Low-expansion silicon-carbon negative electrode sheet, preparation method thereof and lithium ion battery

Abstract

This invention relates to a low-expansion silicon-carbon anode sheet, its preparation method, and a lithium-ion battery. The first and second surfaces of the current collector of the anode sheet are sequentially provided with a first binder layer, a negative electrode active material layer, and a second binder layer from the inside out. The first binder layer strengthens the adhesion between the active material and the current collector, reducing the expansion of the negative electrode during charging and discharging. Simultaneously, during high-temperature and high-pressure shaping of the cell, the second binder layer on the negative electrode surface becomes viscous. Part of it penetrates into the negative electrode active material, fusing with the binder in the active material to tightly bond the separator to the negative electrode surface. Another part permeates through the separator pores, tightly bonding the positive electrode, thus tightly bonding the entire cell together. This not only restrains the horizontal and vertical expansion changes of the negative electrode system but also reduces the interfacial contact resistance between the negative electrode and the separator, and between the separator and the positive electrode, slowing down the capacity decay rate caused by the expansion of the silicon-carbon anode during cycling and improving the battery's cycle performance.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a low-expansion silicon-carbon anode sheet, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, as highly efficient energy storage devices, have been widely used in new energy vehicles, consumer electronics, energy storage systems, and other fields due to their advantages such as high energy density, long cycle life, high charge and discharge efficiency, and low environmental pollution. They are one of the core supporting components for the current development of the new energy industry. Lithium-ion batteries mainly consist of four core components: the positive electrode, the negative electrode, the electrolyte, and the separator. Among them, the negative electrode, as the key carrier for lithium-ion intercalation / deintercalation and charge transport, directly determines the battery's core indicators such as energy density, cycle stability, and charge / discharge rate. Currently, commercially available lithium-ion batteries mostly use graphite-based materials for their negative electrodes, but their performance is gradually becoming insufficient to meet the ever-increasing demands for high energy density in end products. Developing high-performance negative electrode materials has become an important direction for the technological upgrade of lithium-ion batteries.

[0003] Silicon-carbon anode materials, as an ideal alternative to graphite anode materials, have been widely researched and applied in the field of lithium-ion batteries in recent years, exhibiting significant advantages over traditional graphite anodes. The theoretical specific capacity of traditional graphite anodes is only 372 mAh / g, with limited room for improvement, failing to meet the high energy density requirements of high-end fields such as new energy vehicles. In contrast, silicon materials have a theoretical specific capacity as high as 4200 mAh / g, more than 10 times that of graphite. Silicon-carbon anode materials, formed by combining silicon and carbon materials, inherit the high capacity characteristics of silicon while leveraging the excellent conductivity and structural stability of carbon materials to compensate for some of the defects of pure silicon. Applying silicon-carbon anode materials (or silicon-carbon / graphite composite anode materials) to lithium-ion batteries can significantly improve battery energy density, effectively addressing the pain point of insufficient capacity in traditional graphite anodes. This aligns with the development needs of miniaturization, lightweighting, and long-range battery life in end products, and has become the mainstream research and application direction for high-performance lithium-ion battery anode materials.

[0004] However, silicon-carbon anode materials suffer from significant volume expansion issues in practical applications. During lithium-ion intercalation, silicon-carbon anodes undergo a violent alloying reaction, resulting in a volume expansion rate exceeding 300%. Even with silicon-carbon / graphite composite modification, the volume expansion remains high. This drastic volume expansion triggers a series of chain reactions: firstly, repeated volume expansion and contraction lead to pulverization and cracking of the electrode structure, causing the active material to detach from the current collector, thus disrupting the internal conductive network of the battery; secondly, volume expansion causes continuous rupture and regeneration of the solid electrolyte interphase (SEI) film on the anode surface. The SEI film regeneration process continuously consumes active lithium and electrolyte within the battery, ultimately drastically shortening the battery's cycle life and failing to meet the long-term stable operation requirements of end products. Therefore, developing low-expansion silicon-carbon anode sheets to effectively suppress volume expansion during charge and discharge, reduce damage to the battery from expansion, and improve battery cycle stability has become a core technical problem urgently needing to be solved in the field of high-performance anode materials for lithium-ion batteries. Summary of the Invention

[0005] The purpose of this invention is to solve the above-mentioned technical problems existing in the prior art, and to provide a low-expansion silicon-carbon anode sheet, its preparation method, and a lithium-ion battery.

[0006] To address the shortcomings of the aforementioned technical problems, the present invention adopts the following technical solution: A low-expansion silicon-carbon anode sheet includes a current collector having a first surface and a second surface disposed opposite to each other. The first surface and the second surface are each provided with a first adhesive layer, a negative electrode active material layer and a second adhesive layer stacked sequentially from the inside out.

[0007] As a further optimization of the low-expansion silicon-carbon anode sheet of the present invention: the current collector is copper foil or aluminum foil.

[0008] As a further optimization of the low-expansion silicon-carbon anode sheet of the present invention: the thickness of the first binder layer and the second binder layer are both 2-5 μm.

[0009] As a further optimization of the low-expansion silicon-carbon anode sheet of the present invention: the binder in the first binder layer is one or any mixture of sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid and styrene-butadiene rubber.

[0010] As a further optimization of the low-expansion silicon-carbon anode sheet of the present invention: the anode active material layer comprises the following raw materials in parts by mass: 90-95 parts of anode material, 1-2 parts of conductive carbon black and 4-7 parts of binder, wherein the anode material comprises silicon-carbon and graphite in a mass ratio of 1:5-7.

[0011] As a further optimization of the low-expansion silicon-carbon anode sheet of the present invention: the binder is one or any mixture of sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid and styrene-butadiene rubber.

[0012] As a further optimization of the low-expansion silicon-carbon anode sheet of the present invention: the binder in the second binder layer is polyvinylidene fluoride or modified polyvinylidene fluoride.

[0013] A method for preparing a low-expansion silicon-carbon anode sheet includes the following steps: The negative electrode active material layer materials are mixed in a certain proportion to obtain the negative electrode active material layer slurry; The first binder, the negative electrode active material layer slurry, and the second binder are sequentially coated on the first and second surfaces of the current collector, dried, and rolled to obtain a low-expansion silicon-carbon negative electrode sheet.

[0014] A lithium-ion battery includes a cell, which is formed by stacking and winding a positive electrode, a separator, and a negative electrode. The negative electrode is the aforementioned negative electrode, and the wound cell is shaped for 30-150 seconds at 60-90°C and 0.6-2.0 MPa.

[0015] As a further optimization of the lithium-ion battery of the present invention: the wound cell is shaped for 120s at 80°C and 1.0MPa.

[0016] The present invention has the following beneficial effects: The silicon-carbon negative electrode sheet of the present invention has a first binder between the negative electrode active material and the current collector. This binder layer can strengthen the adhesion between the active material and the current collector, reduce the peeling of the negative electrode and the current collector during charge-discharge expansion and volume change. Simultaneously, a second binder is coated on the surface of the negative electrode active material. When the negative electrode sheet / separator / positive electrode are alternately stacked to form a battery cell, and the battery cell is shaped under high temperature and high pressure, the second binder on the surface of the negative electrode is in a viscous flow state. Part of it penetrates into the negative electrode active material, fusing with the binder in the negative electrode active material to tightly bond the separator to the negative electrode surface. Part of it seeps out through the pores of the separator, tightly bonding the positive electrode, making the entire battery cell tightly bonded together. This not only restrains the system expansion changes in the horizontal and vertical directions of the negative electrode, but also reduces the interfacial contact resistance between the negative electrode and the separator, and between the separator and the positive electrode, slowing down the capacity decay rate caused by the expansion of the silicon-carbon negative electrode during cycling, and improving the battery cycle performance. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the silicon-carbon anode sheet of the present invention; Figure 2 The graphs show the 1C / 3C cycle curves of lithium-ion batteries from Example 1 and Comparative Examples 1-4 at 25°C. Figure 3The graphs show the 1C / 3C cycle curves of lithium-ion batteries in Example 1 and Comparative Examples 1-4 at 45°C. Detailed Implementation

[0018] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0019] A Low-Expansion Silicon-Carbon Anode Sheet like Figure 1 As shown, a low-expansion silicon-carbon anode includes a current collector having a first surface and a second surface disposed opposite to each other. The current collector is a copper foil or an aluminum foil.

[0020] Both the first and second surfaces are sequentially stacked from the inside out with a first adhesive layer, a negative electrode active material layer, and a second adhesive layer. The thickness of both the first and second adhesive layers is 2-5 μm.

[0021] The adhesive in the first adhesive layer is one or any mixture of sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid and styrene-butadiene rubber.

[0022] The negative electrode active material layer comprises the following raw materials in parts by mass: 90-95 parts of negative electrode material, 1-2 parts of conductive carbon black, and 4-7 parts of binder. The negative electrode material comprises silicon carbide and graphite in a mass ratio of 1:5-7. The binder is one or any mixture of sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid, and styrene-butadiene rubber.

[0023] The adhesive in the second adhesive layer is polyvinylidene fluoride or modified polyvinylidene fluoride.

[0024] A method for preparing a low-expansion silicon-carbon anode sheet includes the following steps: The negative electrode active material layer materials are mixed in a certain proportion to obtain the negative electrode active material layer slurry; The negative electrode active material layer comprises the following raw materials in parts by mass: 90-95 parts of negative electrode material, 1-2 parts of conductive carbon black, and 4-7 parts of binder. The negative electrode material comprises silicon carbide and graphite in a mass ratio of 1:5-7. The binder is one or any mixture of sodium carboxymethyl cellulose, polyacrylonitrile, polyacrylic acid, and styrene-butadiene rubber.

[0025] The first binder, the negative electrode active material layer slurry, and the second binder are sequentially coated on the first and second surfaces of the current collector, dried, and rolled to obtain a low-expansion silicon-carbon negative electrode sheet.

[0026] A lithium-ion battery comprises a cell, an electrolyte, and a casing. The cell is formed by stacking and winding a positive electrode, a separator, and a negative electrode. The wound cell is shaped for 30-150 seconds at 60-90℃ and 0.6-2.0 MPa. The positive electrode uses lithium metal oxide (such as lithium cobalt oxide or lithium iron phosphate) as the active material and is coated onto an aluminum foil current collector. The negative electrode uses a graphite / silicon-based material and is coated onto a copper foil current collector. A porous polymer separator separates the positive and negative electrodes, preventing short circuits and allowing lithium ions to pass through. The electrolyte is an organic solvent containing lithium salts, serving as the medium for lithium ion migration. The casing seals and protects the internal components, and a safety valve ensures safety. During charging and discharging, lithium ions intercalate and deintercalate between the positive and negative electrodes to achieve energy conversion.

[0027] <Example 1> The method for preparing the silicon-carbon negative electrode and lithium-ion battery in this embodiment includes the following steps: Take the adhesive (sodium carboxymethyl cellulose) solution and apply it evenly to both sides of an 8-micron thick copper foil using a coating machine. After drying in an oven, the total thickness of the coating on both sides is 5 microns.

[0028] The ratio of negative electrode active material is: negative electrode material (silicon-carbon / graphite): conductive carbon black: binder LA133: binder SBR = 94 (15 / 85): 1.5: 3.5: 1.

[0029] Take silicon carbide material, graphite material, conductive carbon black, binder LA133, and binder SBR according to the above mass percentages. Mix the above materials evenly with water as a solvent, pass them through a 150-mesh sieve to obtain a negative electrode active material slurry, and then use a coating machine to evenly coat both sides of the copper foil that has been coated with binder, and dry it in an oven.

[0030] The PVDF binder was dissolved in a solvent and coated onto the surface of the active material from step 2. The coating was then dried in an oven, resulting in a total coating thickness of 4 micrometers.

[0031] The electrode sheet is rolled to the required thickness using a roller press to obtain a silicon-carbon negative electrode sheet.

[0032] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0033] The battery cells are taken and hot-pressed for 120 seconds at 80℃ and 1.0MPa.

[0034] The battery cell is encapsulated with an aluminum-plastic film, vacuum dried, and then injected with electrolyte. After standing for 48 hours until the electrolyte fully wets the positive and negative electrode plates and the separator, the battery is activated by charging and discharging, thus obtaining the lithium-ion battery of this embodiment.

[0035] <Example 2> The method for preparing the silicon-carbon negative electrode and lithium-ion battery in this embodiment includes the following steps: Take the adhesive (polyacrylonitrile) suspension and apply it evenly to both sides of an 8-micron thick copper foil using a coating machine. After drying in an oven, the total thickness of the coating on both sides is 5 microns.

[0036] The ratio of negative electrode active material is: negative electrode material (silicon-carbon / graphite): conductive carbon black: binder LA133: binder SBR = 94 (15 / 85): 1.5: 3.5: 1.

[0037] Take silicon carbide material, graphite material, conductive carbon black, binder LA133, and binder SBR according to the above mass percentages. Mix the above materials evenly with water as a solvent, pass them through a 150-mesh sieve to obtain a negative electrode active material slurry, and then use a coating machine to evenly coat both sides of the copper foil that has been coated with binder, and dry it in an oven.

[0038] Take the dissolved PVDF adhesive and apply it to the surface of the active material from step 2. Then dry it in an oven. The total thickness of the coating layer is 4 micrometers.

[0039] The electrode sheet is rolled to the required thickness using a roller press to obtain a silicon-carbon negative electrode sheet.

[0040] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0041] The battery cells are taken and hot-pressed for 120 seconds at 80℃ and 1.0MPa.

[0042] The battery cell from step 6 is encapsulated with an aluminum-plastic film, vacuum dried, and then injected with electrolyte. After standing for 48 hours until the electrolyte fully wets the positive and negative electrode plates and the separator, the battery is activated by charging and discharging, thus obtaining the lithium-ion battery of this embodiment.

[0043] <Example 3> The method for preparing the silicon-carbon negative electrode and lithium-ion battery in this embodiment includes the following steps: Take the SBR emulsion of adhesive and apply it evenly to both sides of an 8-micron thick copper foil using a coating machine. After drying in an oven, the total thickness of the coating on both sides is 5 microns.

[0044] The ratio of negative electrode active material is: negative electrode material (silicon-carbon / graphite): conductive carbon black: binder LA133: binder SBR = 92 (40 / 60): 1.5: 4.5: 2.

[0045] Take silicon carbide material, graphite material, conductive carbon black, binder LA133, and binder SBR according to the above mass percentages. Mix the above materials evenly with water as a solvent, pass them through a 150-mesh sieve to obtain a negative electrode active material slurry, and then use a coating machine to evenly coat both sides of the copper foil that has been coated with binder, and dry it in an oven.

[0046] Take the dissolved PVDF adhesive and apply it to the surface of the active material from step 2. Then dry it in an oven. The total thickness of the coating layer is 4 micrometers.

[0047] The electrode sheet from step 3 is rolled to the required thickness using a roller press to obtain a silicon-carbon negative electrode sheet.

[0048] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0049] The battery cells are taken and hot-pressed for 120 seconds at 80℃ and 1.0MPa.

[0050] The battery cell is encapsulated with an aluminum-plastic film, vacuum dried, and then injected with electrolyte. After standing for 48 hours until the electrolyte fully wets the positive and negative electrode plates and the separator, the battery is activated by charging and discharging, thus obtaining the lithium-ion battery of this embodiment.

[0051] <Example 4> The method for preparing the silicon-carbon negative electrode and lithium-ion battery in this embodiment includes the following steps: Take the adhesive (SBR) solution and apply it evenly to both sides of a 9-micron thick aluminum foil using a coating machine. After drying in an oven, the total thickness of the coating on both sides is 5 microns.

[0052] The ratio of negative electrode active material is: negative electrode material (silicon-carbon / graphite): conductive carbon black: binder LA133: binder SBR = 90 (15 / 85): 1: 2: 2.5.

[0053] Take silicon carbide material, graphite material, conductive carbon black, binder LA133, and binder SBR according to the above mass percentages. Mix the above materials evenly with water as a solvent, pass them through a 150-mesh sieve to obtain a negative electrode active material slurry, and then use a coating machine to evenly coat both sides of the copper foil that has been coated with binder, and dry it in an oven.

[0054] The binder-modified PVDF was dissolved in a solvent and coated onto the surface of the active material from step 2. The coating was then dried in an oven, with a total coating thickness of 4 micrometers.

[0055] The modified PVDF is titanium dioxide-modified PVDF. The preparation method is as follows: add foamed titanium dioxide to a water-ethanol solution of silane coupling agent, stir at 30~50℃ for 20~28h, filter, wash with anhydrous ethanol, and dry to obtain silane coupling agent-modified foamed titanium dioxide; add 2~5% of the mass of PVDF to silane coupling agent-modified foamed titanium dioxide, mix evenly, and stir at 100~150℃ for 1~2h to obtain modified PVDF.

[0056] The electrode sheet is rolled to the required thickness using a roller press to obtain a silicon-carbon negative electrode sheet.

[0057] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0058] The battery cells are taken and hot-pressed for 30 seconds at 60℃ and 2.0MPa.

[0059] <Example 5> The method for preparing the silicon-carbon negative electrode and lithium-ion battery in this embodiment includes the following steps: Take the adhesive (LA133) solution and apply it evenly to both sides of a 9-micron thick aluminum foil using a coating machine. After drying in an oven, the total thickness of the coating on both sides is 5 microns.

[0060] The ratio of negative electrode active material is: negative electrode material (silicon-carbon / graphite): conductive carbon black: binder LA133: binder SBR = 95 (15 / 85): 2: 3: 4.

[0061] Take silicon carbide material, graphite material, conductive carbon black, binder LA133, and binder SBR according to the above mass percentages. Mix the above materials evenly with water as a solvent, pass them through a 150-mesh sieve to obtain a negative electrode active material slurry, and then use a coating machine to evenly coat both sides of the copper foil that has been coated with binder, and dry it in an oven.

[0062] The binder-modified PVDF was dissolved in a solvent and coated onto the surface of the active material from step 2. The coating was then dried in an oven, with a total coating thickness of 4 micrometers.

[0063] 100 parts of vinylidene fluoride monomer, 0.1-0.5 parts of perfluoropolyether carboxylic acid, 0.01-0.1 parts of ammonium persulfate, 1-2 parts of polyethylene glycol, 2-8 parts of diatomaceous earth, and 0.1-0.2 parts of ethyl acetate are added to water, reacted, coagulated, washed, and dried to obtain modified PVDF.

[0064] The electrode sheet is rolled to the required thickness using a roller press to obtain a silicon-carbon negative electrode sheet.

[0065] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0066] The battery cells are taken and hot-pressed for 150 seconds at 90℃ and 0.6MPa.

[0067] The battery cell is encapsulated with an aluminum-plastic film, vacuum dried, and then injected with electrolyte. After standing for 48 hours until the electrolyte fully wets the positive and negative electrode plates and the separator, the battery is activated by charging and discharging, thus obtaining the lithium-ion battery of this embodiment.

[0068] <Comparative Example 1> The preparation method of this comparative lithium-ion battery is as follows: The negative electrode active material ratio is as follows: negative electrode material (silicon-carbon / graphite): conductive carbon black: binder LA133: binder SBR = 94 (15 / 85): 1.5: 3.5: 1. The above materials are mixed evenly with water as solvent and passed through a 150-mesh sieve to obtain a negative electrode active material slurry. Then, the negative electrode active material slurry is evenly coated on both sides of an 8-micron copper foil through a coating machine, dried in an oven, and rolled to the required electrode thickness by a roller press to obtain a silicon-carbon negative electrode sheet.

[0069] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0070] The battery cells are taken and hot-pressed for 120 seconds at 80℃ and 1.0MPa.

[0071] The battery cell is encapsulated with an aluminum-plastic film, vacuum dried, and then injected with electrolyte. After standing for 48 hours until the electrolyte fully wets the positive and negative electrode plates and the separator, the battery is activated by charging and discharging, thus obtaining the lithium-ion battery of this embodiment.

[0072] <Comparative Example 2> The preparation method of this comparative lithium-ion battery is as follows: Take the PVDF adhesive solution and apply it evenly to both sides of an 8-micron thick copper foil using a coating machine. After drying in an oven, the total thickness of the coating on both sides is 5 microns.

[0073] The negative electrode active material ratio is as follows: negative electrode material (silicon-carbon / graphite): conductive carbon black: binder LA133: binder SBR = 94 (15 / 85): 1.5: 3.5: 1. The above materials are mixed evenly with water as solvent and passed through a 150-mesh sieve to obtain a negative electrode active material slurry. Then, the negative electrode active material slurry is evenly coated on both sides of an 8-micron copper foil through a coating machine, dried in an oven, and rolled to the required electrode thickness by a roller press to obtain a silicon-carbon negative electrode sheet.

[0074] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0075] The battery cells are taken and hot-pressed for 120 seconds at 80℃ and 1.0MPa.

[0076] The battery cell is encapsulated with an aluminum-plastic film, vacuum dried, and then injected with electrolyte. After standing for 48 hours until the electrolyte fully wets the positive and negative electrode plates and the separator, the battery is activated by charging and discharging, thus obtaining the lithium-ion battery of this embodiment.

[0077] <Comparative Example 3> The preparation method of this comparative lithium-ion battery is as follows: The negative electrode active material ratio is as follows: negative electrode material (silicon-carbon / graphite): conductive carbon black: binder LA133: binder SBR = 94 (15 / 85): 1.5: 3.5: 1. The above materials are mixed evenly with water as solvent and passed through a 150-mesh sieve to obtain a negative electrode active material slurry. Then, the negative electrode active material slurry is evenly coated on both sides of an 8-micron copper foil using a coating machine and dried in an oven. The PVDF binder was dissolved in a solvent and coated onto the surface of the active material. The coating was then dried in an oven, resulting in a total coating thickness of 4 micrometers.

[0078] The silicon-carbon negative electrode sheet is obtained by rolling it to the required electrode thickness using a roller press.

[0079] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0080] The battery cells are taken and hot-pressed for 120 seconds at 80℃ and 1.0MPa.

[0081] The battery cell is encapsulated with an aluminum-plastic film, vacuum dried, and then injected with electrolyte. After standing for 48 hours until the electrolyte fully wets the positive and negative electrode plates and the separator, the battery is activated by charging and discharging, thus obtaining the lithium-ion battery of this embodiment.

[0082] <Comparative Example 4> The preparation method of the silicon-carbon negative electrode sheet and lithium-ion battery in this comparative example includes the following steps: The PVDF adhesive was dissolved in a solvent and evenly coated on both sides of an 8-micron thick copper foil using a coating machine. After drying in an oven, the total thickness of the coating on both sides was 5 microns.

[0083] Take silicon carbide material, graphite material, conductive carbon black, binder LA133, and binder SBR according to the above mass percentages. Mix the above materials evenly with water as a solvent, pass them through a 150-mesh sieve to obtain a negative electrode active material slurry, and then use a coating machine to evenly coat both sides of the copper foil that has been coated with binder, and dry it in an oven.

[0084] The PVDF binder was dissolved in a solvent, coated onto the surface of the active material, and then dried in an oven. The total thickness of the coating layer was 4 micrometers.

[0085] The electrode sheet is rolled to the required thickness using a roller press to obtain a silicon-carbon negative electrode sheet.

[0086] A ternary cathode material (nickel-cobalt-manganese 811), conductive carbon black, and PVDF were mixed in a ratio of 97:1:2 using NMP as a solvent to prepare a cathode slurry. This slurry was sieved and coated onto both sides of a 12-micron aluminum foil. After drying in an oven and rolling with a roller press, a cathode sheet was obtained. The cathode sheet, a 12-micron PP separator, and a cathode sheet were alternately stacked to form a battery cell.

[0087] The battery cell is encapsulated with an aluminum-plastic film, vacuum dried, and then injected with electrolyte. After standing for 48 hours until the electrolyte fully wets the positive and negative electrode plates and the separator, the battery is activated by charging and discharging, thus obtaining the lithium-ion battery of this embodiment.

[0088] Performance Testing The batteries from Examples 1-3 and the comparative example were fully charged at the same current and then disassembled. The expansion of the silicon-carbon negative electrode sheet in the horizontal direction and the thickness direction (vertical direction) was measured. The details are shown in the table below.

[0089] The room temperature cycling curves of lithium-ion batteries in Example 1 and Comparative Examples 1-4 are shown below. Figure 1 As shown, the high-temperature cycling curves of lithium-ion batteries in Example 1 and Comparative Examples 1-4 are as follows: Figure 2 As shown.

[0090] The test results of the experimental examples show that, compared with Comparative Example 1, the low-expansion silicon-carbon negative electrode sheet of the present invention exhibits significantly reduced horizontal and vertical expansion during the charge and discharge process. Horizontal expansion decreased from 0.6% to 0.1%, and vertical expansion decreased from 43% to 26%. Furthermore, the lithium-ion battery of Example 1 shows a significant improvement in cycle performance compared to the battery of Comparative Example 1, with room temperature cycles increasing from over 100 to over 1000, and high-temperature cycles increasing from 50 to over 500. This is because the silicon-carbon negative electrode sheet of the present invention has a first binder between the negative electrode active material and the current collector. This binder layer strengthens the adhesion between the active material and the current collector, reducing negative electrode expansion during charge and discharge, and minimizing peeling from the current collector during volume changes. Meanwhile, a second binder is coated on the surface of the negative electrode active material. When the negative electrode sheet / separator / positive electrode are alternately stacked to form a cell, and the cell is shaped under high temperature and high pressure, the second binder on the surface of the negative electrode is in a viscous flow state. Part of it penetrates into the negative electrode active material and fuses with the binder in the negative electrode active material to tightly bond the separator to the surface of the negative electrode. Part of it seeps out through the pores of the separator and tightly bonds the positive electrode, so that the entire cell is tightly bonded together. This can not only restrain the system expansion changes in the horizontal and vertical directions of the negative electrode, but also reduce the interfacial contact resistance between the negative electrode and the separator, and between the separator and the positive electrode, thus slowing down the capacity decay rate caused by the expansion of the silicon-carbon negative electrode during cycling and improving the battery cycle performance.

[0091] Comparative Example 2 only placed a binder between the negative electrode active material and the current collector, while Comparative Example 3 only placed a binder on the surface of the negative electrode active material. Although the expansion of the batteries prepared in Comparative Example 2 was improved compared to Comparative Example 1, it was still significantly worse than that of Example 1. This shows that in this invention, only by placing a binder simultaneously between the negative electrode active material and the current collector, and on the surface of the negative electrode active material, can the two layers of binders work together to achieve a low expansion effect.

[0092] Although Comparative Example 4 had binders between the negative electrode active material and the current collector, as well as on the surface of the negative electrode active material, the cell did not undergo high-temperature and high-pressure shaping. As a result, the expansion of the final battery was almost the same as that of Comparative Examples 2 and 3. This shows that the two layers of binders and the high-temperature and high-pressure shaping process complement each other. Only by optimizing both aspects together can the interfacial contact resistance between the negative electrode and the separator, and between the separator and the positive electrode be reduced, thus slowing down the capacity decay rate caused by the expansion of the silicon-carbon negative electrode during cycling and improving the battery's cycle performance.

[0093] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A low-expansion silicon-carbon anode sheet, comprising a current collector, characterized in that: The current collector has a first surface and a second surface that are disposed opposite to each other; The first surface and the second surface are each provided with a first adhesive layer, a negative electrode active material layer and a second adhesive layer from the inside out.

2. The low-expansion silicon-carbon anode sheet as described in claim 1, characterized in that: The current collector is a copper foil or an aluminum foil.

3. The low-expansion silicon-carbon anode sheet as described in claim 1, characterized in that: The thickness of both the first adhesive layer and the second adhesive layer is 2-5 μm.

4. The low-expansion silicon-carbon anode sheet as described in claim 1, characterized in that: The adhesive in the first adhesive layer is one or any mixture of sodium carboxymethyl cellulose, polyacrylonitrile adhesive, polyacrylic adhesive and styrene-butadiene rubber.

5. The low-expansion silicon-carbon anode sheet as described in claim 1, characterized in that: The negative electrode active material layer comprises the following raw materials in parts by mass: 90-95 parts of negative electrode material, 1-2 parts of conductive carbon black, and 4-7 parts of binder. The negative electrode material comprises silicon carbon and graphite in a mass ratio of 1:5-7.

6. The low-expansion silicon-carbon anode sheet as described in claim 5, characterized in that, The adhesive is one or any mixture of sodium carboxymethyl cellulose, polyacrylonitrile adhesive, polyacrylic adhesive, and styrene-butadiene rubber.

7. The low-expansion silicon-carbon anode sheet as described in claim 1, characterized in that, The adhesive in the second adhesive layer is polyvinylidene fluoride or modified polyvinylidene fluoride.

8. The method for preparing the low-expansion silicon-carbon anode sheet as described in claim 1, characterized in that, Includes the following steps: The negative electrode active material layer materials are mixed in a certain proportion to obtain the negative electrode active material layer slurry; The first binder, the negative electrode active material layer slurry, and the second binder are sequentially coated on the first and second surfaces of the current collector, dried, and rolled to obtain a low-expansion silicon-carbon negative electrode sheet.

9. A lithium-ion battery, comprising a cell, wherein the cell is formed by stacking and winding a positive electrode, a separator, and a negative electrode, characterized in that, The negative electrode is the negative electrode as described in any one of claims 1-7, and the wound cell is shaped for 30-150s under conditions of 60-90℃ and 0.6-2.0MPa.

10. The lithium-ion battery as described in claim 9, characterized in that: The wound battery cell was shaped for 120 seconds at 80°C and 1.0 MPa.

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