Multilayer silicon gradient negative plate and preparation method thereof

Abstract

The invention relates to the technical field of batteries, in particular to a multilayer silicon gradient negative plate which comprises a current collector, a first active layer, a second active layer and a third active layer, the silicon material content of the first active layer is smaller than that of the second active layer, and the silicon material content of the second active layer is smaller than that of the third active layer. The compaction density of the third active layer is smaller than the compaction density of the second active layer, the compaction density of the second active layer is smaller than the compaction density of the first active layer, and the contents of the silicon materials of different layers are set in a gradient manner to control the expansion of the active substance layers at different positions; the first active layer close to the current collector is provided with less silicon material so as to meet the cohesiveness requirement between the active substance layer and the current collector, and the third active layer far away from the first active layer is provided with more silicon material so as to meet the energy density requirement. And different layers are provided with different compaction densities, so that buffer spaces for silicon material expansion can be reserved for different layers.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a multilayer silicon gradient negative electrode sheet and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries are a kind of secondary battery system with two different lithium intercalation compounds that can reversibly deintercalate lithium ions as positive and negative electrodes. With the advantages of high specific capacity, long cycle life, small self-discharge, etc., it is widely used in mobile phones, portable computers, video cameras, cameras and other fields. Large-capacity lithium ion batteries have been used in electric vehicles. In addition to this, the application of lithium ion batteries is also expanding to artificial satellites, aerospace, energy storage and other fields. Lithium ion batteries may become one of the main power sources in the 21st century.

[0003] However, there are still problems such as insufficient battery endurance and low energy density. With the rapid development of society, people are more pursuing lithium ion batteries that combine high energy density and fast charging. Carbon materials have small volume expansion and long cycle life during cycling, and the theoretical specific capacity of silicon is higher than that of carbon materials, but the volume expansion is large and the cycle life is low. Therefore, people will use carbon materials and silicon materials in combination to combine the advantages of carbon materials and silicon materials. However, when adding a large amount of silicon material, the expansion of silicon material can easily lead to a decrease in adhesion between active material and current collector, resulting in detachment of the active material layer from the current collector, leading to negative electrode failure. When adding a small amount of silicon material, the energy density cannot be fully improved.

[0004] Therefore, it is urgent to invent a multilayer silicon gradient negative electrode sheet to solve the above technical problems. SUMMARY

[0005] One of the purposes of the present application is to provide a multilayer silicon gradient negative electrode sheet to solve the problems of the prior art. The multilayer silicon gradient negative electrode sheet can reduce the island effect caused by particle breakage and loss of conductive network due to expansion stress during cycling, has good cycle performance, and can ensure the adhesion between the current collector and the active material layer. At the same time, a high proportion of silicon material content can be set to have high energy density.

[0006] To solve the above technical problems, the present application adopts the following technical solutions:

[0007] A multi-layer silicon gradient negative electrode sheet is provided, comprising: a current collector, a first active layer, a second active layer, and a third active layer, the first active layer is coated on the surface of the current collector, the second active layer is coated on the surface of the first active layer away from the current collector, and the third active layer is coated on the surface of the second active layer away from the current collector, the first active layer, the second active layer and the third active layer all contain silicon material and carbon material, wherein the silicon material content of the first active layer is less than that of the second active layer, the silicon material content of the second active layer is less than that of the third active layer, the compaction density of the third active layer is less than that of the second active layer, and the compaction density of the second active layer is less than that of the first active layer.

[0008] Further, the ratio of the silicon material content between the first active layer, the second active layer and the third active layer is 0.5-10:5-20:1-40.

[0009] Further, the ratio of carbon material and silicon material in the first active layer is 85-95:0.5-10, the ratio of carbon material and silicon material in the second active layer is 75-90:5-20, and the ratio of carbon material and silicon material in the third active layer is 55-85:10-40.

[0010] Further, the ratio of the compaction density of the first active layer, the second active layer and the third active layer is 1.65-1.85:1.55-1.70:0.95-1.20.

[0011] Further, the ratio of the area density of the first active layer, the second active layer and the third active layer is 35-45:35-45:10-25.

[0012] Further, the area density of the third active layer is 10-25 g / cm2.

[0013] Further, the carbon material is at least one of soft carbon, hard carbon, artificial graphite, natural graphite, and composite graphite.

[0014] Further, the carbon material of the first active layer is natural graphite, the carbon material of the second active layer is artificial graphite, and the carbon material of the third active layer is porous carbon.

[0015] Further, the silicon material is at least one of silicon monoxide, composite silicon carbon, and vapor deposition silicon carbon.

[0016] The beneficial effects of the present application are that: the present application sets multiple active material layers, and sets the content gradient of silicon materials in different layers, controls the expansion of the active material layer at different positions, sets less silicon material for the first active layer close to the current collector to meet the bonding requirement between the active material layer and the current collector, sets more silicon material for the third active layer far away to meet the energy density requirement, and sets different compaction densities for different layers to meet the buffer space reserved for the expansion of silicon material in different layers, thereby reducing the island effect caused by particle breakage and loss of conductive network due to expansion stress in the cycle process, reducing irreversible capacity loss, and realizing long cycle.

[0017] The second purpose of the present application is to provide a preparation method of the above-mentioned multi-layer silicon gradient negative electrode sheet, respectively preparing active material slurries of the first active layer, the second active layer and the third active layer, coating the first active layer slurry on the current collector according to a certain surface density, drying, and then rolling according to a certain compaction density to obtain the electrode sheet A, coating the second active layer slurry on the electrode sheet A according to a certain surface density, drying, and then rolling according to a certain compaction density to obtain the electrode sheet B, coating the third active layer slurry on the electrode sheet B according to a certain surface density, drying, and then rolling according to a certain compaction density to obtain the multi-layer silicon gradient negative electrode sheet. BRIEF DESCRIPTION OF DRAWINGS

[0018] The drawings described herein are used to provide further understanding of the present application, and form a part of the present application. The schematic embodiments of the present application and the description thereof are used to explain the present application, and do not constitute improper limitation on the present application. In the drawings:

[0019] Figure 1 It is a structural schematic diagram of the negative electrode sheet of the present application.

[0020] Among them: 1-current collector; 2-first active layer; 3-second active layer; 4-third active layer. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meanings as understood by those skilled in the art in the technical field of the present application. The terms used in the specification of the present application are only for the purpose of describing the specific embodiments of the present application, and are not intended to limit the present application.

[0022] The application will be further described in detail below with reference to the accompanying drawings, but not as a limitation to the application.

[0023] The first aspect of the present application provides a multi-layer silicon gradient negative electrode sheet, comprising: a current collector 1, a first active layer 2, a second active layer 3 and a third active layer 4, the first active layer 2 is coated on the surface of the current collector 1, the second active layer 3 is coated on the surface of the first active layer 2 away from the current collector 1, and the third active layer 4 is coated on the surface of the second active layer 3 away from the current collector 1, the first active layer 2, the second active layer 3 and the third active layer 4 all contain silicon material and carbon material, wherein the silicon material content of the first active layer 2 is less than that of the second active layer 3, the silicon material content of the second active layer 3 is less than that of the third active layer 4, the compaction density of the third active layer 4 is less than that of the second active layer 3, and the compaction density of the second active layer 3 is less than that of the first active layer 2.

[0024] The present application uses silicon material and carbon material in combination, wherein the carbon material functions as a buffer medium, and when the silicon material expands, the carbon material elastically deforms to absorb part of the stress, and at the same time, the carbon material has a large porosity and can slip when subjected to expansion stress to prevent stress concentration, and the provision of the carbon material provides porosity and forms a conductive network to improve the diffusion efficiency of electrolyte and lithium ions and improve the fast-charging performance of the battery. By providing the first active layer 2 closest to the current collector 1 with the smallest silicon material content, the over-expansion of the silicon material is prevented, which can cause the adhesion between the current collector 1 and the first active layer 2 to fail, thereby causing the negative electrode to fail. By providing the outermost third active layer 4 with more silicon material, the energy density of the battery is improved. The second active layer 3 serves as a buffer to prevent the first active layer 2 and the third active layer 4 from being separated due to the large difference in silicon material content and causing uneven expansion. By controlling the compaction density gradient of the first active layer 2, the second active layer 3 and the third active layer 4, the expansion of each active layer is matched, and the third active layer 4 with more silicon material is provided with a smaller compaction density to provide more buffer space, thereby reducing the island effect caused by the particle breakage and loss of conductive network due to the expansion stress during the cycle process, reducing the irreversible capacity loss, and achieving long cycle.

[0025] Preferably, the ratio of the silicon material content between the first active layer 2, the second active layer 3 and the third active layer 4 is 0.5-10:5-20:1-40, which can form a stable gradient expansion and prevent the interface from being separated due to a too large expansion difference. Preferably, the silicon material content of the first active layer 2 is 0.5%-10%.

[0026] Preferably, the ratio of carbon material to silicon material in the first active layer 2 is 85-95:0.5-10, the ratio of carbon material to silicon material in the second active layer 3 is 75-90:5-20, and the ratio of carbon material to silicon material in the third active layer 4 is 55-85:10-40. When the ratio of carbon material to silicon material is controlled within the above range, the expansion requirements of different active layers can be met.

[0027] Preferably, the compaction density ratio of the first active layer 2, the second active layer 3, and the third active layer 4 is 1.65–1.85: 1.55–1.70: 0.95–1.20. Controlling the compaction density ratio helps to form a reasonable gradient of lithium-ion and electrolyte transport channels, which can ensure the buffering of silicon material expansion while satisfying the lithium-ion transport of each active layer, thus ensuring the fast charging performance of the battery. Preferably, the compaction density of the first active layer 2 is 1.65 g / cm3–1.85 g / cm3.

[0028] Preferably, the areal density ratio of the first active layer 2, the second active layer 3, and the third active layer 4 is 35-45:35-45:10-25. When the compaction density and areal density are controlled at the same time, the thickness ratio is also controlled. By controlling the appropriate thickness ratio, the mutual influence of expansion between different layers can be controlled so as not to be too great.

[0029] Preferably, the areal density of the third active layer 4 is 10-25 g / cm2.

[0030] Preferably, the carbon material is at least one of soft carbon, hard carbon, artificial graphite, natural graphite, and composite graphite.

[0031] Preferably, the carbon material of the first active layer 2 is natural graphite, the carbon material of the second active layer 3 is artificial graphite, and the carbon material of the third active layer 4 is porous carbon.

[0032] Natural graphite has higher specific capacity and compaction, but it requires better compatibility with electrolytes and has poor cycle performance. Artificial graphite has advantages over natural graphite in rate capability and cycle performance. Porous carbon can provide fast ion transport channels, but its compaction is lower, resulting in less expansion of the first active layer 2. Using natural graphite as the carbon material for the first active layer 2 can meet the buffering requirements for silicon material expansion while achieving a higher compaction density to improve battery energy density. Using porous carbon as the third active layer 4 provides more lithium-ion transport channels, allowing lithium ions to be transported between the first active layer 2 and the second active layer 3. Furthermore, the higher porosity of porous carbon is more conducive to providing buffer space for more silicon material. Various carbon materials can be selected for porous carbon, as long as the porosity is greater than 50%.

[0033] Preferably, the silicon material is at least one of silicon suboxide, composite silicon-carbon, and vapor-deposited silicon-carbon.

[0034] Preferably, the adhesive can be two or three of PAA, CMC, and SBR.

[0035] Preferably, the design of the multilayer silicon gradient anode of this application is suitable for electrodes with a thick-coated active material layer, because when the thickness is within...

[0036] In some embodiments, the cathode material refers only to ternary cathode materials.

[0037] The second objective of this invention is to provide a method for preparing the aforementioned multilayer silicon gradient anode, characterized in that: active material slurries for a first active layer 2, a second active layer 3, and a third active layer 4 are prepared respectively; the first active layer 2 slurry is coated onto a current collector 1 at a certain areal density; after drying, it is rolled at a certain compaction density to obtain electrode A; the second active layer 3 slurry is coated onto electrode A at a certain areal density; after drying, it is rolled at a certain compaction density to obtain electrode B; the third active layer 4 slurry is coated onto electrode B at a certain areal density; after drying, it is rolled at a certain compaction density to obtain the multilayer silicon gradient anode.

[0038] Example 1

[0039] A silicon multilayer gradient negative electrode sheet and its lithium-ion battery preparation method include the following steps:

[0040] 1. Weigh the materials according to the weight ratio of natural graphite: fumed silicon carbide: SP: SWCNT: PAA: CMC: SBR = 90: 5.5: 1.0: 0.1: 2.0: 0.5: 0.9, add a certain proportion of pure water, and put them into a mixing tank in sequence. Mechanically stir for 4 hours until evenly dispersed, and then slowly stir and vacuum defoam for 30 minutes to obtain slurry A.

[0041] 2. Mix slurry A according to a surface density of 45 g / cm³. 2 After coating onto bright copper foil and drying in a forced-air environment at 100℃, the coating density is 1.7-1.8 g / cm³. 3 The compaction density is rolled to obtain electrode A.

[0042] 3. Weigh the materials according to the weight ratio of artificial graphite: fumed silicon carbide: SP: SWCNT: PAA: CMC: SBR = 85:10:1.0:0.1:2.5:0.5:0.9, add a certain proportion of pure water, and put them into a mixing tank in sequence. Mechanically stir for 4 hours until evenly dispersed, and then slowly stir and vacuum defoam for 30 minutes to obtain slurry B.

[0043] 4. Mix slurry B according to a surface density of 45 g / cm³. 2After coating onto electrode A and drying in a forced-air environment at 100℃, the coating is applied at a ratio of 1.65–1.70 g / cm³. 3 The compaction density is rolled to obtain electrode B.

[0044] 5. Weigh the materials according to the weight ratio of porous carbon: gaseous silicon carbon: SP:SWCNT:PAA:CMC:SBR = 74:20:1.0:0.1:3.5:0.5:0.9, add a certain proportion of pure water, and put them into a mixing tank in sequence. Mechanically stir for 4 hours until evenly dispersed, and then slowly stir and vacuum defoam for 30 minutes to obtain slurry C.

[0045] 6. Mix slurry C according to a surface density of 20 g / cm³. 2 After coating onto electrode B and drying, apply at a rate of 1.1-1.2 g / cm³. 3 The compaction density is rolled to obtain electrode C.

[0046] 7. Cut electrode C and assemble it into a symmetrical cell. Perform EIS testing using an electrochemical workstation to obtain the ionic conductivity and tortuosity of the electrode. Assemble it with a ternary cathode electrode into a single-layer lithium-ion pouch cell and perform a 1C cycle test for 1000 cycles to obtain the capacity retention rate.

[0047] Example 2

[0048] The difference between this embodiment and Embodiment 1 is that the artificial graphite in step (3) is replaced with natural graphite, and the porous carbon in step (5) is replaced with artificial graphite.

[0049] Example 3

[0050] The difference between this embodiment and Embodiment 1 is that the porous carbon in step (5) is replaced with artificial graphite. (Example 4)

[0051] The difference between this embodiment and embodiment 1 is that the gaseous silicon carbide in steps (1), (3), and (5) is replaced with silicon suboxide;

[0052] Example 5

[0053] The difference between this embodiment and Embodiment 1 is that: according to the weight ratio of artificial graphite: fumed silicon carbide: SP: SWCNT: PAA: CMC: SBR = 75: 20: 1.0: 0.1: 2.5: 0.5: 0.9, the materials are weighed, a certain proportion of pure water is added, and the materials are put into a mixing tank and mechanically stirred for 4 hours until they are evenly dispersed. Then, the mixture is slowly stirred and vacuumed for 30 minutes to remove bubbles, thus obtaining slurry B.

[0054] Weigh the materials according to the weight ratio of porous carbon: fumed silicon carbon: SP: SWCNT: PAA: CMC: SBR = 54:40:1.0:0.1:3.5:0.5:0.9, add a certain proportion of pure water, and put them into a mixing tank in sequence. Mechanically stir for 4 hours until evenly dispersed, and then slowly stir and vacuum defoam for 30 minutes to obtain slurry C.

[0055] Comparative Example 1

[0056] 1. Weigh the materials according to the weight ratio of artificial graphite: fumed silicon carbide: SP:SWCNT:PAA:CMC:SBR = 80:15:1.0:0.1:2.5:0.5:0.9, add a certain proportion of pure water, and put them into a mixing tank in sequence. Mechanically stir for 4 hours until evenly dispersed, and then slowly stir and vacuum defoam for 30 minutes to obtain slurry A0.

[0057] 2. The slurry is coated onto the bright copper foil at a surface density of 110 g / cm2. After drying in a forced-air oven at 100℃, it is rolled at a compaction density of 1.60-1.65 g / cm3 to obtain electrode A0.

[0058] 3. Cut the A0 electrode sheet and assemble it into a symmetrical cell. Perform EIS testing using an electrochemical workstation to obtain the ionic conductivity and tortuosity of the electrode sheet. Assemble it with a ternary cathode electrode sheet into a single-layer lithium-ion pouch cell and perform a 1C cycle test for 1000 cycles to obtain the capacity retention rate.

[0059] Comparative Example 2

[0060] The difference between this comparative example and Comparative Example 1 is that the artificial graphite in step (1) is replaced with natural graphite, and the gaseous silicon carbide is replaced with silicon suboxide.

[0061] Comparative Example 3

[0062] The difference between this comparative example and Comparative Example 1 is that: according to the weight ratio of artificial graphite: fumed silicon carbide: SP: SWCNT: PAA: CMC: SBR = 90:5:1.0:0.1:2.5:0.5:0.9, the materials were weighed, a certain proportion of pure water was added, and the materials were put into a mixing tank and mechanically stirred for 4 hours until they were evenly dispersed. Then, the mixture was slowly stirred and vacuumed for 30 minutes to remove bubbles, resulting in slurry A0.

[0063] Comparative Example 4

[0064] The difference between this embodiment and Embodiment 1 is that: according to the weight ratio of artificial graphite: fumed silicon carbide: SP: SWCNT: PAA: CMC: SBR = 54:40:1.0:0.1:3.5:0.5:0.9, the materials are weighed, a certain proportion of pure water is added, and the materials are put into a mixing tank and mechanically stirred for 4 hours until they are evenly dispersed. Then, the mixture is slowly stirred and vacuumed for 30 minutes to remove bubbles, thus obtaining slurry B.

[0065] Weigh the materials according to the weight ratio of porous carbon: fumed silicon carbon: SP: SWCNT: PAA: CMC: SBR = 75: 20: 1.0: 0.1: 2.5: 0.5: 0.9, add a certain proportion of pure water, and put them into a mixing tank in sequence. Mechanically stir for 4 hours until evenly dispersed, and then slowly stir and vacuum defoam for 30 minutes to obtain slurry C.

[0066] Comparative Example 5

[0067] The difference between this embodiment and Embodiment 1 is that: slurry C is prepared according to a surface density of 20 g / cm³. 2 After coating onto electrode B and drying, apply at a rate of 1.65-1.7 g / cm³. 3 The compaction density is rolled to obtain electrode C.

[0068] 1. Porosity testing method:

[0069] 1) Cut the electrode sheet into 40*40mm squares, use a micrometer to measure the electrode sheet thickness R1, and weigh the electrode sheet as W1;

[0070] 2) Immerse the electrode in the electrolyte for 1 hour to fully wet it, wipe off the free electrolyte on the surface with lint-free paper, weigh it, and measure the weight W2 of the electrode.

[0071] 3) The actual porosity of the electrode is obtained through the calculation formula: Porosity = (W2 - W1) / ((R1 - copper foil thickness) * electrode area)

[0072] 2. Ionic conductivity / torsional stiffness τ test method:

[0073] 1) Cut the electrode sheets to 25*35mm to make a stacked symmetrical battery with an electrode-separator-electrode structure;

[0074] 2) After the battery is assembled, let it stand for 10 minutes, then test the EIS.

[0075] 3) The test parameters for AC EIS are frequency 100000~1Hz and disturbance voltage 10mV; ④ By fitting the EIS spectrum, the ionic conductivity σ and tortuosity τ of the electrode are obtained.

[0076] 4) The test formula is as follows:

[0077] Electrical conductivity calculation formula: σ=Rion / d

[0078] Ion impedance Rion: In the Nyquist plot, the low-frequency line segment is extended until it intersects the X-axis, and the difference between this intersection point and the intersection point of the high-frequency line segment and the X-axis is 3 times; d is the electrode thickness.

[0079] Formula for calculating tortuosity: τ=(R ion·A·ε·σ) / d (2)

[0080] τ is the tortuosity; Rion is the ionic resistance; A is the electrode area; ε is the electrode porosity; σ is the electrolyte conductivity; d is the electrode thickness.

[0081] Ion impedance Rion: In the Nyquist plot, the low-frequency line segment is extended until it intersects the X-axis, and the difference between this intersection point and the intersection point of the high-frequency line segment and the X-axis is 3 times.

[0082] 3. Capacity and capacity retention test

[0083] The test was conducted according to the national standard GB / T 18287-2013 "General Specification for Lithium-ion Batteries and Battery Packs for Mobile Phones". Under conditions of 25±2℃, the batteries were charged at 1C constant current and constant voltage to 4.2V, with a cutoff current of 0.05C, and then rested for 10 minutes. The batteries were then discharged at 1C to 3.0V and rested for 10 minutes. This process was repeated 1000 times. The capacity retention rate is the percentage of the discharge capacity in the corresponding cycle to the initial discharge capacity.

[0084] The secondary batteries prepared in Examples 1-4 and the electrodes and secondary batteries prepared in the comparative examples were subjected to electrical performance tests. The test results are shown in Table 1.

[0085] Table 1

[0086]

[0087] As can be seen from the comparison of Example 1, Comparative Example 1, and Comparative Example 3, when the negative electrode does not have multiple active material layers with a silicon content gradient, the cycle performance is poor when the silicon content in the negative electrode active material is set to a high level, and the cell capacity is low when the silicon content is set to a low level. This is because the active material close to the current collector 1 is easily affected by the expansion and contraction of the silicon material during charging and discharging, which leads to the failure of the bond between the current collector 1 and the active material, resulting in battery failure and affecting cycle performance. Although the expansion and contraction of the active material far from the current collector 1 has a smaller impact on the interface between the current collector 1 and the active material, the cell capacity is smaller because the silicon content cannot be gradient-set and a high silicon content cannot be set.

[0088] As can be seen from the comparison between Comparative Example 1 and Comparative Example 2, using silicon materials with less silicon content can improve cycle performance but reduce cell capacity. This is because silicon dioxide has a smaller expansion degree compared to fumed silicon, but provides fewer sites for lithium-ion embedding, resulting in a decrease in cell capacity.

[0089] As can be seen from the comparison of Examples 1-3, the battery performance is better when natural graphite is placed in the first active layer 2, artificial graphite in the second active layer 3, and porous carbon in the third active layer 4. This is because the third active layer 4 has a higher silicon content and fewer pores. By placing porous carbon in the third active layer 4, sufficient ion transport channels can be provided. The first active layer 2 has a lower silicon content. Placing highly compacted natural graphite in the first active layer 2 can improve the energy density of the battery. Placing artificial graphite in the second active layer 3 plays a buffering role.

[0090] As can be seen from the comparison between Example 5 and Comparative Example 4, when the content of silicon material is not set in a gradient, the interface between the two adjacent active layers will separate due to the different expansion conditions, causing the battery to fail.

[0091] As can be seen from the comparison between Example 1 and Comparative Example 1, when the compaction density exceeds the range of this application, the cycle performance of the battery is poor. This is because when the compaction density is too high, the buffer space for the expansion of silicon material is reduced, resulting in excessive expansion stress that causes the silicon material to shatter, which in turn causes changes in the structure of the active material and affects the cycle performance of the battery.

[0092] The foregoing description illustrates and describes several preferred embodiments of the present invention. However, as previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept by means of the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A multilayer silicon gradient anode, characterized in that, include: The current collector (1), a first active layer (2), a second active layer (3), and a third active layer (4) are provided. The first active layer (2) is coated on the surface of the current collector (1). The second active layer (3) is coated on the surface of the first active layer (2) away from the current collector (1). The third active layer (4) is coated on the surface of the second active layer (3) away from the current collector (1). The first active layer (2), the second active layer (3), and the third active layer (4) all contain silicon and carbon materials. The silicon content of the first active layer (2) is less than that of the second active layer (3). The silicon content of the second active layer (3) is less than that of the third active layer (4). The compaction density of the third active layer (4) is less than that of the second active layer (3). The compaction density of the second active layer (3) is less than that of the first active layer (2).

2. The multilayer silicon gradient anode sheet according to claim 1, characterized in that: The ratio of silicon material content among the first active layer (2), the second active layer (3), and the third active layer (4) is 0.5-10:5-20:1-40.

3. The multilayer silicon gradient anode sheet according to claim 2, characterized in that: The ratio of carbon material to silicon material in the first active layer (2) is 85-95: The ratio of carbon material to silicon material in the second active layer (3) is 75-90:5-20, and the ratio of carbon material to silicon material in the third active layer (4) is 55-85:10-40.

4. The multilayer silicon gradient anode sheet according to claim 3, characterized in that: The compaction density ratio of the first active layer (2), the second active layer (3) and the third active layer (4) is 1.65~1.85:1.55~1.70:0.95~1.

20.

5. The multilayer silicon gradient anode sheet according to claim 4, characterized in that: The areal density ratio of the first active layer (2), the second active layer (3), and the third active layer (4) is 35-45:35-45:10-25.

6. The multilayer silicon gradient anode sheet according to claim 5, characterized in that: The areal density of the third active layer (4) is 10-25 g / cm2.

7. The multilayer silicon gradient anode sheet according to claim 6, characterized in that: The carbon material is at least one of soft carbon, hard carbon, artificial graphite, natural graphite, and composite graphite.

8. The multilayer silicon gradient anode sheet according to claim 7, characterized in that: The carbon material of the first active layer (2) is natural graphite, the carbon material of the second active layer (3) is artificial graphite, and the carbon material of the third active layer (4) is porous carbon.

9. The multilayer silicon gradient anode sheet according to claim 8, characterized in that: The silicon material is at least one of silicon suboxide, composite silicon carbon, and vapor-deposited silicon carbon.

10. A method for preparing a multilayer silicon gradient anode as described in claims 1-9, characterized in that: Active material slurries for the first active layer (2), the second active layer (3), and the third active layer (4) are prepared respectively. The first active layer (2) slurry is coated on the current collector (1) with a certain surface density. After drying, it is rolled with a certain compaction density to obtain electrode A. The second active layer (3) slurry is coated on electrode A with a certain surface density. After drying, it is rolled with a certain compaction density to obtain electrode B. The third active layer (4) slurry is coated on electrode B with a certain surface density. After drying, it is rolled with a certain compaction density to obtain a multilayer silicon gradient negative electrode.

Images