Negative electrode sheet, method for preparing the negative electrode sheet, lithium ion battery, and electric vehicle

Abstract

Negative electrode sheet: The negative electrode sheet includes a current collector (1), and a silicon-containing layer (2), an intermediate layer (3), and a carbon layer (4) stacked in this order on at least one surface of the current collector (1), the intermediate layer including a first conductive agent and a first binder.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This disclosure claims priority to and the benefit of Chinese Patent Application No. 202211738679.5, entitled "NEGATIVE ELECTRODE SHEET AND PREPARATION METHOD THEREFOR, LITHIUM-ION BATTERY, AND ELECTRIC VEHICLE," filed on December 30, 2022. The entire contents of the foregoing application are incorporated herein by reference.

[0002] The present disclosure relates to the field of lithium ion battery technology, and in particular to a negative electrode sheet and a method for preparing the same, a lithium ion battery, and an electric vehicle. [Background technology]

[0003] Increasing market demand in the new energy vehicle industry is placing higher requirements on the energy density and fast charging performance of power batteries. In mainstream lithium-ion battery systems, the energy density of conventional graphite anodes (carbon materials) is gradually approaching its theoretical limit (372 mAh / g). As an emerging anode material, silicon materials offer advantages such as a high theoretical capacity (4,200 mAh / g), a low platform for lithium intercalation, and abundant resources, making them the most promising anode material. However, the lithium intercalation process of silicon materials results in a large volume expansion (approximately 300%), which reduces the lithium intercalation capacity, and the low electrical conductivity of silicon also affects the electrochemical performance of silicon. Blending graphite and silicon materials is an effective way to improve the performance of lithium-ion batteries. However, due to the difference in the lithium intercalation performance between silicon and graphite, the compatibility of the blended system is poor, resulting in a reduced cycle life of the ion battery. Therefore, there is an urgent need for a negative electrode to solve the problem of low cycle life of silicon-containing negative electrode batteries. Summary of the Invention

[0004] The objective of the present disclosure is to overcome the problem of low cycle performance of silicon-containing negative electrode lithium ion batteries in the related art, and to provide a negative electrode sheet and a preparation method thereof, a lithium ion battery, and an electric vehicle.

[0005] To achieve the above object, a first aspect of the present disclosure provides a negative electrode sheet. The negative electrode sheet includes a current collector, and a silicon-containing layer, an intermediate layer, and a carbon layer stacked in this order on at least one surface of the current collector. The intermediate layer includes a first conductive agent and a first binder.

[0006] A second aspect of the present disclosure provides a method for preparing a negative electrode sheet, comprising sequentially forming a silicon-containing layer, an intermediate layer, and a carbon layer on at least one surface of a current collector, the intermediate layer comprising a first conductive agent and a first binder.

[0007] A third aspect of the present disclosure provides a lithium-ion battery comprising a negative electrode sheet provided by the present disclosure or a negative electrode sheet obtained by using the method for preparing a negative electrode sheet provided by the present disclosure.

[0008] A fourth aspect of the present disclosure provides an electric vehicle, the electric vehicle including a lithium-ion battery provided by the present disclosure.

[0009] According to the negative electrode sheet provided in the present disclosure, a silicon-containing layer, an intermediate layer, and a carbon layer are sequentially stacked on at least one surface of a current collector. An intermediate layer containing a conductive agent and a binder is introduced between the silicon-containing layer and the carbon layer. This effectively improves the interfacial compatibility between the carbon layer and the silicon-containing layer, enhancing the adhesion and interfacial stress between the two layers and preventing separation of the two layers due to the expansion of the silicon-containing layer. Furthermore, the intermediate layer containing a conductive agent and a binder establishes stable electron and ion transfer channels between the carbon layer and the silicon-containing layer, mitigating the lithium deposition phenomenon caused by uneven distribution of the negative electrode electrochemical reaction during the battery charge and discharge process, thereby extending the cycle life of the lithium-ion battery.

[0010] The accompanying drawings provide a further understanding of the present disclosure, are used to constitute a part of this specification, and are used to explain, but not limit, the present disclosure in conjunction with the following specific embodiments. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a schematic diagram of a negative electrode sheet according to an embodiment of the present disclosure. [Figure 2]1 is a schematic diagram of room temperature rapid charging cycle performance of the lithium ion batteries prepared in Examples 1 to 3 and Comparative Example 2. [Figure 3] 1 is a photograph of the negative electrode sheet of the lithium ion batteries prepared in Example 2 and Comparative Examples 1 and 2, which was disassembled after 100 cycles of rapid charging, when the battery was at 100% charge. DETAILED DESCRIPTION OF THE INVENTION

[0012] The range endpoints and any values ​​disclosed in this specification are not limited to the exact range or value, and should be understood to include values ​​close to these ranges or values. With respect to ranges of values, the range endpoints, the range endpoints and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new value ranges, and these value ranges should be considered to be specifically disclosed in this specification.

[0013] A first aspect of the present disclosure provides a negative electrode sheet. The negative electrode sheet includes a current collector and a silicon-containing layer, an intermediate layer, and a carbon layer stacked in this order on at least one surface of the current collector. The intermediate layer includes a first conductive agent and a first binder.

[0014] According to some embodiments of the present disclosure, as shown in FIG. 1 , a silicon-containing layer 2, an intermediate layer 3, and a carbon layer 4 are sequentially stacked on the upper and lower surfaces of a current collector 1, respectively, to form a negative electrode sheet.

[0015] According to some embodiments of the present disclosure, the current collector 1 comprises copper foil and / or foam copper. In other embodiments, the current collector 1 comprises copper foil.

[0016] According to some embodiments of the present disclosure, the silicon-containing layer 2 includes a silicon material, a second carbon material, a second conductive agent, and a second binder.

[0017] According to some embodiments of the present disclosure, the silicon material includes at least one of silicon, silicon oxide, and silicon-carbon material.

[0018] Furthermore, the silicon material includes silicon oxide and / or silicon-carbon material. In the present disclosure, silicon oxide (SiO x , where 0 < x < 2, and may be, for example, SiO) is an organic compound. In the first lithium intercalation process, SiO x first reacts with lithium to produce elemental silicon, Li2O, and lithium silicate (Li4SiO4, Li2SiO3, Li2SiO5, etc.). Elemental silicon can further react with Li to produce a reversible capacity. The produced Li2O and lithium silicate no longer participate in the reaction during subsequent electrochemical cycle processes, but can have the effect of alleviating the volume expansion of elemental silicon and protecting the active material. Elemental silicon undergoes a lithium intercalation reaction with lithium ions as an active site to provide a high specific capacity. Furthermore, lithium silicate can alleviate the volume expansion after lithium intercalation of elemental silicon during charge / discharge cycles, reduce the particle crushing and pulverization of elemental silicon, and as a result, effectively extend the cycle life of the battery. The silicon-carbon material is a composite material of silicon and carbon. Silicon undergoes a lithium intercalation reaction with lithium ions to provide a high specific capacity. The carbon material has a high electrical conductivity and can improve the ion movement on the surface of the electrode material to improve the rate performance of the lithium-ion battery. Furthermore, the carbon base can further alleviate the particle crushing and pulverization of silicon, and as a result, effectively extend the cycle life of the battery.

[0019] According to some embodiments of the present disclosure, the second carbon material includes at least one of natural graphite, artificial graphite, hard carbon, soft carbon, and mesocarbon microbeads.

[0020] Furthermore, the second carbon material includes natural graphite and / or artificial graphite. The silicon-containing layer 2 including natural graphite and / or artificial graphite can effectively reduce the volume expansion caused by chalking of the silicon material, thereby further improving the initial coulomb efficiency and cycle life of the battery.

[0021] According to some embodiments of the present disclosure, the second conductive agent comprises at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, or arrayed carbon fibers.

[0022] Additionally, the second conductive agent includes a mixture of single-walled carbon nanotubes and conductive carbon black.

[0023] Furthermore, in the mixture of single-walled carbon nanotubes and conductive carbon black, the weight ratio of single-walled carbon nanotubes to conductive carbon black is 1:(2-10), for example, 1:2, 1:5, 1:7, 1:9, or 1:10. The second conductive agent in the silicon-containing layer 2 obtained based on this weight ratio can establish an effective point-line combined conductive network around the silicon material. The conductive network is wrapped around the silicon material, improving the conductivity of the silicon-containing layer 2 and further improving the structural stability of the silicon-containing layer 2.

[0024] According to some embodiments of the present disclosure, the second binder comprises styrene-butadiene rubber and / or polyacrylic acid.

[0025] Furthermore, the second binder includes polyacrylic acid. Polyacrylic acid is used as the second binder for the silicon-containing layer 2, thereby providing a strong binding force that limits the expansion and chalking of the silicon material, reducing the separation of the silicon material from the current collector 1 due to the expansion and chalking, and can promote the transfer of lithium ions and electrons on the negative electrode sheet by binding the silicon material with the second carbon material, the second conductive agent, etc.

[0026] Furthermore, the weight average molecular weight of the polyacrylic acid ranges from 300,000 g / mol to 600,000 g / mol, such as 300,000 g / mol, 350,000 g / mol, 400,000 g / mol, 450,000 g / mol, 500,000 g / mol, 550,000 g / mol, or 600,000 g / mol.

[0027] According to some embodiments of the present disclosure, based on the total weight of the silicon-containing layer 2, in the silicon-containing layer 2, the total content of the silicon material and the second carbon material is in the range of 70 wt% to 97 wt%, the content of the second conductive agent is in the range of 0.1 wt% to 15 wt%, and the content of the second binder is in the range of 3 wt% to 15 wt%. For example, the total content of the silicon material and the second carbon material in the silicon-containing layer 2 may be 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, 95 wt%, 97 wt%, etc.; the content of the second conductive agent may be 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 5 wt%, 7 wt%, 10 wt%, 12 wt%, 15 wt%, etc.; and the content of the second binder may be 3 wt%, 3.5 wt%, 5 wt%, 7 wt%, 10 wt%, 12 wt%, 15 wt%, etc. When the contents of the above components are within these ranges, the silicon-containing layer 2 has a high mass specific capacity while maintaining appropriate binding strength and electrical conductivity, facilitating an improvement in the energy density of the battery.

[0028] According to some embodiments of the present disclosure, the silicon-containing layer 2 further includes a second dispersant. The second dispersant improves the dispersion of the silicon material, the second carbon material, and the second conductive agent in the silicon-containing layer 2 to prevent aggregation of these components, thereby ensuring that the capabilities of these components are fully utilized. When the second binder includes styrene-butadiene rubber, the silicon-containing layer 2 includes a second dispersant.

[0029] Additionally, the second dispersing agent includes at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose.

[0030] According to some embodiments of the present disclosure, when the silicon-containing layer 2 includes a second dispersant, the content of the second dispersant in the silicon-containing layer 2 is 15 wt % or less, based on the total weight of the silicon-containing layer 2.

[0031] According to some embodiments of the present disclosure, the carbon layer 4 includes a third carbon material, a third conductive agent, and a third binder.

[0032] Additionally, the third carbon material includes at least one of natural graphite, artificial graphite, hard carbon, soft carbon, and mesocarbon microbeads.

[0033] Furthermore, the third carbon material includes natural graphite and / or artificial graphite, and the carbon layer 4 obtained by using natural graphite and / or artificial graphite can provide high specific mass capacity and high initial coulombic efficiency, and has better dynamic performance.

[0034] Additionally, the third conductive agent includes at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, or aligned carbon fibers.

[0035] Furthermore, the third conductive agent is conductive carbon black. Conductive carbon black is used as the third conductive agent in the carbon layer 4, so that the conductive network of the carbon layer 4 can be further optimized while maintaining low costs.

[0036] Additionally, the third binder includes styrene-butadiene rubber and / or polyacrylic acid.

[0037] Furthermore, the third binder is styrene-butadiene rubber, which is used as the third binder for the carbon layer 4, so that it can provide an appropriate binding strength while reducing the processing difficulty of the coating process of the carbon layer 4.

[0038] Furthermore, the weight average molecular weight of the styrene-butadiene rubber may range from 200,000 g / mol to 1,000,000 g / mol, such as 200,000 g / mol, 250,000 g / mol, 300,000 g / mol, 350,000 g / mol, 400,000 g / mol, 450,000 g / mol, 500,000 g / mol, 550,000 g / mol, 600,000 g / mol, 650,000 g / mol, 700,000 g / mol, 750,000 g / mol, 800,000 g / mol, 850,000 g / mol, 900,000 g / mol, 950,000 g / mol, or 1,000,000 g / mol.

[0039] According to some embodiments of the present disclosure, based on the total weight of the carbon layer 4, in the carbon layer, the content of the third carbon material 4 is in the range of 70 wt% to 97 wt%, the content of the third conductive agent is in the range of 0.1 wt% to 15 wt%, and the content of the third binder is in the range of 1 wt% to 15 wt%. For example, in the carbon layer 4, the content of the third carbon material is 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, 95 wt%, 97 wt%, etc.; the content of the third conductive agent is 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 5 wt%, 7 wt%, 10 wt%, 12 wt%, 15 wt%, etc.; and the content of the third binder is 1 wt%, 2 wt%, 3 wt%, 3.5 wt%, 5 wt%, 7 wt%, 10 wt%, 12 wt%, 15 wt%, etc. When the contents of the above components are within these ranges, the carbon layer 4 can have a high mass-specific capacitance while maintaining appropriate binding strength and electrical conductivity, facilitating an improvement in the energy density of the battery.

[0040] According to some embodiments of the present disclosure, the carbon layer 4 further includes a third dispersant, which improves the dispersion of the third carbon material and the third conductive agent in the carbon layer to prevent aggregation of these components, thereby ensuring that the components can fully exhibit their capabilities.

[0041] Additionally, the third dispersant comprises at least one of carboxymethylcellulose, sodium carboxymethylcellulose, and lithium carboxymethylcellulose. In some other embodiments, the third dispersant comprises lithium carboxymethylcellulose.

[0042] According to some embodiments of the present disclosure, based on the total weight of the carbon layer 4, the content of the third dispersant in the carbon layer 4 ranges from 0.1 wt% to 15 wt%, for example, 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 5 wt%, 7 wt%, 10 wt%, 12 wt%, or 15 wt%.

[0043] According to some embodiments of the present disclosure, the first conductive agent comprises at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, or aligned carbon fibers.

[0044] Furthermore, the first conductive agent includes single-walled carbon nanotubes and / or conductive carbon black. The single-walled carbon nanotubes and / or conductive carbon black are used as the first conductive agent in the intermediate layer 3, and are intimately combined with the binder in the intermediate layer 3 to form a cross-linked conductive network, which can facilitate carrier transfer between the silicon-containing layer 2 and the carbon layer 4.

[0045] Furthermore, the first conductive agent includes a mixture of single-walled carbon nanotubes and conductive carbon black.

[0046] Furthermore, in the mixture containing single-walled carbon nanotubes and conductive carbon black, the weight ratio of single-walled carbon nanotubes to conductive carbon black is 1:(1-10), for example, 1:1, 1:2, 1:5, 1:7, 1:9, or 1:10. A mixture of single-walled carbon nanotubes and conductive carbon black at this ratio forms a continuous conductive network of point-line connections in the intermediate layer 3, promoting the rapid movement of electrons and lithium ions in the intermediate layer 3 and thereby improving the battery's speed performance; it also reduces the lithium deposition phenomenon caused by uneven distribution of electrochemically active sites on the electrode during battery charging and discharging, thereby improving the cycle performance of the lithium-ion battery and thereby extending the battery's service life. In addition, the structural stability and flexibility of the intermediate layer 3 are enhanced, allowing it to adapt to the stress generated by the structural expansion of the silicon-containing layer 2.

[0047] According to some embodiments of the present disclosure, the first binder comprises polyacrylic acid and / or styrene-butadiene rubber.

[0048] Further, the first binder may comprise a mixture of polyacrylic acid and styrene-butadiene rubber. The weight ratio of polyacrylic acid to styrene-butadiene rubber may be (0.1 to 40:1), such as 0.1:1, 0.5:1, 1:1, 5:1, 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, or 40:1. In some other embodiments, the weight ratio of polyacrylic acid to styrene-butadiene rubber is (2 to 10:1). The first binder obtained based on this weight ratio can significantly enhance the adhesion between the silicon-containing layer 2 and the carbon layer 4.

[0049] Furthermore, the weight average molecular weight of the polyacrylic acid in the first binder is in the range of 300,000 g / mol to 600,000 g / mol, such as 300,000 g / mol, 350,000 g / mol, 400,000 g / mol, 450,000 g / mol, 500,000 g / mol, 550,000 g / mol, or 600,000 g / mol.

[0050] Furthermore, the weight average molecular weight of the styrene-butadiene rubber in the first binder is in the range of 200,000 g / mol to 1,000,000 g / mol, e.g., 200,000 g / mol, 250,000 g / mol, 300,000 g / mol, 350,000 g / mol, 400,000 g / mol, 450,000 g / mol, 500,000 g / mol, 550,000 g / mol, 600,000 g / mol, 650,000 g / mol, 700,000 g / mol, 750,000 g / mol, 800,000 g / mol, 850,000 g / mol, 900,000 g / mol, 950,000 g / mol, or 1,000,000 g / mol.

[0051] According to some embodiments of the present disclosure, the content of the first conductive agent in the intermediate layer 3 is in the range of 10 wt% to 70 wt%, and the content of the first binder in the intermediate layer 3 is in the range of 30 wt% to 90 wt%, based on the total weight of the intermediate layer 3. For example, the content of the first conductive agent is 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, etc.; and the content of the first binder is 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, etc. When the content of the above components is within this range, the electrical conductivity and adhesive effect of the intermediate layer 3 can be effectively exerted, and the interface compatibility between the silicon-containing layer 2 and the carbon layer 4 can be improved.

[0052] According to some embodiments of the present disclosure, the intermediate layer 3 further comprises a first carbon material. In the present disclosure, the first carbon material is introduced into the intermediate layer 3, thereby increasing the proportion of active material in the intermediate layer 3 and further increasing the proportion of active material in the negative electrode sheet. In addition, a good lithium ion transport channel is formed between the silicon-containing layer 2 and the carbon layer 4.

[0053] Additionally, the first carbon material comprises at least one of natural graphite, artificial graphite, hard carbon, soft carbon, and mesocarbon microbeads. In some other embodiments of the present disclosure, the first carbon material comprises natural graphite and / or artificial graphite.

[0054] According to some embodiments of the present disclosure, the content of the first carbon material in the intermediate layer 3 is in the range of 5 wt% to 20 wt%, for example, 5 wt%, 7 wt%, 10 wt%, 12 wt%, 15 wt%, 17 wt%, or 20 wt%, based on the total weight of the intermediate layer 3. When the content of the first carbon material is within this range, the lithium ion transport pathway can be optimized and the energy density of the battery can be increased, while the electrical conduction and adhesive effects of the intermediate layer 3 can be reliably expected.

[0055] In the present disclosure, the appropriate types and proportions of the conductive agent and binder in the intermediate layer 3 are selected, the first carbon material is introduced into the intermediate layer 3, and the compositions and proportions of the silicon-containing layer 2 and the carbon layer 4 are appropriately selected, thereby reducing the DC internal resistance of the lithium ion battery and further improving the rate performance of the lithium ion battery.

[0056] According to some embodiments of the present disclosure, the intermediate layer 3 further comprises a first dispersant.

[0057] Additionally, the first dispersing agent includes at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose.

[0058] According to some embodiments of the present disclosure, based on the total weight of the intermediate layer 3, the content of the first dispersant ranges from 0.1 wt% to 15 wt%, for example, 0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, 5 wt%, 7 wt%, 10 wt%, 12 wt%, or 15 wt%.

[0059] According to some embodiments of the present disclosure, based on the single-side thickness, the total thickness of the silicon-containing layer 2, the intermediate layer 3, and the carbon layer 4 ranges from 40 μm to 100 μm, for example, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, or 100 μm.

[0060] Furthermore, the thickness ratio of the silicon-containing layer 2 to the carbon layer 4 is (1-5):1, for example, 1:1, 2:1, 3:1, 4:1, or 5:1.

[0061] Furthermore, the thickness of the intermediate layer 3 is 1 μm to 20 μm, for example, 1 μm, 5 μm, 7 μm, 10 μm, 12 μm, 15 μm, 17 μm, or 20 μm, based on the thickness of one side. When the thickness of the intermediate layer 3 is within this range, the proportion of the negative electrode active material is not excessively reduced, and the effects of the intermediate layer 3 are effectively exhibited.

[0062] In some other embodiments of the present disclosure, the thickness of the intermediate layer 3 ranges from 1 μm to 5 μm.

[0063] A second aspect of the present disclosure provides a method for preparing a negative electrode sheet, comprising sequentially forming a silicon-containing layer, an intermediate layer, and a carbon layer on at least one surface of a current collector, the intermediate layer comprising a first conductive agent and a first binder.

[0064] According to some embodiments of the present disclosure, the preparation method comprises: (1) preparing a slurry A by mixing a silicon material, a second carbon material, a second conductive agent, and a second binder in a certain ratio; (2) mixing a first conductive agent and a first binder in a certain ratio to prepare a slurry B; (3) mixing a third carbon material, a third conductive agent, and a third binder in a certain ratio to prepare a slurry C; (4) Sequentially applying the slurry A, the slurry B, and the slurry C to the current collector to obtain a negative electrode sheet. Includes:

[0065] According to some embodiments of the present disclosure, a dispersant may be added in each of steps (1) to (3).

[0066] In the method for preparing the negative electrode sheet according to the second embodiment of the present disclosure, the compositions and amounts of the silicon-containing layer, the intermediate layer, the carbon layer, etc. are all completely the same as those of the silicon-containing layer, the intermediate layer, the carbon layer, etc. of the negative electrode sheet according to the first embodiment of the present disclosure, but to avoid repetition, they will not be described in detail again in the second embodiment of the present disclosure, and this should not be construed as a limitation on the present disclosure by those skilled in the art.

[0067] A third aspect of the present disclosure provides a lithium-ion battery comprising a negative electrode sheet provided by the present disclosure or a negative electrode sheet obtained by using the method for preparing a negative electrode sheet provided by the present disclosure.

[0068] A fourth aspect of the present disclosure provides an electric vehicle, the electric vehicle including a lithium-ion battery provided by the present disclosure.

[0069] The present disclosure will now be described in detail using examples. In the following examples, The thickness of the negative electrode sheet is measured by using a myriameter.

[0070] The weight average molecular weight of polyacrylic acid is 400,000 g / mol.

[0071] The weight average molecular weight of styrene-butadiene rubber is 400,000 g / mol. [Example]

[0072] Silicon material (silicon oxide, SiO ) based on a weight ratio of 60:17:3:10 xSlurry A was prepared by dispersing carbon material (single-walled carbon nanotubes, x = 1), graphite (artificial graphite), a conductive agent (single-walled carbon nanotubes to conductive carbon black in a weight ratio of 1:2), and a binder (polyacrylic acid) in deionized water. Slurry A was sieved for standby. Slurry B was prepared by dispersing a conductive agent (single-walled carbon nanotubes), a binder (polyacrylic acid to styrene-butadiene rubber in a weight ratio of 60:15), and a dispersant (lithium carboxymethyl cellulose) in deionized water in a weight ratio of 10:75:15. Slurry B was prepared by dispersing a carbon material (artificial graphite), a conductive agent (conductive carbon black), a binder (styrene-butadiene rubber), and a dispersant (lithium carboxymethyl cellulose) in deionized water in a weight ratio of 96:1:1.5:1.5. Slurry C was prepared by dispersing a carbon material (artificial graphite), a conductive agent (conductive carbon black), a binder (styrene-butadiene rubber), and a dispersant (lithium carboxymethyl cellulose) in deionized water in a weight ratio of 96:1:1.5:1.5. Slurry C was sieved for standby. Slurry A, Slurry B, and Slurry C were successively and evenly coated onto a copper foil current collector to obtain a silicon-containing bottom layer, an intermediate layer, and a graphite top layer, and then coated on two sides. The negative electrode sheet was then dried, wound, rolled, and die-cut to obtain a negative electrode sheet. The total thickness of one side, excluding the current collector (total thickness of the silicon-containing layer, intermediate layer, and carbon layer based on the thickness of one side), was 60 μm. The thickness of the intermediate layer on one side of the negative electrode sheet was 5 μm, and the thickness ratio of the silicon-containing layer to the carbon layer was 4:1.

[0073] In this example, the positive electrode active material is lithium nickel manganese cobalt. A lithium ion battery is a pouch-type battery made of a positive electrode sheet, a separator, and a negative electrode sheet stacked in a Z-configuration. [Example]

[0074] Silicon material (silicon oxide, SiO ) based on a weight ratio of 60:17:3:10 xSlurry A was prepared by dispersing carbon material (single-walled carbon nanotubes, x = 1), graphite (artificial graphite), a conductive agent (single-walled carbon nanotubes to conductive carbon black in a weight ratio of 1:2), and a binder (polyacrylic acid) in deionized water. Slurry A was sieved for standby. Slurry B was prepared by dispersing a conductive agent (single-walled carbon nanotubes), a binder (polyacrylic acid to styrene-butadiene rubber in a weight ratio of 40:15), and a dispersant (sodium carboxymethylcellulose) in deionized water in a weight ratio of 30:55:15. Slurry B was prepared by dispersing a carbon material (artificial graphite), a conductive agent (conductive carbon black), a binder (styrene-butadiene rubber), and a dispersant (lithium carboxymethylcellulose) in deionized water in a weight ratio of 96:1:1.5:1.5. Slurry C ... sieved for standby. Slurries A, B, and C were successively and evenly coated onto a copper foil current collector to obtain a silicon-containing bottom layer, an intermediate layer, and a graphite top layer, and then coated on two sides. The negative electrode sheet was then dried, wound, rolled, and die-cut to obtain a negative electrode sheet. The total thickness of one side, excluding the current collector (total thickness of the silicon-containing layer, intermediate layer, and carbon layer), was 60 μm. The thickness of the intermediate layer on one side of the negative electrode sheet was 5 μm, and the thickness ratio of the silicon-containing layer to the carbon layer was 4:1.

[0075] In this example, the positive electrode active material is lithium nickel manganese cobalt. A lithium ion battery is a pouch-type battery made of a positive electrode sheet, a separator, and a negative electrode sheet stacked in a Z-configuration. [Example]

[0076] A lithium ion battery was prepared according to the method of Example 1, except that the thickness of the intermediate layer on one side of the coated negative electrode sheet was 1 μm. [Example]

[0077] A lithium-ion battery was prepared based on the method of Example 1, except that the weight ratio of polyacrylic acid to styrene-butadiene rubber during the preparation of Slurry B was 2:20. [Example]

[0078] A lithium-ion battery was prepared based on the method of Example 1, except that the weight ratio of polyacrylic acid to styrene-butadiene rubber during the preparation of Slurry B was 10:1. [Example]

[0079] A lithium-ion battery was prepared based on the method of Example 1, except that the weight ratio of polyacrylic acid to styrene-butadiene rubber during the preparation of Slurry B was 40:1. [Example]

[0080] A lithium ion battery was prepared according to the method of Example 1, except that the thickness of the intermediate layer on one side of the negative electrode sheet was 25 μm. [Example]

[0081] A lithium-ion battery was prepared according to the method of Example 1, except for the composition of the raw materials in Slurry B. Specifically, Slurry B was prepared by dispersing a conductive agent (single-walled carbon nanotubes), a binder (polyacrylic acid to styrene-butadiene rubber in a weight ratio of 60:15), and a dispersant (lithium carboxymethyl cellulose) in deionized water at a weight ratio of 0.05:75:20. [Example]

[0082] A lithium-ion battery was prepared according to the method of Example 1, except that artificial graphite was added to Slurry B. Specifically, Slurry B was prepared by dispersing artificial graphite, a conductive agent (single-walled carbon nanotubes), a binder (polyacrylic acid to styrene-butadiene rubber in a weight ratio of 60:15), and a dispersant (lithium carboxymethyl cellulose) in deionized water in a weight ratio of 5:11:69:15. [Example]

[0083] A lithium-ion battery was prepared according to the method of Example 1, except that silicon oxide (SiO x , x=1) was replaced by the same weight of elemental silicon material. [Example]

[0084] A lithium ion battery was prepared according to the method of Example 1, except that the same weight of hard carbon was used instead of artificial graphite during the preparation of slurry A. [Example]

[0085] A lithium ion battery was prepared according to the method of Example 1, except that the same weight of hard carbon was used instead of artificial graphite during the preparation of Slurry C. [Example]

[0086] A 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 during the preparation of Slurry A was 1:20. [Example]

[0087] A lithium-ion battery was prepared according to the method of Example 1, except that the same weight of styrene-butadiene rubber was used instead of polyacrylic acid during the preparation of slurry A.

[0088] Comparative Example 1 A lithium ion battery was prepared based on the method of Example 1, with the difference being that during the preparation of the negative electrode sheet, slurries were prepared based on the formulations of Slurry A and Slurry C in Example 1, and Slurry A and Slurry C were mixed to obtain a mixed slurry that was applied as a layered coating instead.

[0089] Comparative Example 2 A lithium ion battery was prepared according to the method of Example 1, except that Slurry B was not applied during the preparation of the negative electrode sheet, in other words, the negative electrode sheet did not include an intermediate layer.

[0090] Test example 1: Room temperature rapid charge cycle test Test Method: The assembled pouch-type batteries were placed in a test cabinet at LAND Electronic Co., Ltd. and subjected to a charge-discharge test. The charging process included charging to 3.8 V at a rate of 1.3 C; charging to 4.0 V at a rate of 2.2 C; charging to 4.1 V at a rate of 3.1 C; and charging to 4.2 V at a rate of 4.0.5 C. The discharging process involved discharging to 2.75 V at a rate of 1 C. Table 1 shows the results of discharge capacity retention after 300 cycles of room temperature rapid charging for the examples and comparative examples. Figure 2 shows the room temperature rapid charging cycle performance of Examples 1 to 3 and Comparative Example 2. Figure 3 shows the lithium deposition state of the negative electrode sheets of Example 2 and Comparative Examples 1 and 2 when the state of charge (SOC) of the disassembled batteries after 100 cycles of room temperature rapid charging was 100%.

[0091] Test example 2: Room temperature DC internal resistance (DCIR) test Test method: The assembled pouch-type battery was placed in a test cabinet at LAND Electronic Co., Ltd., and the battery's state of charge (SOC) was adjusted to 50%. After leaving the battery for 2 hours, the battery was discharged at a rate of 1.5C for 30 seconds. The DCIR value can be obtained by subtracting the terminal voltage during the rest period before discharge from the discharge terminal voltage and dividing by the discharge current. See Table 1 for the test results.

[0092] [Table 1]

[0093] The results in Table 1 show that Examples 1 to 3 have low DCIR values. The discharge capacity retention rate after 300 cycles of room temperature rapid charging is greater than 94%. The silicon-containing negative electrode sheet of Comparative Example 1 does not use a layered arrangement. Compared with Example 1, the DCIR value is significantly increased, and the discharge capacity retention rate after 300 cycles of room temperature rapid charging is approximately 50%. As shown in Figure 2, the silicon-containing negative electrode sheet of Comparative Example 2 uses a layered arrangement, but does not have an intermediate layer between the silicon-containing layer and the carbon layer. Battery performance begins to decline after more than 100 cycles of room temperature rapid charging. The discharge capacity retention rate after 300 cycles is less than 50%, and the cycle life is lower than Examples 1 to 3. This indicates that the layered arrangement 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 layer and silicon layer composite negative electrode.

[0094] Furthermore, it can be seen from Examples 1, 2, and 4 to 6 that the components and content of the binder in the intermediate layer affect the cycle performance of the battery. Appropriate composition and content of the binder can increase the adhesion between the carbon layer and the silicon layer, thereby increasing the cycle life of the battery.

[0095] From Examples 1, 3, and 7, it can be seen that the thickness of the intermediate layer also affects the adhesion between the carbon layer and the silicon layer. If the intermediate layer is too thick, the ion diffusion distance increases and the internal resistance at the interface increases, resulting in an increase in the DCIR value of the intermediate layer and a decrease in the cycle performance of the battery. If the intermediate layer is too thin, the adhesion is insufficient. It can be seen that an appropriate thickness range is beneficial to increasing the cycle life of the battery.

[0096] Compared with Example 1, the conductive agent in the intermediate layer of Example 8 was reduced, resulting in an increase in the DCIR value of the intermediate layer, which indicates that the low-conductivity intermediate layer can no longer form a good electronic circuit between the two layers, adversely affecting the impedance and cycle performance of the battery and further reducing the cycle life.

[0097] In Example 9, artificial graphite is further added to the intermediate layer based on Example 1, which reduces the DCIR value, improves the discharge capacity retention rate after cycling, and improves the battery performance.

[0098] In Example 10, silicon oxide (SiO x , x=1), elemental silicon material was used at the same weight. Compared with Example 1, the discharge capacity retention rate decreased, indicating that silicon oxide material can provide better cycle performance than silicon.

[0099] In Examples 11 and 12, hard carbon was used instead of graphite in the silicon-containing layer and the carbon layer, respectively. The discharge capacity retention rate after 300 cycles is significantly lower than that of Example 1.

[0100] In Example 13, the content of single-walled carbon nanotubes in the conductive agent of the silicon-containing layer was reduced compared to that in Example 1, resulting in a corresponding increase in the DCIR value, indicating that an appropriate proportion of single-walled carbon nanotubes can effectively reduce the impedance of the battery and further increase the cycle life.

[0101] In Example 14, the binder for all silicon layers contains styrene-butadiene rubber. Compared to Example 1, which uses polyacrylic acid as the binder, the binding strength of styrene-butadiene rubber is weaker than that of polyacrylic acid, and therefore 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.

[0102] 2, it can be seen that in Comparative Example 2, no intermediate layer is included, and the capacity rapidly decreases during fast charge cycles. However, in Examples 1 to 3, high capacity retention is maintained. The introduction of the intermediate layer effectively improves the interfacial compatibility between the silicon-containing layer and the carbon layer, thereby improving battery performance during fast charge cycles.

[0103] 3, it can be seen that in Comparative Example 2, which does not include an intermediate layer, obvious lithium deposition occurs during fast charging cycles (i.e., white lithium metal is deposited on the surface of the negative electrode sheet). However, in Example 2, no obvious lithium deposition occurs at the interface of the negative electrode sheet after cycling, indicating that the introduction of an intermediate layer improves the carrier transport channel at the interface between the two layers.

[0104] In the present disclosure, it can be seen that by selecting the appropriate types and proportions of the conductive agent and binder in the intermediate layer, introducing the first carbon material into the intermediate layer, and properly selecting the compositions and proportions of the silicon-containing layer and the carbon layer, the cycle life of the lithium ion battery can be further improved.

[0105] The above examples merely show some embodiments of the present disclosure. Although the description is relatively specific and detailed, it should not be understood as a limitation on the scope of the present disclosure. It should be noted that those skilled in the art can make some modifications and improvements without departing from the concept of the present disclosure. These modifications and improvements belong to the scope of protection of the present disclosure. Therefore, the patent protection scope of the present disclosure is subject to the appended claims. [Explanation of symbols]

[0106] 1 Current collector 2. Silicon-containing layer 3. Middle class 4 Carbon Layer

Claims

1. A negative electrode sheet comprising a current collector (1), and a silicon-containing layer (2), an intermediate layer (3), and a carbon layer (4) stacked in this order on at least one surface of the current collector (1), wherein the intermediate layer (3) comprises a first conductive agent and a first binder.

2. 2. The negative electrode sheet according to claim 1, wherein the silicon-containing layer (2) comprises a silicon material, a second carbon material, a second conductive agent, and a second binder.

3. the second carbon material comprises at least one of natural graphite, artificial graphite, hard carbon, soft carbon, or mesocarbon microbeads; or the second conductive agent comprises at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, or aligned carbon fibers; The negative electrode sheet according to claim 2 .

4. 4. The negative electrode sheet according to claim 2, wherein, in the silicon-containing layer (2), the total content of the silicon material and the second carbon material is in the range of 70 wt % to 97 wt %, the content of the second conductive agent is in the range of 0.1 wt % to 15 wt %, and the content of the second binder is in the range of 3 wt % to 15 wt %, based on the total weight of the silicon-containing layer (2).

5. The negative electrode sheet according to any one of claims 1 to 4, wherein the carbon layer (4) comprises a third carbon material, a third conductive agent, and a third binder.

6. the third carbon material comprises at least one of natural graphite, artificial graphite, hard carbon, soft carbon, or mesocarbon microbeads; or the third conductive agent comprises at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, or aligned carbon fibers; The negative electrode sheet according to claim 5 .

7. 7. The negative electrode sheet according to claim 5, wherein, in the carbon layer (4), a content of the third carbon material is in the range of 70 wt % to 97 wt %, a content of the third conductive agent is in the range of 0.1 wt % to 15 wt %, and a content of the third binder is in the range of 1 wt % to 15 wt %, based on a total weight of the carbon layer (4).

8. The negative electrode sheet according to any one of claims 1 to 7, wherein the first conductive agent comprises at least one of single-walled carbon nanotubes, conductive graphite, conductive carbon black, graphene, or aligned carbon fibers.

9. The negative electrode sheet according to any one of claims 1 to 8, wherein the first binder contains polyacrylic acid and / or styrene-butadiene rubber.

10. 10. The negative electrode sheet according to claim 1, wherein the first binder comprises a mixture of polyacrylic acid and styrene-butadiene rubber, and the weight ratio of the polyacrylic acid to the styrene-butadiene rubber is (0.1 to 40):1; or the weight ratio of the polyacrylic acid to the styrene-butadiene rubber is (2 to 10):

1.

11. 11. The negative electrode sheet according to claim 1, wherein the content of the first conductive agent in the intermediate layer (3) is 10 wt % to 70 wt %, and the content of the first binder in the intermediate layer (3) is 30 wt % to 90 wt %, based on the total weight of the intermediate layer (3).

12. The negative electrode sheet according to any one of claims 1 to 11, wherein the intermediate layer (3) further comprises a first carbon material.

13. The negative electrode sheet according to claim 12, wherein the content of the first carbon material is 5 wt % to 20 wt % based on the total weight of the intermediate layer (3).

14. The negative electrode sheet according to any one of claims 1 to 13, wherein the total thickness of the silicon-containing layer (2), the intermediate layer (3), and the carbon layer (4) is 40 µm to 100 µm, based on the thickness of one side.

15. The negative electrode sheet according to any one of claims 1 to 14, wherein the thickness ratio of the silicon-containing layer (2) to the carbon layer (4) is (1 to 5):

1.

16. The negative electrode sheet according to any one of claims 1 to 15, wherein the thickness of the intermediate layer (3) is 1 µm to 20 µm based on the thickness of one side.

17. A method for preparing a negative electrode sheet, comprising sequentially forming a silicon-containing layer (2), an intermediate layer (3), and a carbon layer (4) on at least one surface of a current collector (1), wherein the intermediate layer (3) comprises a first conductive agent and a first binder.

18. (1) preparing a slurry A by mixing a silicon material, a second carbon material, a second conductive agent, and a second binder in a certain ratio; (2) preparing a slurry B by mixing a first conductive agent and a first binder in a certain ratio; (3) preparing a slurry C by mixing a third carbon material, a third conductive agent, and a third binder in a certain ratio; (4) Slurry A, Slurry B, and Slurry C are applied sequentially to a current collector to obtain a negative electrode sheet.

18. The method of claim 17, comprising:

19. A lithium ion battery comprising the negative electrode sheet according to any one of claims 1 to 16, or a negative electrode sheet prepared by using the method according to claim 17 or 18.

20. 20. An electric vehicle comprising the lithium ion battery of claim 19.

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