A high energy density lithium ion battery silicon-based negative electrode sheet and preparation method thereof

Abstract

The present invention discloses a high-energy-density silicon-based negative electrode sheet for lithium-ion batteries, comprising a current collector, an inner coating layer applied to the surface of the current collector, a middle coating layer applied to the surface of the inner coating layer, and an outer coating layer applied to the surface of the middle coating layer; the inner coating layer and the middle coating layer both comprise silicon oxide and single-particle graphite, the mass ratio of silicon oxide to single-particle graphite in the inner coating layer being 15-30:70-85, the mass ratio of silicon oxide to single-particle graphite in the middle coating layer being 88-95:5-12, and the outer coating layer being a single-particle graphite layer. The silicon-based negative electrode sheet prepared by the present invention has a high silicon material content, which significantly reduces the weight of the negative electrode sheet and effectively slows down the expansion of the negative electrode sheet, so that the prepared lithium-ion battery has both good energy density and electrical performance. The expansion rate of the silicon-based negative electrode sheet and the expansion rate of the lithium-ion battery before and after cycling are significantly lower than those of conventional single-layer and double-layer structured negative electrode sheets.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a high-energy-density silicon-based negative electrode sheet for lithium ion batteries and a preparation method thereof. Background Art

[0002] At present, graphite-based negative electrode materials are still the mainstream negative electrode materials used in lithium-ion batteries. Their theoretical gram capacity of 372mAh / g seriously limits their ability to improve the energy density of lithium-ion batteries. The actual gram capacity of the currently more excellent artificial graphite is only 340-355mAh / g. Its development potential has reached a bottleneck and can no longer meet the demand of lithium-ion batteries for small size and high energy density.

[0003] Among the many negative electrode materials, silicon-based negative electrodes have extremely high lithium storage capacity (pure silicon theoretically 4200mAh / g) and abundant resources. The gram capacity of silicon oxide can also reach a high capacity of 1400-1500mAh / g, making it the most competitive material to replace graphite as the negative electrode of the next generation of lithium-ion batteries. However, the high expansion and contraction coefficient of silicon-based negative electrodes causes them to continuously pulverize during the lithium insertion and extraction cycle, which destroys the overall structure of the silicon-based negative electrode material, continuously consumes active lithium, and the cycle performance decays rapidly, which limits its large-scale application. In addition, due to the expansion / contraction of silicon-based materials, the coating area and the contact surface of the current collector are easily separated during the cycle, which increases the contact gap between the material area and the current collector, causing the material area to gradually lose electrical contact, significantly increasing the internal resistance of the battery and accelerating the battery cycle decay. Therefore, this poses a huge challenge to the cycle stability of the electrode structure.

[0004] Therefore, the traditional single-layer electrode structure design can no longer meet the demand. In this regard, a large number of researchers have done a lot of work on the electrode structure design of silicon-based negative electrodes, mainly including multi-layer structure design to reduce the expansion of silicon-based negative electrodes. Patent CN 112968148 A reports a lithium-ion battery negative electrode sheet and a lithium-ion battery. The lithium-ion battery negative electrode sheet includes a first negative electrode active material layer and a second negative electrode active material layer. The first layer is a carbon material and the second layer is a silicon material. The DC resistance of the lithium-ion battery constructed with the first negative electrode active material layer is lower than the DC resistance of the lithium-ion battery constructed with the second negative electrode active material layer. Such a structural design can improve the performance of silicon-based materials, but the bottom layer uses an almost pure carbon negative electrode, which has limited effect on improving energy density and cannot solve the expansion problem of the upper silicon layer. It also requires laser drilling, which is difficult to operate and difficult to achieve industrialization; Patent CN 114678490 A reported a lithium-ion battery negative electrode sheet comprising a two-layer structure, a bottom graphite buffer layer and an upper silicon-carbon layer, which achieved excellent first-time efficiency and coating adhesion. However, the expansion problem of silicon material in the electrode structure remains unsolved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a high-energy-density silicon-based negative electrode sheet for lithium-ion batteries and a preparation method thereof. The silicon-based negative electrode sheet has a high silicon material content, which greatly reduces the weight of the negative electrode sheet and effectively slows down the expansion of the negative electrode sheet, so that the prepared lithium-ion battery has both good energy density and electrical performance. The expansion rate of the silicon-based negative electrode sheet before and after cycling and the expansion rate of the lithium-ion battery are significantly lower than those of conventional single-layer and double-layer structured negative electrode sheets.

[0006] The technical solution of the present invention is:

[0007] A high-energy-density silicon-based negative electrode sheet for lithium-ion batteries comprises a current collector, an inner coating applied on the surface of the current collector, a middle coating applied on the surface of the inner coating, and an outer coating applied on the surface of the middle coating; the inner coating and the middle coating both comprise silicon oxide and single-particle graphite, the mass ratio of silicon oxide to single-particle graphite in the inner coating is 15-30:70-85, the mass ratio of silicon oxide to single-particle graphite in the middle coating is 88-95:5-12, and the outer coating is a single-particle graphite layer.

[0008] The particle size D50 of the single graphite particle is 5-7 μm; both sides of the current collector are coated with an inner coating, the surface of each inner coating layer is coated with a middle coating layer, and the surface of each middle coating layer is coated with an outer coating layer; the total surface density of the inner coating layer on both sides is 25-35 g / m 2 The total surface density of the middle coating on both sides is 65-75g / m 2 The double-sided total surface density of the outer coating is 15-23 g / m 2 .

[0009] The inner coating also includes an inner layer conductive agent and an inner layer binder. In the inner coating, silicon dioxide and single-particle graphite are used as the main materials of the inner layer. The mass ratio of the inner layer main material, the inner layer conductive agent and the inner layer binder is 89-94:1-5:3-6. The inner layer conductive agent includes a liquid inner layer conductive agent and a powder inner layer conductive agent. The liquid inner layer conductive agent is formed by mixing and dispersing one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and composite conductive agents in water. The powder inner layer conductive agent is a mixture of one or more of superconductive carbon black, nanocarbon fiber conductive agent and multilayer graphene powder. The inner layer binder includes a polyacrylic acid binder and a styrene-butadiene rubber binder. The mass ratio of the polyacrylic acid binder and the styrene-butadiene rubber binder is 6-10:1. The solid content of the slurry for coating the inner coating is 30-35% and the viscosity is 2500-4000mpas.

[0010] The middle coating also includes a middle layer conductive agent and a middle layer binder. In the middle coating, silicon dioxide and single-particle graphite are used as the main materials of the middle layer. The mass ratio of the middle layer main material, the middle layer conductive agent and the middle layer binder is 84-89:2-4:6-10. The middle layer conductive agent is a liquid middle layer conductive agent, which is formed by mixing and dispersing one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and composite conductive agents in water. The middle layer binder includes a polyacrylic acid binder and a sodium carboxymethyl cellulose binder. The mass ratio of the polyacrylic acid binder and the sodium carboxymethyl cellulose binder is 7-10:1. The slurry for applying the middle layer coating has a solid content of 18-23% and a viscosity of 1500-3000mpas.

[0011] The outer coating also includes an outer layer conductive agent and an outer layer binder. The mass ratio of single-particle graphite, outer layer conductive agent and outer layer binder in the outer coating is 93-96:1-3:1-4. The outer layer conductive agent includes a liquid outer layer conductive agent and a powder outer layer conductive agent. The liquid outer layer conductive agent is formed by mixing and dispersing one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and composite conductive agents in water. The powder outer layer conductive agent is a mixture of one or more of superconductive carbon black, nanocarbon fiber conductive agent and multilayer graphene powder. The outer layer binder includes a polyacrylic acid binder and a styrene-butadiene rubber binder. The mass ratio of the polyacrylic acid binder to the styrene-butadiene rubber binder is 2-4:1. The slurry for applying the outer coating has a solid content of 36-44% and a viscosity of 1000-2500mpas.

[0012] The composite conductive agent is formed by mixing multi-walled carbon nanotubes and carbon black conductive agent, and the mass ratio of the multi-walled carbon nanotubes to the carbon black conductive agent is 1:3-5.

[0013] The current collector is selected from carbon-coated copper foil with a thickness of 4.5 μm, carbon-coated copper foil with a thickness of 6 μm, copper foil with a thickness of 4.5 μm or copper foil with a thickness of 6 μm.

[0014] A method for preparing a high energy density lithium-ion battery silicon-based negative electrode sheet specifically comprises the following steps:

[0015] (1) Preparing the slurry for the inner coating: silicon dioxide, single-grain graphite, liquid inner layer conductive agent, powder inner layer conductive agent and inner layer binder are mixed by a semi-dry slurry process to prepare the negative electrode slurry A;

[0016] (2) Applying the negative electrode slurry A to both sides of the current collector to form a double-layer inner coating, and placing the current collector coated with the double-layer inner coating in an oven for low-temperature and vacuum treatment before use;

[0017] (3) preparing the slurry for coating the middle coating: silicon dioxide, single-grain graphite, liquid middle layer conductive agent and middle layer binder are mixed by wet slurry process to prepare negative electrode slurry B;

[0018] (4) preparing a slurry for coating an outer coating: preparing a negative electrode slurry C by wet slurrying of single-grain graphite, a liquid outer layer conductive agent, a powder outer layer conductive agent and an outer layer binder;

[0019] (5) coating the negative electrode slurry B on the surface of the double-layer inner coating to form a double-layer middle coating, coating the negative electrode slurry C on the surface of the double-layer middle coating, and finally moving the mixture into an oven for vacuum drying to obtain the negative electrode material;

[0020] (6) The dried negative electrode material is subjected to rolling and slitting processes to obtain a silicon-based negative electrode sheet.

[0021] In the step (2), the temperature of the low temperature and vacuum treatment is 45°C, the vacuum pressure is -90KPa, and the time is 12h; in the step (5), the temperature of the vacuum drying treatment is 95°C, the vacuum pressure is -90KPa, and the time is 24h.

[0022] In the step (6), the compaction density of the roller is 1.35-1.55g / m 3 .

[0023] Advantages of the present invention:

[0024] (1) The present invention adopts a three-layer active material coating structure. The silicon content of the active material in the three-layer material coating structure is higher than 80%, so that the energy density of the prepared lithium-ion battery is greater than 380Wh / Kg. The full-charge expansion rate of the silicon-based negative electrode sheet and the lithium-ion battery is low, and the active material coating and the current collector are not easy to fall off, which greatly improves the cycle performance of the silicon-based negative electrode sheet;

[0025] (2) The inner coating of the present invention uses a high proportion of single-particle graphite to match a low proportion of silicon oxide. The inner layer binder includes a polyacrylic acid (PAA) binder and a styrene-butadiene rubber (SBR) binder, which improves the adhesion between the inner coating and the current collector and increases the flexibility of the silicon-based negative electrode sheet. The small-particle graphite can provide more particles at the same mass and is better distributed between the silicon oxide particles, thereby increasing conductivity and buffering expansion.

[0026] (3) The middle coating of the present invention uses a high proportion of silicon oxide matched with a low proportion of single-particle graphite, and at the same time uses a low proportion of polyacrylic acid (PAA) binder and sodium carboxymethyl cellulose (CMC) binder, as well as a liquid middle layer conductive agent, so that the prepared lithium-ion battery takes into account the energy density while ensuring the conductivity of the middle coating, so that it can match the resistivity of the inner coating and the outer coating;

[0027] (4) The outer coating of the present invention uses a high proportion of pure single-particle graphite, and the coating surface density is low, which improves the quality and flexibility of the surface of the silicon-based negative electrode sheet;

[0028] (5) The single-particle graphite of the inner coating and the outer coating of the present invention produces a synergistic effect to inhibit expansion. During the charge and discharge process, the middle coating exhibits high expansion, the low-proportion silicon oxide of the inner coating exhibits low expansion, and the single-particle graphite of the outer coating has an extremely low expansion rate. Therefore, the inner coating and the outer coating can well buffer the expansion of the high-proportion silicon oxide of the middle coating. Compared with the traditional single-layer and double-layer designs, a buffered expansion phenomenon is presented, so that the overall expansion of the silicon-based negative electrode sheet exhibits low expansion;

[0029] (6) The low expansion rate of the silicon-based negative electrode sheet produced by the present invention makes the expansion rate of the lithium-ion battery low, thereby greatly improving the cycle performance of the lithium-ion battery. After the cycle is completed, the silicon-based negative electrode sheet still does not show the phenomenon of active material coating falling off, which is of great significance to the future large-scale promotion of high-silicon negative electrode sheets for lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a cross-sectional view of the silicon-based negative electrode sheet of the present invention.

[0031] Figure 2 This is a diagram of the surface state of the silicon-based negative electrode sheet after the battery cell in Example 1 of the present invention has been cycled 400 times.

[0032] Figure 3 This is a diagram of the surface state of the silicon-based negative electrode sheet after the battery cell in Comparative Example 1 of the present invention has been cycled 400 times.

[0033] Reference numerals: 01 - current collector, 02 - inner coating, 03 - middle coating, 04 - outer coating. DETAILED DESCRIPTION

[0034] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0035] Example 1

[0036] See Figure 1 A method for preparing a high energy density lithium-ion battery silicon-based negative electrode sheet comprises the following steps:

[0037] (1) Preparing a slurry for applying an inner coating: silicon dioxide, single-grain graphite, liquid inner-layer conductive agent, powder inner-layer conductive agent and inner-layer binder are subjected to a semi-dry slurrying process to prepare a negative electrode slurry A; wherein, the mass ratio of the inner layer main material, i.e. silicon dioxide and single-grain graphite, in the negative electrode slurry A is 30:70, the mass ratio of the inner layer main material, the inner layer conductive agent and the inner layer binder is 92:3:5, the inner layer conductive agent is an aqueous solution of single-walled carbon nanotubes (SWCNT), superconductive carbon black SP and nanofibers (VGCF), the mass ratio of SWCNT, SP and VGCF is 1:15:10, the inner layer binder includes PAA binder and SBR binder, the mass ratio of PAA binder to SBR binder is 7:1, the solid content of the negative electrode slurry A is 34.5%, and the viscosity is 3854mpas;

[0038] (2) The negative electrode slurry A is transferred to an extrusion coating transfer kettle, circulated to remove bubbles, and the negative electrode slurry A is coated on both sides of the current collector 01 (carbon-coated copper foil with a thickness of 6 μm) to form a double-layer internal coating 02. The current collector 01 coated with the double-layer internal coating 02 is placed in an oven at a temperature of 45°C and a vacuum pressure of -90Kpa for 12 hours for standby use; wherein, the double-sided total surface density of the internal coating 02 is 30g / m 2 , the dry film thickness after drying is 28μm;

[0039] (3) preparing the slurry for coating the middle coating: silicon dioxide, single-grain graphite, liquid middle-layer conductive agent and middle-layer binder are subjected to a wet slurrying process to prepare the negative electrode slurry B; wherein, the mass ratio of the middle main material, i.e. silicon dioxide and single-grain graphite in the negative electrode slurry B is 92:8, the mass ratio of the middle-layer main material, the middle-layer conductive agent and the middle-layer binder is 86:4:10, the middle-layer conductive agent is a mixed aqueous solution of SWCNT and a composite conductive agent, the mass ratio of SWCNT and the composite conductive agent is 1:3, the composite conductive agent is a mixture of multi-walled carbon nanotubes and superconductive carbon black, the mass ratio of multi-walled carbon nanotubes and superconductive carbon black is 1:3-5, the middle-layer binder includes a PAA binder and a CMC binder, the mass ratio of the PAA binder and the CMC binder is 8:1, the solid content of the negative electrode slurry B is 22.5%, and the viscosity is 2269mpas;

[0040] (4) Preparing a slurry for applying an external coating: single-particle graphite, an outer conductive agent, and an outer binder are subjected to a wet slurrying process to prepare a negative electrode slurry C; wherein the mass ratio of single-particle graphite, outer conductive agent, and outer binder in the negative electrode slurry C is 95.0:1.5:3.5, the outer conductive agent is an aqueous solution of SWCNT and superconductive carbon black SP, the mass ratio of SWCNT to superconductive carbon black SP is 1:15, the outer binder includes a PAA binder and an SBR binder, the mass ratio of PAA binder to SBR binder is 3:1, the solid content in the negative electrode slurry C is 40.6%, and the viscosity is 1780mpas;

[0041] (5) The negative electrode slurry B and the negative electrode slurry C are transferred to a double-layer extrusion coating transfer kettle, circulated to remove bubbles, and then the negative electrode slurry B is coated on the surface of the double-layer inner coating 02 to form a double-layer middle coating 03. The negative electrode slurry C is coated on the surface of the double-layer middle coating 03 to form a double-layer outer coating 04. Finally, the negative electrode material is moved to an oven and treated at a temperature of 95°C and a vacuum pressure of -90Kpa for 24 hours to obtain the negative electrode material; wherein, the total surface density of the middle coating on both sides is 68g / m 2 The total surface density of the outer coating on both sides is 18g / m 2 , the dry film thickness of the negative electrode material is 122μm;

[0042] (6) The dried negative electrode material was rolled (the compaction density was 1.45 g / m 2 ), slitting process and other processing to obtain a silicon-based negative electrode sheet.

[0043] The prepared silicon-based negative electrode sheet and the ternary high-nickel positive electrode sheet with a Ni content of more than 85% are assembled into a soft-pack lithium-ion battery with a battery capacity of 28Ah, an electrochemical test cycle rate of 0.5C, and a voltage range of 2.75-4.25V.

[0044] Example 2

[0045] A method for preparing a high energy density lithium-ion battery silicon-based negative electrode sheet specifically comprises the following steps:

[0046] (1) Preparing a slurry for applying an inner coating: silicon dioxide, single-grain graphite, liquid inner-layer conductive agent, powder inner-layer conductive agent and inner-layer binder are mixed by a semi-dry slurry process to prepare a negative electrode slurry A; wherein, the mass ratio of the inner layer main material, namely silicon dioxide and single-grain graphite in the negative electrode slurry A is 28:72, the mass ratio of the inner layer main material, the inner layer conductive agent and the inner layer binder is 90.5:3.5:6, the inner layer conductive agent is an aqueous solution of single-walled carbon nanotubes (SWCNT), superconductive carbon black SP and nano-carbon fiber (VGCF) conductive agent, the mass ratio of SWCNT, SP and VGCF is 1:15:10, the inner layer binder includes PAA binder and SBR binder, the mass ratio of PAA binder to SBR binder is 9.5:1, the solid content of the negative electrode slurry A is 33.6%, and the viscosity is 3682mpas;

[0047] (2) The negative electrode slurry A is transferred to an extrusion coating transfer kettle, circulated to remove bubbles, and the negative electrode slurry A is coated on both sides of a carbon-coated copper foil with a thickness of 4.5 μm to form a double-layer inner coating. The current collector coated with the double-layer inner coating is placed in an oven at a temperature of 45°C and a vacuum pressure of -90KPa for 12 hours and then used; wherein the total surface density of the inner coating on both sides is 30g / m 2 , the dry film thickness after drying is 28μm;

[0048] (3) preparing a slurry for coating the middle coating: silicon dioxide, single-grain graphite, liquid middle-layer conductive agent and middle-layer binder are subjected to a wet slurrying process to prepare a negative electrode slurry B; wherein, the mass ratio of the middle main material, i.e. silicon dioxide and single-grain graphite in the negative electrode slurry B is 94:6, the mass ratio of the middle-layer main material, the middle-layer conductive agent and the middle-layer binder is 86:4:10, the middle-layer conductive agent is a mixed aqueous solution of SWCNT and a composite conductive agent, the mass ratio of SWCNT and the composite conductive agent is 1:3, the composite conductive agent is a mixture of multi-walled carbon nanotubes and superconductive carbon black, the mass ratio of multi-walled carbon nanotubes and superconductive carbon black is 1:3-5, the middle-layer binder includes a PAA binder and a CMC binder, the mass ratio of the PAA binder to the CMC binder is 8:1, the solid content of the negative electrode slurry B is 23.7%, and the viscosity is 2540mpas;

[0049] (4) Preparing a slurry for applying an external coating: single-particle graphite, an outer conductive agent, and an outer binder are subjected to a wet slurrying process to prepare a negative electrode slurry C; wherein the mass ratio of single-particle graphite, outer conductive agent, and outer binder in the negative electrode slurry C is 95.0:1.5:3.5, the outer conductive agent is an aqueous solution of SWCNT and superconductive carbon black SP, the mass ratio of SWCNT to superconductive carbon black SP is 1:15, the outer binder includes a PAA binder and an SBR binder, the mass ratio of PAA binder to SBR binder is 2:1, the solid content in the negative electrode slurry C is 42.1%, and the viscosity is 1970mpas;

[0050] (5) The negative electrode slurry B and the negative electrode slurry C are transferred to a double-layer extrusion coating transfer kettle, circulated to remove bubbles, and then the negative electrode slurry B is coated on the surface of the double-layer inner coating to form a double-layer middle coating. The negative electrode slurry C is coated on the surface of the double-layer middle coating to form a double-layer outer coating. Finally, it is moved to an oven and treated at a temperature of 95°C and a vacuum pressure of -90KPa for 24 hours to obtain the negative electrode material; wherein, the total surface density of the middle coating on both sides is 72g / m 2 The total surface density of the outer coating on both sides is 22g / m 2 , the dry film thickness of the negative electrode material is 128μm;

[0051] (6) The dried negative electrode material was rolled (the compaction density was 1.5 g / m 2 ), slitting process and other processing to obtain a silicon-based negative electrode sheet.

[0052] The prepared silicon-based negative electrode sheet and the ternary high-nickel positive electrode sheet with a Ni content of more than 85% are assembled into a soft-pack lithium-ion battery with a battery capacity of 28Ah, an electrochemical test cycle rate of 0.5C, and a voltage range of 2.75-4.25V.

[0053] Example 3

[0054] A method for preparing a high energy density lithium-ion battery silicon-based negative electrode sheet specifically comprises the following steps:

[0055] (1) Preparing a slurry for applying an inner coating: silicon dioxide, single-grain graphite, liquid inner-layer conductive agent, powder inner-layer conductive agent and inner-layer binder are mixed by a semi-dry slurry process to prepare a negative electrode slurry A; wherein, the mass ratio of the inner layer main material, i.e. silicon dioxide, and single-grain graphite in the negative electrode slurry A is 25:75, the mass ratio of the inner layer main material, the inner layer conductive agent and the inner layer binder is 90.5:3.5:6, the inner layer conductive agent is an aqueous solution of SWCNT and a VGCF conductive agent, the mass ratio of SWCNT and VGCF is 1:10, the inner layer binder includes a PAA binder and an SBR binder, the mass ratio of the PAA binder to the SBR binder is 10:1, the solid content of the negative electrode slurry A is 32.6%, and the viscosity is 3451mpas;

[0056] (2) The negative electrode slurry A is transferred to an extrusion coating transfer kettle, circulated to remove bubbles, and the negative electrode slurry A is coated on both sides of a copper foil with a thickness of 6 μm to form a double-layer inner coating. The current collector coated with the double-layer inner coating is placed in an oven at a temperature of 45°C and a vacuum pressure of -90KPa for 12 hours and then used; wherein the double-sided total surface density of the inner coating is 32g / m 2 , the dry film thickness after drying is 29μm;

[0057] (3) Preparing the slurry for coating the middle coating: silicon dioxide, single-grain graphite, liquid middle-layer conductive agent and middle-layer binder are subjected to a wet slurrying process to prepare the negative electrode slurry B; wherein, the mass ratio of the middle main material, i.e. silicon dioxide and single-grain graphite in the negative electrode slurry B is 94:6, the mass ratio of the middle-layer main material, the middle-layer conductive agent and the middle-layer binder is 87:3.5:9.5, the middle-layer conductive agent is a mixed aqueous solution of SWCNT and a composite conductive agent, the mass ratio of SWCNT and the composite conductive agent is 1:4, the composite conductive agent is a mixture of multi-walled carbon nanotubes and superconductive carbon black, the mass ratio of multi-walled carbon nanotubes and superconductive carbon black is 1:3-5, the middle-layer binder includes a PAA binder and a CMC binder, the mass ratio of the PAA binder and the CMC binder is 6:1, the solid content of the negative electrode slurry B is 22.7%, and the viscosity is 2640mpas;

[0058] (4) Preparing a slurry for applying an external coating: single-particle graphite, an outer conductive agent, and an outer binder are subjected to a wet slurrying process to prepare a negative electrode slurry C; wherein the mass ratio of single-particle graphite, outer conductive agent, and outer binder in the negative electrode slurry C is 94.5:2:3.5, the outer conductive agent is an aqueous solution of SWCNT and superconductive carbon black SP, the mass ratio of SWCNT to superconductive carbon black SP is 1:15, the outer binder includes a PAA binder and an SBR binder, the mass ratio of PAA binder to SBR binder is 3:1, the solid content in the negative electrode slurry C is 41.6%, and the viscosity is 1870mpas;

[0059] (5) The negative electrode slurry B and the negative electrode slurry C are transferred to a double-layer extrusion coating transfer kettle, circulated to remove bubbles, and then the negative electrode slurry B is coated on the surface of the double-layer inner coating to form a double-layer middle coating. The negative electrode slurry C is coated on the surface of the double-layer middle coating to form a double-layer outer coating. Finally, it is moved to an oven and treated at a temperature of 95°C and a vacuum pressure of -90KPa for 24 hours to obtain the negative electrode material; wherein, the double-sided total surface density of the middle coating is 68g / m 2 The total surface density of the outer coating on both sides is 23g / m 2 , the dry film thickness of the negative electrode material is 126μm;

[0060] (6) The dried negative electrode material was rolled (the compaction density was 1.5 g / m 2 ), slitting process and other processing to obtain a silicon-based negative electrode sheet.

[0061] The prepared silicon-based negative electrode sheet and the ternary high-nickel positive electrode sheet with a Ni content of more than 85% are assembled into a soft-pack lithium-ion battery with a battery capacity of 28Ah, an electrochemical test cycle rate of 0.5C, and a voltage range of 2.75-4.25V.

[0062] Comparative Example 1

[0063] A method for preparing a double-layer coated lithium-ion battery silicon-based negative electrode sheet, the same as steps (1) to (3) of Example 1, the difference being that after the negative electrode slurry B is coated on the surface of the double-layer inner coating, the negative electrode slurry C is not coated, and the negative electrode material is directly moved to an oven and treated at a temperature of 95°C and a vacuum pressure of -90KPa for 24 hours to obtain a negative electrode material. The negative electrode material is roller-pressed (with a compaction density of 1.45g / m 2 ), slitting process and other processing to obtain a silicon-based negative electrode sheet.

[0064] The prepared silicon-based negative electrode sheet and the ternary high-nickel positive electrode sheet with a Ni content of more than 85% are assembled into a soft-pack lithium-ion battery with a battery capacity of 28Ah, an electrochemical test cycle rate of 0.5C, and a voltage range of 2.75-4.25V.

[0065] Comparative Example 2

[0066] A method for preparing a single-layer coated silicon-based negative electrode sheet for a lithium-ion battery comprises preparing a negative electrode slurry B according to step (3) of Example 2, coating the negative electrode slurry B on both sides of a carbon-coated copper foil having a thickness of 4.5 μm, and then directly transferring the coating to an oven and treating the copper foil at a temperature of 95° C. and a vacuum pressure of -90 kPa for 24 hours to obtain a negative electrode material. The total surface density of the negative electrode slurry B on both sides is 72 g / m 2 The negative electrode material was rolled as in Example 2 (the compaction density was 1.45 g / m 2), slitting process and other processing to obtain a silicon-based negative electrode sheet.

[0067] The prepared silicon-based negative electrode sheet and the ternary high-nickel positive electrode sheet with a Ni content of more than 85% are assembled into a soft-pack lithium-ion battery with a battery capacity of 28Ah, an electrochemical test cycle rate of 0.5C, and a voltage range of 2.75-4.25V.

[0068] The lithium-ion batteries prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to charge and discharge experiments. The experimental results are shown in Table 1 below.

[0069] Table 1

[0070]

[0071] From the experimental results data in Table 1, it can be seen that the silicon-based negative electrode sheets prepared in Examples 1-3 have a lower expansion rate, which makes the expansion rate of the lithium-ion battery cells low, and after 400 cycles of charge and discharge, the ACR growth rate is lower than that of the lithium-ion batteries prepared in Comparative Examples 1 and 2, and the capacity retention rate is higher.

[0072] from Figure 2 and Figure 3 From the surface state diagram of the silicon-based negative electrode sheet, it can be seen that after 400 cycles of charge and discharge, the silicon-based negative electrode sheet prepared in Comparative Example 1 showed obvious powder separation from the current collector, while the surface of the silicon-based negative electrode sheet prepared in Example 1 remained basically unchanged.

[0073] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A high energy density lithium-ion battery silicon-based negative electrode sheet, characterized by: The invention comprises a current collector, an inner coating coated on the surface of the current collector, a middle coating coated on the surface of the inner coating, and an outer coating coated on the surface of the middle coating; the inner coating and the middle coating both comprise silicon oxide and single-particle graphite, the mass ratio of silicon oxide to single-particle graphite in the inner coating is 15-30:70-85, the mass ratio of silicon oxide to single-particle graphite in the middle coating is 88-95:5-12, and the outer coating is a single-particle graphite layer; The middle coating also includes a middle layer conductive agent and a middle layer binder. In the middle coating, silicon dioxide and single-particle graphite are used as the main materials of the middle layer. The mass ratio of the middle layer main material, the middle layer conductive agent and the middle layer binder is 84-89:2-4:6-10. The middle layer conductive agent is a liquid middle layer conductive agent, which is formed by mixing and dispersing one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and composite conductive agents in water. The middle layer binder includes a polyacrylic acid binder and a sodium carboxymethyl cellulose binder; the composite conductive agent is a mixture of multi-walled carbon nanotubes and superconductive carbon black, and the mass ratio of multi-walled carbon nanotubes to superconductive carbon black is 1:3-5.

2. The high energy density lithium-ion battery silicon-based negative electrode sheet according to claim 1, characterized in that: The particle size D50 of the single graphite particle is 5-7 μm; both sides of the current collector are coated with an inner coating, the surface of each inner coating layer is coated with a middle coating layer, and the surface of each middle coating layer is coated with an outer coating layer; the total surface density of the inner coating layer on both sides is 25-35 g / m 2 The total surface density of the middle coating on both sides is 65-75g / m 2 The double-sided total surface density of the outer coating is 15-23 g / m 2 .

3. The high energy density lithium-ion battery silicon-based negative electrode sheet according to claim 1, characterized in that: The inner coating also includes an inner layer conductive agent and an inner layer binder. In the inner coating, silicon dioxide and single-particle graphite are used as the main materials of the inner layer. The mass ratio of the inner layer main material, the inner layer conductive agent and the inner layer binder is 89-94:1-5:3-6. The inner layer conductive agent includes a liquid inner layer conductive agent and a powder inner layer conductive agent. The liquid inner layer conductive agent is formed by mixing and dispersing one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and composite conductive agents in water. The powder inner layer conductive agent is a mixture of one or more of superconductive carbon black, nanocarbon fiber conductive agent and multilayer graphene powder. The inner layer binder includes a polyacrylic acid binder and a styrene-butadiene rubber binder. The mass ratio of the polyacrylic acid binder and the styrene-butadiene rubber binder is 6-10:

1. The solid content of the slurry for coating the inner coating is 30-35% and the viscosity is 2500-4000mpas.

4. The high energy density lithium-ion battery silicon-based negative electrode sheet according to claim 1, characterized in that: The mass ratio of the polyacrylic acid binder to the sodium carboxymethyl cellulose binder is 7-10:1, and the solid content of the slurry for applying the middle layer coating is 18-23% and the viscosity is 1500-3000mpas.

5. The high energy density lithium-ion battery silicon-based negative electrode sheet according to claim 1, characterized in that: The outer coating also includes an outer layer conductive agent and an outer layer binder. The mass ratio of single-particle graphite, outer layer conductive agent and outer layer binder in the outer coating is 93-96:1-3:1-4. The outer layer conductive agent includes a liquid outer layer conductive agent and a powder outer layer conductive agent. The liquid outer layer conductive agent is formed by mixing and dispersing one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes and composite conductive agents in water. The powder outer layer conductive agent is a mixture of one or more of superconductive carbon black, nanocarbon fiber conductive agent and multilayer graphene powder. The outer layer binder includes a polyacrylic acid binder and a styrene-butadiene rubber binder. The mass ratio of the polyacrylic acid binder and the styrene-butadiene rubber binder is 2-4:

1. The solid content of the slurry for coating the outer coating is 36-44% and the viscosity is 1000-2500mpas.

6. The high energy density silicon-based negative electrode sheet for lithium-ion batteries according to claim 1, characterized in that: The current collector is made of carbon-coated copper foil with a thickness of 4.5 μm or carbon-coated copper foil with a thickness of 6 μm.

7. The method for preparing a high energy density lithium-ion battery silicon-based negative electrode sheet according to claim 1, characterized in that: The specific steps include: (1) Prepare the slurry for coating the inner coating: prepare the negative electrode slurry A by mixing silicon dioxide, single-grain graphite, liquid inner layer conductive agent, powder inner layer conductive agent and inner layer binder through a semi-dry slurry process; (2) Apply the negative electrode slurry A to both sides of the current collector to form a double-layer inner coating, and place the current collector coated with the double-layer inner coating in an oven for low temperature and vacuum treatment before use; (3) Prepare the slurry for coating the middle coating: prepare the negative electrode slurry B by wet slurrying process of silicon dioxide, single-grain graphite, liquid middle layer conductive agent and middle layer binder; (4) Prepare the slurry for coating the outer coating: prepare the negative electrode slurry C by wet slurrying process of single-grain graphite, liquid outer layer conductive agent, powder outer layer conductive agent and outer layer binder; (5) Apply the negative electrode slurry B on the surface of the double-layer inner coating to form a double-layer middle coating. The negative electrode slurry C is coated on the surface of the double-layer middle coating, and finally moved to an oven for vacuum drying to obtain the negative electrode material; (6) The dried negative electrode material is subjected to rolling and slitting processes to obtain a silicon-based negative electrode sheet.

8. The method for preparing a high energy density silicon-based negative electrode sheet for lithium-ion batteries according to claim 7, characterized in that: In step (2), the temperature of the low temperature and vacuum treatment is 45°C, the vacuum pressure is -90KPa, and the time is 12h; in step (5), the temperature of the vacuum drying treatment is 95°C, the vacuum pressure is -90KPa, and the time is 24h.

9. The method for preparing a high energy density lithium-ion battery silicon-based negative electrode sheet according to claim 7, characterized in that: In step (6), the compaction density of the roller is 1.35-1.55g / m 3 .

Images