 wt %.

[0061] When the first negative electrode active material layer 21 includes the first silicon-based negative electrode active material in an amount equal to the above range, the first negative electrode active material layer 21 may have a higher capacity and prevent the conductivity from being reduced due to volume change due to the inclusion of the silicon-based active material.

[0062] In an embodiment, the first negative electrode active material layer 21 may further include a carbon-based negative electrode active material. The carbon-based negative electrode active material may include one or more selected from artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, SuperP and graphene.

[0063] In one embodiment, the content of the carbon-based negative electrode active material in the first negative electrode active material layer 21 may be 70-99 wt %, specifically 80-98 wt %, and more specifically 90-97 wt %, based on the total weight of the first negative electrode active material layer 21. Within the above range, the binding force of the negative electrode may be increased, and the conductivity in the negative electrode may be improved.

[0064] Based on the total weight of the first negative electrode active material layer 21, the sum of the contents of the first silicon-based negative electrode active material and the carbon-based negative electrode active material in the first negative electrode active material layer 21 may be 80-99.5% by weight, specifically 80-99% by weight, and more specifically 85-98% by weight. Within the above range, the energy density and capacity of the negative electrode can be increased, and the long cycle performance can be improved.

[0065] In the embodiment, the first negative electrode active material layer 21 may further include a conductive material. The conductive material may impart conductivity to the negative electrode 100 .

[0066] The conductive material may include, for example, one or more selected from metal-based conductive materials, carbon-based conductive materials, and conductive polymers. The metal-based conductive material may be, for example, metal powder or metal fiber such as copper, nickel, aluminum, silver, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; or conductive metal oxides such as titanium oxide. The carbon-based conductive material may be, for example, graphite, carbon black, graphene, or carbon nanotubes. The conductive polymer may be, for example, a polyphenylene derivative, etc.

[0067] In one embodiment, the first negative electrode active material layer 21 may include carbon nanotubes as a conductive material. Carbon nanotubes form a conductive network between silicon-based materials, thereby minimizing the problem of isolated conductive paths caused by volume expansion of silicon-based materials due to long-term use of the secondary battery. Carbon nanotubes may include single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0068] In one embodiment, the first negative electrode active material layer 21 may contain a conductive material other than the single-walled carbon nanotube in consideration of economic aspects. That is, the first negative electrode active material layer 21 contains a silicon-based active material having a large volume during charge and discharge in a relatively small proportion, and thus may be advantageous in terms of conductivity, and thus other conductive materials may be used instead of the single-walled carbon nanotube having high conductivity. For example, multi-walled carbon nanotubes, graphite, carbon black, etc. may be contained as the conductive material.

[0069] Multi-walled carbon nanotubes refer to carbon nanotubes having multiple single-walled carbon nanotubes. Compared with single-walled carbon nanotubes, multi-walled carbon nanotubes are more economical, and multi-walled carbon nanotubes can be arranged on the surface of a silicon-based material together with single-walled carbon nanotubes, so that a conductive network with adjacent silicon-based materials can be formed. The average diameter of the multi-walled carbon nanotubes can be 5-200nm, 5-100nm or 5-50nm. When the above range is met, it can be easily dispersed in the negative electrode active material composition, and a conductive network can be effectively formed between silicon-based materials. The average diameter is the average value calculated after measuring the diameter of 100 multi-walled carbon nanotubes in the negative electrode active material layer by SEM. The average length of the multi-walled carbon nanotubes can be 0.1-100μm, 0.1-50μm or 0.1-3μm. When the above range is met, even if the content of the multi-walled carbon nanotubes in the negative electrode active material composition is less, the formation of the conductive network can be maximized. The average length is an average value calculated by measuring the lengths of 100 multi-walled carbon nanotubes in the negative electrode active material layer by SEM.

[0070] In the embodiment, the content of the conductive material in the first negative electrode active material layer 21 may be 0.01-3.0 wt %, specifically 0.02-2.5 wt %, and more specifically 0.05-2.0 wt %, based on the total weight of the first negative electrode active material layer 21. Within the above range, even if the content of the conductive material is small, the binding force and conductivity of the negative electrode can be greatly improved, and a secondary battery with excellent power characteristics and life characteristics can be obtained.

[0071] In an embodiment, the first negative electrode active material layer 21 may further include a binder. The binder may improve the binding force between the first negative electrode active material layer 21 and the negative electrode current collector 10 and the binding force between the negative electrode active materials. The binder may include, for example, one or more selected from polyvinylidene fluoride, carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyimide, polyamide-imide, polyvinyl alcohol, hydroxypropyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyethylene, polypropylene, acrylated styrene butadiene rubber and epoxy resin. In one embodiment, carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol may be used as a binder.

[0072] In the embodiment, the binder may further include a thickener that can enhance the cohesion of the binder. That is, in this specification, the binder and the thickener may be collectively referred to as a binder. The thickener can improve the problem of cracks on the surface of the negative electrode by enhancing the cohesion of the binder. The thickener may include, for example, one or more selected from carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose, methyl ethyl hydroxyethyl cellulose and cellulose gum.

[0073] In the embodiment, the content of the binder in the first negative electrode active material layer 21 may be 0.05-10.0 wt %, 0.1-8.0 wt %, 0.2-7.0 wt % or 0.3-5.0 wt % based on the total weight of the first negative electrode active material layer 21. When the above range is met, the increase in the resistance of the negative electrode can be minimized, and excellent negative electrode binding force can be achieved.

[0074] The second negative electrode active material layer 22 is formed on at least one side of the negative electrode current collector 10. The second negative electrode active material layer 22 includes a second silicon-based negative electrode active material.

[0075] In an embodiment, the second silicon-based negative electrode active material may include a material selected from SiO x (0≤x<2), Si / C composite and Si alloy. x corresponds to SiO x In one embodiment, SiO x The average particle size (D50) of (0≤x<2) can be 4-9 μm or 5-7 μm. When the above range is met, the life of the secondary battery can be prevented from being reduced due to the volume expansion of the silicon-based material, and the side reaction on the surface of the silicon-based material can be suppressed. In one embodiment, SiO x The specific surface area (0≤x<2) can be 4-9m 2 / g or 5-8m 2 / g. When the above range is met, the conductive network with the conductive material can be improved. The Si / C composite can be obtained by combining carbon with SiO x (0≤x<2) The particles are heat treated (sintered) in a bonded state so that the carbon material is coated on the SiO x (0≤x<2) in the form of particle surface or carbon dispersed in SiO x (0≤x<2) The form inside the particle. The Si alloy may be a form in which Si is alloyed with one or more metals selected from Zn, Al, Mn, Ti, Fe and Sn.

[0076] Compared with carbon-based materials, silicon-based negative electrode active materials show higher capacity, so high energy density can be achieved for the same unit volume. In the embodiment, the second silicon-based negative electrode active material can be the same material as the first silicon-based negative electrode active material, but is not limited thereto, and the first silicon-based negative electrode active material and the second silicon-based negative electrode active material can also be different from each other.

[0077] In the embodiment, the content of the second silicon-based negative electrode active material in the second negative electrode active material layer 22 may be 6-35 wt % based on the total weight of the second negative electrode active material layer 22. Specifically, the content of the second silicon-based negative electrode active material in the second negative electrode active material layer 22 may be 7-25 wt %, more specifically 8-20 wt %, based on the total weight of the second negative electrode active material layer 22.

[0078] In an embodiment, the weight percentage of the second silicon-based negative electrode active material in the second negative electrode active material layer 22 based on the total weight of the second negative electrode active material layer 22 can be greater than the weight percentage of the first silicon-based negative electrode active material in the first negative electrode active material layer 21 based on the total weight of the first negative electrode active material layer 21.

[0079] When the second negative electrode active material layer 22 contains the second silicon-based negative electrode active material in the same content as the above range, it can have a higher capacity due to the inclusion of the silicon-based active material. At the same time, by controlling the stress increase caused by the volume expansion of the silicon-based active material within an appropriate range, it has the effect of improving the long cycle performance.

[0080] In an embodiment, the second negative electrode active material layer 22 may further include a carbon-based negative electrode active material. The carbon-based negative electrode active material may include one or more selected from artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, SuperP and graphene.

[0081] In one embodiment, based on the total weight of the second negative electrode active material layer 22 , the content of the carbon-based negative electrode active material in the second negative electrode active material layer 22 may be 60-95 wt %, specifically 65-93 wt %, and more specifically 70-92 wt %.

[0082] In an embodiment, the weight percentage of the carbon-based negative electrode active material in the second negative electrode active material layer 22 based on the total weight of the second negative electrode active material layer 22 may be less than the weight percentage of the carbon-based negative electrode active material in the first negative electrode active material layer 21 based on the total weight of the first negative electrode active material layer 21.

[0083] Within the above range, the energy density of the negative electrode can be improved, the negative electrode binding force can be increased, and the electrical conductivity in the negative electrode can be improved.

[0084] Based on the total weight of the second negative electrode active material layer 22, the sum of the contents of the second silicon-based negative electrode active material and the carbon-based negative electrode active material in the second negative electrode active material layer 22 can be 80-99.5% by weight, specifically 80-99% by weight, and more specifically 85-98% by weight. Within the above range, the energy density and capacity of the negative electrode can be increased, and the long cycle performance can be improved.

[0085] In the embodiment, the second negative electrode active material layer 22 may further include a conductive material. The conductive material may impart conductivity to the negative electrode 100 .

[0086] The conductive material may include, for example, one or more selected from metal-based conductive materials, carbon-based conductive materials, and conductive polymers. The metal-based conductive material may be, for example, metal powder or metal fiber such as copper, nickel, aluminum, silver, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; or conductive metal oxides such as titanium oxide. The carbon-based conductive material may be, for example, graphite, carbon black, graphene, or carbon nanotubes. The conductive polymer may be, for example, a polyphenylene derivative, etc.

[0087] In one embodiment, the second negative electrode active material layer 22 may include carbon nanotubes as a conductive material. Carbon nanotubes form a conductive network between silicon-based materials, thereby minimizing the problem of isolated conductive paths caused by volume expansion of silicon-based materials due to long-term use of the secondary battery. Carbon nanotubes may include single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0088] In one embodiment, the second negative active material layer 22 may include single-walled carbon nanotubes as a conductive material.

[0089] Compared with multi-walled carbon nanotubes, single-walled carbon nanotubes have a smaller diameter and a longer length, so when using the same content of single-walled carbon nanotubes, the conductive effect can be maximized, and due to high flexibility, a conductive network can be more effectively formed between silicon-based materials, so the energy capacity retention rate of the secondary battery can be improved. The average diameter of the single-walled carbon nanotubes can be 0.1-8.0nm, 1.5-5.0nm or 1.5-3.0nm. The average diameter is the average value calculated after measuring the diameter of 100 single-walled carbon nanotubes in the negative electrode active material layer by SEM. The average length of the single-walled carbon nanotubes can be 3-20μm, 4-20μm or 5-20μm. When the above range is met, even if the volume of the silicon-based material changes excessively, the conductive network connecting the silicon-based materials can be maintained, so the energy capacity retention rate of the secondary battery can be further improved. The average length is the average value calculated after measuring the length of 100 single-walled carbon nanotubes in the negative electrode active material layer by SEM.

[0090] In the embodiment, the content of the conductive material in the second negative electrode active material layer 22 may be 0.01-3.0 wt %, specifically 0.02-2.5 wt %, and more specifically 0.05-2.0 wt %, based on the total weight of the second negative electrode active material layer 22. Within the above range, even if the content of the conductive material is small, the binding force and conductivity of the negative electrode can be greatly improved, and a secondary battery with excellent power characteristics and life characteristics can be obtained.

[0091] In an embodiment, the second negative electrode active material layer 22 may further include a binder. The binder can improve the binding force between the negative electrode active materials. The binder may include, for example, one or more selected from polyvinylidene fluoride, carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyimide, polyamide-imide, polyvinyl alcohol, hydroxypropyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyethylene, polypropylene, acrylated styrene butadiene rubber and epoxy resin. In one embodiment, carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol can be used as a binder.

[0092] In an embodiment, the binder may further include a thickener that can enhance the cohesion of the binder. The thickener can improve the problem of cracks on the surface of the negative electrode by enhancing the cohesion of the binder. The thickener may include, for example, one or more selected from carboxymethyl cellulose, methyl cellulose, hydroxypropyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose, methyl ethyl hydroxyethyl cellulose and cellulose gum.

[0093] In an embodiment, the content of the binder in the second negative electrode active material layer 22 may be 0.01-8.0 wt %, 0.05-6.0 wt %, 0.1-5.0 wt % or 0.5-3.0 wt % based on the total weight of the second negative electrode active material layer 22 .

[0094] In an embodiment, the weight percentage of the binder in the second negative active material layer 22 based on the total weight of the second negative active material layer 22 may be less than the weight percentage of the binder in the first negative active material layer 21 based on the total weight of the first negative active material layer 21 .

[0095] When the above range is satisfied, the increase in resistance of the negative electrode may be minimized, and at the same time, excellent negative electrode binding force may be achieved.

[0096] In the embodiment, the thickness of the second negative active material layer 22 is greater than that of the first negative active material layer 21. Specifically, the ratio of the thickness of the first negative active material layer 21 to the thickness of the second negative active material layer 22 may be 4.5:5.5 to 0.5:9.5, more specifically 4:6 to 1:9.

[0097] That is, the thickness of the second negative electrode active material layer 22 having a relatively high content of silicon-based negative electrode active material can be greater than the thickness of the first negative electrode active material layer 21 having a relatively low content of silicon-based negative electrode active material. Therefore, even if the second negative electrode active material layer 22 contains a large amount of silicon-based negative electrode active material, the distance between the silicon-based negative electrode active materials in the second negative electrode active material layer 22 can be relatively increased. If the thickness of the second negative electrode active material layer 22 having a relatively high content of silicon-based negative electrode active material is thinner than the thickness of the first negative electrode active material layer 21 having a relatively low content of silicon-based negative electrode active material, the distance between the silicon-based negative electrode active materials in the second negative electrode active material layer 22 may become extremely short. In this case, a reduction in long cycle characteristics may occur due to the volume change of the silicon-based active material during charge and discharge.

[0098] By setting the thickness of the second negative electrode active material layer 22 having a relatively high content of silicon-based negative electrode active material to be greater than the thickness of the first negative electrode active material layer 21 having a relatively low content of silicon-based negative electrode active material, the long cycle characteristics of the secondary battery can be improved by minimizing the change in porosity even if the volume of the silicon-based negative electrode active material changes during charging and discharging.

[0099] In the embodiment, the total content of the first silicon-based negative electrode active material and the second silicon-based negative electrode active material contained in the entire negative electrode active material layer 20 may be 5-20% by weight, specifically 6-15% by weight, based on the total weight of the negative electrode active material layer 20. When the above range is met, the energy density of the negative electrode can be increased, and the energy capacity retention rate of the secondary battery can be increased.

[0100] In the embodiment, the content of the carbon-based negative electrode active material contained in the entire negative electrode active material layer 20 may be 75-95 wt %, specifically 80-93 wt %. When the above range is met, the energy density of the negative electrode can be improved, the negative electrode binding force can be increased, and the conductivity in the negative electrode can be improved.

[0101] Method for manufacturing negative electrode for secondary battery

[0102] The present invention provides a method for manufacturing a negative electrode for a secondary battery, which comprises the following steps: coating a first composition containing a first silicon-based negative electrode active material on a negative electrode current collector to form a first layer; and coating a second composition containing a second silicon-based negative electrode active material on the first layer to form a second layer thicker than the first layer, wherein, based on the total weight of solids contained in the first composition, the content of the first silicon-based negative electrode active material in the first composition is less than 5 weight %, and the weight percentage of the second silicon-based negative electrode active material in the second composition based on the total weight of the solids contained in the second composition is greater than the weight percentage of the first silicon-based negative electrode active material in the first composition based on the total weight of the solids contained in the first composition.

[0103] Figure 2 1 is a flowchart for explaining a method for manufacturing a negative electrode for a secondary battery according to an embodiment of the present invention. Figure 2 , a method for manufacturing a negative electrode for a secondary battery according to an embodiment of the present invention is described.

[0104] refer to Figure 2 In step S100, the first composition is coated on the negative electrode current collector. In more detail, the first composition can be coated on at least one side of the negative electrode current collector. By coating the first composition, a first layer can be formed.

[0105] The first composition includes a first silicon-based negative electrode active material. For the first silicon-based negative electrode active material, all the contents mentioned in the description of the negative electrode for the secondary battery can be applied.

[0106] In one embodiment, based on the total weight of the solids contained in the first composition, the content of the first silicon-based negative electrode active material in the first composition may be 5% by weight or less. Specifically, based on the total weight of the solids contained in the first composition, the content of the first silicon-based negative electrode active material in the first composition may be 0.2-4.5% by weight, more specifically 0.5-4% by weight.

[0107] In an embodiment, the first composition may further include a carbon-based negative electrode active material. For the carbon-based negative electrode active material, all the contents mentioned in the description of the negative electrode for the secondary battery may be applied.

[0108] In one embodiment, based on the total weight of the solids contained in the first composition, the content of the carbon-based negative electrode active material in the first composition may be 70-99 wt %, specifically 80-98 wt %, and more specifically 90-97 wt %.

[0109] In the embodiment, the first composition may further include a conductive material. For the conductive material, all of the contents mentioned in the description of the negative electrode for the secondary battery may be applied.

[0110] In one embodiment, based on the total weight of the solids contained in the first composition, the content of the conductive material in the first composition may be 0.01-3.0 wt %, specifically 0.02-2.5 wt %, and more specifically 0.05-2.0 wt %.

[0111] In the embodiment, the first composition may further include a binder. For the binder, all the contents mentioned in the description of the negative electrode for secondary batteries can be applied.

[0112] In one embodiment, the binder content in the first composition may be 0.05-10.0 wt %, 0.1-8.0 wt %, 0.2-7.0 wt % or 0.3-5.0 wt % based on the total weight of the solids contained in the first composition.

[0113] In an embodiment, the first composition may be in the form of a solvent comprising the first silicon-based negative electrode active material, the carbon-based negative electrode active material, the conductive material and the binder, and in one embodiment, may be a composition in the form of a slurry. The solvent may include, for example, one or more substances selected from water, methanol, ethanol, ethylene glycol, diethylene glycol and glycerol.

[0114] Next, in step S200, the second composition is coated on the first layer. By coating the second composition, the second layer can be formed.

[0115] The second composition includes a second silicon-based negative electrode active material. For the second silicon-based negative electrode active material, all the contents mentioned in the description of the negative electrode for the secondary battery can be applied.

[0116] In one embodiment, based on the total weight of the solids contained in the second composition, the content of the second silicon-based negative electrode active material in the second composition may be 6-35% by weight. Specifically, based on the total weight of the solids contained in the second composition, the content of the second silicon-based negative electrode active material in the second composition may be 7-25% by weight, more specifically 8-20% by weight.

[0117] In an embodiment, the weight percentage of the second silicon-based negative electrode active material in the second composition based on the total weight of solids contained in the second composition may be greater than the weight percentage of the first silicon-based negative electrode active material in the first composition based on the total weight of solids contained in the first composition.

[0118] In an embodiment, the second composition may further include a carbon-based negative electrode active material. For the carbon-based negative electrode active material, all the contents mentioned in the description of the negative electrode for the secondary battery may be applied.

[0119] In one embodiment, based on the total weight of the solids contained in the second composition, the content of the carbon-based negative electrode active material in the second composition may be 60-95% by weight, specifically 65-93% by weight, and more specifically 70-92% by weight.

[0120] In an embodiment, the weight percentage of the carbon-based negative electrode active material in the second composition based on the total weight of the solids contained in the second composition may be less than the weight percentage of the carbon-based negative electrode active material in the first composition based on the total weight of the solids contained in the first composition.

[0121] In the embodiment, the second composition may further include a conductive material. For the conductive material, all of the contents mentioned in the description of the negative electrode for the secondary battery can be applied.

[0122] In one embodiment, based on the total weight of the solids contained in the second composition, the content of the conductive material in the second composition may be 0.01-3.0 wt %, specifically 0.02-2.5 wt %, and more specifically 0.05-2.0 wt %.

[0123] In the embodiment, the second composition may further include a binder. For the binder, all the contents mentioned in the description of the negative electrode for secondary batteries can be applied.

[0124] In one embodiment, the binder content in the second composition may be 0.01-8.0 wt %, 0.05-6.0 wt %, 0.1-5.0 wt % or 0.5-3.0 wt % based on the total weight of the solids contained in the second composition.

[0125] In an embodiment, the weight percentage of the binder in the second composition based on the total weight of solids contained in the second composition may be less than the weight percentage of the binder in the first composition based on the total weight of solids contained in the first composition.

[0126] In an embodiment, the second composition may be in the form of a second silicon-based negative electrode active material, a carbon-based negative electrode active material, a conductive material, and a binder in a solvent, and in one embodiment, may be a composition in the form of a slurry. The solvent may include, for example, one or more substances selected from water, methanol, ethanol, ethylene glycol, diethylene glycol, and glycerol.

[0127] The step S100 of coating the first composition and the step S200 of coating the second composition may be performed sequentially or simultaneously. In one embodiment, the first composition and the second composition may be coated on the negative electrode current collector simultaneously using a double-layer coating device.

[0128] After step S200 , the method may further include drying the first composition and the second composition coated on at least one side of the negative electrode current collector.

[0129] Therefore, by drying the first layer prepared from the first composition and the second layer prepared from the second composition, the first negative electrode active material layer and the second negative electrode active material layer can be formed.

[0130] In an embodiment, the ratio of the thickness of the first layer to the second layer before drying may be 4.5:5.5 to 0.5:9.5, and more specifically 4:6 to 1:9.

[0131] Alternatively, the thickness ratio of the first negative electrode active material layer to the second negative electrode active material layer after drying may be 4.5:5.5 to 0.5:9.5, and more specifically 4:6 to 1:9.

[0132] Secondary battery

[0133] In addition, in another aspect of the present invention, there may be provided a secondary battery including the above-mentioned negative electrode for a secondary battery.

[0134] As described above, the negative electrode for a secondary battery may include a negative electrode collector and a negative electrode active material layer, and the negative electrode active material layer may include a first negative electrode active material layer and a second negative electrode active material layer.

[0135] In the secondary battery, in addition to the negative electrode according to the present invention, a positive electrode and a separator are also included. The positive electrode may include a positive electrode current collector and an active material layer disposed on the positive electrode current collector. The active material layer may contain an active material. For example, the positive electrode active material layer may contain a positive electrode active material, and the positive electrode active material may be a material that can insert and extract lithium ions.

[0136] The positive electrode active material may be a lithium metal oxide. For example, the positive electrode active material may be one of a lithium manganese-based oxide, a lithium nickel-based oxide, a lithium cobalt-based oxide, a lithium nickel manganese-based oxide, a lithium nickel cobalt aluminum-based oxide, a lithium iron phosphate-based compound, a lithium manganese phosphate-based compound, a lithium cobalt phosphate-based compound, and a lithium vanadium phosphate-based compound, and is not necessarily limited to a specific example.

[0137] The separator can be between the negative electrode and the positive electrode. The separator is provided to prevent a short circuit between the negative electrode and the positive electrode and to generate the flow of ions. The separator can include a porous polymer film or a porous non-woven fabric. Among them, the porous polymer film can be composed of a single layer or multiple layers of polyolefin-based polymers such as ethylene polymers, propylene polymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers. The porous non-woven fabric can include high melting point glass fibers, polyethylene terephthalate fibers. However, it is not limited thereto. According to the embodiment, the separator can be a high heat-resistant separator (ceramic coated separator, CCS) containing ceramic.

[0138] The negative electrode, the positive electrode and the separator can be made into an electrode assembly by winding, lamination, folding or zigzag stacking. In addition, the electrode assembly is provided together with an electrolyte, so that a secondary battery according to the present invention can be made. The secondary battery can be any one of a cylindrical, prismatic, pouch and coin-shaped can, but is not limited thereto.

[0139] The electrolyte may be a non-aqueous electrolyte. The electrolyte may include a lithium salt and an organic solvent. The organic solvent may include at least one of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), dipropyl carbonate (DPC), vinylene carbonate (VC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, gamma-butyrolactone, propylene sulfide or tetrahydrofuran.

[0140] Example

[0141] Hereinafter, based on the examples and comparative examples, the negative electrode for a secondary battery and the secondary battery according to the present invention will be described in more detail. However, the following examples and comparative examples are merely illustrative for explaining the present invention in more detail, and the present invention is not limited to the following examples and comparative examples.

[0142] Example 1

[0143] <Preparation of the first composition>

[0144] A carbon-based negative electrode active material composed of artificial graphite (D 50 : 13 μm) and natural graphite (D 50 : 10 μm), silicon oxide (SiO), multi-walled carbon nanotubes (MWCNT), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed at 93.4:2.0:1.0:1.2:2.4 (by weight) in water as a solvent to prepare the first composition.

[0145] <Preparation of the second composition>

[0146] A carbon-based negative electrode active material composed of artificial graphite (D 50 : 13 μm) and natural graphite (D 50 : 10 μm), silicon oxide (SiO), single-walled carbon nanotubes (SWCNT), carboxymethyl cellulose, and styrene-butadiene rubber were mixed at 86.63:10.57:1.0:1.2:0.6 (by weight) in water as a solvent to prepare the second composition.

[0147] <Manufacture of the negative electrode>

[0148] Using a double-layer slot die, the prepared first composition and second composition were simultaneously coated on a copper thin film, so that a first layer derived from the first composition was formed on the copper thin film as the negative electrode current collector, and a second layer derived from the second composition was formed on the first layer. Then, it was dried under vacuum at 130 °C for 1 hour to manufacture a negative electrode having a first negative electrode active material layer and a second negative electrode active material layer, where the first negative electrode active material layer was formed by drying the first layer, and the second negative electrode active material layer was formed by drying the second layer.

[0149] Among them, the first composition and the second composition were coated such that the thickness ratio of the first layer to the second layer was 3:7. Refer to Figure 1 The structure of the negative electrode of Example 1 can be understood. That is, in Figure 1 , t1 < t2, more specifically, t1:t2 = 3:7.

[0150] <Manufacture of the positive electrode>

[0151] Li[Ni0.88 Co 0.10 Mn 0.02 ]O 2 , carbon black, and polyvinylidene fluoride (PVdF) were mixed at a ratio of 96:3:1 (based on weight) to prepare a slurry. The slurry was uniformly coated on an aluminum foil and vacuum dried to manufacture a positive electrode.

[0152] <Manufacturing of Secondary Battery>

[0153] The negative electrode and positive electrode are cut (notching) into the specified size and stacked, a polyethylene (PE) separator is placed between the negative electrode and the positive electrode to form a battery cell, and then the tabs of the negative electrode and the positive electrode are welded. The welded negative electrode / separator / positive electrode assembly is placed in a soft package and the three sides except the electrolyte injection part are sealed.

[0154] The electrolyte was injected through the electrolyte injection part, and the remaining surface was sealed, and then immersed for more than 12 hours. The electrolyte was prepared by dissolving 1M LiPF in a mixed solvent of EC / EMC / DEC. 6 of electrolyte.

[0155] After that, pre-charging was performed for 36 minutes at a current corresponding to 0.25C. Degassing was performed after 1 hour, and aging was performed for more than 24 hours, and then formation charge and discharge were performed (charging conditions were CC-CV0.2C 4.2V 0.05C cut-off, and discharge conditions were CC 0.2C 2.5V cut-off). After that, standard charge and discharge were performed to manufacture a secondary battery (charging conditions were CC-CV 0.33C 4.2V 0.05C cut-off, and discharge conditions were CC 0.33C 2.5V cut-off).

[0156] Comparative Example 1

[0157] <Preparation of First Composition>

[0158] Will be made of artificial graphite (D 50 :13μm) and natural graphite (D 50 A carbon-based negative electrode active material (above 10 μm), silicon oxide (SiO), multi-walled carbon nanotubes, carboxymethyl cellulose and styrene-butadiene rubber were mixed in water as a solvent at a ratio of 94.15:1.25:1.0:1.2:2.4 (based on weight) to prepare a first composition.

[0159] <Preparation of Second Composition>

[0160] Will be made of artificial graphite (D 50:13μm) and natural graphite (D 50 A second composition was prepared by mixing a carbon-based negative electrode active material (aqueous material having a thickness of 10 μm and a thickness of 10 μm), silicon oxide (SiO), single-walled carbon nanotubes, carboxymethyl cellulose and styrene-butadiene rubber in a ratio of 62.2:35.0:1.0:1.2:0.6 (based on weight) in water as a solvent.

[0161] <Manufacturing of negative electrode>

[0162] The negative electrode was manufactured by the same method as in Example 1, except that the first composition and the second composition were applied so that the thickness ratio of the first layer to the second layer was 8:2. Figure 3 The structure of the negative electrode of Comparative Example 1 can be understood. Figure 3 , t1>t2, more specifically, t1:t2=8:2.

[0163] <Manufacturing of positive electrode>

[0164] A positive electrode was manufactured by the same method as in Example 1.

[0165] <Manufacturing of Secondary Battery>

[0166] A secondary battery was manufactured by the same method as in Example 1, except that the above-mentioned negative electrode was used.

[0167] Comparative Example 2

[0168] <Preparation of First Composition>

[0169] Will be made of artificial graphite (D 50 :13μm) and natural graphite (D 50 A carbon-based negative electrode active material (above 10 μm), silicon oxide (SiO), multi-walled carbon nanotubes, carboxymethyl cellulose and styrene-butadiene rubber were mixed in water as a solvent at a ratio of 93.4:2.0:1.0:1.2:2.4 (based on weight) to prepare a first composition.

[0170] <Preparation of Second Composition>

[0171] Will be made of artificial graphite (D 50 :13μm) and natural graphite (D 50 A second composition was prepared by mixing a carbon-based negative electrode active material (aqueous material having a thickness of 10 μm and a thickness of 10 μm), silicon oxide (SiO), single-walled carbon nanotubes, carboxymethyl cellulose and styrene-butadiene rubber in a ratio of 83.2:14.0:1.0:1.2:0.6 (based on weight) in water as a solvent.

[0172] <Manufacturing of negative electrode>

[0173] The negative electrode was fabricated in the same manner as in Example 1, except that the first composition and the second composition were coated such that the thickness ratio of the first layer to the second layer was 5:5. Refer to Figure 4 The structure of the negative electrode of Comparative Example 2 can be understood. That is, in Figure 4 , t1 = t2, and more specifically, t1:t2 = 5:5.

[0174] <Fabrication of the Positive Electrode>

[0175] The positive electrode was fabricated in the same manner as in Example 1.

[0176] <Fabrication of the Secondary Battery>

[0177] The secondary battery was fabricated in the same manner as in Example 1, except that the above negative electrode was used.

[0178] Comparative Example 3

[0179] <Preparation of the First Composition>

[0180] A carbon-based negative electrode active material composed of artificial graphite (D 50 : 13 μm) and natural graphite (D 50 : 10 μm), silicon oxide (SiO), multi-walled carbon nanotubes, carboxymethyl cellulose, and styrene-butadiene rubber were mixed in water as a solvent at 87.4:8.0:1.0:1.2:2.4 (by weight) to prepare the first composition.

[0181] <Preparation of the Second Composition>

[0182] A carbon-based negative electrode active material composed of artificial graphite (D 50 : 13 μm) and natural graphite (D 50 : 10 μm), silicon oxide (SiO), single-walled carbon nanotubes, carboxymethyl cellulose, and styrene-butadiene rubber were mixed in water as a solvent at 89.2:8.0:1.0:1.2:0.6 (by weight) to prepare the second composition.

[0183] <Fabrication of the Negative Electrode>

[0184] The negative electrode was fabricated in the same manner as in Example 1, except that the above first composition and second composition were used. Refer to Figure 1 The structure of the negative electrode of Comparative Example 3 can be understood. That is, in Figure 1 , t1 < t2, and more specifically, t1:t2 = 3:7.

[0185] <Fabrication of the Positive Electrode>

[0186] The positive electrode was fabricated in the same manner as in Example 1.

[0187] <Manufacturing of Secondary Battery>

[0188] A secondary battery was manufactured by the same method as in Example 1, except that the above-mentioned negative electrode was used.

[0189] Comparative Example 4

[0190] <Manufacturing of negative electrode>

[0191] Will be made of artificial graphite (D 50 :13μm) and natural graphite (D 50 A carbon-based negative electrode active material (anode material with a thickness of 10 μm and a diameter of 10 μm) was mixed with silicon oxide (SiO), multi-walled carbon nanotubes, carboxymethyl cellulose and styrene-butadiene rubber in a ratio of 88.30:8.0:1.0:1.2:1.5 (based on weight) in water as a solvent to prepare a negative electrode active material composition.

[0192] The prepared negative electrode active material composition was coated on a copper thin film and then dried under vacuum at 130° C. for 1 hour to manufacture a negative electrode. Figure 5 The structure of the negative electrode of Comparative Example 4 can be understood.

[0193] <Manufacturing of positive electrode>

[0194] A positive electrode was manufactured by the same method as in Example 1.

[0195] <Manufacturing of Secondary Battery>

[0196] A secondary battery was manufactured by the same method as in Example 1, except that the above-mentioned negative electrode was used.

[0197] <Experimental Example 1> Evaluation of Characteristics of Negative Electrode Active Material Layer

[0198] The components contained in the negative electrode active material layer in the dried negative electrodes in Example 1 and Comparative Examples 1 to 4 are summarized as shown in Table 1 below.

[0199] [Table 1]

[0200]

[0201] Referring to Table 1, it can be confirmed that the contents of the components contained in each layer of Example 1 and Comparative Examples 1 to 4 are set to be different, but in Example 1 and Comparative Examples 1 to 4, the contents of graphite and silicon oxide contained in the entire negative electrode active material layer are almost the same.

[0202] In addition, it can be confirmed that in the case of Example 1 and Comparative Examples 1 to 3 in which a relatively large amount of SBR is contained in the first layer in contact with the current collector, the adhesion between the current collector and the negative electrode active material layer is excellent, so that the negative electrode does not fall off, but in the case of Comparative Example 4 in which the first layer and the second layer are not distinguished and formed into one layer, due to the migration of SBR as a binder during the drying process, the adhesion between the negative electrode active material layer and the negative electrode current collector is relatively low, so that the negative electrode falls off.

[0203] <Experimental Example 2> Evaluation of porosity uniformity

[0204] After measuring the negative electrodes of Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 4 with a three-dimensional X-ray microscope (3D X-ray Microscope, XRM), the change in porosity in the negative electrode was simulated by simulating the volume expansion of graphite and silicon oxide, assuming that the x-axis and y-axis are fixed and the volume expansion occurs only in the z-axis as the thickness direction. At this time, the volume expansion of graphite is assumed to be 20%, and the volume expansion of silicon oxide is assumed to be 150%. The simulation results thus obtained are shown in Figures 6 to 9 In. Figures 6 to 9 In the graph, the y-axis direction (longitudinal direction) is the thickness direction of the electrode, and the smaller the color deviation is, the more uniform the porosity is.

[0205] See also Fig. 9 It can be confirmed that the porosity is most uniform when graphite and silicon oxide are uniformly contained throughout the electrode as shown in Comparative Example 4. However, as described above, the negative electrode of Comparative Example 4 has a disadvantage of reduced adhesion between the negative electrode active material layer and the negative electrode current collector due to migration of SBR as a binder during the drying process, and thus the application may be limited.

[0206] In addition, see Figure 6 It can be confirmed that even when the second layer having a relatively large silicon oxide content is thick as shown in Example 1, the porosity is uniform.

[0207] On the other hand, see Figure 7 and Figure 8 It can be confirmed that in the case of Comparative Examples 1 and 2, the porosity is not uniform.

[0208] That is, the results of Experimental Example 2 show that, when the thickness of the second layer having a relatively large silicon oxide content is set thicker than that of the first layer, the uniformity of the porosity can be improved.

[0209] <Experimental Example 3> Evaluation of Cycle Characteristics of Secondary Batteries

[0210] First, the secondary battery of Comparative Example 1 and the secondary battery of Comparative Example 2 were charged and discharged with the rapid charging time set to 22 minutes at a state of charge (SOC) of 80%. The results of the capacity retention rate according to the charge and discharge cycle are shown in FIG. Fig.10 (Comparative Example 1_1, Comparative Example 2_1).

[0211] See also Fig.10 It can be confirmed that in Comparative Example 1, the capacity decreases more with the progress of charge and discharge than in Comparative Example 2. That is, it can be confirmed that in the case of Comparative Example 1 where the thickness of the second layer is relatively thin, the cycle characteristics are worse than in Comparative Example 2 where the thickness of the first layer and the second layer are the same.

[0212] In addition, the secondary battery of Example 1 and the secondary battery of Comparative Example 2 were charged and discharged with the rapid charging time set to 27 minutes at a state of charge (SOC) of 80%. The results of the capacity retention rate according to the charge and discharge cycles are shown in FIG. Fig.10 (Example 1_1, Comparative Example 2_2).

[0213] See also Fig.10 It can be confirmed that in Example 1, the capacity hardly decreases even when charging and discharging are performed, and the capacity is larger than that of the secondary battery of Comparative Example 2. That is, it can be confirmed that in the case of Example 1 in which the thickness of the second layer is relatively thick, the cycle characteristics are better than those of Comparative Example 2 in which the thickness of the first layer and the second layer are the same.

[0214] In addition, the secondary battery of Example 1 and the secondary battery of Comparative Example 3 were charged and discharged with the rapid charging time set to 80% of the state of charge (SOC) at 27 minutes, and the results of the capacity retention rate according to the charge and discharge cycle are shown in FIG. Fig.11 middle.

[0215] See also Fig.11 It can be confirmed that in Example 1, the capacity hardly decreases even when charging and discharging are performed, and the capacity is larger than that of the secondary battery of Comparative Example 3. That is, it can be confirmed that in the case of Example 1 in which the silicon oxide content of the first layer is relatively small, the cycle characteristics are better than those of Comparative Example 3 in which the silicon oxide content of the first layer is relatively large.

Claims

1. A negative electrode for a secondary battery, comprising: Anode current collector; a first negative electrode active material layer, the first negative electrode active material layer being formed on at least one side of the negative electrode current collector and comprising a first silicon-based negative electrode active material; and a second negative electrode active material layer, the second negative electrode active material layer is formed on the first negative electrode active material layer and contains a second silicon-based negative electrode active material, the second negative electrode active material layer is thicker than the first negative electrode active material layer, Wherein, based on the total weight of the first negative electrode active material layer, the content of the first silicon-based negative electrode active material in the first negative electrode active material layer is less than 5 weight %, The weight percentage of the second silicon-based negative electrode active material in the second negative electrode active material layer based on the total weight of the second negative electrode active material layer is greater than the weight percentage of the first silicon-based negative electrode active material in the first negative electrode active material layer based on the total weight of the first negative electrode active material layer.

2. The negative electrode for a secondary battery according to claim 1, wherein The first silicon-based negative electrode active material and the second silicon-based negative electrode active material include a material selected from SiO x , one or more of Si / C composite and Si alloy, wherein 0≤x<2.

3. The negative electrode for a secondary battery according to claim 1, wherein The first negative electrode active material layer and the second negative electrode active material layer further include a carbon-based negative electrode active material.

4. The negative electrode for a secondary battery according to claim 3, wherein The carbon-based negative electrode active material includes at least one selected from artificial graphite, natural graphite, hard carbon, soft carbon, carbon black and graphene.

5. The negative electrode for a secondary battery according to claim 4, wherein The carbon black includes acetylene black, Ketjen black and SuperP.

6. The negative electrode for a secondary battery according to claim 1, wherein Based on the total weight of the first negative electrode active material layer and the second negative electrode active material layer, the total content of the first silicon-based negative electrode active material and the second silicon-based negative electrode active material contained in the entire first negative electrode active material layer and the second negative electrode active material layer is 5-20 weight %.

7. The negative electrode for a secondary battery according to claim 3, wherein: Based on the total weight of the first negative electrode active material layer and the second negative electrode active material layer, the content of the carbon-based negative electrode active material contained in the entire first negative electrode active material layer and the second negative electrode active material layer is 75-95% by weight.

8. The negative electrode for a secondary battery according to claim 1, wherein A ratio of a thickness of the first negative electrode active material layer to a thickness of the second negative electrode active material layer is 4:6 to 1:

9.

9. The negative electrode for a secondary battery according to claim 1, wherein Based on the total weight of the second negative electrode active material layer, the content of the second silicon-based negative electrode active material in the second negative electrode active material layer is 6-35% by weight.

10. The negative electrode for a secondary battery according to claim 1, wherein The first negative electrode active material layer and the second negative electrode active material layer further include a conductive material.

11. The negative electrode for a secondary battery according to claim 1, wherein The first negative electrode active material layer and the second negative electrode active material layer further include a binder.

12. The negative electrode for a secondary battery according to claim 11, wherein A weight percentage of the binder in the first negative electrode active material layer based on the total weight of the first negative electrode active material layer is greater than a weight percentage of the binder in the second negative electrode active material layer based on the total weight of the second negative electrode active material layer.

13. The negative electrode for a secondary battery according to claim 3, wherein: The weight percentage of the carbon-based negative electrode active material in the first negative electrode active material layer based on the total weight of the first negative electrode active material layer is greater than the weight percentage of the carbon-based negative electrode active material in the second negative electrode active material layer based on the total weight of the second negative electrode active material layer.

14. The negative electrode for a secondary battery according to claim 13, wherein Based on the total weight of the first negative electrode active material layer, the sum of the contents of the first silicon-based negative electrode active material and the carbon-based negative electrode active material in the first negative electrode active material layer is 85-98% by weight. Based on the total weight of the second negative electrode active material layer, the sum of the contents of the second silicon-based negative electrode active material and the carbon-based negative electrode active material in the second negative electrode active material layer is 85-98 weight %.

15. A method for manufacturing a negative electrode for a secondary battery, comprising the following steps: coating a first composition including a first silicon-based negative electrode active material on a negative electrode current collector to form a first layer; and coating a second composition including a second silicon-based negative electrode active material on the first layer to form a second layer thicker than the first layer, Wherein, based on the total weight of the solids contained in the first composition, the content of the first silicon-based negative electrode active material in the first composition is less than 5 weight %, The weight percentage of the second silicon-based negative electrode active material in the second composition based on the total weight of solids contained in the second composition is greater than the weight percentage of the first silicon-based negative electrode active material in the first composition based on the total weight of solids contained in the first composition.

16. The method for manufacturing a negative electrode for a secondary battery according to claim 15, wherein: The first composition and the second composition further include a carbon-based negative electrode active material.

17. The method for manufacturing a negative electrode for a secondary battery according to claim 16, wherein: The weight percentage of the carbon-based negative electrode active material in the first composition based on the total weight of solids contained in the first composition is greater than the weight percentage of the carbon-based negative electrode active material in the second composition based on the total weight of solids contained in the second composition.

18. The method for manufacturing a negative electrode for a secondary battery according to claim 15, wherein: The first composition and the second composition further comprise a binder.

19. The method for manufacturing a negative electrode for a secondary battery according to claim 18, wherein: The weight percentage of the binder in the first composition based on the total weight of solids contained in the first composition is greater than the weight percentage of the binder in the second composition based on the total weight of solids contained in the second composition.

20. The method for manufacturing a negative electrode for a secondary battery according to claim 15, wherein: A ratio of a thickness of the first layer to a thickness of the second layer is 4:6 to 1:9.

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