Sodium-ion battery negative electrode sheet, preparation method thereof and battery

Abstract

This invention relates to a sodium-ion battery negative electrode sheet, its preparation method, and the battery itself, belonging to the field of sodium-ion battery technology. It solves the problem that existing sodium-ion battery negative electrodes cannot simultaneously meet the requirements of high rate of charge and high energy density. The negative electrode sheet includes: a current collector, a first active layer, and a second active layer; the first active layer is disposed on at least one side of the current collector; the second active layer is disposed on the side of the first active layer away from the current collector; wherein, the ratio of the specific surface area of ​​the first negative electrode active material to the second negative electrode active material is a, the ratio of the D50 of the first negative electrode active material to the second negative electrode active material is b, the ratio of the areal density of the second active layer to the first active layer is c, and the ratio of the porosity of the first active layer to the second active layer is d, where a, b, c, and d are all greater than 1, and 3.76 < a × b + c × d < 7.42. The negative electrode sheet of this invention can significantly improve the cycle performance of the battery cell under high rate charge and discharge conditions while ensuring high energy density of the battery cell.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a sodium-ion battery negative electrode sheet, its preparation method, and the battery itself. Background Technology

[0002] Sodium-ion batteries are considered an ideal choice for next-generation large-scale energy storage and low-speed electric vehicles due to the abundance and low cost of sodium resources and their similar electrochemical working principle to lithium-ion batteries. The anode material, a key component of sodium-ion batteries, directly determines the battery's energy density and cycle life. Anode active materials are generally hard carbon materials, with a disordered, graphite-like layered structure containing numerous micropores, microcrystalline defects, and a large specific surface area, significantly impacting battery capacity and rate performance.

[0003] Currently, research on improving the performance of sodium-ion battery anodes mainly focuses on modifying active materials, such as through nano-sizing, elemental doping, or compositing with carbon materials to enhance conductivity and structural stability. However, simply modifying the anode material is usually insufficient to simultaneously meet the demands for high rate capability and high energy density. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a sodium-ion battery anode sheet, its preparation method, and the battery itself, in order to solve the problem that existing sodium-ion battery anodes cannot simultaneously meet the requirements of high rate capability and high energy density.

[0005] On one hand, the present invention provides a negative electrode sheet for a sodium-ion battery, comprising: a current collector; a first active layer disposed on at least one side of the current collector, the first active layer being formed by coating the current collector with a first active slurry and drying; a second active layer disposed on the side of the first active layer away from the current collector, the second active layer being formed by coating the surface of the first active layer with a second active slurry and drying; the first active slurry comprising a first negative electrode active material, a conductive agent, and a binder; the second active slurry comprising a second negative electrode active material, a conductive agent, and a binder; wherein, the ratio of the specific surface area of ​​the first negative electrode active material and the second negative electrode active material is a, the ratio of the D50 of the first negative electrode active material and the second negative electrode active material is b, the ratio of the areal density of the second active layer and the first active layer is c, the ratio of the porosity of the first active layer and the second active layer is d, and a, b, c, and d are all greater than 1, and a, b, c, and d satisfy 3.76 < a × b + c × d < 7.42.

[0006] Furthermore, the specific surface area of ​​the first negative electrode active material is 6.3 m². 2 / g~9.4m 2 / g; and / or, the specific surface area of ​​the second negative electrode active material is 3.9m². 2 / g~7.7m2 / g.

[0007] Furthermore, the areal density of the first active layer is 0.82 g / cm³. 2 ~0.88g / cm 2 ; and / or, the areal density of the second active layer is 0.85 g / cm³. 2 ~0.93g / cm 2 .

[0008] Furthermore, the porosity of the first active layer is 25% to 35%; and / or, the porosity of the second active layer is 18% to 28%.

[0009] Furthermore, the particle size D50 of the first negative electrode active material is 5~8 μm, and / or the particle size D50 of the second negative electrode active material is 3~6 μm.

[0010] Furthermore, the coating thickness of the first active layer is 40~50μm, and / or the coating thickness of the second active layer is 40~50μm.

[0011] Furthermore, the first negative electrode active material includes hard carbon, and the second negative electrode active material includes hard carbon.

[0012] Furthermore, the conductive agent includes one or more of conductive carbon black, carbon nanotubes, and graphene; and / or, the binder includes one or more of styrene-butadiene rubber, carboxymethyl cellulose, and sodium polyacrylate.

[0013] On the other hand, the present invention also provides a method for preparing a sodium-ion battery negative electrode sheet, used to prepare the negative electrode sheet in the above embodiments, comprising the following steps: S1, the first negative electrode active material, conductive agent and binder are mixed evenly in the dispersant to obtain the first active layer slurry; S2, the second negative electrode active material, conductive agent and binder are mixed evenly in the dispersant to obtain the second active layer slurry; S3, the first active layer slurry is coated onto the blank current collector and dried to obtain a current collector coated with the first active layer; S4, the second active layer slurry is coated onto the first active layer and dried to obtain a negative electrode sheet coated with the first active layer and the second active layer. S5, Roll forming of the negative electrode sheet; Wherein, the ratio of the specific surface area of ​​the first negative electrode active material to the second negative electrode active material is a, the ratio of the D50 of the first negative electrode active material to the second negative electrode active material is b, the ratio of the areal density of the second active layer to the first active layer is c, and the ratio of the porosity of the first active layer to the second active layer is d. Then a, b, c, and d satisfy: a, b, c, and d are all greater than 1, and 3.76 < a × b + c × d < 7.42.

[0014] Thirdly, the present invention also provides a sodium-ion battery, comprising: the negative electrode sheet in the above embodiments or the negative electrode sheet prepared by the preparation method in the above embodiments.

[0015] This invention can achieve at least one of the following beneficial effects: 1. The negative electrode sheet of the battery of the present invention adopts a double-layer coating method. By coating a first active layer on the surface of the current collector and coating a second active layer on the surface of the first active layer, the negative electrode sheet has a distinct layered structure. The first active layer uses an active material with a high specific surface area, while controlling the coating surface density to be low, which can improve the kinetic velocity and increase the ion conduction rate, thereby improving the fast charging performance; while the second active layer uses a higher coating surface density, which can increase the amount of active material, thereby increasing the energy density of the cell.

[0016] 2. This invention, by controlling the ratio of the specific surface area (a) of the first active layer and the second active layer of the negative electrode, the ratio of the particle size (b), the ratio of the coating surface density (c) of the second active layer and the first active layer, and the ratio of the porosity (d) of the first active layer and the second active layer, all to be greater than 1; and simultaneously ensuring that 3.76 < a × b + c × d < 7.42, can significantly improve the cycle performance of the battery cell under high-rate charge and discharge conditions and enhance the fast-charging performance of the battery cell while maintaining high energy density.

[0017] 3. The sodium-ion battery assembled using the negative electrode sheet of the present invention has excellent rate performance and energy density. After 500 cycles at 3C, the battery can retain more than 81.5% of its capacity.

[0018] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0019] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0020] Figure 1 This is a schematic diagram of the structure of a negative electrode sheet according to an embodiment of the present invention.

[0021] Figure 2 This is a schematic diagram of the structure of a negative electrode sheet according to another embodiment of the present invention.

[0022] Figure label: 1. First active layer; 2. Second active layer; 3. Current collector. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the present invention clearer, exemplary embodiments of the present invention will be described below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. For clarity and brevity, not all features of actual embodiments are described in the specification.

[0024] Embodiments of the present invention provide a negative electrode sheet for a sodium-ion battery, such as... Figure 1 and Figure 2 As shown, the negative electrode includes: a current collector 3, a first active layer 1, and a second active layer 2. The first active layer 1 is disposed on at least one side of the current collector 3, and is formed by coating the current collector 3 with a first active slurry and drying it. The second active layer 2 is disposed on the side of the first active layer 1 away from the current collector 3, and is formed by coating the surface of the first active layer 1 with a second active slurry and drying it. The first active slurry includes a first negative electrode active material, a conductive agent, and a binder; the second active slurry includes a second negative electrode active material, a conductive agent, and a binder.

[0025] In this design, the specific surface area of ​​the first negative electrode active material is greater than that of the second negative electrode active material; the particle size D50 of the first negative electrode active material is greater than that of the second negative electrode active material; the coating areal density of the first active layer 1 is less than that of the second active layer 2; and the porosity of the first active layer 1 is greater than that of the second active layer 2. Specifically, if the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is *a*, the ratio of the D50 of the first negative electrode active material to that of the second negative electrode active material is *b*, the ratio of the areal density of the second active layer to that of the first active layer is *c*, and the ratio of the porosity of the first active layer to that of the second active layer is *d*, then *a*, *b*, *c*, and *d* are all greater than 1. For example, *a*, *b*, *c*, and *d* can be independently ranges of 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.2, 2.5, 2.8, 3, or any combination thereof.

[0026] In embodiments of the present invention, a first active layer and a second active layer are sequentially coated onto the negative electrode current collector, forming a distinct layered structure. The first active layer uses an active material with a high specific surface area, while maintaining a low areal density and high porosity, providing ample space and channels for the rapid migration of sodium ions, thereby increasing kinetic velocity and ion conduction rate, and thus improving fast-charging performance. The second active layer uses an active material with a larger particle size and smaller specific surface area, and a higher areal density, allowing for increased active material usage and stable storage of a large number of sodium ions, thereby increasing the energy density of the battery cell and ensuring high capacity. Therefore, the negative electrode sheet of the present invention can significantly improve the cycle performance of the battery cell at high rates while maintaining high energy density.

[0027] Simultaneously, a, b, c, and d need to satisfy 3.76 < a×b + c×d < 7.42. For example, a×b + c×d can be a range of 3.78, 3.8, 3.85, 3.9, 3.95, 4, 4.2, 4.5, 4.7, 4.8, 5, 5.2, 5.4, 5.5, 5.7, 5.9, 6, 6.2, 6.5, 6.8, 7, 7.2, 7.4, or any combination thereof. This invention optimizes sodium ion transport kinetics, suppresses sodium deposition, improves safety, enhances interfacial stress distribution and cycle stability, and reduces charge transfer impedance by precisely controlling the relationship between the specific surface area, particle size, areal density, and porosity of the active material between the two active layers. This allows the battery to achieve both high energy density and high-rate fast charging performance.

[0028] In some embodiments, the first negative electrode active material is hard carbon, including one or more of resin-based hard carbon, biomass-based hard carbon, carbon black, and pitch-based hard carbon, such as one or more of phenolic resin, epoxy resin, polyacrylonitrile, polyvinyl alcohol, polyvinyl chloride, acetylene black, biomass carbon, pitch, and coal tar.

[0029] In some embodiments, the second negative electrode active material is hard carbon, including one or more of resin-based hard carbon, biomass-based hard carbon, carbon black, and pitch-based hard carbon, such as one or more of phenolic resin, epoxy resin, polyacrylonitrile, polyvinyl alcohol, polyvinyl chloride, acetylene black, biomass carbon, pitch, and coal tar.

[0030] In some embodiments, the specific surface area of ​​the first negative electrode active material is 6.3 m². 2 / g ~ 9.4m 2 / g, for example 6.3m 2 / g, 6.5m 2 / g, 6.7m 2 / g, 6.9m 2 / g, 7.1m 2 / g, 7.3m 2 / g, 7.5m 2 / g, 7.7m 2 / g、8m 2 / g, 8.3m 2 / g, 8.5m 2 / g, 8.8m 2 / g、9m 2 / g, 9.2m 2 / g, 9.4m 2 / g or a range consisting of any two of them.

[0031] In some embodiments, the specific surface area of ​​the second negative electrode active material is 3.9 m². 2 / g~7.7m 2 / g, for example, 3.9m 2 / g、4m 2 / g, 4.2m 2 / g, 4.4m 2 / g, 4.6m 2 / g, 4.8m 2 / g、5m 2 / g, 5.2m 2 / g, 5.4m 2 / g, 5.6m 2 / g, 5.8m 2 / g、6m 2 / g, 6.2m 2 / g, 6.4m 2 / g, 6.5m 2 / g, 6.8m 2 / g、7m 2 / g, 7.2m 2 / g, 7.5m 2 / g, 7.7m 2 / g or a range consisting of any two of them.

[0032] This invention utilizes a first active layer with a large specific surface area to ensure rapid ion conduction at high rates, giving the battery cell excellent rate performance; at the same time, it uses a second active layer with a smaller specific surface area to suppress electrode side reactions and improve cycle performance.

[0033] In some embodiments, the particle size D50 of the first negative electrode active material is 5~8 μm, for example, a range of 5 μm, 5.2 μm, 5.4 μm, 5.5 μm, 5.6 μm, 5.8 μm, 6 μm, 6.2 μm, 6.4 μm, 6.5 μm, 6.6 μm, 6.8 μm, 7 μm, 7.2 μm, 7.4 μm, 7.5 μm, 7.6 μm, 7.8 μm, 8 μm, or any combination thereof.

[0034] In some embodiments, the particle size D50 of the second negative electrode active material is 3~6 μm, for example, a range of 3 μm, 3.2 μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.8 μm, 4 μm, 4.2 μm, 4.4 μm, 4.5 μm, 4.6 μm, 4.8 μm, 5 μm, 5.2 μm, 5.4 μm, 5.5 μm, 5.6 μm, 5.8 μm, 6 μm, or any combination thereof.

[0035] In this invention, particle size D50 refers to particles smaller than this size comprising 50% of the total. The smaller the particle size of the negative electrode active material, the shorter the distance sodium ions travel from the particle surface to the center, resulting in better rate performance of the battery cell. Conversely, larger particle sizes lead to higher compaction density and thus higher energy density. This invention uses a larger particle size active material in the first active layer and a smaller particle size active material in the second active layer, enabling the two active layers to work synergistically. This achieves both rapid sodium ion conduction and increased energy density of the battery cell, thereby realizing fast charging and high capacity in sodium-ion batteries.

[0036] In some embodiments, the areal density of the first active layer is 0.82 g / cm³. 2 ~0.88g / cm 2 For example, 0.82 g / cm³ 2 0.83g / cm 2 0.84 g / cm 2 0.85g / cm 2 0.86 g / cm 2 0.87g / cm 2 0.88g / cm 2 Or a range consisting of any two of them.

[0037] In some embodiments, the areal density of the second active layer is 0.85 g / cm³. 2 ~0.93g / cm 2 For example, 0.85 g / cm³ 2 0.86 g / cm 2 0.87g / cm 2 0.88g / cm 2 0.89 g / cm 2 0.9g / cm 2 0.91g / cm 2 0.92g / cm 2 0.93g / cm 2 Or a range consisting of any two of them.

[0038] The lower the areal density of the active layer, the faster the ion and electron conduction, resulting in better cell rate performance; conversely, the higher the areal density, the higher the energy density, but the longer the ion and electron transport paths, leading to poorer cell rate performance. This invention employs a first active layer with a lower areal density and a second active layer with a higher areal density, simultaneously ensuring rapid ion conduction and high energy density. It should be noted that the areal density in this invention refers to the areal density of a single side of the current collector.

[0039] In some embodiments, the porosity of the first active layer is 25% to 35%, for example, a range of 25%, 28%, 30%, 32%, 35%, or any combination thereof.

[0040] In some embodiments, the porosity of the second active layer is 18-28%, for example, a range of 18%, 20%, 22%, 24%, 26%, 28%, or any combination thereof.

[0041] Increasing porosity, to a certain extent, facilitates electrolyte penetration and improves ion mobility during cycling; conversely, decreasing porosity, to a certain extent, can increase compaction density and energy density. This invention, by controlling the porosity of the first and second active layers and their ratio, can improve energy density while enhancing the fast charging capability of the battery cell.

[0042] In some embodiments, such as Figure 1 As shown, the first active layer 1 and the second active layer 2 can be disposed on one side of the current collector 3; as Figure 2 As shown, the first active layer 1 and the second active layer 2 can also be disposed on both sides of the current collector 3.

[0043] In some embodiments, the coating thickness of the first active layer is 40-50 μm, for example, a range of 40 μm, 41 μm, 42 μm, 43 μm, 44 μm, 45 μm, 46 μm, 47 μm, 48 μm, 49 μm, 50 μm or any combination thereof.

[0044] In some embodiments, the coating thickness of the second active layer is 40-50 μm, for example, 40 μm, 41 μm, 42 μm, 43 μm, 44 μm, 45 μm, 46 μm, 47 μm, 48 μm, 49 μm, 50 μm or any combination thereof.

[0045] A thicker active layer results in higher energy density but poorer rate performance and increased ion transport distance and resistance; a thinner coating results in better rate performance and reduced ion transport distance and resistance, but lower energy density. This invention optimizes the capacity-rate balance, improves interfacial stress management, and enhances liquid-phase ion transport by controlling the coating thickness of the two active layers, thus facilitating a balance between high energy density and high-rate fast charging performance.

[0046] In some implementations, the current collector includes aluminum foil.

[0047] In some embodiments, the conductive agent in the first active slurry includes one or more of conductive carbon black (SP), carbon nanotubes, and graphene; the binder used in the first active slurry includes one or more of styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and sodium polyacrylate (PAA).

[0048] In some embodiments, the mass ratio of the first negative electrode active material in the first active slurry is 92.5% to 94.5%, for example, 92.5%, 93%, 93.5%, 94%, 94.5%, or any combination thereof; the mass ratio of the conductive agent is 1%; and the mass ratio of the binder is 4.5% to 6.5%, for example, 4.5%, 5%, 5.5%, 6%, 6.5%, or any combination thereof.

[0049] In some embodiments, the conductive agent in the second active slurry includes one or more of conductive carbon black (SP), carbon nanotubes, and graphene; the binder used in the second active slurry includes one or more of styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and polyacrylic acid (PAA).

[0050] In some embodiments, the mass ratio of the second negative electrode active material in the second active slurry is 92.5% to 94.5%, the mass ratio of the conductive agent is 1%, and the mass ratio of the binder is 4.5% to 6.5%.

[0051] In some embodiments, the viscosity of the first active slurry is 4000~5000 mPa. s; The viscosity of the second active slurry is 4000~5000 mPa For example, the viscosities of the first and second active slurries are each independently 4000 mPa. s, 4100mPa s, 4200mPa s, 4300mPa s, 4400mPa s, 4500mPa s, 4600mPa s, 4700mPa s, 4800mPa s, 4900mPa s, 5000mPa The range of s or any two thereof. By controlling the viscosity of the first and second active slurries, this invention can increase the storage time of the slurry, improve the coating yield, and reduce phenomena such as slurry segregation.

[0052] The present invention also provides a method for preparing a sodium-ion battery negative electrode sheet, which can be used to prepare the negative electrode sheet of the above embodiments, specifically including the following steps: S1, the first negative electrode active material, conductive agent and binder are mixed evenly in a solvent to obtain the first active layer slurry; S2, the second negative electrode active material, conductive agent and binder are mixed evenly in a solvent to obtain the second active layer slurry; S3, the first active layer slurry is coated onto the blank current collector and dried to obtain a current collector coated with the first active layer; S4, the second active layer slurry is coated onto the first active layer and dried to obtain a negative electrode sheet coated with the first active layer and the second active layer. S5, roll-press the negative electrode sheet after the first and second active layers have been coated.

[0053] Wherein, the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is a, the ratio of the D50 of the first negative electrode active material to that of the second negative electrode active material is b, the ratio of the areal density of the second active layer to that of the first active layer is c, and the ratio of the porosity of the first active layer to that of the second active layer is d. Then a, b, c, and d satisfy: a, b, c, and d are all greater than 1, and 3.76 < a × b + c × d < 7.42.

[0054] The negative electrode obtained by the preparation method of the present invention can improve the rate performance of the battery cell while ensuring high energy density, so that it still has excellent cycle performance at high rates.

[0055] Specifically, in step S1, the negative electrode material is hard carbon; the conductive agent is one or more of conductive carbon black, carbon nanotubes, and graphene; the binder includes styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC), wherein the addition ratio of SBR is 3%~4% and the addition ratio of CMC is 1.5%~2.5%; the solvent is N-methylpyrrolidone (NMP) and water.

[0056] In step S1, the viscosity of the first active layer slurry is controlled at 4000~5000 mPa. s.

[0057] Specifically, in step S2, the negative electrode material is hard carbon; the conductive agent is one or more of conductive carbon black, carbon nanotubes, and graphene; the binder includes styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC), wherein the addition ratio of SBR is 3%~4% and the addition ratio of CMC is 1.5%~2.5%; the solvent is N-methylpyrrolidone (NMP) and water.

[0058] In step S2, the viscosity of the second active layer slurry is controlled at 4000~5000 mPa. s.

[0059] Specifically, in step S3, the drying temperature is 70~80℃, for example, 70℃, 71℃, 72℃, 73℃, 74℃, 75℃, 76℃, 77℃, 78℃, 79℃, or 80℃. The coating thickness of the first active layer is 40~50μm.

[0060] Specifically, in step S4, the drying temperature is 70~80℃, for example, 70℃, 71℃, 72℃, 73℃, 74℃, 75℃, 76℃, 77℃, 78℃, 79℃, or 80℃. The coating thickness of the second active layer is 40~50μm.

[0061] In some embodiments, in step S5, the negative electrode sheet is subjected to a peel force test before and after rolling, thereby ensuring that the first active layer and the second active layer on the negative electrode sheet are firmly adhered to the current collector and can remain stable when applied in sodium-ion batteries.

[0062] An embodiment of the present invention also provides a sodium-ion battery, which includes the negative electrode sheet described in the above embodiment.

[0063] The sodium-ion battery of this invention uses the above-mentioned negative electrode sheet with double-layer active material, which improves the battery energy density and has excellent cycle performance at high rates, which is beneficial for fast charging of the battery.

[0064] The technical solution of the present invention will be further illustrated below with specific embodiments.

[0065] Example 1 The preparation process of the sodium-ion battery negative electrode sheet in this embodiment is as follows: Step S1: Hard carbon, SP, SBR, and CMC are mixed uniformly in NMP and water at a mass ratio of 93.5:1:3.5:2, and stirred at room temperature for 3 hours to obtain the first active layer slurry; wherein, the specific surface area of ​​the first negative electrode active material, hard carbon, is 6.9 m². 2 / g, with a particle size D50 of 5.967μm; Step S2: Another type of hard carbon, SP, SBR, and CMC are mixed uniformly in NMP and water at a mass ratio of 93.5:1:3.5:2, and stirred at room temperature for 3 hours to obtain the second active layer slurry; wherein, the specific surface area of ​​the second negative electrode active material, hard carbon, is 4.54 m². 2 / g, particle size D50 is 4.42μm; Step S3: Apply the first active layer slurry to the blank aluminum foil current collector at a speed of 10 m / min, and dry it in an oven at 75°C. The coating thickness of the first active layer is 45 μm, and the areal density is 0.83 g / cm³. 2 The porosity is 31.68%; Step S4: Apply the second active layer onto the first active layer at a speed of 10 m / min, and dry it in an oven at 75°C. The coating thickness of the second active layer is 45 μm, and the areal density is 0.93 g / cm³. 2 The porosity is 18%; Step S5: Under a preset pressure, roll the negative electrode sheet after the first and second active layers have been coated.

[0066] In this embodiment, a=1.52, b=1.35, c=1.12, d=1.76, a×b+c×d=4.0232, which satisfies 3.76<a×b+c×d<7.42.

[0067] Example 2 The preparation method in this embodiment is the same as that in Example 1, the only difference being: The specific surface area of ​​the first negative electrode active material, hard carbon, is 8.78 m². 2 / g, with a particle size D50 of 7.78μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 4.12 m². 2 / g, particle size D50 is 3.74μm; The areal density of the first active layer is 0.86 g / cm³. 2 The porosity is 34.6%; The areal density of the second active layer is 0.93 g / cm³. 2 The porosity is 18.8%.

[0068] In this embodiment, a=2.13, b=2.08, c=1.08, d=1.84, a×b+c×d=6.4176, which satisfies 3.76<a×b+c×d<7.42.

[0069] Example 3 The preparation method in this embodiment is the same as that in Example 1, the only difference being: The specific surface area of ​​the first negative electrode active material, hard carbon, is 7.54 m².2 / g, particle size D50 is 7.18μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 4.12 m². 2 / g, particle size D50 is 3.74μm; The areal density of the first active layer is 0.82 g / cm³. 2 The porosity is 34.7%; The areal density of the second active layer is 0.9 g / cm³. 2 The porosity is 25.3%.

[0070] In this embodiment, a=1.83, b=1.92, c=1.1, d=1.37, a×b+c×d=5.0206, which satisfies 3.76<a×b+c×d<7.42.

[0071] Example 4 The preparation method in this embodiment is the same as that in Example 1, the only difference being: The specific surface area of ​​the first negative electrode active material, hard carbon, is 9.4 m². 2 / g, particle size D50 is 5.012μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 4.54 m². 2 / g, particle size D50 is 4.42μm; The areal density of the first active layer is 0.83 g / cm³. 2 The porosity is 25.1%; The areal density of the second active layer is 0.93 g / cm³. 2 The porosity is 18%.

[0072] In this embodiment, a=2.07, b=1.13, c=1.12, d=1.39, a×b+c×d=3.8959, which satisfies 3.76<a×b+c×d<7.42.

[0073] Example 5 The preparation method in this embodiment is the same as that in Example 1, the only difference being: The specific surface area of ​​the first negative electrode active material, hard carbon, is 6.3 m². 2 / g, particle size D50 is 8μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 3.9 m². 2 / g, particle size D50 is 3.11μm; The areal density of the first active layer is 0.88 g / cm³. 2 The porosity is 34.2%; The areal density of the second active layer is 0.93 g / cm³. 2 The porosity is 28%.

[0074] In this embodiment, a=1.62, b=2.57, c=1.06, d=1.22, a×b+c×d=5.4566, which satisfies 3.76<a×b+c×d<7.42.

[0075] Example 6 The preparation method in this embodiment is the same as that in Example 1, the only difference being: The specific surface area of ​​the first negative electrode active material, hard carbon, is 9.34 m². 2 / g, particle size D50 is 7.95μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 5.12 m². 2 / g, particle size D50 is 6μm; The areal density of the first active layer is 0.82 g / cm³. 2 The porosity is 35%; The areal density of the second active layer is 0.85 g / cm³. 2 The porosity is 20.5%.

[0076] In this embodiment, a=1.82, b=1.325, c=1.04, d=1.71, a×b+c×d=4.1899, which satisfies 3.76<a×b+c×d<7.42.

[0077] Comparative Example 1 The preparation method of this comparative example is the same as that of Example 1, except that: The specific surface area of ​​the first negative electrode active material, hard carbon, is 5.03 m². 2 / g, particle size D50 is 5.31μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 4.12 m². 2 / g, particle size D50 is 3.74μm; The areal density of the first active layer is 0.85 g / cm³. 2 The porosity is 29.8%; The areal density of the second active layer is 0.92 g / cm³. 2 The porosity is 22.4%.

[0078] In this comparative example, the specific surface area of ​​the first negative electrode active material is too small, a=1.22, b=1.42, c=1.08, d=1.33, resulting in a×b+c×d=3.1688 being relatively small, which does not satisfy 3.76<a×b+c×d<7.42.

[0079] Comparative Example 2 The preparation method of this comparative example is the same as that of Example 1, except that: The specific surface area of ​​the first negative electrode active material, hard carbon, is 9.4 m². 2 / g, particle size D50 is 7.96μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 3.95 m². 2 / g, particle size D50 is 3.04μm; The areal density of the first active layer is 0.83 g / cm³. 2 The porosity is 33.1%; The areal density of the second active layer is 0.93 g / cm³. 2 The porosity is 18.7%.

[0080] In this comparative example, a=2.38, b=2.62, c=1.12, d=1.77, and a×b+c×d=8.218 is relatively large, which does not satisfy 3.76<a×b+c×d<7.42.

[0081] Comparative Example 3 The preparation method of this comparative example is the same as that of Example 1, except that: The specific surface area of ​​the first negative electrode active material, hard carbon, is 6.5 m². 2 / g, particle size D50 is 7.83μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 7.4 m². 2 / g, particle size D50 is 3.21μm; The areal density of the first active layer is 0.82 g / cm³. 2 The porosity is 32%; The areal density of the second active layer is 0.93 g / cm³. 2 The porosity is 18.6%.

[0082] In this comparative example, a = 0.88 (not satisfying > 1), b = 2.44, c = 1.13, d = 1.72, a × b + c × d = 4.0908, which satisfies 3.76 < a × b + c × d < 7.42.

[0083] Comparative Example 4 The preparation method of this comparative example is the same as that of Example 1, except that: The specific surface area of ​​the first negative electrode active material, hard carbon, is 8.78 m². 2 / g, particle size D50 is 5.12μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 4.12 m². 2 / g, with a particle size D50 of 5.96μm; The areal density of the first active layer is 0.86 g / cm³. 2 The porosity is 34.6%; The areal density of the second active layer is 0.93 g / cm³. 2 The porosity is 18.8%.

[0084] In this embodiment, a=2.13, b=0.86 (not satisfying >1), c=1.08, d=1.84, a×b+c×d=3.819, which satisfies 3.76<a×b+c×d<7.42.

[0085] Comparative Example 5 The specific surface area of ​​the first negative electrode active material, hard carbon, is 8.78 m². 2 / g, with a particle size D50 of 7.78μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 4.12 m². 2 / g, particle size D50 is 3.74μm; The areal density of the first active layer is 0.88 g / cm³. 2 The porosity is 34.6%; The areal density of the second active layer is 0.85 g / cm³. 2 The porosity is 18.8%.

[0086] In this embodiment, a=2.13, b=2.08, c=0.96 (not satisfying >1), d=1.84, a×b+c×d=6.1968, which satisfies 3.76<a×b+c×d<7.42.

[0087] Comparative Example 6 The specific surface area of ​​the first negative electrode active material, hard carbon, is 8.78 m². 2 / g, with a particle size D50 of 7.78μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 4.12 m². 2 / g, particle size D50 is 3.74μm; The areal density of the first active layer is 0.86 g / cm³. 2 The porosity is 25%; The areal density of the second active layer is 0.93 g / cm³. 2 The porosity is 28%.

[0088] In this embodiment, a=2.13, b=2.08, c=1.08, d=0.89 (not satisfying >1), a×b+c×d=5.3916, which satisfies 3.76<a×b+c×d<7.42.

[0089] Comparative Example 7 The preparation method of this comparative example is the same as that of Example 1, except that: The specific surface area of ​​the first negative electrode active material, hard carbon, is 7 m². 2 / g, particle size D50 is 8μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 7.4 m². 2 / g, particle size D50 is 3.21μm; The areal density of the first active layer is 0.88 g / cm³. 2 The porosity is 34.9%; The areal density of the second active layer is 0.85 g / cm³. 2 The porosity is 18.2%.

[0090] In this comparative example, a = 0.94 (not satisfying > 1), b = 2.49, c = 0.97 (not satisfying > 1), d = 1.92, a × b + c × d = 4.203, which satisfies 3.76 < a × b + c × d < 7.42.

[0091] Comparative Example 8 The preparation method of this comparative example is the same as that of Example 1, except that: The specific surface area of ​​the first negative electrode active material, hard carbon, is 7 m². 2 / g, particle size D50 is 5μm; The specific surface area of ​​the second negative electrode active material, hard carbon, is 7.4 m². 2 / g, particle size D50 is 5.8μm; The areal density of the first active layer is 0.86 g / cm³. 2 The porosity is 25%; The areal density of the second active layer is 0.85 g / cm³. 2 The porosity is 25.4%.

[0092] In this comparative example, a=0.95, b=0.86, c=0.99, d=0.98, none of which satisfy >1, and a×b+c×d=1.7872, which does not satisfy 3.76<a×b+c×d<7.42.

[0093] Comparative Example 9 The preparation method of this comparative example is the same as that of Example 1, except that: The specific surface area of ​​the first negative electrode active material, hard carbon, is 9.5 m². 2 / g, the specific surface area of ​​hard carbon, the second negative electrode active material, is 4.54 m². 2 / g.

[0094] In this comparative example, the specific surface area of ​​the first active negative electrode material is too large, with a=20.9, b=1.35, c=1.12, and d=1.76. The sum of a×b+c×d=4.4927, which satisfies the condition 3.76<a×b+c×d<7.42.

[0095] Comparative Example 10 The preparation method of this comparative example is the same as that of Example 1, except that: The particle size D50 of the first negative electrode active material, hard carbon, is 4.8 μm; the particle size D50 of the second negative electrode active material, hard carbon, is 4.42 μm. Among them, the particle size of the first negative electrode active material is too small, a=1.01, b=1.35, c=1.12, d=1.76, a×b+c×d=3.3347, which does not satisfy 3.76<a×b+c×d<7.42.

[0096] Comparative Example 11 The preparation method of this comparative example is the same as that of Example 1, except that: The areal density of the first active layer is 0.83 g / cm³. 2 The areal density of the second active layer is 0.95 g / cm³. 2 .

[0097] Among them, the areal density of the second active layer is too large; a=1.52, b=1.35, c=1.14, d=1.76, a×b+c×d=4.0584, which satisfies 3.76<a×b+c×d<7.42.

[0098] Comparative Example 12 The preparation method of this comparative example is the same as that of Example 1, except that: The porosity of the first active layer is 28.2%, and the porosity of the second active layer is 17.8%.

[0099] Among them, the porosity of the second active layer is too small, a=1.52, b=1.35, c=1.12, d=1.58, a×b+c×d=3.8216, which satisfies 3.76<a×b+c×d<7.42.

[0100] The negative electrode sheets from the above embodiments and comparative examples were assembled into battery cells: The electrode sheets were assembled into battery cells under controlled environmental conditions with a dew point ≤ -30℃. Before electrolyte injection, the battery cells were baked at 90℃ for 24 hours to control moisture content. The positive electrode sheet of the battery cell was a layered oxide oil-based sodium-ion battery positive electrode sheet; the electrolyte was a 1 mol / L solution obtained by uniformly dissolving NaPF6 in equal volumes of ethylene carbonate (EC) and propylene carbonate (PC) organic solvents.

[0101] The battery cell performance was tested using the following specific methods: Cyclic performance test: At 25℃, constant current and constant voltage charge-discharge cycles were performed at 1C / 1C, 2C / 2C, and 3C / 3C rates, respectively. The initial discharge capacity was recorded as C0. After 500 charge-discharge cycles, the discharge capacity on the 500th cycle was recorded as C1. The capacity retention rate was calculated using the following formula: Capacity retention rate = C1 / C0 × 100%. The test results are shown in Table 1.

[0102] Energy density test: At 25℃, the battery was charged at a constant current and constant voltage of 0.1C and then discharged at a constant current. The energy density of the discharge was calculated based on the test results. The test results are shown in Table 1.

[0103] Table 1. Cell performance test results for each embodiment and comparative example.

[0104] As can be seen from Table 1, in Examples 1 to 6, the capacity retention rate of the battery cells after 500 cycles at 1C is above 90.5%, the capacity retention rate after 500 cycles at 2C is above 86%, and the capacity retention rate after 500 cycles at 3C is above 81.5%. Compared with the comparative examples, at 25°C, the capacity retention rate of the battery cells after 500 cycles at 1C, 2C, and 3C is significantly improved, and the energy density of the battery cells is also significantly improved.

[0105] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A negative electrode sheet for a sodium-ion battery, characterized in that, include: current collector; A first active layer is disposed on at least one side of the current collector, and the first active layer is formed by coating the current collector with a first active slurry and drying it. The second active layer is disposed on the side of the first active layer away from the current collector, and the second active layer is formed by coating the surface of the first active layer with a second active slurry and drying it. The first active slurry includes a first negative electrode active material, a conductive agent, and a binder; the second active slurry includes a second negative electrode active material, a conductive agent, and a binder. Wherein, the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is a, the ratio of the D50 of the first negative electrode active material to that of the second negative electrode active material is b, the ratio of the areal density of the second active layer to that of the first active layer is c, and the ratio of the porosity of the first active layer to that of the second active layer is d. a, b, c, and d are all greater than 1, and a, b, c, and d satisfy 3.76 < a × b + c × d < 7.

42.

2. The negative electrode sheet according to claim 1, characterized in that, The specific surface area of ​​the first negative electrode active material is 6.3 m². 2 / g~9.4m 2 / g; and / or, the specific surface area of ​​the second negative electrode active material is 3.9m². 2 / g~7.7m 2 / g.

3. The negative electrode sheet according to claim 1, characterized in that, The areal density of the first active layer is 0.82 g / cm³. 2 ~0.88g / cm 2 ; and / or, the areal density of the second active layer is 0.85 g / cm³. 2 ~0.93g / cm 2 .

4. The negative electrode sheet according to claim 1, characterized in that, The porosity of the first active layer is 25% to 35%; and / or the porosity of the second active layer is 18% to 28%.

5. The negative electrode sheet according to claim 1, characterized in that, The particle size D50 of the first negative electrode active material is 5~8μm, and / or the particle size D50 of the second negative electrode active material is 3~6μm.

6. The negative electrode sheet according to claim 1, characterized in that, The coating thickness of the first active layer is 40~50μm, and / or the coating thickness of the second active layer is 40~50μm.

7. The negative electrode sheet according to any one of claims 1-6, characterized in that, The first negative electrode active material includes hard carbon, and the second negative electrode active material includes hard carbon.

8. The negative electrode sheet according to any one of claims 1-6, characterized in that, The conductive agent includes one or more of conductive carbon black, carbon nanotubes, and graphene; and / or the binder includes one or more of styrene-butadiene rubber, carboxymethyl cellulose, and sodium polyacrylate.

9. A method for preparing a sodium-ion battery negative electrode, characterized in that, The method for preparing the negative electrode sheet according to any one of claims 1-8 includes the following steps: S1, the first negative electrode active material, conductive agent and binder are mixed evenly in the dispersant to obtain the first active layer slurry; S2, the second negative electrode active material, conductive agent and binder are mixed evenly in the dispersant to obtain the second active layer slurry; S3, the first active layer slurry is coated onto a blank current collector and dried to obtain a current collector coated with the first active layer; S4, the second active layer slurry is coated onto the first active layer and dried to obtain a negative electrode sheet coated with the first active layer and the second active layer. S5, the negative electrode sheet is rolled; Wherein, the ratio of the specific surface area of ​​the first negative electrode active material to that of the second negative electrode active material is a, the ratio of the D50 of the first negative electrode active material to that of the second negative electrode active material is b, the ratio of the areal density of the second active layer to that of the first active layer is c, and the ratio of the porosity of the first active layer to that of the second active layer is d. Then a, b, c, and d satisfy: a, b, c, and d are all greater than 1, and 3.76 < a × b + c × d < 7.

42.

10. A sodium-ion battery, characterized in that, include: The negative electrode sheet according to any one of claims 1-8 or the negative electrode sheet prepared by the method according to claim 9.

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