Negative electrode for lithium secondary battery, method for manufacturing negative electrode for lithium secondary battery, and lithium secondary battery comprising negative electrode

Abstract

The present application relates to a negative electrode for a lithium secondary battery, a method for manufacturing a negative electrode for a lithium secondary battery, and a lithium secondary battery comprising the negative electrode.

Description

Technical Field

[0001] This application claims priority and benefit to Korean Patent Application No. 10-2021-0090580, filed with the Korean Intellectual Property Office on July 9, 2021, and Korean Patent Application No. 10-2021-0189600, filed with the Korean Intellectual Property Office on December 28, 2021, the entire contents of which are incorporated herein by reference.

[0002] This application relates to a negative electrode for lithium secondary batteries, a method for preparing a negative electrode for lithium secondary batteries, and a lithium secondary battery comprising the negative electrode. Background Technology

[0003] With the rapid increase in the use of fossil fuels, the demand for alternative or clean energy sources is growing, and as part of this trend, the most active research area is the generation and storage of electricity using electrochemical reactions.

[0004] Currently, representative examples of electrochemical devices using this type of electrochemical energy include secondary batteries, and their applications are expanding.

[0005] With the technological advancements in mobile devices and the increasing demands they generate, the need for secondary batteries as an energy source is rapidly growing. Among these secondary batteries, lithium-ion batteries, characterized by high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used. Furthermore, methods for preparing high-density electrodes with even higher energy density per unit volume have been actively researched for use in these high-capacity lithium-ion batteries.

[0006] Typically, a secondary battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material for inserting and deintercalating lithium ions from the positive electrode, and silicon-based particles with a large discharge capacity can be used as the negative electrode active material.

[0007] In particular, in recent years, in response to the demand for high-density energy batteries, there has been active research on methods to increase capacity by using silicon-based compounds, such as Si / C or SiOx, which have a capacity more than 10 times that of graphite-based materials, as anode active materials. However, while silicon-based compounds, as high-capacity materials, possess superior capacity characteristics compared to graphite used in existing technologies, their rapid volume expansion during charging disrupts the conductive path, leading to battery performance degradation and a decrease in capacity from the initial stage. Furthermore, for silicon-based anodes, uniform lithium-ion charging is not achieved in the depth direction of the anode during repeated charge and discharge cycles, and the reaction occurs on the surface. This results in accelerated surface degradation, necessitating improvements in battery cycle performance.

[0008] Therefore, to address the aforementioned problems when using silicon-based compounds as anode active materials, various measures have been discussed, such as adjusting the driving potential; measures to suppress volume expansion itself, such as methods for further coating the active material layer with a thin film and methods for controlling the particle size of the silicon-based compound; or developing adhesives capable of suppressing the volume expansion of the silicon-based compound to prevent the breakage of the conductive path. Furthermore, research has been conducted on limiting the proportion of silicon-based active material used during initial charging and discharging through pre-lithiation of the silicon-based active material layer, thereby imparting storage functionality to supplement the lifespan characteristics of silicon-based anodes.

[0009] However, the above measures may actually degrade the performance of the battery, thus limiting their application and hindering the commercialization of negative electrode batteries with high silicon content. Furthermore, as the proportion of silicon active material in the silicon active material layer increases, pre-lithiation concentrates on the surface of the negative electrode, thus damaging the surface-side silicon active material. With the occurrence of uneven pre-lithiation, issues arise regarding the improvement of lifespan characteristics.

[0010] Therefore, it is necessary to study the improvement of the cycle performance and capacity characteristics of lithium secondary batteries, even when using silicon-based compounds as active materials, to prevent electrode surface degradation during charge and discharge cycles and improve uniformity during pre-lithiation.

[0011] [Existing technical documents]

[0012] [Patent Literature]

[0013] Japanese Patent Application Publication No. 2009-080971 Summary of the Invention

[0014] [Technical Issues]

[0015] This invention aims to provide a negative electrode for a lithium secondary battery, a method for preparing the negative electrode for a lithium secondary battery, and a lithium secondary battery comprising the negative electrode. The negative electrode for a lithium secondary battery can prevent the degradation of the electrode surface during charge and discharge cycles (a problem that exists when using silicon-based active materials on the negative electrode). In addition, it can improve the cycle performance and capacity characteristics of the lithium secondary battery by improving the uniformity during pre-lithiation.

[0016] [Technical Solution]

[0017] Exemplary embodiments of the present specification provide an anode for a lithium secondary battery, the anode comprising: an anode current collector layer; a first anode active material layer on one or both surfaces of the anode current collector layer; and a second anode active material layer on a surface of the first anode active material layer opposite to the surface facing the anode current collector layer, wherein the first anode active material layer comprises a first anode active material layer composition containing a first anode active material, the second anode active material layer comprises a second anode active material layer composition, the second anode active material layer composition comprises a second anode active material, a second anode conductive material and a second anode binder, the first anode active material comprises one or more selected from the group consisting of SiOx (x=0) and SiOx (0<x<2), and SiOx (x=0) is contained in an amount of 95 parts by weight or more based on 100 parts by weight of the first anode active material, the second anode conductive material comprises one or more selected from the group consisting of point-shaped conductive materials, linear conductive materials and planar conductive materials, the second anode active material comprises one or more selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming an alloy with lithium, and lithium-based nitrides, and the silicon-based active material is present in an amount of 50 parts by weight or more and 100 parts by weight or less based on 100 parts by weight of the second anode active material.

[0018] Furthermore, the first anode active material layer may be in contact with part or all of the surface of the anode current collector layer, and the second anode active material layer may be in contact with part or all of the surface of the first anode active material layer.

[0019] Another exemplary embodiment provides a method for preparing an anode for a lithium secondary battery, the method comprising: providing an anode current collector layer; forming a first anode active material layer by applying a first anode active material layer composition comprising a first anode active material onto one or both surfaces of the anode current collector layer; and forming a second anode active material layer by applying a second anode active material layer composition comprising a second anode active material, a second anode conductive material and a second anode binder onto a surface of the first anode active material layer opposite to the surface facing the anode current collector layer, wherein the first anode active material comprises one or more selected from the group consisting of SiOx (x=0) and SiOx (0<x<2), and based on 100 parts by weight of the first anode active material, SiOx (x=0) is included in an amount of 95 parts by weight or more, the second anode conductive material comprises one or more selected from the group consisting of point-shaped conductive materials, linear conductive materials and planar conductive materials, the second anode active material comprises one or more selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming an alloy with lithium, and lithium-based nitrides, and based on 100 parts by weight of the second anode active material, the silicon-based active material is present in an amount of 50 parts by weight or more and 100 parts by weight or less.

[0020] Furthermore, the first anode active material layer may be applied in contact with part or all of the surface of the anode current collector layer, and the second anode active material layer may be applied in contact with part or all of the surface of the first anode active material layer.

[0021] Yet another exemplary embodiment provides a lithium secondary battery, comprising: a cathode, an anode for a lithium secondary battery according to the present application, a separator disposed between the cathode and the anode, and an electrolyte.

[0022] [Beneficial Effects]

[0023] The anode for a lithium secondary battery according to an exemplary embodiment of the present invention has a double-layer active material layer, wherein the double-layer active material layer comprises a first anode active material layer and a second anode active material layer. In particular, the first anode active material comprised in the first anode active material layer comprises one or more selected from the group consisting of SiOx (x=0) and SiOx (0<x<2), and based on 100 parts by weight of the first anode active material, SiOx (x=0) is comprised in an amount of 95 parts by weight or more; the second anode active material comprised in the second anode active material layer comprises one or more selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming an alloy with lithium, and lithium-based nitrides, and based on 100 parts by weight of the second anode active material, the silicon-based active material is present in an amount of 50 parts by weight or more and 100 parts by weight or less.

[0024] In particular, the second anode active material may comprise one or more selected from the group consisting of carbon-based active materials, SiOx (0<x<2), SiC and Si alloys, and may particularly comprise SiOx (0<x<2).

[0025] The anode for a lithium secondary battery according to the present application has a double-layer active material layer with the specific composition and content as described above. In particular, since the first anode active material layer comprises a high content of SiOx (x=0), it can have the advantages of being favorable for high capacity, high density and fast charging. In addition, by comprising a silicon-based active material, a carbon-based active material, and the like in the second anode active material layer, electrode surface deterioration during charge and discharge cycles can be prevented, and uniformity during prelithiation can also be improved.

[0026] In addition, the anode is an anode wherein the second anode conductive material comprises at least one selected from the group consisting of point-shaped conductive materials, linear conductive materials and planar conductive materials; in particular, since the anode inevitably comprises a linear conductive material, the anode does not significantly affect the service life characteristics of existing lithium secondary batteries, and due to the increase in the number of sites where charge and discharge can occur, it has the characteristic of excellent output performance at high C-rates.

[0027] In particular, by having the second negative electrode active material layer with the composition described above, the degradation of the negative electrode surface during charging and discharging can be reduced. Furthermore, since a buffer layer can be imparted during the pre-lithiation process of pre-charging the negative electrode for lithium secondary batteries, the problems encountered when using a single-layer silicon-based active material layer can be solved. Therefore, a key feature of the present invention is that the negative electrode for lithium secondary batteries exhibits excellent lifespan characteristics in addition to high capacity, high density, and fast charging. As an example, the second negative electrode active material layer of the present invention can function as a buffer layer. Si electrodes have superior capacity characteristics compared to SiO electrodes and carbon-based electrodes. However, in Si electrodes, surface degradation occurs rapidly due to the rapid reaction with Li ions during charging / discharging. This also occurs during the pre-lithiation process when Li ions are pre-coated to the negative electrode. In the pre-lithiation process, a buffer layer is used to prevent direct contact between the Si electrode and lithium and to prevent surface degradation. Therefore, the second negative electrode active material layer of the present invention is described as having the same effect as the buffer layer during the pre-lithiation process.

[0028] In summary, the negative electrode for lithium secondary batteries according to this application is characterized in that a first negative electrode active material layer and a second negative electrode active material layer are included as a double layer, wherein a specific composition and content are applied to the double layer in order to take advantage of the electrode using a high content of Si particles as a single layer active material, while solving the disadvantages of the electrode when it has the above advantages, namely, surface degradation problem, uniformity problem during pre-lithiation problem and service life characteristic problem. Attached Figure Description

[0029] Figure 1 This is a diagram illustrating the stacked structure of a negative electrode for a lithium secondary battery according to an exemplary embodiment of this application.

[0030] Figure 2 A graph showing the RPT capacity retention rate of embodiments and comparative examples according to this application is presented.

[0031] Figure 3 A graph showing the RPT resistance increase rate according to embodiments and comparative examples of this application is presented.

[0032] Figure 4 This is a diagram illustrating the stacked structure of a negative electrode for a lithium secondary battery according to an exemplary embodiment of this application.

[0033] Figure 5 This is a flowchart illustrating a wet-drying process according to an exemplary embodiment of this application.

[0034] Figure 6 This is a flowchart illustrating a wet hood wet process according to an exemplary embodiment of this application.

[0035] Figure 7 It is a SEM image showing the interface formed on the negative electrode of this application through a wet-drying process.

[0036] Figure 8 It is a SEM image showing the interface formed on the negative electrode of this application through the wet-mask wet process.

[0037] <Explanation of Symbols and Marks>

[0038] 10: Second negative electrode active material layer

[0039] 20: First negative electrode active material layer

[0040] 30: Negative electrode current collector layer Detailed Implementation

[0041] Before describing the present invention, some terms will be defined.

[0042] In this specification, when a part "includes" a constituent element, unless otherwise specifically described, this does not mean that other constituent elements are excluded, but rather that other constituent elements may be further included.

[0043] In this specification, "p to q" means the range of "above p and below q".

[0044] In this specification, "specific surface area" is measured by the BET method, specifically calculated using a BELSORP-mini II manufactured by BEL Japan Co., Ltd., from the amount of nitrogen adsorbed at liquid nitrogen temperature (77K). In other words, in this application, BET specific surface area can refer to the specific surface area measured by the aforementioned method.

[0045] In this specification, "Dn" refers to particle size distribution, specifically the particle size at the n% point of the cumulative particle number distribution based on particle size. That is, D50 is the particle size (average particle size) at the 50% point of the cumulative particle number distribution based on particle size, D90 is the particle size at the 90% point of the cumulative particle number distribution based on particle size, and D10 is the particle size at the 10% point of the cumulative particle number distribution based on particle size. Alternatively, particle size distribution can be measured using laser diffraction. Specifically, after dispersing the powder to be measured in a dispersion medium, the resulting dispersion is introduced into a commercially available laser diffraction particle size measurement device (e.g., Microtrac S3500) to measure the difference in diffraction patterns based on particle size as particles pass through a laser beam, thereby calculating the particle size distribution.

[0046] In the present specification, the fact that a polymer contains a monomer as a monomer unit means that the monomer participates in a polymerization reaction and is thus contained in the polymer as a repeating unit. In the present specification, the case where a polymer contains a monomer is interpreted to be the same as the case where a polymer contains a monomer as a monomer unit.

[0047] In the present specification, unless otherwise specified as "homopolymer", the term "polymer" should be understood to be used in a broad sense and includes copolymers.

[0048] In the present specification, the weight-average molecular weight (Mw) and number-average molecular weight (Mn) are polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using commercially available monodisperse polystyrene polymers (standard samples) with various degrees of polymerization for measuring molecular weight as reference materials. In the present specification, unless otherwise described, molecular weight means weight-average molecular weight.

[0049] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings, so that those skilled in the art to which the present invention pertains can easily implement the present invention. However, the present invention can be implemented in various different forms and is not limited to the following description.

[0050] An exemplary embodiment of the present specification provides an anode for a lithium secondary battery, the anode comprising: an anode current collector layer; a first anode active material layer on at least one surface or both surfaces of the anode current collector layer; and a second anode active material layer on a surface of the first anode active material layer opposite to the surface facing the anode current collector layer, wherein the first anode active material layer includes a first anode active material layer composition containing a first anode active material, the second anode active material layer includes a second anode active material layer composition, the second anode active material layer composition contains a second anode active material, a second anode conductive material and a second anode binder, the first anode active material includes one or more selected from the group consisting of SiOx (x=0) and SiOx (0<x<2), and contains 95 parts by weight or more of SiOx (x=0) based on 100 parts by weight of the first anode active material, the second anode conductive material includes one or more selected from the group consisting of point-shaped conductive materials, linear conductive materials and planar conductive materials, the second anode active material includes one or more selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming an alloy with lithium, and lithium-based nitrides, and the silicon-based active material is present in an amount of 50 parts by weight or more and 100 parts by weight or less based on 100 parts by weight of the second anode active material.

[0051] The negative electrode for lithium secondary batteries according to this application can be a negative electrode in which a first negative electrode active material layer and a second negative electrode active material layer are included as a double layer, wherein a specific composition and content are applied to the double layer in order to take advantage of the electrode using a high content of Si particles as a single layer active material, while solving the disadvantages of the electrode having the above advantages, namely, surface degradation problem, uniformity problem during pre-lithiation problem and service life characteristic problem.

[0052] Figure 1 This is a diagram illustrating the stacked structure of a negative electrode for a lithium secondary battery according to an exemplary embodiment of this application. Specifically, it can be confirmed that the negative electrode 100 for a lithium secondary battery includes a first negative electrode active material layer 20 and a second negative electrode active material layer 10 on one surface of the negative electrode current collector layer 30, and Figure 1 A first negative electrode active material layer is shown on one surface, but the first negative electrode active material layer may be contained on both surfaces of the negative electrode current collector layer. As shown, the first negative electrode active material layer may be in contact with the entire surface of the negative electrode current collector layer, and the second negative electrode active material layer may be in contact with the entire surface of the first negative electrode active material layer.

[0053] like Figure 4 As shown, the first negative electrode active material layer 20 and the second negative electrode active material layer 10 can be formed on both surfaces of the negative electrode current collector layer 30. Figure 4 As shown, the arrangement can be 10 > 20 > 30 > 20 > 10. Other arrangements can be 10 > 20 > 30 > 20, 10 > 20 > 30 > 10, or 10 > 20 > 30 > 10 > 20. Furthermore, the compositions of the active material layers coated on both sides can be the same or different from each other. Preferably, the negative electrode current collector layer has the same composition on both sides, for example, 10 > 20 > 30 > 20 > 10.

[0054] The negative electrode for lithium secondary batteries of the present invention will be described in more detail below.

[0055] In an exemplary embodiment of this application, a negative electrode for a lithium secondary battery is provided, the negative electrode comprising: a negative electrode current collector layer; a first negative electrode active material layer on one or both surfaces of the negative electrode current collector layer; and a second negative electrode active material layer on the surface of the first negative electrode active material layer opposite to the surface facing the negative electrode current collector layer.

[0056] In an exemplary embodiment of the present application, the negative electrode current collector layer generally has a thickness of 1 μm to 100 μm. The negative electrode current collector layer is not particularly limited as long as it has high electrical conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, materials obtained by surface-treating surfaces of copper or stainless steel with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy and the like can be used. In addition, the negative electrode current collector layer can also increase the bonding strength of the negative electrode active material by forming fine uneven irregularities on its surface, and the negative electrode current collector layer can be used in various forms such as films, sheets, foils, meshes, porous bodies, foamed bodies and non-woven fabrics.

[0057] In an exemplary embodiment of the present application, the negative electrode current collector layer can have a thickness of 1 μm or more and 100 μm or less.

[0058] However, the thickness can be variously modified according to the type and application of the negative electrode used, and is not limited thereto.

[0059] In an exemplary embodiment of the present application, the first negative electrode active material comprises one or more selected from the group consisting of SiOx (x=0) and SiOx (0<x<2), and based on 100 parts by weight of the first negative electrode active material, 95 parts by weight or more of SiOx (x=0) can be comprised.

[0060] In an exemplary embodiment of the present application, the first negative electrode active material comprises one or more selected from the group consisting of SiOx (x=0) and SiOx (0<x<2), and based on 100 parts by weight of the first negative electrode active material, it can comprise 95 parts by weight or more, preferably 97 parts by weight or more, more preferably 99 parts by weight or more of SiOx (x=0), and can comprise 100 parts by weight or less of SiOx (x=0).

[0061] In an exemplary embodiment of the present application, specifically, pure silicon (Si) particles can be used as the first negative electrode active material. Using pure silicon (Si) as the first negative electrode active material can mean that, as described above, based on a total of 100 parts by weight of the first negative electrode active material, pure Si particles (SiOx (x=0)) that are not bound to other particles or elements are comprised within the above range.

[0062] The first negative electrode active material used in the first negative electrode active material layer of the present invention may undergo very complex crystal changes during the electrochemical absorption, storage, and release of lithium atoms. As the electrochemical absorption, storage, and release of lithium ions proceed, the composition and crystal structure of the silicon particles change to Si (crystal structure: Fd3m), LiSi (crystal structure: I41 / a), Li2Si (crystal structure: C2 / m), Li7Si2 (Pbam), Li22Si5 (F23), etc. Furthermore, with the complex changes in crystal structure, the volume of the silicon particles expands by approximately four times. Therefore, during repeated charge and discharge cycles, the silicon particles are destroyed, and due to the formation of bonds between lithium atoms and silicon particles, the initial lithium atom insertion sites on the silicon particles are disrupted, resulting in a significant deterioration in cycle life.

[0063] In an exemplary embodiment of this application, the first negative electrode active material may be composed of SiOx (x = 0).

[0064] The first negative electrode active material layer according to this application may comprise the first negative electrode active material, and specifically may comprise pure silicon particles containing 95 parts by weight or more of SiOx (x = 0). In this case, when a high content of pure silicon particles is included, the capacity characteristics are excellent, and in order to solve the degradation of service life caused by the resulting surface inhomogeneity reaction, the above-mentioned problem is solved by including the second negative electrode active material layer according to the present invention.

[0065] On the other hand, the first negative electrode active material of the present invention can have an average particle size (D50) of 3 μm to 10 μm, specifically 4 μm to 8 μm, and more specifically 5 μm to 7 μm. When the average particle size is within the range of 3 μm to 10 μm, the specific surface area of ​​the particles is within a suitable range, thus forming a negative electrode slurry viscosity within a suitable range. Therefore, the dispersion of the particles constituting the negative electrode slurry is promoted. Furthermore, the size of the first negative electrode active material has a value equal to or greater than the lower limit range mentioned above, and since the composite containing conductive material and binder in the negative electrode slurry results in an excellent contact area between the silicon particles and the conductive material, the possibility of a continuous conductive network is increased, thereby increasing the capacity retention rate. On the other hand, when the average particle size meets the above range, excessively large silicon particles are eliminated, thus forming a smooth surface of the negative electrode, thereby preventing non-uniformity of current density during charging and discharging.

[0066] In an exemplary embodiment of this application, the first negative electrode active material typically has a characteristic BET specific surface area. The BET specific surface area of ​​the first negative electrode active material is preferably 0.01 m². 2 / g to 150.0m 2 / g, more preferably 0.1m 2 / g to 100.0m 2 / g, with 0.2m being particularly preferred. 2 / g to 80.0m 2 / g, and the optimal value is 0.2m. 2 / g to 18.0m 2 / g. The BET specific surface area was measured using DIN 66131 (using nitrogen).

[0067] In an exemplary embodiment of this application, the first negative electrode active material may exist, for example, in a crystalline or amorphous form, and is preferably not porous. The silicon particles are preferably spherical or fragmented. Alternatively, but less preferably, the silicon particles may also have a fibrous structure or exist in the form of a silicon-containing film or coating.

[0068] In an exemplary embodiment of this application, the first negative electrode active material may have a non-spherical form and its sphericity is, for example, less than 0.9, such as 0.7 to 0.9, such as 0.8 to 0.9, and such as 0.85 to 0.9.

[0069] In this application, the sphericity is determined by Equation 1 below, where A is the area and P is the boundary line.

[0070] [Formula 1]

[0071] 4πA / P 2

[0072] An exemplary embodiment of this application provides a negative electrode for a lithium secondary battery, wherein a first negative electrode active material layer composition is based on 100 parts by weight, and the first negative electrode active material is present in an amount of 60 parts by weight or more.

[0073] In another exemplary embodiment, based on 100 parts by weight of the first negative electrode active material layer composition, the amount of the first negative electrode active material may be 60 parts by weight or more, preferably 65 parts by weight or more, more preferably 70 parts by weight or more, and may be 95 parts by weight or less, preferably 90 parts by weight or less, more preferably 80 parts by weight or less.

[0074] Even when using a first negative electrode active material with significantly high capacity within the aforementioned range, the first negative electrode active material layer composition according to this application can also solve one or more of the problems of surface degradation during charging and discharging, uniformity during pre-lithiation, and lifetime characteristics by using a second negative electrode active material layer as described below, without degrading the overall capacity performance of the negative electrode.

[0075] In this field, graphite-based compounds are typically used as negative electrode active materials. However, in recent years, with the increasing demand for high-capacity batteries, there has been an increase in attempts to mix and use silicon-based compounds to increase capacity. However, with silicon-based compounds, there is a limitation: rapid volume expansion during charging / discharging can damage the conductive paths formed in the negative electrode active material layer, thus leading to battery performance degradation.

[0076] Therefore, in an exemplary embodiment of this application, the first negative electrode active material layer composition may further comprise one or more selected from the group consisting of a first negative electrode conductive material and a first negative electrode adhesive.

[0077] In this case, the first negative electrode conductive material and the first negative electrode binder contained in the first negative electrode active material layer composition can be those used in the art without limitation.

[0078] In an exemplary embodiment of this application, the first negative electrode conductive material can be any material commonly used in the art, without limitation. Specifically, it can include one or more materials selected from the group consisting of point conductive materials, planar conductive materials and linear conductive materials.

[0079] The contents of the first negative electrode conductive material are the same as those of the second negative electrode conductive material, which will be described below, and therefore will be described below.

[0080] In an exemplary embodiment of this application, the first negative electrode adhesive may comprise at least one selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials whose hydrogen is replaced by Li, Na, Ca, etc., and may also comprise various copolymers thereof.

[0081] The first negative electrode adhesive according to the exemplary embodiment of this application serves to support the active material and the conductive material, so as to prevent the negative electrode structure from twisting and deforming during the volume expansion and mitigation of the first negative electrode active material. When the above function is satisfied, all common adhesives can be applied. Specifically, water-based adhesives can be used, and more specifically, PAM-based adhesives can be used.

[0082] In an exemplary embodiment of the present application, based on 100 parts by weight of the first negative electrode active material layer composition, the content of the first negative electrode binder may be 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less, and may be 5 parts by weight or more, or 10 parts by weight or more.

[0083] In an exemplary embodiment of the present application, the second negative electrode active material may comprise one or more selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming an alloy with lithium, and lithium-based nitrides.

[0084] In this case, based on 100 parts by weight of the second negative electrode active material, the content of the silicon-based active material may be 50 parts by weight or more and 100 parts by weight or less, preferably 60 parts by weight or more and 100 parts by weight or less, more preferably 65 parts by weight or more and 100 parts by weight or less.

[0085] In the second negative electrode active material layer, when the parts by weight of the silicon-based active material is less than the above range of 50 parts by weight or more and 100 parts by weight or less, the second negative electrode active material layer反而 acts as a resistive layer during charge and discharge cycles, leading to a decrease in capacity retention, thus causing the problem of an increase in the resistance increase rate of the negative electrode.

[0086] In an exemplary embodiment of the present application, the silicon-based active material comprised in the second negative electrode active material may comprise one or more selected from the group consisting of SiOx (0 < x < 2), SiC and Si alloys.

[0087] In an exemplary embodiment of the present application, the silicon-based active material provides a negative electrode for a lithium secondary battery comprising SiOx (0 < x < 2) or SiC.

[0088] In another exemplary embodiment, the silicon-based active material comprised in the second negative electrode active material may comprise SiOx (0 < x < 2).

[0089] In another exemplary embodiment, the silicon-based active material comprised in the second negative electrode active material may comprise SiC.

[0090] The negative electrode for a lithium secondary battery according to the present application may be composed of two layers, and comprises the second negative electrode active material in the second negative electrode active material layer as described above, and by comprising the above first negative electrode active material, while maintaining high capacity and high density characteristics, it solves the problems of surface degradation during charging and discharging, uniformity problems during prelithiation, and service life characteristics problems.

[0091] In exemplary embodiments of this application, representative examples of the carbon-based active materials include natural graphite, artificial graphite, expandable graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotubes, fullerenes, activated carbon, etc., and the carbon-based active materials can be used without limitation, as long as the carbon-based active materials are generally used in carbon materials for lithium secondary batteries, and specifically can be processed into spherical or dot-shaped forms for use.

[0092] In an exemplary embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the carbon-based active material comprises graphite, the graphite comprises artificial graphite and natural graphite, and the weight ratio of the artificial graphite to the natural graphite is 5:5 to 9.5:0.5.

[0093] Artificial graphite according to an exemplary embodiment of the present invention may be in the form of primary particles, or may be in the form of secondary particles obtained by aggregating multiple primary particles.

[0094] As used in this invention, the term "primary particle" means the original particle from which other kinds of particles are formed, and multiple primary particles can be aggregated, combined, or assembled to form secondary particles.

[0095] As used in this invention, the term "secondary particle" refers to a physically distinguishable large particle formed by aggregating, combining, or assembling individual primary particles.

[0096] The artificial graphite of the primary particles can be prepared by heat treatment of one or more selected from the group consisting of needle coke, mosaic coke, and coal tar pitch.

[0097] The artificial graphite is typically prepared by carbonizing raw materials (such as coal tar, coal tar pitch, and petroleum-based heavy oil) to above 2,500°C. After this graphitization, the particles, which have undergone conditioning (such as crushing and secondary particle formation), can also be used as negative electrode active materials. In the case of artificial graphite, crystals are randomly distributed within the particles, have a lower sphericity than natural graphite, and are slightly sharper in shape.

[0098] Examples of artificial graphite used in exemplary embodiments of the present invention include commercially widely used mesophase carbon microspheres (MCMB) and mesophase pitch-based carbon fibers (MPCF), block graphitized artificial graphite, powdered graphitized artificial graphite, etc. The artificial graphite may have a sphericity of less than 0.91, or 0.6 to 0.91, or 0.7 to 0.9.

[0099] Furthermore, the artificial graphite may have a particle size of 5 μm to 30 μm, preferably 10 μm to 25 μm.

[0100] Specifically, the artificial graphite primary particles can have a D50 of 6 μm to 15 μm, or 6 μm to 10 μm, or 6 μm to 9 μm. When the D50 of the primary particles meets such a range of 6 μm to 15 μm, the primary particles can be formed with high graphitization, appropriately ensuring the orientation index of the negative electrode active material particles, thereby improving fast charging performance.

[0101] The artificial graphite secondary particles can be formed by assembling primary particles. That is, the secondary particles can be a structure formed by aggregating primary particles together through an assembly process. The secondary particles may contain a carbonaceous matrix that aggregates the primary particles. The carbonaceous matrix may contain at least one of soft carbon and graphite. The soft carbon can be formed by heat-treating pitch.

[0102] The carbon matrix content in the secondary particles can be from 8% to 16% by weight, specifically from 9% to 12% by weight. This range is lower than the carbon matrix content used in typical artificial graphite secondary particles. Within this range, the particle size of the primary particles in the secondary particles can be controlled, allowing for the preparation of structurally stable secondary particles even with a small amount of the carbon matrix required for assembly, and ensuring that the amount of primary particles constituting the secondary particles is uniform.

[0103] The surface of the artificial graphite secondary particles comprises a carbon coating, and the carbon coating may comprise at least one of amorphous carbon and crystalline carbon.

[0104] The crystalline carbon can further improve the conductivity of the negative electrode active material. The crystalline carbon may contain at least one selected from the group consisting of fullerenes and graphene.

[0105] The amorphous carbon can suppress the expansion of natural graphite by appropriately maintaining the strength of the coating. The amorphous carbon can be a carbide selected from at least one of the groups consisting of tar, pitch and other organic materials, or it can be a carbon-based material formed using hydrocarbons as a source in chemical vapor deposition.

[0106] The other organic materials may be carbonides of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldose, or ketose, as well as carbonides of organic materials selected from combinations thereof.

[0107] The artificial graphite secondary particles may have a D50 of 10 μm to 25 μm, specifically 12 μm to 22 μm, and more specifically 13 μm to 20 μm. When the above range of 10 μm to 25 μm is met, the artificial graphite secondary particles can be uniformly dispersed in the slurry and can also improve the charging performance of the battery.

[0108] The artificial graphite secondary particles can have a tap density of 0.85 g / cc to 1.30 g / cc, specifically 0.90 g / cc to 1.10 g / cc, and more specifically 0.90 g / cc to 1.07 g / cc. When the above range of 0.85 g / cc to 1.30 g / cc is met, the artificial graphite secondary particles can be smoothly filled in the negative electrode, which means that the adhesion of the negative electrode can be improved.

[0109] The natural graphite is typically in the form of plate-like aggregates before processing, and the plate-like particles can be processed into spheres with smooth surfaces through post-processing (such as particle crushing and reassembly processes) to be used as active materials for preparing electrodes.

[0110] The natural graphite used in the exemplary embodiments of the present invention may have a sphericity greater than 0.91 and less than 0.97, or 0.93 to 0.97, or 0.94 to 0.96.

[0111] The natural graphite may have a particle size of 5 μm to 30 μm or 10 μm to 25 μm.

[0112] According to an exemplary embodiment of the present invention, the weight ratio of the artificial graphite to the natural graphite can be 5:5 to 9.5:0.5, or 5:5 to 9.3:0.7, or 5:5 to 9:1, or 6:4 to 9:1. When the weight ratio of the artificial graphite to the natural graphite meets such a range, better output can be exhibited, which can be advantageous in terms of service life and fast charging performance.

[0113] In an exemplary embodiment of this application, the planar conductive material used as the negative electrode conductive material has a different structure and function than the carbon-based active material typically used as the negative electrode active material. Specifically, the carbon-based active material used as the negative electrode active material can be artificial graphite or natural graphite, and refers to a material processed into spherical or dot-like shapes and used to promote the storage and release of lithium ions.

[0114] In contrast, planar conductive materials used as negative electrode conductive materials are materials with planar or plate-like shapes, and can be described as plate-like graphite. That is, the planar conductive material refers to a material included in order to maintain the conductive path in the negative electrode active material layer, and means a material used to ensure the planar conductive path in the negative electrode active material layer rather than to store and release lithium.

[0115] In other words, the fact that plate-shaped graphite is used as a conductive material in this application means that the plate-shaped graphite is processed into a planar or plate-like shape and used as a material to ensure a conductive path rather than to store and release lithium. In this case, the included negative electrode active material has high capacity characteristics for lithium storage and release, and plays the role of storing and releasing all lithium ions transferred from the positive electrode.

[0116] In contrast, in this application, the fact that carbon-based active materials are used as active materials means that carbon-based active materials are processed into dots or spheres and used as materials for storing or releasing lithium.

[0117] In other words, in the exemplary embodiments of this application, the BET specific surface area of ​​artificial graphite or natural graphite, which is a carbon-based active material, can meet the requirement of 0.1 m². 2 / g or more and 4.5m 2 The range is below / g. Furthermore, plate-shaped graphite, as a planar conductive material, is in a planar form and can have a thickness of 5m. 2 BET specific surface area above / g.

[0118] Representative examples of the metal-based active materials may be compounds containing one or more metallic elements selected from the group consisting of Al, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, Ti, Sb, Ga, Mn, Fe, Co, Ni, Cu, Sr, and Ba. These metal compounds can be used in any form, such as elements, alloys, oxides (TiO2, SnO2, etc.), nitrides, sulfides, borides, and alloys with lithium; however, elements, alloys, oxides, and alloys with lithium can increase capacity.

[0119] In an exemplary embodiment of this application, the second negative electrode active material comprises one or more but less than two selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming alloys with lithium, and lithium-based nitrides. Based on 100 parts by weight of the second negative electrode active material, the silicon-based active material may be present in amounts of 50 parts by weight or more but less than 100 parts by weight, 60 parts by weight or more but less than 100 parts by weight, and 65 parts by weight or more but less than 100 parts by weight.

[0120] In an exemplary embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the second negative electrode active material comprises two or more selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming alloys with lithium, and lithium-based nitrides, and the silicon-based active material is present in an amount of 50 parts by weight or more and 95 parts by weight or less based on 100 parts by weight of the second negative electrode active material.

[0121] In another exemplary embodiment, the second negative electrode active material comprises two or more selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming an alloy with lithium, and lithium-based nitrides, and based on 100 parts by weight of the second negative electrode active material, the silicon-based active material may be included in an amount of 50 parts by weight or more and 95 parts by weight or less, preferably 60 parts by weight or more and 90 parts by weight or less, more preferably 65 parts by weight or more and 80 parts by weight or less.

[0122] In an exemplary embodiment of the present application, for a negative electrode for a lithium secondary battery, the second negative electrode active material comprises a silicon-based active material, and based on 100 parts by weight of the second negative electrode active material layer composition, the silicon-based active material may be present in an amount of 50 parts by weight or more and 100 parts by weight or less, preferably 60 parts by weight or more and 100 parts by weight or less, more preferably 70 parts by weight or more and 100 parts by weight or less.

[0123] In an exemplary embodiment of the present application, the negative electrode for a lithium secondary battery comprises a silicon-based active material and a carbon-based active material as the second negative electrode active material, and based on 100 parts by weight of the second negative electrode active material layer composition, the silicon-based active material may be present in an amount of 40 parts by weight or more and 95 parts by weight or less, preferably 45 parts by weight or more and 80 parts by weight or less, more preferably 50 parts by weight or more and 75 parts by weight or less.

[0124] As described above, when the second negative electrode active material satisfies the above composition and content, a lithium secondary battery with further improved various properties (such as cycle life characteristics) can be prepared. That is, in the present application, the second negative electrode active material layer functions as a buffer layer. In order to solve the problems of surface degradation during charging and discharging, uniformity problem during prelithiation, and service life characteristics problem, by including the above SiOx (0<x<2) and / or carbon-based active material, the violent reaction with lithium ions on the surface of the second negative electrode active material layer can be suppressed.

[0125] In summary, the second negative electrode active material layer according to the present application has the above composition and content, thereby solving the problem of surface degradation when charge and discharge cycles continue, and can achieve the prelithiation effect even if prelithiation to the first negative electrode active material layer is not achieved during prelithiation, and at the same time has the characteristic of being capable of providing a negative electrode for high-capacity and high-density lithium secondary batteries.

[0126] In an exemplary embodiment of the present application, there is provided a negative electrode for a lithium secondary battery, wherein based on 100 parts by weight of the second negative electrode active material layer composition, the second negative electrode active material is present in an amount of 60 parts by weight or more.

[0127] In another exemplary embodiment, based on 100 parts by weight of the second negative electrode active material layer composition, the amount of the second negative electrode active material may be 60 parts by weight or more, preferably 63 parts by weight or more, and may be 95 parts by weight or less, preferably 90 parts by weight or less, more preferably 70 parts by weight or less.

[0128] The second negative electrode active material layer composition according to this application has the characteristic of improving service life by using a second negative electrode active material within the above-mentioned range without degrading the capacity performance of the negative electrode. The second negative electrode active material has a lower capacity characteristic compared with the first negative electrode active material, but has less particle cracking during charge and discharge cycles or during pre-lithiation.

[0129] In an exemplary embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the second negative electrode active material layer comprises a second negative electrode active material, a second negative electrode conductive material, and a second negative electrode binder.

[0130] In this case, the second negative electrode conductive material may include one or more selected from the group consisting of point-like conductive materials, linear conductive materials, and planar conductive materials.

[0131] In an exemplary embodiment of this application, the dot-shaped conductive material can be used to improve the conductivity of the negative electrode, and means a conductive material that is conductive without causing chemical change and has a dot-shaped or spherical shape. Specifically, the dot-shaped conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, conductive fiber, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and preferably contains carbon black in order to achieve high conductivity and excellent dispersibility.

[0132] In an exemplary embodiment of this application, the dot-shaped conductive material may have a density of 40m. 2 / g or more and 70m 2 / g or less, preferably 45m 2 / g or more and 65m 2 / g or less, more preferably 50m 2 / g or more and 60m 2 BET specific surface area below / g.

[0133] In an exemplary embodiment of this application, the dot-shaped conductive material may have a particle size of 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 20 nm to 60 nm.

[0134] In an exemplary embodiment of this application, the second negative electrode conductive material may comprise a planar conductive material.

[0135] The planar conductive material can increase the surface contact between silicon particles in the negative electrode to improve conductivity, while suppressing the breakage of the conductive path due to volume expansion, and can be represented as a plate-shaped conductive material or a bulk conductive material.

[0136] In an exemplary embodiment of this application, the planar conductive material may comprise at least one selected from the group consisting of plate graphite, graphene, graphene oxide, and graphite flakes, and may preferably be plate graphite.

[0137] In an exemplary embodiment of this application, the planar conductive material may have an average particle size (D50) of 2 μm to 7 μm, specifically 3 μm to 6 μm, and more specifically 4 μm to 5 μm. When the average particle size meets the above range, sufficient particle size is beneficial for dispersion without causing an excessive increase in the viscosity of the negative electrode slurry. Therefore, when the particles are dispersed using the same equipment and time, the dispersion effect is excellent.

[0138] In an exemplary embodiment of this application, the planar conductive material provides a negative electrode composition having a D10 of 0.5 μm or more and 1.5 μm or less, a D50 of 2.5 μm or more and 3.5 μm or less, and a D90 of 7.0 μm or more and 15.0 μm or less.

[0139] In an exemplary embodiment of this application, a high specific surface area planar conductive material with a high BET specific surface area or a low specific surface area planar conductive material can be used as the planar conductive material.

[0140] In exemplary embodiments of this application, high specific surface area planar conductive materials or low specific surface area planar conductive materials can be used as the planar conductive material without limitation. However, in particular, the planar conductive material according to this application may be affected to some extent by the dispersion effect in terms of electrode performance, so it is particularly desirable to use a low specific surface area planar conductive material that does not cause dispersion problems.

[0141] In an exemplary embodiment of this application, the planar conductive material may have a 5m² surface area. 2 BET specific surface area above / g.

[0142] In another exemplary embodiment, the planar conductive material may have a 5m 2 / g or more and 500m 2 / g or less, preferably 5m 2 / g or more and 300m 2 / g or less, preferably 5m2 / g or more and 250m 2 BET specific surface area below / g.

[0143] In yet another exemplary embodiment, the planar conductive material is a high specific surface area planar conductive material, and the BET specific surface area can meet 50m². 2 / g or more and 500m 2 / g or less, preferably 80m 2 / g or more and 300m 2 / g or less, more preferably 100m 2 / g or more and 300m 2 The range below / g.

[0144] In yet another exemplary embodiment, the planar conductive material is a low specific surface area planar conductive material, and the BET specific surface area can meet the requirement of 5m². 2 / g or more and 40m 2 / g or less, preferably 5m 2 / g or more and 30m 2 / g or less, preferably 5m 2 / g or more and 25m 2 The range below / g.

[0145] Other conductive materials include linearly conductive materials such as carbon nanotubes. These carbon nanotubes can be bundled carbon nanotubes. A bundled carbon nanotube can contain multiple carbon nanotube units. Specifically, unless otherwise stated, the term "bundled" as used herein refers to a secondary shape in the form of a bundle or rope, wherein multiple carbon nanotube units are arranged side-by-side or coiled together with substantially the same orientation as the longitudinal axis of the carbon nanotube units. Within the carbon nanotube unit, the graphite sheet has a cylindrical shape with a nanometer-sized diameter and an sp2 bond structure. In this case, the carbon nanotube unit can exhibit conductive or semiconductor properties depending on the structure and angle of the graphite sheet coiling. Compared to coiled carbon nanotubes, bundled carbon nanotubes can be uniformly dispersed during the fabrication of the negative electrode, and the conductivity of the negative electrode can be improved by smoothly forming a conductive network within it.

[0146] In particular, the linear conductive material according to the exemplary embodiments of this application may be a single-walled carbon nanotube (SWCNT).

[0147] The single-walled carbon nanotubes are materials in which carbon atoms are arranged in a hexagonal pattern to form a tubular shape. Due to their unique chirality, they exhibit non-conductor, conductor, or semiconductor properties. Furthermore, because the carbon atoms are connected by strong covalent bonds, they have tensile strength that is more than 100 times higher than that of steel, excellent flexibility and elasticity, and chemical stability.

[0148] The single-walled carbon nanotubes have an average diameter of 0.5 nm to 15 nm. According to an exemplary embodiment of the invention, the single-walled carbon nanotubes may have an average diameter of 1 nm to 10 nm, or 1 nm to 5 nm, or 1 nm to 2 nm. When the average diameter of the single-walled carbon nanotubes meets such a range, the conductivity of the negative electrode can be maintained even if the content of single-walled carbon nanotubes is very small, and preferred viscosity and solid content can be obtained when preparing the conductive material dispersion. In the conductive material dispersion, the single-walled carbon nanotubes aggregate with each other, and therefore can exist in a tangled state (aggregate). Thus, after confirming the diameter of any tangled single-walled carbon nanotube aggregate extracted from the conductive material dispersion by SEM or TEM, the average diameter is obtained by dividing the diameter of the aggregate by the number of single-walled carbon nanotubes constituting the aggregate.

[0149] The single-walled carbon nanotubes can have a diameter of 500m. 2 / g to 1,500m 2 / g、900m 2 / g to 1,200m 2 / g, and specifically 250m 2 / g to 330m 2 / g BET specific surface area. When the above range is met, a conductive material dispersion with a preferred solid content is obtained, and excessive increase in the viscosity of the negative electrode slurry is prevented. The BET specific surface area can be measured by the nitrogen adsorption BET method.

[0150] The single-walled carbon nanotubes can have an aspect ratio of 50 to 20,000, or a length of 5 μm to 100 μm, or 5 μm to 50 μm. When the aspect ratio or length meets such ranges, the specific surface area is at a high level, allowing the single-walled carbon nanotubes in the negative electrode to be adsorbed to the active material particles by strong attraction. Therefore, even when the volume of the negative electrode active material expands, the conductive network can be maintained stably. When observing the single-walled carbon nanotube powder by SEM, the aspect ratio can be confirmed by obtaining the average aspect ratio of 15 single-walled carbon nanotubes with a high aspect ratio and 15 single-walled carbon nanotubes with a low aspect ratio.

[0151] Because single-walled carbon nanotubes have a higher aspect ratio than multi-walled and double-walled carbon nanotubes, they have a long length and a large volume, which gives them the advantage of being able to build conductive networks even with only a small amount.

[0152] In an exemplary embodiment of this application, based on 100 parts by weight of the second negative electrode active material layer composition, the second negative electrode conductive material is present in an amount of more than 1 part by weight and less than 40 parts by weight.

[0153] In another exemplary embodiment, based on 100 parts by weight of the second negative electrode active material layer composition, the amount of the second negative electrode conductive material may be more than 1 part by weight and less than 40 parts by weight, preferably more than 10 parts by weight and less than 30 parts by weight, more preferably more than 15 parts by weight and less than 25 parts by weight.

[0154] In an exemplary embodiment of this application, the second negative electrode conductive material comprises a dot-shaped conductive material, a planar conductive material, and a linear conductive material, and the ratio of the dot-shaped conductive material:planar conductive material:linear conductive material can satisfy a ratio of 1:1:0.01 to 1:1:1.

[0155] In an exemplary embodiment of this application, based on 100 parts by weight of the second negative electrode conductive material, the dot-shaped conductive material can satisfy the range of more than 1 part by weight and less than 60 parts by weight, preferably more than 5 parts by weight and less than 50 parts by weight, and more preferably more than 10 parts by weight and less than 50 parts by weight.

[0156] In an exemplary embodiment of this application, based on 100 parts by weight of the second negative electrode conductive material, the planar conductive material may satisfy the range of 1 part by weight or more and 60 parts by weight or less, preferably 5 parts by weight or more and 50 parts by weight or less, and more preferably 10 parts by weight or more and 50 parts by weight or less.

[0157] In an exemplary embodiment of this application, based on 100 parts by weight of the second negative electrode conductive material, the linear conductive material may be in the range of 0.01 parts by weight to 10 parts by weight, preferably 0.05 parts by weight to 8 parts by weight, and more preferably 0.1 parts by weight to 5 parts by weight.

[0158] In an exemplary embodiment of this application, the second negative electrode conductive material may include linear conductive materials and planar conductive materials.

[0159] In an exemplary embodiment of this application, the second negative electrode conductive material comprises a linear conductive material and a planar conductive material, and the ratio of the linear conductive material to the planar conductive material can satisfy 0.01:1 to 0.1:1.

[0160] In an exemplary embodiment of this application, since the second negative electrode conductive material specifically comprises linear conductive material and planar conductive material, and each satisfies the composition and ratio, the lifespan characteristics of existing lithium secondary batteries are not significantly affected, and the number of points where the battery can be charged and discharged is increased, thereby the second negative electrode conductive material has excellent output characteristics at high C-rate.

[0161] A negative electrode for a lithium secondary battery is provided, wherein the first negative electrode conductive material according to the present application comprises at least a linearly conductive material.

[0162] A negative electrode for a lithium secondary battery is provided, wherein the second negative electrode conductive material according to this application comprises at least a linearly conductive material.

[0163] In an exemplary embodiment of this application, the second negative electrode conductive material may be composed of a linear conductive material.

[0164] In this case, based on 100 parts by weight of the second negative electrode active material, the content of the linear conductive material can be 0.1 to 2 parts by weight, or 0.1 to 0.7 parts by weight, or 0.1 to 0.3 parts by weight. When the content of the linear conductive material meets such a range, a conductive network can be sufficiently constructed in the negative electrode active material layer, which is advantageous in terms of mixing and coating processability during electrode preparation. Furthermore, in the negative electrode according to an exemplary embodiment of the invention, since both the first and second negative electrode active material layers contain linear conductive material, the conductive network between the active materials accompanying the volume expansion and contraction of the Si electrode can be maintained, which is advantageous in terms of lifetime and can maintain fast charging performance. Basically, fast charging performance is advantageous because, compared to graphite, the Si-containing negative electrode can be coated with a thin film, but the linear conductive material also contributes to fast charging by maintaining a strong conductive network. In addition, the linear conductive material can further help improve the initial drop in the lifetime of the Si-containing negative electrode and maintain its lifetime.

[0165] The second negative electrode conductive material according to this application has a completely different structure from the positive electrode conductive material applied to the positive electrode. That is, the second negative electrode conductive material according to this application is used to support the contact points between silicon-based active materials in which the volume expansion of the electrodes is very large due to charging and discharging, and the positive electrode conductive material plays a buffering role, such as mitigation, during rolling, while also imparting partial conductivity, and its structure and function are completely different from the negative electrode conductive material of this invention.

[0166] Furthermore, the second negative electrode conductive material according to this application is applied to a silicon-based active material and has a completely different structure from the conductive material applied to a graphite-based active material. That is, the conductive material used in the electrode with the graphite-based active material has only small particles relative to the active material, thus possessing properties that improve output characteristics and impart partial conductivity, and its structure and function are completely different from the first negative electrode conductive material used together with the silicon-based active material in this invention.

[0167] In this case, the same content as that of the second negative electrode conductive material can be applied independently to the content of the first negative electrode conductive material.

[0168] In an exemplary embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the first negative electrode active material layer has a thickness of 10 μm or more and 200 μm or less, and the second negative electrode active material layer has a thickness of 10 μm or more and 100 μm or less. When the first negative electrode active material layer and the second negative electrode active material layer are present on both sides of the current collector, each layer has the thickness range defined above.

[0169] In an exemplary embodiment of this application, a negative electrode for a lithium secondary battery is provided, wherein the loading amount (a) of the first negative electrode active material layer composition is more than twice the loading amount (b) of the second negative electrode active material layer composition.

[0170] In another exemplary embodiment, the loading amount (a) of the first negative electrode active material layer composition can be in the range of more than 1.5 times and less than 10 times, preferably more than 2.2 times and less than 6 times, of the loading amount (b) of the second negative electrode active material layer composition.

[0171] The loading amount may refer to the weight of the composition for forming the negative electrode active material layer, and specifically, the loading amount of the composition may have the same meaning as the loading amount of the slurry containing the composition.

[0172] In an exemplary embodiment of this application, the loading (a) of the first negative electrode active material layer composition can meet the requirement of 2 mg / cm³. 2 Above and 5mg / cm 2 The following is preferred: 2.2 mg / cm³ 2 Above and 4mg / cm 2 The following range.

[0173] In an exemplary embodiment of this application, the loading (b) of the second negative electrode active material layer composition can meet the requirement of 0.5 mg / cm³. 2 Above and 1.5 mg / cm 2 The following is preferred: 0.8 mg / cm³ 2 Above and 1.3 mg / cm 2 The following range.

[0174] The first negative electrode active material layer composition and the second negative electrode active material layer composition may have the loading amounts as described, so as to adjust the ratio of the active materials comprised in the first negative electrode active material layer and the second negative electrode active material layer. That is, the capacity characteristic can be optimized by adjusting the amount of the first negative electrode active material comprised in the first negative electrode active material layer, meanwhile, the amount of the second negative electrode active material comprised in the second negative electrode active material layer can be set and adjusted to suppress the surface reaction of the negative electrode without deteriorating the capacity characteristic, thereby having the characteristic of improving service life characteristics.

[0175] In an exemplary embodiment of the present application, the negative electrode for a lithium secondary battery may comprise two layers, and is particularly characterized in that the second negative electrode active material layer acts as a buffer layer during prelithiation to prevent the first negative electrode active material layer from being prelithiated.

[0176] In an exemplary embodiment of the present application, the negative electrode for a lithium secondary battery may be a prelithiated negative electrode.

[0177] In an exemplary embodiment of the present application, there is provided a method of preparing a negative electrode for a lithium secondary battery, the method comprising: providing a negative electrode current collector layer; forming a first negative electrode active material layer by applying a first negative electrode active material layer composition comprising a first negative electrode active material onto one surface or both surfaces of the negative electrode current collector layer; and forming a second negative electrode active material layer by applying a second negative electrode active material layer composition comprising a second negative electrode active material, a second negative electrode conductive material and a second negative electrode binder onto a surface of the first negative electrode active material layer opposite to the surface facing the negative electrode current collector layer, wherein the first negative electrode active material comprises one or more selected from the group consisting of SiOx (x=0) and SiOx (0<x<2), and based on 100 parts by weight of the first negative electrode active material, SiOx (x=0) is comprised in an amount of 95 parts by weight or more, the second negative electrode conductive material comprises one or more selected from the group consisting of point-shaped conductive materials, linear conductive materials and planar conductive materials, the second negative electrode active material comprises one or more selected from the group consisting of carbon-based active materials, silicon-based active materials, metal-based active materials capable of forming an alloy with lithium and lithium-based nitrides, and based on 100 parts by weight of the second negative electrode active material, the silicon-based active material is present in an amount of 50 parts by weight or more and 100 parts by weight or less.

[0178] In the method for preparing a negative electrode, the above content can be applied to the compositions and contents included in each step.

[0179] In an exemplary embodiment of the present application, forming a first negative electrode active material layer is provided by applying a first negative electrode active material layer composition onto one surface or both surfaces of a negative electrode current collector layer.

[0180] In other words, the step involves forming an active material layer on the negative electrode current collector layer, and may refer to forming the active material layer on the surface (lower part) facing the current collector layer in a double-layer structure.

[0181] In an exemplary embodiment of this application, the application of the first negative electrode active material layer composition includes applying a first negative electrode slurry comprising the first negative electrode active material layer composition and a negative electrode slurry solvent, and drying the applied first negative electrode slurry.

[0182] In this case, the solid content of the first negative electrode slurry can be in the range of 10% to 40%.

[0183] In an exemplary embodiment of this application, the formation of the first negative electrode active material layer may include: mixing a first negative electrode slurry; and coating one or both surfaces of the negative electrode current collector layer with the mixed first negative electrode slurry, and for coating, a coating method commonly used in the art may be used.

[0184] In an exemplary embodiment of this application, a second negative electrode active material is formed by applying a second negative electrode active material layer composition to a surface of the first negative electrode active material layer opposite to the surface facing the negative electrode current collector layer.

[0185] In other words, the step involves forming a second negative electrode active material layer on the first negative electrode active material layer, and may refer to forming the active material layer on the surface (upper part) away from the current collector layer in the double-layer structure. If the first negative electrode active material layer is formed on both surfaces of the current collector layer, the second negative electrode active material layer can be applied to one or both of such first negative electrode active material layers.

[0186] In an exemplary embodiment of this application, the application of the second negative electrode active material layer composition includes applying a second negative electrode slurry containing the second negative electrode active material layer composition and a negative electrode slurry solvent, and drying the applied second negative electrode slurry.

[0187] In this case, the solid content of the second negative electrode slurry can be in the range of 10% to 40%.

[0188] In an exemplary embodiment of this application, a method for preparing a negative electrode for a lithium secondary battery is provided, wherein the formation of the second negative electrode active material layer includes: mixing a second negative electrode slurry; and coating the surface of the first negative electrode active material layer opposite to the surface facing the negative electrode current collector layer with the mixed second negative electrode slurry.

[0189] For the coating, a coating method commonly used in the art can be used.

[0190] The description of forming the first negative electrode active material layer can be applied equally to forming the second negative electrode active material layer.

[0191] In an exemplary embodiment of this application, a method for preparing a negative electrode for a lithium secondary battery is provided, wherein forming a second negative electrode active material layer on a first negative electrode active material layer includes a wet-cover dry process or a wet-cover wet process.

[0192] In an exemplary embodiment of this application, the wet-drying process may refer to a process in which a first negative electrode active material layer composition is applied, then the applied composition is partially or completely dried, and a second negative electrode active material layer composition is applied on top of it. An exemplary wet-drying process is shown in Figure 5 In the flowchart, during the wet-on-dry process, a first negative electrode slurry mixture is prepared and then applied to the current collector. The first negative electrode slurry mixture is dried to form a first layer. Then, a second negative electrode slurry mixture is prepared and then applied onto the first layer. The second negative electrode slurry mixture is dried to form a second layer. These layers can be calendered and pressed to form a negative electrode. The negative electrode can then be slit twice using a single coating die. In another exemplary embodiment of this application, the wet-on-dry process refers to the process of applying a first negative electrode active material layer composition and then applying a second negative electrode active material layer composition on top of it without drying the applied first negative electrode active material layer composition. An exemplary wet-on-dry process is shown in... Figure 6 In the flowchart, during the wet-coating process, a first negative electrode slurry mixture is prepared and then applied as a first layer onto the current collector. A second negative electrode slurry mixture is then prepared and applied onto the first layer. The second negative electrode slurry mixture is dried to form a second layer. These layers can be calendered and pressed to form a negative electrode. The negative electrode can then be cut twice using a single coating die.

[0193] Specifically, the wet-drying process involves applying a first negative electrode active material layer composition, then partially or completely drying the applied composition, and then applying a second negative electrode active material layer composition on top of it. Through the process described above, the first and second negative electrode active material layers can have clear or well-defined boundaries. Therefore, the characteristic feature is that the compositions contained in the first and second negative electrode active material layers are not mixed, and a double layer may be included.

[0194] In an exemplary embodiment of this application, the negative electrode slurry solvent can be used without limitation, as long as the solvent can dissolve the first negative electrode active material layer composition and the second negative electrode active material layer composition. Specifically, water or NMP can be used.

[0195] As a result of the wet-on-wet process, a bonding region can be formed. To perform the wet-on-wet process, the viscosity of the first negative electrode active material layer composition can be lower than the viscosity of the second negative electrode active material layer composition, allowing for mutual mixing in the bonding region and enabling the process to proceed. Figure 7 As shown, after drying the first negative electrode active material layer (i.e., the wet-drying process), an interface is clearly formed by applying a second negative electrode active material layer composition. And as... Figure 8 As shown, by applying the second negative electrode active material layer composition (while simultaneously applying the first negative electrode active material layer composition and the second negative electrode active material layer composition) while the first negative electrode active material layer composition is not completely dry, they mix at the interface to form a bonding region.

[0196] In an exemplary embodiment of this application, a method for preparing a negative electrode for a lithium secondary battery is provided. The method includes: subjecting a negative electrode on which a first negative electrode active material layer and a second negative electrode active material layer are formed on a negative electrode current collector to pre-lithiation, wherein the negative electrode is subjected to at least one of the following four pre-lithiation processes: lithium plating process, lithium metal transfer process, lithium metal deposition process, or stabilized lithium metal powder (SLMP) coating process.

[0197] The lithium-ion battery anode described above comprises a first anode active material layer of SiOx (x=0) to improve capacity characteristics, and a second anode active material layer having a specific composition of the aforementioned silicon-based and / or carbon-based active materials. Therefore, it directly offers the advantage of fast charging. Furthermore, since the second anode active material has the aforementioned composition and is therefore highly irreversible, it provides particularly advantageous results even in pre-lithiation processes where the anode is pre-charged. Compared to simply applying the first anode active material layer, the second anode active material has the composition described above, thereby enabling a uniform pre-lithiation process on the upper part of the anode. Therefore, the anode exhibits characteristics that can further improve its lifespan.

[0198] In an exemplary embodiment of this application, the porosity of the first and second negative electrode active material layers can be in the range of 10% or more and 60% or less.

[0199] In another exemplary embodiment, the porosity of the first and second negative electrode active material layers can be in the range of 10% or more and 60% or less, preferably 20% or more and 50% or less, and more preferably 30% or more and 45% or less.

[0200] The porosity varies depending on the composition and content of the active material, conductive material, and binder contained in the first and second negative electrode active material layers; therefore, the conductivity and resistance in the electrode are characterized by having an appropriate range.

[0201] In an exemplary embodiment of this application, a lithium secondary battery is provided, comprising: a positive electrode, a negative electrode for a lithium secondary battery according to this application, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0202] The secondary battery according to the exemplary embodiments of this specification may specifically include the negative electrode for a lithium secondary battery described above. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator inserted between the positive and negative electrodes, and an electrolyte, and the negative electrode is the same as the negative electrode described above. Since the negative electrode has already been described in detail, its specific description will be omitted.

[0203] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and containing positive electrode active material.

[0204] In the positive electrode, the positive current collector is not particularly limited, as long as it is conductive and will not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or materials in which the surface of aluminum or stainless steel has been surface-treated with carbon, nickel, titanium, silver, etc., can be used. Furthermore, the positive current collector can typically have a thickness of 3 μm to 500 μm, and the adhesion of the positive electrode active material can be improved by forming micro-irregularities on the surface of the current collector. For example, the positive current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0205] The positive electrode active material can be a commonly used positive electrode active material. Specifically, the positive electrode active material includes: layered compounds, such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) or compounds replaced by one or more transition metals; lithium iron oxide, such as LiFe3O4; lithium manganese oxide, such as Li... 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides, such as LiV3O8, V2O5 and Cu2V2O7; Ni-site type lithium nickel oxide, represented by the chemical formula LiNi 1-c2 M c2 O2 (here, M is selected from at least one of the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and c2 satisfies 0.01≤c2≤0.3); lithium manganese composite oxide, represented by the chemical formula LiMn 2-c3 M c3O2 (here, M is selected from at least one of the group consisting of Co, Ni, Fe, Cr, Zn and Ta, and c3 satisfies 0.01≤c3≤0.1) or Li2Mn3MO8 (here, M is selected from at least one of the group consisting of Fe, Co, Ni, Cu and Zn); LiMn2O4, wherein the Li in the chemical formula is partially replaced by alkaline earth metal ions; etc., but not limited thereto. The positive electrode can be Li metal.

[0206] The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder, in addition to the aforementioned positive electrode active material.

[0207] In this context, the positive electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitations, provided that the positive electrode conductive material is electronically conductive without causing chemical changes in the constructed battery. Specific examples include graphite, such as natural or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one or a mixture of two or more of these can be used.

[0208] In addition, the positive electrode adhesive is used to improve the bonding force between positive electrode active material particles and the adhesion force between the positive electrode active material and the positive electrode current collector. Specific examples may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof may be used.

[0209] The separator separates the negative and positive electrodes and provides a channel for lithium ion movement. It can be used without particular limitations, as long as it is typically used in secondary batteries. In particular, separators with excellent electrolyte retention and low resistance to ion movement in the electrolyte are preferred. Specifically, porous polymer membranes can be used, such as porous polymer membranes formed from polyolefin polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers), or laminates of two or more layers thereof. Alternatively, conventional porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, separators coated with ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and can be selectively used as single-layer or multi-layer structures.

[0210] Examples of the electrolytes include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used to prepare lithium secondary batteries.

[0211] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0212] As the non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate can be used.

[0213] In particular, among carbonate-based organic solvents, ethylene carbonate and propylene carbonate, as cyclic carbonates, are high-viscosity organic solvents with high dielectric constants, thereby effectively dissociating lithium salts, and are therefore preferred for use. Furthermore, such cyclic carbonates can be mixed in appropriate ratios with low-viscosity and low-dielectric-constant linear carbonates such as dimethyl carbonate and diethyl carbonate, and used to prepare electrolytes with high conductivity, and are therefore even more preferred for use.

[0214] As the metal salt, a lithium salt can be used, which is a material that is readily soluble in non-aqueous electrolytes. For example, as the anion of the lithium salt, one or more of the following can be used: F - Cl- I - NO3 - N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - .

[0215] In order to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, the electrolyte may contain one or more additives in addition to the above-mentioned electrolyte components. These additives may include, for example, alkylene carbonate halide compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexamethylphosphoryltriamine, nitrobenzene derivatives, sulfur, quinone imine dyes, and N-substituted compounds. Zolpidemone, N,N-substituted imidazolidinyl ether, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride.

[0216] An exemplary embodiment of the present invention provides a battery module comprising the secondary battery as a unit cell and a battery pack comprising the battery module. The battery module and battery pack contain the secondary battery having high capacity, high rate performance, and high cycle performance, and therefore can be used as a power source for medium to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems.

[0217] The mode of the present invention

[0218] Preferred embodiments will be presented below to aid in understanding the invention; however, these embodiments are provided merely to illustrate the invention, and it will be apparent to those skilled in the art that various changes and modifications may be made within the scope and spirit of the invention, and naturally, such changes and modifications also fall within the scope of the appended claims.

[0219] <Example>

[0220] <Preparation of the negative electrode>

[0221] Example 1: Preparation of the negative electrode

[0222] Preparation of the first negative electrode active material layer

[0223] A first negative electrode active material layer composition was prepared, comprising Si (average particle size (D50): 5 μm) as a silicon-based active material, a first conductive material, a second conductive material, a third conductive material, and polyacrylamide as a binder, in a weight ratio of 70:9.8:10:0.2:10. A first negative electrode slurry (solid concentration 25% by weight) was prepared by adding this first negative electrode active material layer composition to distilled water as a solvent for forming the negative electrode slurry.

[0224] The first conductive material is carbon black C (specific surface area: 58 m²). 2 / g, diameter: 37nm), the second conductive material is plate-shaped graphite (specific surface area: 17m²). 2 / g, average particle size (D50): 3.5μm), and the third conductive material is carbon nanotubes.

[0225] After dispersing the first conductive material, the second conductive material, the third conductive material, the binder and water at 2500 rpm for 30 minutes using a homogenizer as the mixing method, an active material is added thereto, and the resulting mixture is then dispersed at 2500 rpm for 30 minutes to prepare the first negative electrode slurry.

[0226] The two surfaces of the copper current collector (thickness: 8 μm), which serves as the negative electrode current collector, are coated with the first negative electrode slurry at a concentration of 2.75 mg / cm². 2 The copper current collector is coated with a loading amount and then rolled and dried in a vacuum oven at 130°C for 10 hours to form a first negative electrode active material layer (thickness: 33μm).

[0227] Preparation of the second negative electrode active material layer

[0228] A second negative electrode active material layer composition was prepared, comprising SiO (average particle size (D50): 3.5 μm) as a silicon-based active material, a second conductive material, a third conductive material, and polyacrylamide as a binder, in a weight ratio of 70:19.8:0.2:10. A second negative electrode slurry (solid concentration 25% by weight) was prepared by adding this second negative electrode active material layer composition to distilled water as a solvent for forming the negative electrode slurry.

[0229] The second conductive material is plate-shaped graphite (specific surface area: 17m²). 2 / g, average particle size (D50): 3.5μm), and the third conductive material is carbon nanotubes.

[0230] After dispersing the second conductive material, the third conductive material, the binder and water at 2500 rpm for 30 minutes using a homogenizer as the mixing method, an active material is added to it, and the resulting mixture is then dispersed at 2500 rpm for 30 minutes to prepare the second negative electrode slurry.

[0231] One surface of the first negative electrode active material layer is coated with the second negative electrode slurry at a concentration of 1 mg / cm². 2 The coating is applied with a loading rate, rolled, and dried in a vacuum oven at 130°C for 10 hours to form a second negative electrode active material layer (thickness: 15μm).

[0232] Then, pre-lithiation is performed by transferring lithium metal onto the upper part of the second negative electrode active material layer. The pre-lithiation ratio is 10% to 15% based on the negative electrode charging capacity.

[0233] Example 1-1: Preparation of the negative electrode

[0234] The negative electrode was prepared in the same manner as in Example 1, except that the negative electrode was not pre-lithiated as in Example 1.

[0235] Example 2: Preparation of the negative electrode

[0236] The negative electrode (solid concentration of 25% by weight) was prepared under the same conditions as in Example 1, except that in the preparation of the second negative electrode active material layer in Example 1, a second negative electrode active material layer composition was prepared, which contained SiC (average particle size (D50): 3.5 μm) as a silicon-based active material, a second conductive material, a third conductive material, and polyacrylamide as a binder in a weight ratio of 70:19.8:0.2:10, and was added to distilled water as a solvent for forming the negative electrode slurry to prepare the second negative electrode slurry.

[0237] Example 3: Preparation of the negative electrode

[0238] The negative electrode (solid concentration of 25% by weight) was prepared under the same conditions as in Example 1, except that in the preparation of the second negative electrode active material layer in Example 1, a second negative electrode active material layer composition was prepared, which contained SiO (average particle size (D50): 3.5 μm) as a silicon-based active material, a second conductive material, a third conductive material, and polyacrylamide as a binder in a weight ratio of 63:17:0.3:19.7, and was added to distilled water as a solvent for forming the negative electrode slurry to prepare the second negative electrode slurry.

[0239] Example 4: Preparation of the negative electrode

[0240] Preparation of the first negative electrode active material layer

[0241] The first negative electrode active material layer was prepared in the same manner as in Example 1.

[0242] Preparation of the second negative electrode active material layer

[0243] A second negative electrode active material layer composition was prepared, comprising SiO (average particle size (D50): 3.5 μm) as a silicon-based active material, artificial graphite, a second conductive material, a third conductive material, and polyacrylamide as a binder in a weight ratio of 50:20:10:10:10. A second negative electrode slurry (solid concentration 25% by weight) was prepared by adding this second negative electrode active material layer composition to distilled water as a solvent for forming the negative electrode slurry.

[0244] The second conductive material is plate-shaped graphite (specific surface area: 17m²). 2 / g, average particle size (D50): 3.5μm), and the third conductive material is carbon nanotubes.

[0245] After dispersing the second conductive material, the third conductive material, the binder and water at 2500 rpm for 30 minutes using a homogenizer as the mixing method, an active material is added thereto, and the resulting mixture is then dispersed at 2500 rpm for 30 minutes to prepare a slurry.

[0246] The first negative electrode active material layer was coated with the second negative electrode slurry at a concentration of 2.8 mg / cm³. 2 The coating is applied with a loading rate, rolled, and dried in a vacuum oven at 130°C for 10 hours to form a second negative electrode active material layer (thickness: 15μm).

[0247] Then, pre-lithiation is performed by transferring lithium metal onto the top of the second negative electrode active material layer.

[0248] Example 5: Preparation of the negative electrode

[0249] A first negative electrode slurry is prepared by mixing 100 parts by weight of Si with a D50 of 5 μm, 13 parts by weight of carbon black as a first conductive material, 0.3 parts by weight of single-walled carbon nanotubes (SWCNTs) with an average diameter of 1 nm, 15 parts by weight of polyacrylic acid (PAA) as a first binder polymer, and 1.1 parts by weight of carboxymethyl cellulose (CMC), and adding water as a first dispersion medium. In this case, the solid content of the first negative electrode slurry is 25% by weight.

[0250] A second negative electrode slurry was prepared by mixing 63 parts by weight of SiO, 7 parts by weight of natural graphite, 30 parts by weight of secondary particles of artificial graphite with a D50 of 16.7 μm and a tap density of 0.91 g / cc, 1.0 part by weight of carbon black as a second conductive material, 3.0 parts by weight of styrene-butadiene rubber (SBR) as a second binder polymer, and 1.1 parts by weight of carboxymethyl cellulose (CMC), and water was added as a second dispersion medium. In this case, the solid content of the second negative electrode slurry was 49% by weight.

[0251] Using a dual-groove die, while coating one surface of a 10 μm thick copper (Cu) film serving as the negative electrode current collector with a first negative electrode slurry, the first negative electrode slurry is simultaneously coated with a second negative electrode slurry. In this case, the loading rates of the first and second negative electrode slurries are both 3 mg / cm³. 2 and 1mg / cm 2 .

[0252] Subsequently, the coated first and second negative electrode slurries are simultaneously dried using an apparatus that combines hot air drying and infrared drying methods to form an active material layer.

[0253] The resulting negative electrode active material layer is simultaneously rolled using a rolling method to prepare a negative electrode with a double active material layer (having a double structure with a thickness of 78 μm).

[0254] Example 6: Preparation of the negative electrode

[0255] The negative electrode was prepared in the same manner as in Example 5, except that in the second negative electrode slurry of Example 5, the negative electrode active material was changed to "31.5 parts by weight of artificial graphite, 3.5 parts by weight of natural graphite and 65 parts by weight of SiO".

[0256] Example 7: Preparation of the negative electrode

[0257] The negative electrode was prepared in the same manner as in Example 5, except that in the second negative electrode slurry of Example 5, the negative electrode active material was changed to "63 parts by weight of SiC, 7 parts by weight of natural graphite and 30 parts by weight of artificial graphite".

[0258] Example 8: Preparation of the negative electrode

[0259] The negative electrode was prepared in the same manner as in Example 5, except that in the second negative electrode slurry of Example 5, the negative electrode active material was changed to "31.5 parts by weight of artificial graphite, 3.5 parts by weight of natural graphite and 65 parts by weight of SiC".

[0260] Methods for measuring the average diameter of single-walled carbon nanotubes

[0261] After measuring single-walled carbon nanotubes using a transmission electron microscope (TEM) (manufacturer: Hitachi, model name: H7650) at a magnification of 150,000x or higher, the average diameter of the single-walled carbon nanotubes identified in the measured images within an arbitrary sampling range was determined. In this case, the average diameter was obtained after measuring single-walled carbon nanotubes by setting the number of measurements to at least 10.

[0262] Comparative Example 1: Preparation of the negative electrode

[0263] An active material layer composition was prepared, comprising Si (average particle size (D50): 5 μm) as a silicon-based active material, a first conductive material, and polyacrylamide as a binder in a weight ratio of 70:20:10. A negative electrode slurry (solid concentration 25% by weight) was prepared by adding the active material layer composition to distilled water as a solvent for forming the negative electrode slurry.

[0264] Carbon black C (specific surface area: 58 m²) was used as the first conductive material. 2 / g, diameter: 37nm).

[0265] After dispersing the first conductive material, binder and water at 2500 rpm for 30 minutes using a homogenizer as the mixing method, an active material is added thereto, and the resulting mixture is then dispersed at 2500 rpm for 30 minutes to prepare a slurry.

[0266] The two surfaces of the copper current collector (thickness: 8μm), which serves as the negative electrode current collector, are coated with the negative electrode slurry at a concentration of 85mg / 25cm. 2 The copper current collector is coated with a loading amount and then rolled and dried in a vacuum oven at 130°C for 10 hours to form a negative electrode active material layer (thickness: 33μm).

[0267] Then, pre-lithiation is performed by transferring lithium metal onto the upper part of the negative electrode active material layer.

[0268] Comparative Example 2: Preparation of the Negative Electrode

[0269] The negative electrode was prepared in the same manner as in Comparative Example 1, except that pre-lithiation was not performed as in Comparative Example 1.

[0270] Comparative Example 3: Preparation of the Negative Electrode

[0271] The negative electrode was prepared in the same manner as in Example 1, except that the stacking order of the first negative electrode active material layer and the second negative electrode active material layer in Example 1 was changed, so that the two sides of the current collector were coated with the second negative electrode slurry, and then the surface of the second negative electrode layer was coated with the first negative electrode slurry.

[0272] Comparative Example 4: Preparation of the Negative Electrode

[0273] The negative electrode was prepared in the same manner as in Example 1, except that in Example 1, the second negative electrode slurry was prepared by adding SiO (average particle size (D50): 3.5 μm) as a silicon-based active material, artificial graphite as a carbon-based active material, a first conductive material, a second conductive material, and polyacrylamide as a binder in a weight ratio of 30:50:5:5:10 to distilled water as a solvent for forming the negative electrode slurry.

[0274] Comparative Example 4-1: Preparation of the Negative Electrode

[0275] The negative electrode was prepared in the same manner as in Comparative Example 4, except that pre-lithiation was not performed as in Comparative Example 4.

[0276] Comparative Example 5: Preparation of the Negative Electrode

[0277] The negative electrode was prepared in the same manner as in Comparative Example 1, except that the active material layer composition was prepared in Comparative Example 1, which included Si (average particle size (D50): 5 μm) and SiO (average particle size (D50): 3.5 μm) as the active material layer composition in a weight ratio of 52.5:17.5:9.8:10:0.2:10, a first conductive material, a second conductive material, a third conductive material, and polyacrylamide as a binder.

[0278] The first conductive material is carbon black C (specific surface area: 58 m²). 2 / g, diameter: 37nm), the second conductive material is plate-shaped graphite (specific surface area: 17m²). 2 / g, average particle size (D50): 3.5μm), and the third conductive material is carbon nanotubes.

[0279] <Preparation of Secondary Batteries>

[0280] By using LiNi as the positive electrode active material 0.6 Co 0.2 Mn 0.2 O2 (average particle size (D50): 15 μm), carbon black (product name: Super C65, manufacturer: Timcal) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent for forming the positive electrode slurry in a weight ratio of 97:1.5:1.5 to prepare a positive electrode slurry (solid concentration of 78% by weight).

[0281] The two surfaces of the aluminum current collector (thickness: 12μm), which serves as the positive electrode current collector, were coated with the positive electrode slurry at a concentration of 537mg / 25cm. 2 The aluminum current collector was coated with a loading amount and rolled and dried in a vacuum oven at 130°C for 10 hours to form a positive electrode active material layer (thickness: 65 μm), thereby preparing the positive electrode (thickness: 77 μm, porosity: 26%).

[0282] The secondary battery of Example 1 was prepared by inserting a polyethylene diaphragm between the positive electrode and the negative electrode of Example 1 and injecting electrolyte therein.

[0283] The electrolyte is obtained by adding 3% by weight of vinylene carbonate based on the total weight of the electrolyte to an organic solvent in which fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) are mixed in a volume ratio of 30:70, and by adding LiPF6 as a lithium salt at a concentration of 1M.

[0284] Secondary batteries were manufactured in the same manner as described above, except that the negative electrode used in the examples and comparative examples was employed.

[0285] Experimental Example 1: Evaluation of Service Life Characteristics

[0286] The lifespan and capacity retention of secondary batteries containing the negative electrodes prepared in Examples 1 to 4 and 1-1, and Comparative Examples 4-1 and 1 to 5 were evaluated using an electrochemical charge and discharge apparatus. The secondary batteries were cycle-tested at 4.2–3.0 V and 1C / 0.5C, and capacity retention was measured during the test by charging / discharging the secondary batteries every 50 cycles at 0.33C / 0.33C (4.2–3.0 V).

[0287] Capacity retention (%) = {(Discharge capacity in the Nth cycle) / (Discharge capacity in the 1st cycle)} × 100

[0288] Figure 2A graph showing the RPT capacity retention rate according to the embodiments and comparative examples is presented.

[0289] Experiment Example 2: Measurement and Evaluation of Resistance Increase Rate

[0290] During the testing of Experimental Example 1, the capacity retention rate was measured by charging / discharging the secondary battery at 0.33C / 0.33C (4.2-3.0V) every 50 cycles. Then, the resistance was measured by discharging the secondary battery at SOC50 with a 2.5C pulse to compare and analyze the resistance increase rate.

[0291] Figure 3 A graph showing the increase rate of RPT resistance according to the embodiments and comparative examples is presented.

[0292] In addition, for the evaluation of service life characteristics and the measurement of resistance increase rate, data were calculated for 200 cycles each, and the results are shown in Table 1 below.

[0293] [Table 1]

[0294]

[0295] As can be confirmed in Table 1, the negative electrodes of Examples 1 to 8 and 1-1 are superior to the negative electrodes of Comparative Examples 1 to 5 and 4-1 in terms of capacity retention and resistance increase rate.

[0296] In particular, the negative electrodes of Examples 1 to 8 and 1-1 have a double-layer active material layer with a specific composition and content. In particular, it has been confirmed that, especially since the first negative electrode active material layer contains a high content of SiOx (x = 0), the negative electrode can have the advantages of high capacity, high density and fast charging. In addition, it has been confirmed that by including one or more of the group consisting of silicon-based active materials and carbon-based active materials in the second negative electrode active material layer, the uniformity on the surface of the negative electrode during charging and discharging can be adjusted, thereby improving cycle characteristics.

[0297] Furthermore, in Examples 1 and 1-1, cases with the same negative electrode composition but with pre-lithiation and cases without pre-lithiation were compared. Although the capacity retention rate of the case without pre-lithiation, as in Example 1-1, was not as good as that of Example 1, it was confirmed that when the composition of the bilayer negative electrode according to this application was met, the capacity retention rate was better and the resistance increase rate was lower compared to Comparative Examples 1 to 5 and 4-1. This is because accelerated degradation on the negative electrode surface can be prevented even during charge and discharge cycles. Moreover, it was confirmed that the above advantages became more pronounced when pre-lithiation was performed as in Example 1, resulting in a significantly higher capacity retention rate.

[0298] In other words, the results from Examples 1 to 8 and 1-1 confirm that the negative electrode of the present invention, having a bilayer structure with specific composition and content, exhibits superior capacity retention and resistance characteristics compared to other negative electrodes (Comparative Examples 1 to 5 and 4-1) by preventing surface degradation during charging and discharging. Furthermore, when comparing Examples 1 and 1-1, it is confirmed that even during pre-lithiation, the negative electrode of the present application, by including a second negative electrode active material layer, is able to prevent uneven pre-lithiation on the surface, thus exhibiting a significantly high capacity retention rate and a similar level of resistance increase.

[0299] In Comparative Example 1, a monolayer active material layer containing Si particles was used as the negative electrode active material. Although the initial capacity was excellent, it was confirmed that the capacity retention rate decreased and the resistance increase rate was high due to the cracking of Si particles on the electrode surface. Therefore, it can be confirmed that the second negative electrode active material layer according to this application acts as a buffer layer.

[0300] Comparative Example 2 is a negative electrode in which a monolayer active material layer containing Si particles is used as the negative electrode active material and no pre-lithiation is performed. In this case, it can be confirmed that, compared with the example, the capacity retention rate and the rate of increase in resistance are also worse due to the cracking of Si particles on the electrode surface during charging and discharging.

[0301] Comparative Example 3 corresponds to a negative electrode in which the order of the first and second negative electrode active material layers of this application is changed. In this case, it can be confirmed that Si particle cracking still occurs on the electrode surface, therefore, the capacity retention rate decreases and the resistance increase rate is also high.

[0302] Comparative Example 4 is a case where the content of silicon-based active material in the second negative electrode active material layer of this application is below the lower limit. That is, Comparative Example 4 is a case where the second negative electrode active material layer contains more graphite-based active material than silicon-based active material. In this case, as can be confirmed in Table 1, it can be confirmed that the second negative electrode active material layer functions as a resistive layer during charging and discharging. Therefore, the capacity retention rate is not as good as that of Examples 1 to 8, and thus the resistance increase rate is high.

[0303] Comparative Example 5 has a single-layer negative electrode active material layer, wherein the negative electrode is a blend of Si active material and SiO active material. In this case, it can be confirmed that, compared with the case in which a two-layer negative electrode active material layer is provided as in the present invention, the optimal Si and SiO content is not met, therefore the capacity retention is reduced and the resistance increase rate is also high.

Claims

1. An anode for a lithium secondary battery, comprising: an anode current collector layer; a first anode active material layer on one or both surfaces of the anode current collector layer; and a second anode active material layer on a surface of the first anode active material layer opposite to the surface facing the anode current collector layer, wherein the first anode active material layer comprises a first anode active material layer composition containing a first anode active material, and the second anode active material layer comprises a second anode active material layer composition, the second anode active material layer composition containing a second anode active material, a second anode conductive material and a second anode binder, the first anode active material comprises one or more selected from the group consisting of SiOx where x=0 and SiOx where 0<x<2, and based on 100 parts by weight of the first anode active material, 95 parts by weight or more of the SiOx where x=0 is comprised, the second anode conductive material comprises at least one selected from the group consisting of a point-shaped conductive material, a linear conductive material and a planar conductive material, and the second anode active material comprises one or more selected from the group consisting of a carbon-based active material, a silicon-based active material, a metal-based active material capable of forming an alloy with lithium and a lithium-based nitride, and based on 100 parts by weight of the second anode active material, the silicon-based active material is present in an amount of 50 parts by weight or more and 100 parts by weight or less.

2. The anode according to claim 1, wherein the silicon-based active material comprises one or more selected from the group consisting of SiOx where 0<x<2, SiC and Si alloys.

3. The anode according to claim 1, wherein the silicon-based active material comprises SiOx where 0<x<2 or SiC.

4. The negative electrode according to claim 1, wherein, based on 100 parts by weight of the first anode active material layer composition, the first anode active material is present in an amount of 60 parts by weight or more.

5. The anode according to claim 1, wherein the first anode active material layer has a thickness of 10 µm or more and 200 µm or less, and the second anode active material layer has a thickness of 10 µm or more and 100 µm or less.

6. The anode according to claim 1, wherein the loading (a) of the first anode active material layer composition satisfies 2 times or more of the loading (b) of the second anode active material layer composition.

7. The anode according to claim 1, wherein the first anode active material layer composition further comprises at least one selected from the group consisting of a first anode conductive material and a first anode binder.

8. The anode according to claim 1, wherein the carbon-based active material comprises graphite, the graphite comprises artificial graphite and natural graphite, and the weight ratio of the artificial graphite to the natural graphite is from 5:5 to 9.5:0.

5.

9. The anode according to claim 1, wherein the second anode conductive material comprises at least the linear conductive material.

10. The anode according to claim 1, wherein the first anode active material layer is in contact with the entire surface of the anode current collector layer, and wherein the second negative electrode active material layer is in contact with the entire surface of the first negative electrode active material layer.

11. A method for producing a negative electrode for a lithium secondary battery, the method comprising: providing a negative electrode current collector layer; forming a first negative electrode active material layer by applying a first negative electrode active material layer composition comprising a first negative electrode active material onto one surface or both surfaces of the negative electrode current collector layer; and forming a second negative electrode active material layer by applying a second negative electrode active material layer composition comprising a second negative electrode active material, a second negative electrode conductive material and a second negative electrode binder onto a surface of the first negative electrode active material layer opposite to the surface facing the negative electrode current collector layer, wherein the first negative electrode active material comprises at least one selected from the group consisting of SiOx where x=0 and SiOx where 0<x<2, and based on 100 parts by weight of the first negative electrode active material, comprises 95 parts by weight or more of SiOx where x=0, the second negative electrode conductive material comprises at least one selected from the group consisting of a point-shaped conductive material, a linear conductive material and a planar conductive material, and the second negative electrode active material comprises at least one selected from the group consisting of a carbon-based active material, a silicon-based active material, a metal-based active material capable of forming an alloy with lithium, and a lithium-based nitride, and based on 100 parts by weight of the second negative electrode active material, the silicon-based active material is present in an amount of 50 parts by weight or more and 100 parts by weight or less.

12. The method according to claim 11, further comprising: subjecting a negative electrode, wherein the first negative electrode active material layer and the second negative electrode active material layer are present on a surface of the negative electrode current collector, to prelithiation, wherein subjecting the negative electrode to prelithiation comprises at least one of the following processes: a lithium electroplating process, a lithium metal transfer process, a lithium metal deposition process, or a stabilized lithium metal powder (SLMP) coating process.

13. The method according to claim 11, wherein the first negative electrode active material layer is in contact with the entire surface of the negative electrode current collector layer, and wherein the second negative electrode active material layer is in contact with the entire surface of the first negative electrode active material layer.

14. The method according to claim 11, wherein the second negative electrode active material layer is formed on the first negative electrode active material layer by a wet-on-dry process, wherein the wet-on-dry process comprises: applying the first negative electrode active material layer composition, partially or completely drying the applied first negative electrode active material layer composition, and applying the second negative electrode active material layer composition onto the first negative electrode active material layer.

15. The method according to claim 11, wherein the second negative electrode active material layer is formed on the first negative electrode active material layer by a wet-on-wet process, wherein the wet-on-wet process comprises: applying the first negative electrode active material layer composition, and applying the second negative electrode active material layer composition onto the first negative electrode active material layer composition without drying the applied first negative electrode active material layer composition.

16. A lithium secondary battery, comprising: positive electrode; Negative electrode for lithium secondary batteries according to any one of claims 1 to 10; A membrane disposed between the positive electrode and the negative electrode; and Electrolytes.

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