A negative electrode sheet and its preparation method, and a battery cell
By constructing a double-layer coating technology with conductive gradient and active material gradient on the surface of the negative electrode, the stress peak problem caused by the volume expansion of silicon-carbon materials during charging and discharging is solved, improving the cycle life and energy density of the battery, and achieving efficient electrochemical reaction and structural stability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HUNAN JUPITER TIMES NEW ENERGY TECHNOLOGY CO LTD
- Filing Date
- 2026-03-09
- Publication Date
- 2026-05-26
AI Technical Summary
In existing technologies, in pursuit of high energy density, the silicon-carbon content and areal density are increased, which makes the negative electrode prone to breakage, resulting in a shortened battery cycle life. However, in order to avoid breakage, a conservative design is necessary, which fails to fully realize the performance potential of silicon-carbon materials.
A double-layer coating technique is used to construct carbon nanotubes and graphene conductive agents with better conductivity on the surface of the negative electrode, and Super P conductive agent with lower conductivity on the bottom layer. The thickness ratio of the two layers is controlled to be 0.8-0.9:1 to form a conductivity gradient and an active material gradient, which preferentially allows lithium insertion/delithiation reaction to occur on the surface layer and buffers volume expansion stress.
It effectively reduces the instantaneous peak value of stress during charging and discharging, reduces mechanical damage to the electrode structure, improves the cycle life and first coulombic efficiency of the battery, and enhances the energy density and stability of the battery.
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Abstract
Description
Technical Field
[0001] This invention belongs to the field of secondary battery technology. More specifically, it relates to a negative electrode sheet, its preparation method, and a battery cell. Background Technology
[0002] Lithium-ion batteries are a core power source for new energy vehicles, energy storage systems, and other fields, and their energy density and cycle life are of paramount importance. Using high-capacity silicon-carbon materials as the negative electrode active material is an inevitable choice to overcome the theoretical specific capacity bottleneck of traditional graphite negative electrodes and improve battery energy density.
[0003] However, silicon-based materials undergo dramatic volume expansion and contraction during lithium insertion / extraction (expansion rates can exceed 300%). This inherent volume effect poses a severe challenge to the structural integrity and reliability of batteries, especially the negative electrode. When the silicon-carbon content, particularly the areal density, is high, repeated and enormous stresses directly act on the negative electrode, causing irreversible mechanical damage. One of the most serious problems is that during battery cycling, cracks may appear in the negative electrode along the longitudinal direction (perpendicular to the coating direction) or laterally, or even completely break.
[0004] While using elastic binders or increasing the amount of binder can alleviate the problem to some extent, this often comes at the cost of energy density and conductivity, and cannot fundamentally solve the stress accumulation problem. In addition, the industry has tried to overcome this issue by reducing silicon content / area density, but this directly limits the upper limit of battery energy density improvement and fails to fully utilize the high capacity advantages of silicon-carbon materials.
[0005] Therefore, existing technologies are caught in a dilemma: in order to pursue high energy density and increase silicon-carbon content and areal density, they have to bear the extremely high risk of "band breakage", which leads to a sharp reduction in battery cycle life or even premature failure; on the other hand, in order to avoid "band breakage", they have to design electrode parameters conservatively, thus failing to fully realize the performance potential of silicon-carbon materials. Summary of the Invention
[0006] The technical problem this invention aims to solve is that, in pursuit of high energy density, increasing the silicon-carbon content and areal density of existing battery negative electrodes inevitably exposes the battery to a high risk of "band breakage," leading to a sharp reduction in cycle life or even premature failure. Conversely, to avoid "band breakage," electrode parameters must be conservatively designed, thus failing to fully realize the performance potential of silicon-carbon materials. Based on these challenges, this invention provides a negative electrode, its preparation method, and a battery cell.
[0007] The purpose of this invention is to provide a negative electrode.
[0008] Another object of the present invention is to provide a method for preparing a negative electrode sheet.
[0009] Another object of the present invention is to provide a battery cell.
[0010] The above-mentioned objective of this invention is achieved through the following technical solution:
[0011] A negative electrode includes a current collector;
[0012] The current collector includes a first surface and a second surface that are opposite to each other;
[0013] A negative electrode active material coating is provided on the first surface and the second surface respectively;
[0014] The negative electrode active material coating includes a bottom active material layer close to the current collector and a surface active material layer away from the current collector.
[0015] The conductive agent in the bottom active material layer is Super P;
[0016] The conductive agents in the surface active material layer are carbon nanotubes and graphene.
[0017] Furthermore, the conductive agent content in the bottom active material layer is 1.2-2.0%;
[0018] The conductive agent content in the surface active material layer is 1.8-3.0%.
[0019] The beneficial effects of the above technical solution are as follows:
[0020] The aforementioned technical solution constructs a surface active material with superior conductivity, while the underlying material has relatively low conductivity. Under an electric field, electrons will seek the path of least impedance, resulting in a preferential influx and distribution of electrons into and on the surface during charging and discharging, leading to less electrochemical polarization on the surface compared to the underlying material. According to the Butler-Wolmer equation, the reaction rate is exponentially related to the overpotential; therefore, the surface active material (regardless of its type and content) will preferentially and rapidly undergo lithium insertion / extraction reactions. Due to the preferential reaction on the surface, its silicon-carbon particles will expand in volume first. At this time, the underlying material remains in a state of reaction lag, with minimal volume change. This means that the maximum expansion stress borne by the electrode does not peak instantaneously at the beginning of charging and discharging, but rather occurs in two phases: the expansion period of the surface and the expansion period of the underlying material. This "staggered peak" mechanism avoids the instantaneous superposition of stress, greatly reducing the impact of stress peaks on the interface structure.
[0021] The preferential expansion of the surface layer primarily releases stress towards the diaphragm, away from the current collector. This relatively spacious area provides a valuable "pressure relief zone" for stress. When the bottom layer begins to expand with hysteresis, some of the resulting inward stress is borne and buffered by the already expanded and shaped surface structure, rather than being entirely rigidly applied to the current collector interface.
[0022] Furthermore, the thickness ratio of the bottom active material layer to the surface active material layer is 0.8-0.9:1.
[0023] The further technical effects of the above technical solution are as follows:
[0024] The slightly thicker surface layer provides a sufficient "main load-bearing area" for preferential and intense electrochemical reactions and volume changes. Its thicker structure can better disperse the expansion stress within its own layer and direct it to the membrane side.
[0025] The thinner underlayer's primary function is adhesion and conductivity, rather than a reaction with the host material. The thinner thickness means less absolute displacement due to expansion strain and less tearing force on the interface. Simultaneously, the thinner coating helps improve its adhesion to the current collector.
[0026] Furthermore, the Si content in the bottom active material layer is 12-15%; and the Si content in the surface active material layer is 10-13%.
[0027] Furthermore, the Si content in the surface active material layer is lower than the Si content in the bottom active material layer.
[0028] The further technical effects of the above solution are as follows:
[0029] The coating-current collector interface is the weakest link in mechanical failure. As a high-silicon region, the underlying layer has a huge inherent absolute expansion. By forcibly delaying and reducing its actual expansion through low conductivity, the direct link between the maximum destructive force and the weakest interface is severed, significantly reducing the driving force of interface peeling.
[0030] Placing the low-silicon layer in the high-conductivity region means that the surface layer primarily undergoes reactions with low expansion rates, such as graphite. This makes the preferential reaction process on the surface layer rapid, stable, and controllable, preventing new problems (such as ion channel blockage) caused by its own excessive expansion, thus perfectly playing the role of a "pioneer reaction zone" and a "stress buffer layer."
[0031] Furthermore, the mass ratio of the carbon nanotubes to the graphene is 1.2-1.4:1.
[0032] Furthermore, the negative electrode active material coating includes a negative electrode active material; the negative electrode active material is composed of artificial graphite and silicon carbon.
[0033] Furthermore, the D50 of the artificial graphite is 13-17 μm; the D50 of the silicon carbide is 6-8 μm.
[0034] A method for preparing a negative electrode sheet, the specific preparation steps of which include:
[0035] The negative electrode active material, conductive agent Super P, binder and solvent water are dispersed evenly to obtain the bottom active material slurry;
[0036] The negative electrode active material, conductive agent carbon nanotubes and graphene, binder and solvent water are uniformly dispersed to obtain a surface active material slurry;
[0037] A double-layer extrusion coating device is used to coat the bottom active material slurry and the top active material slurry onto the first and second surfaces of the current collector.
[0038] After drying, rolling, slitting and die cutting, the negative electrode sheet is obtained.
[0039] Furthermore, the specific preparation steps also include:
[0040] After mixing the adhesive and water, stir and disperse evenly to obtain the adhesive solution;
[0041] First, the conductive agent Super P is dispersed in the adhesive solution. After the dispersion is uniform, the negative electrode active material is added and the dispersion is continued to be uniform to obtain the bottom active material slurry.
[0042] Conductive carbon nanotubes and graphene are dispersed in a colloid solution. After the dispersion is uniform, a negative electrode active material is added to obtain a surface active material slurry.
[0043] A double-layer extrusion coating device is used to coat the bottom active material slurry and the top active material slurry onto the first and second surfaces of the current collector.
[0044] After drying, rolling, slitting and die-cutting, the negative electrode sheet is obtained;
[0045] In the bottom active material layer, the mass content of the negative electrode active material is 95%, the content of the conductive agent is 1.2-2.0%, and the remainder is binder;
[0046] In the surface active material layer, the mass content of negative electrode active material is 95%, the content of conductive agent is 1.8-3.0%, and the balance is binder.
[0047] A battery cell includes the aforementioned negative electrode, positive electrode, and a separator sandwiched between the negative electrode and the positive electrode. Detailed Implementation
[0048] The present invention will be further illustrated below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field.
[0049] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.
[0050] Example 1
[0051] Raw material preparation:
[0052] An 8μm thick electrolytic copper foil was used as the negative electrode current collector;
[0053] Artificial graphite and silicon carbide are used as the negative electrode active materials; the D50 of artificial graphite is 13 μm, and the D50 of silicon carbide is 6 μm; the silicon content of silicon carbide is 20%.
[0054] Super P is used as the bottom conductive agent, and carbon nanotubes and graphene are used as the surface conductive agents.
[0055] SBR emulsion with a solid content of 45% and sodium carboxymethyl cellulose were used as binders;
[0056] Deionized water was used as the solvent.
[0057] Slurry preparation:
[0058] Sodium carboxymethyl cellulose and deionized water were mixed at a mass ratio of 1:20 and dissolved evenly. Then, SBR emulsion was added and stirred continuously to obtain a glue solution with a solid content of 2%.
[0059] Take a portion of the adhesive solution, and add 1.2% (by mass) of conductive agent Super P based on the total dry matter mass of the bottom coating. Then, disperse the mixture at 2000 rpm for 30 minutes in a high-speed disperser to ensure that Super P is fully dispersed. Subsequently, add the negative electrode active material, which is composed of artificial graphite and silicon-carbon composite material. The ratio of the negative electrode active material is calculated so that the silicon content in the final bottom active material layer is 12%, and the total amount of negative electrode active material accounts for 95% of the total dry matter mass of the bottom coating; thus obtaining the bottom slurry.
[0060] Take another portion of the adhesive solution and add a conductive agent based on the total mass of the surface coating dry matter. The conductive agent is a composite of carbon nanotubes and graphene with a mass ratio of 1.2:1, and the total amount of conductive agent added is 1.8%. Disperse the mixture at 3000 rpm for 60 minutes in a high-speed disperser to ensure that the carbon nanotubes and graphene are fully depolymerized. Subsequently, add a negative electrode active material, which is composed of artificial graphite and silicon-carbon composite material. The ratio of the negative electrode active material is calculated so that the silicon content in the final surface active material layer is 10%, which is lower than the silicon content in the bottom layer. The total amount of the negative electrode active material accounts for 95% of the total mass of the surface coating dry matter. The surface slurry is obtained.
[0061] Electrode preparation:
[0062] A double-layer extrusion coating machine is used to simultaneously coat the bottom layer slurry and the top layer slurry onto both surfaces of the copper foil. The bottom layer is the part closer to the current collector, while the top layer is the part relatively farther away from the current collector. By precisely controlling the delivery rate of the extrusion pump, the wet film thickness of the bottom and top layers is controlled, so that after drying and rolling, the thickness ratio of the bottom active material layer to the top active material layer is 0.8:1; the total areal density of single-sided coating is 150 g / m².
[0063] The coated electrode sheets were dried in stages at 80°C to remove the solvent. Subsequently, they were cold-pressed at room temperature using a roller press to control the compaction density of the electrode sheets to 1.65 g / cm³.
[0064] The rolled electrode sheets are slit and die-cut to obtain negative electrode sheets of the required size.
[0065] Example 2
[0066] Raw material preparation:
[0067] An 8μm thick electrolytic copper foil was used as the negative electrode current collector;
[0068] Artificial graphite and silicon carbide are used as the negative electrode active materials; the D50 of artificial graphite is 15 μm, and the D50 of silicon carbide is 7 μm; the silicon content of silicon carbide is 20%.
[0069] Super P is used as the bottom conductive agent, and carbon nanotubes and graphene are used as the surface conductive agents.
[0070] SBR emulsion with a solid content of 45% and sodium carboxymethyl cellulose were used as binders;
[0071] Deionized water was used as the solvent.
[0072] Slurry preparation:
[0073] Sodium carboxymethyl cellulose and deionized water were mixed at a mass ratio of 1:20 and dissolved evenly. Then, SBR emulsion was added and stirred continuously to obtain a glue solution with a solid content of 2%.
[0074] Take a portion of the adhesive solution, and add 1.6% (by mass) of conductive agent Super P based on the total dry matter mass of the bottom coating. Then, disperse the mixture at 2000 rpm for 30 minutes using a high-speed disperser to ensure that Super P is fully dispersed. Subsequently, add the negative electrode active material, which is composed of artificial graphite and silicon-carbon composite material. The ratio of the negative electrode active material is calculated so that the silicon content in the final bottom active material layer is 14%, and the total amount of the negative electrode active material accounts for 95% of the total dry matter mass of the bottom coating; thus obtaining the bottom slurry.
[0075] Take another portion of the adhesive solution and add a conductive agent based on the total mass of the surface coating dry matter. The conductive agent is a composite of carbon nanotubes and graphene with a mass ratio of 1.3:1, and the total amount of conductive agent added is 2.0%. Disperse the mixture at 3000 rpm for 60 minutes in a high-speed disperser to ensure that the carbon nanotubes and graphene are fully depolymerized. Subsequently, add a negative electrode active material, which is composed of artificial graphite and silicon-carbon composite material. The ratio of the negative electrode active material is calculated so that the silicon content in the final surface active material layer is 12%, which is lower than the silicon content in the bottom layer. The total amount of the negative electrode active material accounts for 95% of the total mass of the surface coating dry matter. The surface slurry is obtained.
[0076] Electrode preparation:
[0077] A double-layer extrusion coating machine is used to simultaneously coat the bottom layer and the top layer with slurry on both surfaces of the copper foil. The bottom layer is the part closer to the current collector, while the top layer is the part relatively farther away from the current collector. By precisely controlling the delivery rate of the extrusion pump, the wet film thickness of the bottom and top layers is controlled, so that after drying and rolling, the thickness ratio of the bottom active material layer to the top active material layer is 0.86:1; the total areal density of the coating on one side is 150 g / m².
[0078] The coated electrode sheets were dried in stages at 80°C to remove the solvent. Subsequently, they were cold-pressed at room temperature using a roller press to control the compaction density of the electrode sheets to 1.65 g / cm³.
[0079] The rolled electrode sheets are slit and die-cut to obtain negative electrode sheets of the required size.
[0080] Example 3
[0081] Raw material preparation:
[0082] An 8μm thick electrolytic copper foil was used as the negative electrode current collector;
[0083] Artificial graphite and silicon carbide are used as the negative electrode active materials; the D50 of artificial graphite is 17 μm, and the D50 of silicon carbide is 8 μm; the silicon content of silicon carbide is 20%.
[0084] Super P is used as the bottom conductive agent, and carbon nanotubes and graphene are used as the surface conductive agents.
[0085] SBR emulsion with a solid content of 45% and sodium carboxymethyl cellulose were used as binders;
[0086] Deionized water was used as the solvent.
[0087] Slurry preparation:
[0088] Sodium carboxymethyl cellulose and deionized water were mixed at a mass ratio of 1:20 and dissolved evenly. Then, SBR emulsion was added and stirred continuously to obtain a glue solution with a solid content of 2%.
[0089] Take a portion of the adhesive solution, and add 2.0% (by mass) of conductive agent Super P based on the total dry matter mass of the bottom coating. Then, disperse the mixture at 2000 rpm for 30 minutes using a high-speed disperser to ensure that Super P is fully dispersed. Subsequently, add the negative electrode active material, which is composed of artificial graphite and silicon-carbon composite material. The ratio of the negative electrode active material is calculated so that the silicon content in the final bottom active material layer is 15%, and the total amount of the negative electrode active material accounts for 95% of the total dry matter mass of the bottom coating; thus obtaining the bottom slurry.
[0090] Take another portion of the adhesive solution and add a conductive agent based on the total mass of the surface coating dry matter. The conductive agent is a composite of carbon nanotubes and graphene with a mass ratio of 1.4:1, and the total amount of conductive agent added is 3.0%. Disperse the mixture at 3000 rpm for 60 minutes in a high-speed disperser to ensure that the carbon nanotubes and graphene are fully depolymerized. Subsequently, add a negative electrode active material, which is composed of artificial graphite and silicon-carbon composite material. The ratio of the negative electrode active material is calculated so that the silicon content in the final surface active material layer is 13%, which is lower than the silicon content in the bottom layer. The total amount of the negative electrode active material accounts for 95% of the total mass of the surface coating dry matter. The surface slurry is obtained.
[0091] Electrode preparation:
[0092] A double-layer extrusion coating machine is used to simultaneously coat the bottom layer and the top layer with slurry on both surfaces of the copper foil. The bottom layer is the part closer to the current collector, while the top layer is the part relatively farther away from the current collector. By precisely controlling the delivery rate of the extrusion pump, the wet film thickness of the bottom and top layers is controlled, so that after drying and rolling, the thickness ratio of the bottom active material layer to the top active material layer is 0.9:1; the total areal density of single-sided coating is 150 g / m².
[0093] The coated electrode sheets were dried in stages at 80°C to remove the solvent. Subsequently, they were cold-pressed at room temperature using a roller press to control the compaction density of the electrode sheets to 1.65 g / cm³.
[0094] The rolled electrode sheets are slit and die-cut to obtain negative electrode sheets of the required size.
[0095] Example 4
[0096] The difference between this embodiment and Embodiment 1 is as follows:
[0097] The Si content in the surface active material layer is the same as that in the bottom active material layer, both being 10%, with all other conditions remaining unchanged.
[0098] Example 5
[0099] The difference between this embodiment and Embodiment 1 is as follows:
[0100] The Si content in the surface active material layer is the same as that in the bottom active material layer, both being 12%, with all other conditions remaining unchanged.
[0101] Example 6
[0102] The difference between this embodiment and Embodiment 1 is as follows:
[0103] The thickness ratio of the bottom active material layer to the top active material layer is 0.7:1, and all other conditions remain unchanged.
[0104] Example 7
[0105] The difference between this embodiment and Embodiment 1 is as follows:
[0106] The thickness ratio of the bottom active material layer to the surface active material layer is 1:1, and all other conditions remain unchanged.
[0107] Comparative Example 1
[0108] The difference between this comparative example and Example 1 is as follows:
[0109] Both the bottom and top layers use Super P as the conductive agent, and the content of the conductive agent in the bottom and top layers is consistent at 1.2%, while other conditions remain unchanged.
[0110] Comparative Example 2
[0111] The difference between this comparative example and Example 1 is as follows:
[0112] Both the bottom and top layers use a composite of carbon nanotubes and graphene as conductive agents, and the content of conductive agents in the bottom and top layers remains the same at 1.8%, while other conditions remain unchanged.
[0113] The performance tests were conducted on the products obtained from the above embodiments and comparative examples. The specific test plan and test results are as follows:
[0114] Positive electrode preparation:
[0115] LiCoO2 was used as the positive electrode active material.
[0116] LiCoO2, conductive agent Super P, and binder polyvinylidene fluoride (PVDF) were mixed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 96.5:1.5:2.0 and then stirred at high speed to prepare a uniform positive electrode slurry.
[0117] The slurry was uniformly coated onto a 15μm thick aluminum foil current collector, and after drying and rolling, a positive electrode sheet was obtained. The areal density of the positive electrode was controlled at 200 g / m², and the compaction density was 3.5 g / cm³.
[0118] Diaphragm:
[0119] A porous ceramic-coated diaphragm of polyethylene (PE) with a thickness of 16μm is used.
[0120] Electrolyte:
[0121] A conventional electrolyte was used, consisting of a 1.0 M LiPF6 mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) in a volume ratio of 1:1:1, with 2% vinylene carbonate (VC) added as a film-forming additive.
[0122] Battery assembly:
[0123] In a dry room (dew point < -40°C) environment, the positive electrode, separator, and negative electrode are stacked in sequence and wound into a battery cell.
[0124] The wound battery cells are placed in an aluminum-plastic packaging shell and vacuum-baked at 75°C for 24 hours to remove moisture.
[0125] Inject the prepared electrolyte and then vacuum seal it.
[0126] After sealing, the battery is first placed to allow the electrolyte to fully impregnate it, and then undergoes its first charge-discharge activation (formation). The formation process is as follows: constant current charging at 0.05C to 4.2V, then constant voltage charging until the current drops to 0.02C, and then constant current discharging at 0.1C to 3.0V. After formation, the battery is vented and resealed.
[0127] Ultimately, a soft-pack lithium-ion battery with a rated capacity of 5Ah was prepared for subsequent performance testing.
[0128] First Coulomb efficiency test:
[0129] Take the newly assembled battery and charge it at a constant current of 0.1C to 4.2V at 25°C. Then switch to constant voltage charging until the current reaches 0.02C, and record the charging capacity (C_charge). Subsequently, discharge it at a constant current of 0.1C to 3.0V, and record the discharging capacity (C_discharge). The initial coulombic efficiency (ICE) = (C_discharge / C_charge) × 100%.
[0130] Cyclic life test:
[0131] The activated battery was charged at 25°C with a constant current and constant voltage of 1C to 4.2V (cutoff current 0.02C), and then discharged with a constant current of 1C to 3.0V. This constitutes one cycle. This process was repeated 500 times. The discharge capacity C_N of the Nth cycle was recorded. Capacity retention = (C_N / C_1) × 100%, where C_1 is the discharge capacity of the first cycle.
[0132] Detailed test results are shown in Table 1.
[0133] Table 1: Product Performance Evaluation Results
[0134]
[0135] The initial coulombic efficiencies of the embodiments were significantly higher than those of the comparative examples. This indicates that the gradient structure of the present invention, especially the highly efficient conductive network on the surface and the optimized interface, effectively promotes the formation of a stable SEI film, reduces irreversible decomposition of the electrolyte during the first cycle, and reduces the loss of active lithium.
[0136] After 500 cycles, the battery capacity retention rate of the embodiment of the present invention is much higher than that of the comparative example. This strongly demonstrates that the "conductivity gradient" and "active material reverse gradient" design successfully manages and buffers the volume expansion of silicon-carbon materials, greatly mitigating capacity decay caused by electrode structure damage (such as coating peeling, pulverization, "band breakage") and interface instability.
[0137] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.
Claims
1. A negative electrode sheet, characterized in that, Including current collectors; The current collector includes a first surface and a second surface that are opposite to each other; A negative electrode active material coating is provided on the first surface and the second surface respectively; The negative electrode active material coating includes a bottom active material layer close to the current collector and a surface active material layer away from the current collector. The conductive agent in the bottom active material layer is Super P; The conductive agents in the surface active material layer are carbon nanotubes and graphene. Furthermore, the conductive agent content in the bottom active material layer is 1.2-2.0%; The conductive agent content in the surface active material layer is 1.8-3.0%; The Si content in the bottom active material layer is 12-15%; and the Si content in the surface active material layer is 10-13%. Furthermore, the Si content in the surface active material layer is lower than the Si content in the bottom active material layer.
2. The negative electrode sheet according to claim 1, characterized in that, The thickness ratio of the bottom active material layer to the top active material layer is 0.8-0.9:
1.
3. The negative electrode sheet according to claim 1, characterized in that, The mass ratio of the carbon nanotubes to the graphene is 1.2-1.4:
1.
4. The negative electrode sheet according to claim 1, characterized in that, The negative electrode active material coating includes a negative electrode active material; the negative electrode active material is a composite of artificial graphite and silicon carbon.
5. A negative electrode sheet according to claim 4, characterized in that, The artificial graphite has a D50 of 13-17 μm; the silicon carbide has a D50 of 6-8 μm.
6. A method for preparing a negative electrode sheet as described in any one of claims 1-5, characterized in that, The specific preparation steps include: The negative electrode active material, conductive agent Super P, binder and solvent water are dispersed evenly to obtain the bottom active material slurry; The negative electrode active material, conductive agent carbon nanotubes and graphene, binder and solvent water are uniformly dispersed to obtain a surface active material slurry; A double-layer extrusion coating device is used to coat the bottom active material slurry and the top active material slurry onto the first and second surfaces of the current collector. After drying, rolling, slitting and die cutting, the negative electrode sheet is obtained.
7. The method for preparing a negative electrode sheet according to claim 6, characterized in that, The specific preparation steps also include: After mixing the adhesive and water, stir and disperse evenly to obtain the adhesive solution; First, the conductive agent Super P is dispersed in the adhesive solution. After the dispersion is uniform, the negative electrode active material is added and the dispersion is continued to be uniform to obtain the bottom active material slurry. Conductive carbon nanotubes and graphene are dispersed in a colloid solution. After the dispersion is uniform, a negative electrode active material is added to obtain a surface active material slurry. A double-layer extrusion coating device is used to coat the bottom active material slurry and the top active material slurry onto the first and second surfaces of the current collector. After drying, rolling, slitting and die-cutting, the negative electrode sheet is obtained; In the bottom active material layer, the mass content of the negative electrode active material is 95%, the content of the conductive agent is 1.2-2.0%, and the remainder is binder; In the surface active material layer, the mass content of negative electrode active material is 95%, the content of conductive agent is 1.8-3.0%, and the balance is binder.
8. A battery cell, characterized in that, It includes a positive electrode, a negative electrode as described in any one of claims 1-5, and a separator sandwiched between the negative electrode and the positive electrode.