Negative electrode sheet, method for manufacturing the same, lithium ion battery, and electric vehicle

Abstract

The application relates to the technical field of lithium ion batteries, and discloses a negative electrode sheet, a preparation method of the negative electrode sheet, a lithium ion battery and an electric vehicle. The negative electrode sheet comprises a current collector and a silicon-containing layer, an intermediate layer and a carbon layer which are sequentially stacked on at least one side of the current collector, wherein the intermediate layer contains a first conductive agent and a first binder. The negative electrode sheet provided by the application introduces the intermediate layer composed of the conductive agent and the binder, can effectively improve the interface compatibility of the carbon layer and the silicon-containing layer, enhance the adhesion between the two layers, avoid separation of the two layers due to expansion of the silicon-containing layer, also enhance the electrical conductivity between the interfaces, slow down the occurrence of lithium precipitation, prolong the cycle life of the lithium ion battery and improve the kinetic performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a negative electrode sheet and its preparation method, a lithium-ion battery, and an electric vehicle. Background Technology

[0002] The growing market demand in the new energy vehicle industry has placed higher requirements on the energy density and fast-charging performance of power batteries. In mainstream lithium-ion battery systems, the energy density of existing graphite anode systems (a type of carbon material) is gradually approaching its theoretical limit (372 mAh / g). Silicon, as an emerging anode material, has become the most promising anode material due to its advantages such as high theoretical capacity (4200 mAh / g), low lithium intercalation platform, and abundant resources. However, silicon undergoes large-scale volume expansion (around 300%) during lithium intercalation, leading to a decrease in lithium intercalation capacity. Furthermore, silicon's low conductivity also affects its electrochemical performance. Mixing graphite and silicon is one effective strategy to improve lithium-ion battery performance, but due to the differences in lithium intercalation performance between silicon and graphite, the compatibility of the mixed system is poor, resulting in a reduced cycle life of the lithium-ion battery. Therefore, there is an urgent need for an anode material that can solve the problem of low cycle life in silicon-containing anode batteries. Summary of the Invention

[0003] The purpose of this invention is to overcome the problem of low cycle life of silicon-containing lithium-ion batteries in the prior art, and to provide a negative electrode sheet, its preparation method, lithium-ion batteries, and electric vehicles.

[0004] To achieve the above objectives, a first aspect of the present invention provides a negative electrode sheet comprising a current collector and a silicon-containing layer, an intermediate layer and a carbon layer sequentially stacked on at least one side of the current collector, wherein the intermediate layer contains a first conductive agent and a first binder.

[0005] A second aspect of the present invention provides a method for preparing a negative electrode sheet, the method comprising the following steps: sequentially coating a silicon-containing layer, an intermediate layer and a carbon layer on at least one surface of a current collector; wherein the intermediate layer contains a first conductive agent and a first binder.

[0006] A third aspect of the present invention provides a lithium-ion battery, wherein the lithium-ion battery comprises a negative electrode sheet provided by the present invention or a negative electrode sheet prepared by the preparation method provided by the present invention.

[0007] A fourth aspect of the present invention provides an electric vehicle comprising the lithium-ion battery provided by the present invention.

[0008] The beneficial effects of the present invention through the above technical solution are:

[0009] The negative electrode sheet provided by this invention comprises a silicon-containing layer, an intermediate layer, and a carbon layer stacked sequentially on at least one side of the current collector. By introducing an intermediate layer containing a conductive agent and a binder between the silicon-containing layer and the carbon layer, on the one hand, the interfacial compatibility between the carbon layer and the silicon-containing layer is effectively improved, the adhesion and interfacial stress between the two layers are enhanced, and separation of the two layers due to the expansion of the silicon-containing layer is avoided. On the other hand, the intermediate layer containing the conductive agent and the binder constructs a stable electron and ion transport channel between the carbon layer and the silicon-containing layer, which mitigates the lithium plating phenomenon caused by the non-uniform distribution of the negative electrode electrochemical reaction during battery charging and discharging, and extends the cycle life of the lithium-ion battery.

[0010] In a preferred embodiment of the present invention, the present invention reduces the DC internal resistance of lithium-ion batteries and thereby increases the rate performance of lithium-ion batteries by selecting appropriate types and proportions of conductive agents and binders in the intermediate layer, introducing a first carbon material in the intermediate layer, and making reasonable selections of the composition and proportions of silicon-containing layers and carbon layers. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the negative electrode sheet according to a preferred embodiment of the present invention;

[0012] Figure 2 The graph shows the room temperature fast charge cycle performance of the lithium-ion batteries prepared in Examples 1-3 and Comparative Example 2.

[0013] Figure 3 These are photographs of the negative electrode plates of the lithium-ion batteries prepared in Example 2 and Comparative Examples 1-2 after 100 fast-charging cycles, when the batteries were disassembled and the state of charge was 100%.

[0014] Explanation of reference numerals in the attached figures

[0015] 1 Current collector 2 Silicon-containing layer 3 Intermediate layer 4 Carbon layer Detailed Implementation

[0016] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0017] The first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a current collector, and a silicon-containing layer, an intermediate layer and a carbon layer sequentially stacked on at least one side of the current collector, wherein the intermediate layer comprises a first conductive agent and a first binder.

[0018] According to a preferred embodiment of the present invention, such as Figure 1As shown, a silicon-containing layer, an intermediate layer, and a carbon layer are sequentially stacked on the upper surface and the lower surface of the current collector to form the negative electrode sheet.

[0019] According to the present invention, preferably, the current collector is selected from copper foil and / or copper foam, and preferably is copper foil.

[0020] According to the present invention, preferably, the silicon-containing layer contains a silicon material, a second carbon material, a second conductive agent, and a second binder.

[0021] Preferably, the silicon material is selected from at least one of silicon, silicon monoxide, and silicon-carbon materials.

[0022] Further, the silicon material is selected from silicon monoxide and / or silicon-carbon materials. The silicon monoxide (SiO x , 0 < x < 2, for example, it can be SiO) in the present invention is an inorganic compound. During the first lithium intercalation process, SiO x reacts with lithium first to generate elemental silicon, Li2O, and lithium silicates (such as Li4SiO4, Li2SiO3, and Li2SiO5); the elemental silicon can further react with Li to generate reversible capacity, and the generated Li2O and lithium silicates do not participate in the reaction during subsequent electrochemical cycling, but can play a role in buffering the volume expansion of elemental silicon and protecting the active material. The elemental silicon serves as an active site to react with lithium ions for lithium intercalation reaction, providing a high specific capacity, while the lithium silicates can buffer the volume expansion of elemental silicon after lithium intercalation, reducing the particle breakage and pulverization of elemental silicon during charge-discharge cycling, and effectively extending the cycle life of the battery. The silicon-carbon material is a composite material of silicon and carbon. Among them, silicon reacts with lithium ions for lithium intercalation reaction to provide a high specific capacity; the carbon material has high conductivity, which can improve the ion transfer on the surface of the electrode material, thereby increasing the rate performance of the lithium-ion battery; at the same time, the carbon substrate can also buffer the breakage and pulverization of silicon particles, and can effectively extend the cycle life of the battery.

[0023] Preferably, the second carbon material is selected from at least one of natural graphite, artificial graphite, hard carbon, soft carbon, and mesophase carbon microspheres.

[0024] Further, the second carbon material is selected from natural graphite and / or artificial graphite. The silicon-containing layer containing natural graphite and / or artificial graphite can effectively alleviate the volume expansion caused by the pulverization of the silicon material, and thus can improve the first Coulombic efficiency and cycle life of the battery.

[0025] Preferably, the second conductive agent is selected from at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, and arrayed carbon fibers.

[0026] Further, the second conductive agent is a mixture of single-walled carbon nanotubes and conductive carbon black.

[0027] Preferably, the weight ratio of the single-walled carbon nanotubes to the conductive carbon black is 1:(2-10). The second conductive agent for the silicon-containing layer obtained with this weight ratio can construct an effective point-to-line conductive network around the silicon material. This conductive network wraps around the silicon material, improving the conductivity of the silicon-containing layer while further increasing its structural stability.

[0028] Preferably, the second adhesive is selected from styrene-butadiene rubber and / or polyacrylic acid.

[0029] Furthermore, the second binder is preferably polyacrylic acid. Using polyacrylic acid as the second binder for the silicon-containing layer can provide strong adhesion to limit the expansion and pulverization of the silicon material, reduce the detachment of the silicon material from the current collector due to expansion and pulverization, and promote the transport of lithium ions and electrons on the negative electrode by bonding the silicon material together with the second carbon material, the second conductive agent, etc.

[0030] Preferably, the polyacrylic acid has a weight-average molecular weight of 300,000 to 600,000 g / mol.

[0031] According to the present invention, preferably, based on the total weight of the silicon-containing layer, the total content of the silicon material and the second carbon material in the silicon-containing layer is 70-97 wt%, the content of the second conductive agent is 0.1-15 wt%, and the content of the second binder is 3-15 wt%. When the contents of the above components are within this range, the silicon-containing layer has a high specific capacity while maintaining suitable adhesion and conductivity, which helps to improve the energy density of the battery.

[0032] According to the present invention, preferably, the silicon-containing layer further comprises a second dispersant. The second dispersant primarily enhances the dispersibility of the silicon material, the second carbon material, and the second conductive agent in the silicon-containing layer, preventing agglomeration among the components and thus affecting the performance of each component. When the second binder comprises styrene-butadiene rubber, the silicon-containing layer comprises the second dispersant.

[0033] Preferably, the second dispersant is selected from at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose.

[0034] According to the present invention, preferably, when the silicon-containing layer contains a second dispersant, the content of the second dispersant in the silicon-containing layer is not greater than 15 wt%, based on the total weight of the silicon-containing layer.

[0035] According to the present invention, preferably, the carbon layer comprises a third carbon material, a third conductive agent, and a third binder.

[0036] Preferably, the third carbon material is selected from at least one of natural graphite, artificial graphite, hard carbon, soft carbon, and mesophase carbon microspheres.

[0037] Furthermore, the third carbon material is selected from natural graphite and / or artificial graphite. Carbon layers obtained from natural graphite and / or artificial graphite can provide higher specific capacity, first coulombic efficiency, and better kinetic performance.

[0038] Preferably, the third conductive agent is selected from at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, and arrayed carbon fibers.

[0039] Furthermore, the third conductive agent is conductive carbon black. Using conductive carbon black as the third conductive agent in the carbon layer allows for further optimization of the conductive network within the carbon layer while maintaining a low cost.

[0040] Preferably, the third adhesive is selected from styrene-butadiene rubber and / or polyacrylic acid.

[0041] Furthermore, the third adhesive is styrene-butadiene rubber (SBR). Using SBR as the third adhesive for the carbon layer can reduce the processing difficulty of the carbon layer coating process while providing suitable adhesion.

[0042] Preferably, the weight-average molecular weight of the styrene-butadiene rubber is 200,000 to 1,000,000 g / mol.

[0043] According to the present invention, preferably, based on the total weight of the carbon layer, the content of the third carbon material in the carbon layer is 70-97 wt%, the content of the third conductive agent is 0.1-15 wt%, and the content of the third binder is 1-15 wt%. When the contents of the above components are within this range, a high specific capacity can be achieved while maintaining suitable adhesion and conductivity, which helps to improve the energy density of the battery.

[0044] According to the present invention, preferably, the carbon layer further comprises a third dispersant. The third dispersant mainly improves the dispersibility of the third carbon material and the third conductive agent in the carbon layer, prevents agglomeration between components, and avoids affecting the performance of each component.

[0045] Preferably, the third dispersant is selected from at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose, and is preferably lithium carboxymethyl cellulose.

[0046] According to the present invention, preferably, the content of the third dispersant in the carbon layer is 0.1-15 wt%, based on the total weight of the carbon layer.

[0047] According to the present invention, preferably, the first conductive agent is selected from at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, and arrayed carbon fibers.

[0048] Furthermore, the first conductive agent is selected from single-walled carbon nanotubes and / or conductive carbon black. Using single-walled carbon nanotubes and / or conductive carbon black as the first conductive agent in the intermediate layer can tightly bond with the binder of the intermediate layer to form a cross-linked conductive network, promoting the transfer of charge carriers between the silicon-containing layer and the carbon layer.

[0049] Furthermore, the first conductive agent is a mixture of single-walled carbon nanotubes and conductive carbon black.

[0050] Preferably, the weight ratio of the single-walled carbon nanotubes to the conductive carbon black is 1:(1-10).

[0051] The mixture of single-walled carbon nanotubes and conductive carbon black in a specific ratio can, on the one hand, form a continuous conductive network with point-line bonding in the intermediate layer, promoting the rapid transfer of electrons and lithium ions in the intermediate layer and enhancing the rate performance of the battery; on the other hand, it can mitigate the lithium plating phenomenon caused by the non-uniform distribution of electrochemical active sites of the electrodes during battery charging and discharging, prolonging the cycle performance of lithium-ion batteries and thus increasing the battery's lifespan; at the same time, it can also enhance the structural stability and flexibility of the intermediate layer to adapt to the stress generated by the expansion of the silicon-containing layer structure.

[0052] According to the present invention, preferably, the first adhesive is selected from polyacrylic acid and / or styrene-butadiene rubber.

[0053] According to the present invention, preferably, the first adhesive is a mixture of polyacrylic acid and styrene-butadiene rubber, wherein the weight ratio of polyacrylic acid to styrene-butadiene rubber is (0.1-40):1, more preferably (2-10):1. The first adhesive obtained with this weight ratio can significantly enhance the adhesion between the silicon-containing layer and the carbon layer.

[0054] Preferably, the polyacrylic acid has a weight-average molecular weight of 300,000 to 600,000 g / mol.

[0055] Preferably, the weight-average molecular weight of the styrene-butadiene rubber is 200,000 to 1,000,000 g / mol.

[0056] According to the present invention, preferably, based on the total weight of the intermediate layer, the content of the first conductive agent in the intermediate layer is 10-70 wt%, and the content of the first binder is 30-90 wt%. When the contents of the above components are within this range, the conductive and adhesive functions of the intermediate layer can be effectively utilized, and the interfacial compatibility between the silicon-containing layer and the carbon layer can be improved.

[0057] According to the present invention, preferably, the intermediate layer further contains a first carbon material. The present invention introduces a first carbon material into the intermediate layer, increasing the proportion of active material in the intermediate layer, further increasing the proportion of active material in the negative electrode, and simultaneously forming a good lithium-ion transport channel between the silicon-containing layer and the carbon layer.

[0058] Preferably, the first carbon material is selected from at least one of natural graphite, artificial graphite, hard carbon, soft carbon, and mesophase carbon microspheres; more preferably, it is selected from natural graphite and / or artificial graphite.

[0059] According to the present invention, preferably, the content of the first carbon material in the intermediate layer is 5-20 wt%, based on the total weight of the intermediate layer. When the content of the first carbon material is within this range, the lithium-ion transport path can be optimized while ensuring that the intermediate layer performs its expected conductivity and bonding function, thereby increasing the energy density of the battery.

[0060] According to the present invention, preferably, the intermediate layer further comprises a first dispersant.

[0061] Preferably, the first dispersant is selected from at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose.

[0062] According to the present invention, preferably, the content of the first dispersant is 0.1-15 wt%, based on the total weight of the intermediate layer.

[0063] According to the present invention, preferably, the total thickness of the silicon-containing layer, the intermediate layer and the carbon layer is 40-100 μm based on the thickness on one side.

[0064] Preferably, the thickness ratio of the silicon-containing layer to the carbon layer is (1-5):1.

[0065] Preferably, the thickness of the intermediate layer is 1-20 μm, measured on a single side. When the thickness of the intermediate layer is within this range, it is possible to effectively utilize the function of the intermediate layer without excessively reducing the proportion of the negative electrode active material.

[0066] Furthermore, the thickness of the intermediate layer is 1-5 μm.

[0067] A second aspect of the present invention provides a method for preparing a negative electrode sheet, the method comprising the following steps: sequentially coating a silicon-containing layer, an intermediate layer and a carbon layer on at least one surface of a current collector; wherein the intermediate layer contains a first conductive agent and a first binder.

[0068] According to the present invention, preferably, the preparation method comprises the following steps:

[0069] (1) Mix silicon material, second carbon material, second conductive agent and second binder in proportion to prepare slurry A;

[0070] (2) The first conductive agent and the first binder are mixed in a certain proportion to prepare slurry B;

[0071] (3) Mix the third carbon material, the third conductive agent and the third binder in a certain proportion to prepare slurry C;

[0072] (4) The slurry A, the slurry B and the slurry C are sequentially coated on the current collector to obtain the negative electrode sheet.

[0073] According to the present invention, preferably, a dispersant is optionally added in steps (1)-(3).

[0074] In the second aspect of the present invention, the composition and amount of the silicon layer, intermediate layer and carbon layer in the method for preparing the negative electrode sheet are exactly the same as those in the first aspect of the present invention. In order to avoid repetition, the present invention will not repeat the description in this second aspect, and those skilled in the art should not understand it as a limitation of the present invention.

[0075] A third aspect of the present invention provides a lithium-ion battery, wherein the lithium-ion battery comprises a negative electrode sheet provided by the present invention or a negative electrode sheet prepared by the preparation method provided by the present invention.

[0076] A fourth aspect of the present invention provides an electric vehicle comprising the lithium-ion battery provided by the present invention.

[0077] The present invention will be described in detail below through embodiments. In the following embodiments,

[0078] The thickness of the negative electrode sheet was measured using a micrometer.

[0079] The weight-average molecular weight of polyacrylic acid is 400,000 g / mol;

[0080] The weight-average molecular weight of styrene-butadiene rubber is 400,000 g / mol.

[0081] Example 1

[0082] Silicon materials (silicon suboxide, SiO2) were mixed in a weight ratio of 60:17:3:10. xSlurry A is prepared by dispersing the following components in deionized water: graphite (artificial graphite), conductive agent (single-walled carbon nanotubes and conductive carbon black in a weight ratio of 1:2), and binder (polyacrylic acid), with x=1; slurry B is prepared by dispersing the following components in deionized water: conductive agent (single-walled carbon nanotubes), binder (polyacrylic acid and styrene-butadiene rubber in a weight ratio of 60:15), and dispersant (lithium carboxymethyl cellulose) in a weight ratio of 10:75:15; slurry C is prepared by dispersing the following components in deionized water: carbon material (artificial graphite), conductive agent (conductive carbon black), binder (styrene-butadiene rubber), and dispersant (lithium carboxymethyl cellulose) in a weight ratio of 96:1:1.5:1.5; slurry C is prepared by dispersing the following components in deionized water: carbon material (artificial graphite), conductive agent (conductive carbon black), binder (styrene-butadiene rubber), and dispersant (lithium carboxymethyl cellulose) in a weight ratio of 96:1:1.5:1.5; slurries A, B, and C are then uniformly coated onto a copper foil current collector to obtain a silicon-containing bottom layer, an intermediate layer, and a graphite top layer, followed by double-sided coating. Then, the negative electrode sheet is obtained by drying, winding, rolling and die-cutting. The total thickness of the single side of the negative electrode sheet is 60 μm, excluding the current collector. The thickness of the single side intermediate layer of the negative electrode sheet is 5 μm, and the thickness ratio of the silicon layer to the carbon layer is 4:1.

[0083] In this embodiment, lithium nickel cobalt manganese oxide is selected as the positive electrode active material, and the lithium-ion battery is a soft pack battery made by stacking the positive electrode sheet, the separator and the negative electrode sheet in a Z-shaped manner.

[0084] Example 2

[0085] Silicon materials (silicon suboxide, SiO2) were mixed in a weight ratio of 60:17:3:10. x Slurry A is prepared by dispersing the following components in deionized water: graphite (artificial graphite), conductive agent (single-walled carbon nanotubes and conductive carbon black in a weight ratio of 1:2), and binder (polyacrylic acid), with x=1; slurry B is prepared by dispersing the following components in deionized water at a weight ratio of 30:55:15; slurry C is prepared by dispersing the following components in deionized water at a weight ratio of 96:1:1.5:1.5; and slurry A, B, and C are then uniformly coated onto a copper foil current collector to obtain a silicon-containing bottom layer, an intermediate layer, and a graphite top layer, followed by double-sided coating. Then, the negative electrode sheet is obtained by drying, winding, rolling and die-cutting. The total thickness of the single side of the negative electrode sheet is 60 μm, excluding the current collector. The thickness of the single side intermediate layer of the negative electrode sheet is 5 μm, and the thickness ratio of the silicon layer to the carbon layer is 4:1.

[0086] In this embodiment, lithium nickel cobalt manganese oxide is selected as the positive electrode active material, and the lithium-ion battery is a soft pack battery made by stacking the positive electrode sheet, the separator, and the negative electrode sheet in a Z-shaped manner.

[0087] Example 3

[0088] The lithium-ion battery was prepared according to the method of Example 1, except that the thickness of the single-sided intermediate layer of the coated negative electrode sheet was 1 μm.

[0089] Example 4

[0090] The lithium-ion battery was prepared according to the method of Example 1, except that the weight ratio of polyacrylic acid and styrene-butadiene rubber was 2:20 when preparing slurry B.

[0091] Example 5

[0092] The lithium-ion battery was prepared according to the method of Example 1, except that the weight ratio of polyacrylic acid and styrene-butadiene rubber was 10:1 when preparing slurry B.

[0093] Example 6

[0094] The lithium-ion battery was prepared according to the method of Example 1, except that the weight ratio of polyacrylic acid and styrene-butadiene rubber was 40:1 when preparing slurry B.

[0095] Example 7

[0096] The lithium-ion battery was prepared according to the method of Example 1, except that the thickness of the single-sided intermediate layer of the negative electrode sheet was 25 μm.

[0097] Example 8

[0098] Lithium-ion batteries were prepared according to the method of Example 1, except that the composition of each raw material in slurry B was different. Specifically, conductive agent (single-walled carbon nanotubes), binder (polyacrylic acid and styrene-butadiene rubber in a weight ratio of 60:15) and dispersant (lithium carboxymethyl cellulose) were dispersed in deionized water at a weight ratio of 0.05:75:20 to prepare slurry B.

[0099] Example 9

[0100] Lithium-ion batteries were prepared according to the method of Example 1, except that artificial graphite was added to slurry B. Specifically, artificial graphite, conductive agent (single-walled carbon nanotubes), binder (polyacrylic acid and styrene-butadiene rubber in a weight ratio of 60:15), and dispersant (lithium carboxymethyl cellulose) were dispersed in deionized water in a weight ratio of 5:11:69:15 to prepare slurry B.

[0101] Example 10

[0102] Lithium-ion batteries were prepared according to the method of Example 1, except that elemental silicon was used instead of an equal weight of silicon suboxide (SiO) when preparing slurry A. x (x = 1).

[0103] Example 11

[0104] Lithium-ion batteries were prepared according to the method of Example 1, except that hard carbon was used instead of an equal weight of artificial graphite when preparing slurry A.

[0105] Example 12

[0106] Lithium-ion batteries were prepared according to the method of Example 1, except that hard carbon was used instead of an equal weight of artificial graphite when preparing slurry C.

[0107] Example 13

[0108] The lithium-ion battery was prepared according to the method of Example 1, except that the weight ratio of single-walled carbon nanotubes to conductive carbon black was 1:20 when preparing slurry A.

[0109] Example 14

[0110] Lithium-ion batteries were prepared according to the method of Example 1, except that styrene-butadiene rubber was used instead of an equal weight of polyacrylic acid when preparing slurry A.

[0111] Comparative Example 1

[0112] The lithium-ion battery was prepared according to the method of Example 1, except that when preparing the negative electrode sheet, the slurry was prepared according to the formula of slurry A and slurry C in Example 1, slurry A and slurry C were mixed to obtain a mixed slurry, and then coated, without using layer coating.

[0113] Comparative Example 2

[0114] The lithium-ion battery was prepared according to the method of Example 1, except that the slurry B was not coated when preparing the negative electrode sheet, that is, the negative electrode sheet did not contain an intermediate layer.

[0115] Test Example 1: Room Temperature Fast Charging Cycle Test

[0116] Test Method: The assembled pouch battery was placed in the Blue Battery Test Cabinet for charge-discharge testing. The charging process was stepped charging, including: ① charging at a 3C rate to 3.8V; ② charging at a 2C rate to 4.0V; ③ charging at a 1C rate to 4.1V; ④ charging at a 0.5C rate to 4.2V. The discharging process was discharging at a 1C rate to 2.75V. The discharge capacity retention results after 300 cycles of room temperature fast charging for each embodiment and comparative example are shown in Table 1. The room temperature fast charging cycle performance of Examples 1-3 and Comparative Example 2 is shown in Table 2. Figure 2 The lithium plating on the negative electrode of the batteries disassembled after 100 cycles of fast charging at room temperature in Examples 2 and 1-2, when the state of charge (SOC) was 100%, is shown in the figure. Figure 3 .

[0117] Test Example 2: Room Temperature DC Internal Resistance (DCIR) Test

[0118] Test method: The assembled pouch battery was placed in the Blue Battery Test Cabinet. The state of charge (SOC) of the battery was adjusted to 50% and left to stand for 2 hours. Then, it was discharged at a rate of 1.5C for 30 seconds. The DCIR value was obtained by subtracting the termination voltage of the resting period before discharge from the discharge termination voltage and dividing by the discharge current. The test results are shown in Table 1.

[0119] Table 1

[0120] Number DCIR (mΩ) Discharge capacity retention rate (%) after 300 cycles Example 1 44.37 95.6 Example 2 43.69 94.7 Example 3 43.25 94.1 Example 4 47.20 92.3 Example 5 44.20 93.9 Example 6 46.60 92.1 Example 7 53.69 74.8 Example 8 57.32 78.2 Example 9 43.25 97.2 Example 10 44.53 84.2 Example 11 44.87 86.3 Example 12 44.58 90.4 Example 13 45.54 92.2 Example 14 54.32 78.3 Comparative Example 1 59.27 52.3 Comparative Example 2 56.25 Below 50

[0121] As shown in Table 1, Examples 1-3 have lower DCIR values; after 300 fast-charging cycles at room temperature, the discharge capacity retention rate is above 94%. In Comparative Example 1, the silicon-containing negative electrode did not employ a layered arrangement, resulting in a significantly higher DCIR value compared to Example 1, with a discharge capacity retention rate of approximately 50% after 300 fast-charging cycles at room temperature. Figure 2 As shown, in Comparative Example 2, the silicon-containing anode sheet is layered, but no intermediate layer is provided between the silicon-containing layer and the carbon layer. The discharge capacity starts to drop after more than 100 cycles of fast charging at room temperature, and the discharge capacity retention rate is less than 50% after 300 cycles. The cycle life is lower than that of Examples 1-3. This shows that the layered design and the introduction of an intermediate layer between the silicon-containing layer and the carbon layer can effectively extend the cycle life of the carbon and silicon composite anode.

[0122] Furthermore, the composition and content of the binder in the visible intermediate layer provided in Examples 1-2 and Examples 4-6 affect the battery cycle performance. A suitable composition and content of the binder can increase the adhesion of the carbon layer and silicon layer and increase the cycle life of the battery.

[0123] Examples 1, 3, and 7 show that the thickness of the intermediate layer also affects the adhesion of the carbon and silicon layers. An excessively thick intermediate layer leads to an increase in ion diffusion distance and interfacial resistance, resulting in an increase in DCIR and a decrease in battery cycle performance. An excessively thin intermediate layer has poor adhesion. Therefore, a suitable thickness range is beneficial to increasing the cycle life of the battery.

[0124] Compared to Example 1, the amount of conductive agent in the intermediate layer of Example 8 was reduced, resulting in an increase in its DCIR. This indicates that the intermediate layer with low conductivity could no longer form a good electronic circuit between the two layers, which had an adverse effect on the battery impedance and cycle performance, and thus reduced the cycle life.

[0125] Example 9 further incorporates artificial graphite into the intermediate layer based on Example 1, resulting in a lower DCIR value, improved discharge capacity retention after cycling, and enhanced battery performance.

[0126] In Example 10, when preparing slurry A, elemental silicon material was used instead of an equal weight of silicon suboxide (SiO2). x(x=1), compared with Example 1, the discharge capacity retention rate decreased, indicating that silicon-oxygen materials can provide better cycle performance than silicon.

[0127] In Examples 11 and 12, hard carbon was used to replace the graphite containing silicon and carbon layers, respectively, and the discharge capacity retention rate after 300 cycles was significantly lower than that in Example 1.

[0128] In Example 13, the content of the conductive agent single-walled carbon nanotubes in the silicon layer was reduced compared to Example 1, resulting in a corresponding increase in the DCIR value. This indicates that an appropriate proportion of single-walled carbon nanotubes can effectively reduce battery impedance and thus increase cycle life.

[0129] In Example 14, the adhesive for the silicon layer was entirely composed of styrene-butadiene rubber, compared to Example 1 where the adhesive was polyacrylic acid. Because the adhesive strength of styrene-butadiene rubber is weaker than that of polyacrylic acid, it cannot withstand the expansion and contraction process of the silicon-containing top layer during charging and discharging, resulting in a decrease in battery capacity.

[0130] pass Figure 2 It can be seen that Comparative Example 2, which does not contain an intermediate layer, experiences rapid capacity decay during fast charging cycles; while Examples 1-3 maintain a high capacity retention rate. The introduction of the intermediate layer effectively improves the interfacial compatibility between the silicon layer and the carbon layer, thereby enhancing battery performance under fast charging cycles.

[0131] pass Figure 3 It can be seen that Comparative Example 2, which does not contain an intermediate layer, exhibited significant lithium plating (white lithium metal was deposited on the surface of the negative electrode) during fast charging cycling; while Example 2 showed no significant lithium plating at the interface of the negative electrode after cycling, indicating that the introduction of the intermediate layer improved the carrier transport channel at the interface between the two layers.

[0132] Therefore, this invention further improves the cycle life of lithium-ion batteries by selecting appropriate types and proportions of conductive agents and binders in the intermediate layer, introducing a first carbon material into the intermediate layer, and making reasonable selections of the composition and proportions of the silicon-containing layer and the carbon layer.

[0133] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a current collector and a silicon-containing layer, an intermediate layer, and a carbon layer sequentially stacked on at least one side of the current collector. The intermediate layer contains a first carbon material, a first conductive agent, and a first binder. Based on the total weight of the intermediate layer, the content of the first carbon material is 5-20 wt%, the content of the first conductive agent is 10-70%, and the content of the first binder is 30-90%. The first carbon material is natural graphite and / or artificial graphite. The first conductive agent is a mixture of single-walled carbon nanotubes and conductive carbon black, with a mass ratio of 1:1 to 10. The first binder is a mixture of polyacrylic acid and styrene-butadiene rubber, with a mass ratio of 0.1 to 40:

1. The silicon-containing layer comprises a silicon material, a second carbon material, a second conductive agent, and a second binder. The silicon material is silicon suboxide and / or silicon-carbon material. The second carbon material is natural graphite and / or artificial graphite. The second conductive agent is a mixture of single-walled carbon nanotubes and conductive carbon black, with a mass ratio of 1:2 to 10 between the single-walled carbon nanotubes and the conductive carbon black. The second binder is polyacrylic acid. The carbon layer comprises a third carbon material, a third conductive agent, and a third binder. The third carbon material is natural graphite and / or artificial graphite, the third conductive agent is conductive carbon black, and the third binder is styrene-butadiene rubber. The total thickness of the silicon-containing layer, the intermediate layer and the carbon layer is 40-100 μm based on the thickness of one side, the thickness ratio of the silicon-containing layer to the carbon layer is (1-5):1, and the thickness of the intermediate layer is 1-20 μm.

2. The negative electrode sheet according to claim 1, characterized in that, Based on the total weight of the silicon-containing layer, the total content of the silicon material and the second carbon material in the silicon-containing layer is 70-97%, the content of the second conductive agent is 0.1-15%, and the content of the second binder is 3-15%.

3. The negative electrode sheet according to claim 1, characterized in that, Based on the total weight of the carbon layer, the content of the third carbon material in the carbon layer is 70-97%, the content of the third conductive agent is 0.1-15%, and the content of the third binder is 1-15%.

4. The negative electrode sheet according to any one of claims 1-3, characterized in that, In the first adhesive, the weight ratio of the polyacrylic acid and the styrene-butadiene rubber is (2-10):

1.

5. The negative electrode sheet according to any one of claims 1-3, characterized in that, The thickness of the intermediate layer is 1-5 μm based on the thickness of one side.

6. A method for preparing a negative electrode sheet according to any one of claims 1-5, characterized in that, The preparation method includes the following steps: sequentially coating a silicon-containing layer, an intermediate layer, and a carbon layer on at least one surface of the current collector.

7. The preparation method according to claim 6, characterized in that, The preparation method includes the following steps: (1) Silicon material, second carbon material, second conductive agent and second binder are mixed in proportion to prepare slurry A; (2) The first carbon material, the first conductive agent and the first binder are mixed in proportion to prepare slurry B; (3) Mix the third carbon material, the third conductive agent and the third binder in proportion to prepare slurry C; (4) The slurry A, the slurry B and the slurry C are sequentially coated on the current collector to obtain the negative electrode sheet.

8. A lithium-ion battery, characterized in that, The lithium-ion battery comprises the negative electrode sheet as described in any one of claims 1-5.

9. An electric vehicle, characterized in that, The electric vehicle includes the lithium-ion battery as described in claim 8.

Images