Negative electrode sheet and manufacturing method therefor, and lithium-ion battery

Abstract

Disclosed in the present invention are a negative electrode sheet and a manufacturing method therefor, and a lithium-ion battery. The negative electrode sheet comprises a current collector, a first active material layer applied to coat at least one surface of the current collector, and a second active material layer applied to coat the surface of the first active material layer. The first active material layer comprises a silicon-based material, and the second active material layer comprises vertically oriented graphite. According to the present invention, the first active material layer increases the energy density of a battery and avoids the demolding risk of the second active material layer; a second active material accelerates lithium ion transport, thereby adapting to a large volume change in a charging and discharging process, shortening the transport path of electrons in the vertical direction, and improving the electrical conductivity of an electrode and the electron collection efficiency of the battery; and an electrolyte is in full contact with the electrode, thereby increasing active sites, providing a certain buffer space for the expansion of the silicon-based material in the first active material layer, reducing the expansion rate of a battery cell, ensuring the structural stability, and improving the cycle performance and charging capability of the battery cell.

Description

Negative electrode sheet, preparation method thereof, and lithium ion battery Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a negative electrode sheet and a preparation method thereof, and a lithium ion battery. Background Art

[0002] As the market demands higher and higher energy density for batteries, the theoretical capacity of a simple graphite negative electrode is only 372 mAh g -1 , which can no longer meet market demand. Developing negative electrode materials with high specific capacity is an effective strategy, among which silicon materials have a high theoretical capacity of 4200mAh g -1 However, the large volume expansion of silicon (400%) leads to silicon particle cracking, electrode expansion, SEI destruction, low Coulombic efficiency, and rapid capacity decay, posing severe challenges to commercial applications.

[0003] Therefore, in addition to optimizing silicon material modification and compounding strategies such as coating, nano-crystallization, and composites with other materials, another approach is to address the volume expansion of silicon-based electrodes by optimizing battery cell production processes. Selecting high-performance silicon-based materials, combined with optimized battery cell production processes, can effectively address the adverse effects of silicon volume expansion that can lead to structural damage to the electrode and consequently, poor battery cell performance.

[0004] In the existing technology, graphite is placed in the first layer to achieve fast charging, and silicon particles are placed in the second layer to improve energy density; and the size of silicon-based particles and graphite particles is designed to suppress the volume expansion of the battery cell. However, these methods cannot effectively solve the problem of increased battery cell thickness caused by the volume expansion of silicon. In addition, there is currently technology that designs for graphite orientation. By designing vertically oriented graphite, the ion transfer rate is accelerated, thereby improving the charging capacity. However, after coating a single layer of vertically oriented graphite with the current collector, it is easy to have demolding problems. Therefore, some people increase and reduce the risk of demolding by coating conventional graphite on the bottom layer and vertically oriented graphite on the upper layer, but this method does not further increase the energy density of the battery cell.

[0005] Summary of the Invention

[0006] The purpose of the present invention is: in order to reduce the risk of demolding and increase the energy density of the battery cell, the present invention designs a double-layer coating process, the first active material layer serves as the bottom layer, including silicon-based granular material, the second active material layer serves as the surface layer, including vertically oriented graphite, and the gaps between the vertical graphite on the surface layer provide expansion buffer space for the expansion of the silicon particles in the bottom layer.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A negative electrode sheet includes a current collector, a first active material layer coated on at least one surface of the current collector, and a second active material layer coated on the surface of the first active material layer, wherein the first active material layer includes a silicon-based material and the second active material layer includes vertically oriented graphite.

[0009] Preferably, the viscosity η1 of the first active material layer and the viscosity η2 of the second active material layer satisfy the relationship: 0.5≤η1 / η2≤2.5.

[0010] Preferably, the viscosity η1 of the first active material layer is 3000 to 7000 mPa·s; and the viscosity η2 of the second active material layer is 3000 to 5000 mPa·s.

[0011] Preferably, the mass ratio of the silicon-based material to the vertically oriented graphite is (2-6):(4-8); and the silicon content in the silicon-based material is 2-15%.

[0012] Preferably, the current collector includes at least one of copper foil, carbon paper, and nickel foil; and the thickness of the current collector is 3 μm to 20 μm.

[0013] Preferably, the silicon-based material includes at least one of silicon particles, silicon nanowires, and silicon-carbon skeleton composite materials; more preferably, silicon particles.

[0014] Preferably, the vertically oriented graphite includes at least one of magnetic graphite and ordinary graphite magnetized by magnetic fluid; wherein the magnetic fluid includes at least one of ferrosoferric oxide, ferrous oxide and ferric oxide.

[0015] The present invention also provides a method for preparing the above-mentioned negative electrode sheet, comprising the following steps:

[0016] (1) mixing a silicon-based material, a binder, a thickener, and a conductive agent, adding a solvent, and stirring uniformly to obtain a first active material layer slurry;

[0017] (2) mixing the magnetic graphite, the binder, and the thickener, adding a solvent, and stirring uniformly to obtain a second active material layer slurry;

[0018] (3) The first active material layer slurry and the second active material layer slurry are coated on the current collector and dried to obtain a negative electrode sheet.

[0019] Preferably, the mass ratio of the silicon-based material, the binder, the thickener and the conductive agent is (92-97%): (2.0-3.5%): (0.1-0.5%): (0.05-0.2%).

[0020] Preferably, the mass ratio of the magnetic graphite, the binder and the thickener is (95% to 98%): (0.6 to 1.5%): (1.0 to 1.5%).

[0021] Preferably, the binder includes at least one of PPA and SBR; the thickener includes at least one of CMC, PAA, PAN and polyacrylate; the conductive agent includes at least one of carbon nanotubes and graphene; the solvent is an aqueous solvent or an oil solvent, and the oil solvent includes at least one of N-methylpyrrolidone, N,N-dimethylformamide and dimethyl sulfoxide.

[0022] Preferably, in step (3), the drying bellows is provided with a magnetic field emission device for converting part or all of the magnetic graphite material in the second active material layer slurry into vertically oriented graphite and vertically distributing it on the current collector.

[0023] Preferably, in step (3), the drying temperature is 60°C to 80°C.

[0024] The present invention also provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator between the positive electrode sheet and the negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet described above.

[0025] Compared with the prior art, the present invention has at least the following beneficial effects:

[0026] (1) In order to reduce the risk of demolding and increase the energy density of the battery cell, the present invention designs a double-layer coating process. The first active material layer serves as the bottom layer and includes a silicon-based granular material. The second active material layer serves as the surface layer and includes vertically oriented graphite. The gaps between the vertical graphite on the surface layer provide expansion buffer space for the expansion of the silicon particles in the bottom layer.

[0027] (2) The double-layer coating structure and magnetic field deflection process of the negative electrode sheet of the present invention, the bottom silicon-based material layer increases the energy density of the battery and avoids the risk of demolding of the vertically oriented graphite on the surface, while the vertically oriented graphite layer on the surface accelerates the lithium ion transmission speed, adapts to the large volume changes during charging and discharging, shortens the electron transmission path in the vertical direction, improves the electrode conductivity and the electron collection efficiency of the battery, and ensures sufficient contact between the electrolyte and the electrode, increasing the active sites and providing a certain buffer space for the expansion of the bottom silicon-based material, which is beneficial to reducing the expansion rate of the battery cell. At the same time, it also ensures structural stability, which is beneficial to improving the cycle performance and charging capacity of the battery cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG1 is a schematic structural diagram of a negative electrode sheet according to an embodiment of the present invention;

[0029] Among them, 1-current collector, 2-silicon-based material, 3-vertically oriented graphite. DETAILED DESCRIPTION

[0030] To make the technical solutions and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0031] According to the first aspect of the present invention, the present invention provides a negative electrode sheet, including a current collector, a first active material layer coated on at least one surface of the current collector, and a second active material layer coated on the surface of the first active material layer, the first active material layer includes a silicon-based material, and the second active material layer includes vertically oriented graphite.

[0032] In one embodiment according to the present invention, the viscosity η1 of the first active material layer and the viscosity η2 of the second active material layer satisfy the relationship: 0.5≤η1 / η2≤2.5; η1 / η2 can specifically be 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5; when the viscosity η1 of the first active material layer and the viscosity η2 of the second active material layer are controlled within the above range, the adhesion between the first active material layer and the second active material layer can be ensured to be better, so that the electrode is not prone to stratification, and the battery can form a complete path for electrons and ions, thereby solving the problem of battery performance deterioration due to electrode stratification, which affects the capacity retention rate of the battery during cycling.

[0033] In one embodiment according to the present invention, the viscosity η1 of the first active material layer is 3000~7000mPa·s, specifically 3000mPa·s, 3500mPa·s, 4000mPa·s, 4500mPa·s, 5000mPa·s, 5500mPa·s, 6000mPa·s, 6500mPa·s, and 7000mPa·s; when the viscosity η1 of the first active material layer is controlled within the above range, a uniform and fixed coating can be formed to ensure that the battery has good electrochemical properties; if the viscosity is too small, the fluidity will be too large and a uniform and fixed coating cannot be formed; if the viscosity is too large, the fluidity will lack, and it is also easy to make the slurry uneven, affecting the electrochemical properties of the battery cell.

[0034] In one embodiment of the present invention, the viscosity η2 of the second active material layer is 3000~5000mPa·s, specifically 3000mPa·s, 3200mPa·s, 3500mPa·s, 3800mPa·s, 4000mPa·s, 4200mPa·s, 4500mPa·s, 4800mPa·s, and 5000mPa·s; when the viscosity η2 of the second active material layer is controlled within the above range, it can be better bonded to the first active material layer, making it difficult for the two coatings to be stratified, so that the battery can form a complete path for electrons and ions, thereby solving the problem of battery performance deterioration due to electrode stratification, affecting the capacity retention rate of the battery during cycling.

[0035] In one embodiment of the present invention, the mass ratio of silicon-based material to vertically oriented graphite is (2-6):(4-8), specifically 2:8, 3:7, 4:6, 5:5, 6:4. When the mass ratio of silicon-based material to vertically oriented graphite is controlled within the above range, while ensuring that the battery has a high energy density, the vertically oriented graphite can also protect the underlying silicon-based material, effectively inhibiting the volume expansion caused by the silicon-based material; if the content of silicon-based material is too much, the volume expansion of the battery will increase seriously, thereby deteriorating the battery and affecting the performance of the battery; if the content of silicon-based material is too little, the battery cannot be guaranteed to have a high energy density.

[0036] In one embodiment of the present invention, the silicon content in the silicon-based material is 2% to 15%, specifically 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, and 15%. When the silicon content in the silicon-based material is controlled within the above range, the second active material layer can effectively control the volume expansion of the silicon-based material, thereby ensuring that the battery has better performance while having a higher energy density.

[0037] In one embodiment of the present invention, the current collector includes at least one of copper foil, carbon paper, and nickel foil; the thickness of the current collector is 3μm to 20μm, specifically 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, and 20μm.

[0038] In one embodiment of the present invention, the silicon-based material includes at least one of silicon particles, silicon nanowires, and silicon-carbon skeleton composite materials; preferably, silicon particles.

[0039] In one embodiment of the present invention, the vertically oriented graphite includes at least one of magnetic graphite and ordinary graphite magnetized by a magnetic fluid; wherein the magnetic fluid includes at least one of ferrosoferric oxide, ferrous oxide, and ferric oxide.

[0040] According to a second aspect of the present invention, the present invention further provides a method for preparing the above-mentioned negative electrode sheet, comprising the following steps:

[0041] (1) mixing a silicon-based material, a binder, a thickener, and a conductive agent, adding a solvent, and stirring uniformly to obtain a first active material layer slurry;

[0042] (2) mixing the magnetic graphite, the binder, and the thickener, adding a solvent, and stirring uniformly to obtain a second active material layer slurry;

[0043] (3) The first active material layer slurry and the second active material layer slurry are coated on the current collector and dried to obtain a negative electrode sheet.

[0044] In order to reduce the risk of demolding and increase the energy density of the battery core, the present invention designs a double-layer coating process. The bottom coating is a silicon-based granular material slurry layer, and the surface coating is a vertically oriented graphite slurry layer. The gaps between the vertical graphite on the surface provide expansion buffer space for the expansion of the bottom silicon particles.

[0045] The double-layer coating structure and magnetic field deflection process of the negative electrode sheet of the present invention, the bottom silicon-based material layer increases the energy density of the battery and avoids the risk of demolding of the vertically oriented graphite on the surface, while the vertically oriented graphite layer on the surface accelerates the lithium ion transmission speed, adapts to the large volume changes during charging and discharging, shortens the electron transmission path in the vertical direction, improves the electrode conductivity and the electron collection efficiency of the battery, and the electrolyte and the electrode are in full contact, which increases the active sites and provides a certain buffer space for the expansion of the bottom silicon-based material, which is beneficial to reducing the expansion rate of the battery cell, while also ensuring structural stability, which is beneficial to improving the cycle performance and charging capacity of the battery cell.

[0046] In one embodiment of the present invention, the mass ratio of the silicon-based material, the binder, the thickener, and the conductive agent is (92-97%): (2.0-3.5%): (0.1-0.5%): (0.05-0.2%); preferably 96.9%: 2.7%: 0.3%: 0.1%;

[0047] In one embodiment of the present invention, the mass ratio of magnetic graphite, binder and thickener is (95% to 98%): (0.6 to 1.5%): (1.0 to 1.5%); preferably 97.7%: 1.3%: 1.0%.

[0048] In one embodiment of the present invention, the binder includes at least one of PPA and SBR; the thickener includes at least one of CMC, PAA, PAN and polyacrylate; the conductive agent includes at least one of carbon nanotubes and graphene; the solvent is an aqueous solvent or an oil solvent, and the oil solvent includes at least one of N-methylpyrrolidone, N,N-dimethylformamide and dimethyl sulfoxide.

[0049] In one embodiment of the present invention, in step (3), a magnetic field emission device is provided in the bellows during drying, for converting part or all of the magnetic graphite material in the second active material layer slurry into vertically oriented graphite and vertically distributing it on the current collector.

[0050] In one embodiment of the present invention, in step (3), the drying temperature is 60°C to 80°C, specifically 60°C, 61°C, 62°C, 63°C, 64°C, 65°C, 66°C, 67°C, 68°C, 69°C, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, or 80°C.

[0051] According to the third aspect of the present invention, the present invention further provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator between the positive electrode sheet and the negative electrode sheet, wherein the negative electrode sheet is the negative electrode sheet described above.

[0052] The positive active material in the positive electrode sheet may be, but is not limited to, LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiMnPO4, LiFePO4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2、LiNi 0.5 Co 0.2 Mn 0.3 O2、LiNi 0.6 Co 0.2 Mn 0.2 O2、LiNi 0.8 Co 0.1 Mn 0.1 O2、LiNi 0.6 Co 0.1 Mn 0.3 O2、LiNi 0.85 Co 0.15 Al 0.05 At least one of O2.

[0053] The diaphragm may be at least one of, but not limited to, polyethylene, polypropylene, polyvinylidene fluoride, and multilayer composite films thereof;

[0054] The electrolyte comprises an electrolyte salt and an organic solvent, wherein the specific types and compositions of the electrolyte salt and the organic solvent are not subject to specific restrictions, and include positive electrode film-forming additives, negative electrode film-forming additives, and cycle-improving and low-temperature additives, etc.;

[0055] The present invention will be further described in detail below through specific embodiments, but the embodiments of the present invention are not limited thereto.

[0056] Example 1

[0057] This embodiment provides a negative electrode sheet, including a current collector 1, a first active material layer coated on at least one surface of the current collector 1, and a second active material layer coated on the surface of the first active material layer, wherein the first active material layer includes a silicon-based material 2, and the second active material layer includes vertically oriented graphite 3.

[0058] This embodiment also provides a method for preparing the above-mentioned negative electrode sheet, comprising the following steps:

[0059] (1) Preparation of the first active material layer slurry: silicon-based active material: carbon nanotubes: PPA: SBR: CMC = 96.9%: 0.1: 2.4: 0.3: 0.3, mixed evenly, added with deionized water, and stirred evenly to obtain the first active material layer slurry, i.e., the bottom coating slurry; wherein the silicon doping amount in the silicon-based active material is 3%; and the viscosity η1 of the slurry is 3000 mPa·s;

[0060] (2) Preparation of the second active material layer slurry: Graphite active material: CMC: SBR = 97.7%: 1.3%: 1.0% were mixed evenly, deionized water was added, and the mixture was stirred evenly to obtain the second active material layer slurry, i.e., the surface coating slurry; wherein the viscosity η2 of the slurry was 3000 mPa·s;

[0061] (3) Slurry coating: The first active material layer slurry and the second active material layer slurry are coated on a copper foil with a thickness of 5 μm in a mass ratio of 2:8, and dried in an oven at 60°C to obtain a negative electrode sheet; wherein, the oven is equipped with a magnetic field reflection device, and during the drying process, the surface magnetic graphite device is acted upon by the magnetic field, becoming vertically oriented graphite and being vertically distributed on the copper foil.

[0062] Example 2

[0063] The difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of the first active material layer slurry to the second active material layer slurry is 3:7; the rest is the same as embodiment 1 and will not be repeated here.

[0064] Example 3

[0065] The difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of the first active material layer slurry to the second active material layer slurry is 4:6; the rest is the same as embodiment 1 and will not be repeated here.

[0066] Example 4

[0067] The difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of the first active material layer slurry to the second active material layer slurry is 5:5; the rest is the same as embodiment 1 and will not be repeated here.

[0068] Example 5

[0069] The difference between this embodiment and embodiment 1 is that in step (3), the mass ratio of the first active material layer slurry to the second active material layer slurry is 6:4; the rest is the same as embodiment 1 and will not be repeated here.

[0070] Example 6

[0071] The difference between this embodiment and embodiment 1 is that, in step (1), the doping amount of silicon in the silicon-based active material is 5%; the rest is the same as embodiment 1 and will not be repeated here.

[0072] Example 7

[0073] The difference between this embodiment and embodiment 2 is that in step (1), the doping amount of silicon in the silicon-based active material is 5%; the rest is the same as embodiment 2 and will not be repeated here.

[0074] Example 8

[0075] The difference between this embodiment and embodiment 3 is that in step (1), the doping amount of silicon in the silicon-based active material is 5%; the rest is the same as embodiment 3 and will not be repeated here.

[0076] Example 9

[0077] The difference between this embodiment and embodiment 4 is that in step (1), the doping amount of silicon in the silicon-based active material is 5%; the rest is the same as embodiment 4 and will not be repeated here.

[0078] Example 10

[0079] The difference between this embodiment and embodiment 5 is that in step (1), the doping amount of silicon in the silicon-based active material is 5%; the rest is the same as embodiment 5 and will not be repeated here.

[0080] Comparative Example 1

[0081] The difference between this comparative example and Example 1 is that in step (3), the mass ratio of the first active material layer slurry to the second active material layer slurry is 7:3; the rest is the same as Example 1 and will not be repeated here.

[0082] Comparative Example 2

[0083] The difference between this comparative example and Example 1 is that in step (3), the mass ratio of the first active material layer slurry to the second active material layer slurry is 1:9; the rest is the same as Example 1 and will not be repeated here.

[0084] Comparative Example 3

[0085] The difference between this comparative example and Example 1 is that the viscosity η1 of the first active material layer slurry is 2450 mPa·s; the viscosity η2 of the second active material layer slurry is 5000 mPa·s, that is, η1 / η2=0.49; the rest is the same as Example 1 and will not be repeated here.

[0086] Comparative Example 4

[0087] The difference between this comparative example and Example 1 is that the viscosity η1 of the first active material layer slurry is 7600 mPa·s, that is, η1 / η2=2.53; the rest is the same as Example 1 and will not be repeated here.

[0088] The negative electrode sheets and positive electrode sheets prepared in Examples 1-10 and Comparative Examples 1-4 were processed into finished batteries through rolling, slitting, tab welding, winding, packaging, baking, liquid injection, formation, degassing and other post-processing.

[0089] The prepared battery was subjected to performance testing, and the test results are shown in Table 1:

[0090] Table 1

[0091] As can be seen from the test results in Table 1, the present invention overcomes the problem of easy demolding of the vertically oriented graphite coating by using an underlying silicon-based coating. A comparison of the test results of Examples 1-5 shows that the more underlying silicon-based coating, the lower the capacity retention rate and the higher the volume expansion of the battery cell. Furthermore, a comparison of the test results of Examples 1-5 with Comparative Examples 1-2 shows that when the mass ratio of the underlying silicon-based coating to the vertically oriented graphite coating is not within the range of (2-6):(4-8), the battery capacity retention rate in Comparative Example 1 is less than 80% and the expansion rate is greater than 10%. This is because excessive silicon content can lead to a significant increase in the volume expansion of the battery, thereby deteriorating the battery and affecting its performance. While the battery capacity retention rate in Comparative Example 2 can reach 80% and the expansion is relatively small, the silicon-based material content in the silicon-based coating is too low, making it impossible to ensure that the battery has a high energy density.

[0092] It can be seen from the test results of Examples 1-5 and the corresponding Examples 6-10 that the higher the silicon doping content of the silicon-based coating at the bottom of the battery cell, the lower the capacity retention rate of the battery cell and the greater the volume expansion of the battery cell; it can be seen from the test results of Example 1 and Comparative Examples 3-4 that when the viscosity η1 of the first active material layer slurry and the viscosity η2 of the second active material layer slurry do not satisfy the relationship 0.5≤η1 / η2≤2.5, the capacity retention rate of the battery after cycling is lower than 80%. The reason is that when the viscosity η1 of the first active material layer slurry and the viscosity η2 of the second active material layer slurry differ too much, the first active material layer and the second active material layer cannot be well bonded, so that the two coatings are prone to delamination, so that the battery cannot form a complete path for electrons and ions, resulting in delamination of the electrode and deterioration of the battery performance, affecting the capacity retention rate of the battery during cycling; further, when the viscosity η1 of the first active material layer slurry is too large or too small, the fluidity of the first active material layer slurry is too small or too large to form a uniform and fixed coating, thereby affecting the performance of the battery.

[0093] In summary, the double-layer coating structure and magnetic field deflection process of the negative electrode sheet of the present invention, the bottom silicon-based material layer increases the energy density of the battery and avoids the risk of demolding of the vertically oriented graphite on the surface, while the vertically oriented graphite layer on the surface accelerates the transmission speed of lithium ions, adapts to the large volume changes during charging and discharging, shortens the transmission path of electrons in the vertical direction, improves the electrode conductivity and the electron collection efficiency of the battery, and ensures sufficient contact between the electrolyte and the electrode, increases the active sites, and provides a certain buffer space for the expansion of the bottom silicon-based material, which is beneficial to reducing the expansion rate of the battery cell. At the same time, it also ensures structural stability, which is beneficial to improving the cycle performance and charging capacity of the battery cell.

[0094] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention fall within the scope of protection of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A negative electrode sheet, characterized in that: It includes a current collector, a first active material layer coated on at least one surface of the current collector, and a second active material layer coated on the surface of the first active material layer, wherein the first active material layer includes a silicon-based material, and the second active material layer includes vertically oriented graphite.

2. The negative electrode sheet according to claim 1, characterized in that: The viscosity η1 of the first active material layer and the viscosity η2 of the second active material layer satisfy the relationship: 0.5≤η1 / η2≤2.

5.

3. The negative electrode sheet according to claim 1 or 2, characterized in that: The viscosity η1 of the first active material layer is 3000 to 7000 mPa·s; the viscosity η2 of the second active material layer is 3000 to 5000 mPa·s.

4. The negative electrode sheet according to claim 1, characterized in that: The mass ratio of the silicon-based material to the vertically oriented graphite is (2-6):(4-8); the content of silicon in the silicon-based material is 2-15%.

5. The negative electrode sheet according to claim 1, characterized in that: The current collector includes at least one of copper foil, carbon paper, and nickel foil; The silicon-based material includes at least one of silicon particles, silicon nanowires, and silicon-carbon skeleton composite materials; The vertically oriented graphite includes at least one of magnetic graphite and ordinary graphite magnetized by magnetic fluid; wherein the magnetic fluid includes at least one of ferroferric oxide, ferrous oxide and ferric oxide.

6. A method for preparing a negative electrode sheet according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) mixing a silicon-based material, a binder, a thickener, and a conductive agent, adding a solvent, and stirring uniformly to obtain a first active material layer slurry; (2) mixing magnetic graphite, a binder, and a thickener, adding a solvent, and stirring evenly to obtain a second active material layer slurry; (3) The first active material layer slurry and the second active material layer slurry are coated on the current collector and dried to obtain a negative electrode sheet.

7. The method for preparing the negative electrode sheet according to claim 6, characterized in that: The mass ratio of the silicon-based material, binder, thickener and conductive agent is (92-97%): (2.0-3.5%): (0.1-0.5%): (0.05-0.2%); the mass ratio of the magnetic graphite, binder and thickener is (95%-98%): (0.6-1.5%): (1.0-1.5%).

8. The method for preparing the negative electrode sheet according to claim 6, characterized in that: In step (3), the drying wind box is provided with a magnetic field emission device for converting part or all of the magnetic graphite material in the second active material layer slurry into vertically oriented graphite and vertically distributing it on the current collector.

9. The method for preparing the negative electrode sheet according to claim 6, characterized in that: In step (3), the drying temperature is 60°C to 80°C.

10. A lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator between the positive electrode sheet and the negative electrode sheet, characterized in that: The negative electrode sheet is the negative electrode sheet described in any one of claims 1 to 5.

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