Sodium ion battery

Abstract

The invention relates to the technical field of sodium-ion batteries, and particularly discloses a sodium-ion battery, which is characterized in that a gradient pore negative plate is internally provided with a multi-stage gradient structure of which the porosity is gradually increased from a current collector to the surface of the negative plate; the high-viscosity binder system takes polyacrylic acid as a main body, comprises carboxymethyl chitosan and nano silicon dioxide particles, and further comprises a single-walled carbon nanotube conductive network, according to the three-layer composite structure diaphragm, the middle layer is a high-porosity polyethylene nanofiber membrane, the two sides are polypropylene / polyimide composite layers, and the composite layers are filled with functional components. The gradient pore negative plate structure is adopted, and the porosity gradient is formed by controlling the coating process so as to buffer the volume change stress; a binder system takes polyacrylic acid as a main body, carboxymethyl chitosan and nano silicon dioxide particles are added, and high viscosity is ensured; a single-walled carbon nanotube conductive network is matched to enhance the conductivity; the diaphragm adopts a three-layer composite structure, so that the ionic conductivity is improved, and the SEI membrane forming ability is stabilized.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and specifically to a sodium-ion battery. Background Technology

[0002] Sodium-ion battery technology has developed rapidly, making significant progress in areas such as material interface control and cell structure optimization. For example, the thickness of the high-voltage layered cathode interface film has been reduced from 50nm to 20nm, and the volume expansion rate of the hard carbon anode has been reduced to below 12%. However, the energy density of commercially available sodium-ion batteries is still generally below 160Wh / kg, which is significantly lower than the application threshold of power batteries above 170Wh / kg. This performance bottleneck directly restricts the promotion of sodium-ion batteries in fields with stringent energy density requirements, such as electric vehicles. As a result, their market applications are currently mainly concentrated in scenarios with lower energy density sensitivity, such as energy storage power stations and two-wheeled electric vehicles. In the energy storage field, the insufficient energy density of a single cell can be compensated for by large-scale battery pack design. Two-wheeled vehicles, due to their limited range requirements (usually <100km), can still be adapted to the performance of existing sodium batteries. However, in power scenarios such as passenger cars with range requirements exceeding 300km, the energy density gap of sodium batteries makes it difficult to achieve substantial application breakthroughs.

[0003] To meet the design requirements of sodium-ion batteries with a capacity of 170Wh / kg or higher, novel positive and negative electrodes with ultra-high capacity exceeding conventional capacities are needed.

[0004] The development of positive electrodes is mainly towards high-voltage layered positive electrodes, but this approach faces challenges related to interface stability and rate performance. High-voltage layered positive electrodes above 4.1V are prone to severe oxidative decomposition reactions in the electrolyte, forming an impedance-type interface film with a thickness exceeding 50nm and an interface resistance of 800-1200Ω. The increased overpotential during charging, exceeding 0.3V, occurs due to the increased cm² area. When the rate is increased to 2C, the positive electrode polarization voltage jumps sharply from 0.15V at 0.1C to 0.6V, the energy efficiency drops from 92% to 78%, and the voltage hysteresis reduces the actual usable capacity by more than 20%. Simultaneously, the reaction between the positive electrode material and the electrolyte releases… It will form a gas mixture with the decomposition products of the electrolyte, generating an expansion pressure of 5-8 kPa inside the cell, which will damage the adhesion between the electrode and the separator, further aggravate the ion transport resistance, and limit the rate performance of high energy density cells.

[0005] To meet the design requirements of sodium-ion batteries with a capacity of 170Wh / kg or higher, alloyed anodes with ultra-high capacity (350mAh / g or higher) such as hard carbon or partially phosphorus-based anodes are needed. However, the high-capacity hard carbon anode presents challenges in terms of volume effect and cell compatibility. The significant anisotropic expansion during sodium intercalation in this anode design results in a 15%-50% change in electrode thickness. This periodic deformation generates 10-15MPa of mechanical stress within the cell. When matched with an aluminum current collector, microcracks easily appear at the current collector-active material interface at stress concentration points, causing the interface resistance to increase at a rate of 120mΩ per 100 cycles, resulting in a 40% increase in the cell's DC internal resistance after 500 cycles. Furthermore, the high specific surface area of ​​the hard carbon anode leads to 30% more electrolyte wetting than conventional lithium-ion anodes. Excess electrolyte not only increases the cell weight (by 18%-22%) but also generates free nitrogen through continuous decomposition. Consumption caused the cell's coulombic efficiency to fall below 85% for the first time, and the energy density design value deviated from the actual value by more than 15%.

[0006] Based on this technical background, the present invention studies a sodium-ion battery. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a sodium-ion battery. The battery cell adopts a gradient porosity negative electrode structure. By controlling the coating process, a porosity gradient is formed inside the electrode to buffer the stress caused by volume changes. The binder system is mainly composed of polyacrylic acid, with the addition of carboxymethyl chitosan and nano-silica particles to ensure high viscosity and strong adhesion. It is combined with a single-walled carbon nanotube conductive network to enhance conductivity. The separator adopts a three-layer composite structure, which has both high ionic conductivity and stable SEI film formation capability.

[0008] To achieve the above objectives, the present invention provides a sodium-ion battery, comprising: Gradient porosity negative electrode sheet has a multi-level gradient structure with gradually increasing porosity from the current collector to the surface of the negative electrode sheet, which is used to buffer the volume change stress during the charging and discharging process of sodium-ion battery. The high-viscosity binder system is based on polyacrylic acid, including carboxymethyl chitosan and nano-silica particles to ensure high viscosity and strong adhesion performance, and also includes a single-walled carbon nanotube conductive network to enhance conductivity. The membrane has a three-layer composite structure, with a middle layer of high-porosity polyethylene nanofiber membrane and two sides of polypropylene / polyimide composite layers. The polypropylene / polyimide composite layers are filled with functional components to improve ion conductivity and stabilize SEI membrane formation.

[0009] The beneficial effects of this invention include: (1) The sodium-ion battery proposed in this invention adopts a gradient pore negative electrode structure. By controlling the coating process, a porosity gradient is formed inside the electrode to buffer the stress of volume change. The binder system is mainly composed of polyacrylic acid, with the addition of carboxymethyl chitosan and nano silica particles to ensure high viscosity and strong adhesion performance. It is combined with a single-walled carbon nanotube conductive network to enhance conductivity. The separator adopts a three-layer composite structure, which has both high ion conductivity and stable SEI film formation capability.

[0010] (2) The sodium-ion battery proposed in this invention has a high-porosity polyethylene nanofiber membrane in the middle of the separator and polypropylene sodium-ion battery / sodium-ion battery polyimide composite layers on both sides, filled with functional components such as ionic liquid and nano magnesium oxide, which have high ion conductivity, the ability to capture acidic substances and stabilize SEI film formation; its cell achieves an energy density of 178~188Wh / kg, significantly improving the overall performance of the battery.

[0011] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0012] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings.

[0013] Figure 1 This is a schematic diagram of the structure of the sodium-ion battery negative electrode sheet proposed in this invention. Detailed Implementation

[0014] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein.

[0015] This invention provides a sodium-ion battery, such as... Figure 1 As shown, it includes: Gradient porosity negative electrode sheet has a multi-level gradient structure with gradually increasing porosity from the current collector to the surface of the negative electrode sheet, which is used to buffer the volume change stress during the charging and discharging process of sodium-ion battery. The high-viscosity binder system is based on polyacrylic acid, including carboxymethyl chitosan and nano-silica particles to ensure high viscosity and strong adhesion performance, and also includes a single-walled carbon nanotube conductive network to enhance conductivity. The membrane has a three-layer composite structure, with a high-porosity polyethylene nanofiber membrane in the middle layer and polypropylene / polyimide composite layers on both sides. The polypropylene / polyimide composite layers are filled with functional components to improve ion conductivity and stabilize SEI membrane formation.

[0016] In this invention, the battery cell adopts a gradient porosity electrode structure. By controlling the coating process, a porosity gradient is formed inside the electrode to buffer the stress caused by volume change. The binder system is mainly composed of polyacrylic acid, with the addition of carboxymethyl chitosan and nano-silica particles to ensure high viscosity and strong adhesion. It is combined with a single-walled carbon nanotube conductive network to enhance conductivity. The separator adopts a three-layer composite structure, which has both high ionic conductivity and stable SEI film formation capability.

[0017] According to the present invention, the porosity near the current collector of the sodium-ion battery gradient pore negative electrode is 20-30%, and the surface porosity is 40-50%.

[0018] According to the present invention, the multi-level gradient structure of the sodium-ion battery includes: Small-diameter hard carbon products are located near the current collector side of sodium-ion batteries, including bottom small-diameter particles, bottom medium-diameter particles, and bottom large-diameter particles. Large-diameter hard carbon products, located near the surface of the sodium-ion battery negative electrode, include upper layer small-diameter particles, upper layer medium-diameter particles, and upper layer large-diameter particles.

[0019] According to the present invention, the particle size of the small-diameter particles at the bottom layer of the sodium-ion battery is in the range of 5μm≤D10≤2.5μm, which is used to ensure that a tightly packed structure is formed near the current collector of the sodium-ion battery, reduce the contact resistance between the current collector of the sodium-ion battery and the active material, and at the same time keep the porosity near the current collector of the sodium-ion battery in the target range of 20~30%. The particle size range of the median particle size in the bottom layer of sodium-ion batteries is 5μm≤D50≤7μm. It is used to cooperate with the small particle size in the bottom layer of sodium-ion batteries to build a dense bottom layer structure on the surface of the current collector of sodium-ion batteries, providing stable support for the large particle size hard carbon products of sodium-ion batteries on the upper layer, and maintaining a low porosity in this area. The particle size range of the large-diameter particles in the bottom layer of sodium-ion batteries is 8μm≤D90≤10μm. Their proportion is small, which is used to avoid the formation of excessively large pores near the current collector of sodium-ion batteries, maintain the compactness and stability of the bottom layer structure, and meet the requirements of porosity and structural stability near the current collector of sodium-ion batteries. The specific surface area of ​​small-diameter hard carbon products for sodium-ion batteries is 2m² / g to 4m² / g, and the tap density is 0.8g / cm³ to 1.0g / cm³.

[0020] According to the present invention, the particle size of the small-diameter particles in the upper layer of the sodium-ion battery is in the range of 3μm≤D10≤5μm, which is used to form larger initial pores near the surface of the sodium-ion battery negative electrode, providing more channels for electrolyte wetting and ion transport, and helping to improve the porosity of the negative electrode surface. The particle size range of the median particle size in the upper layer of sodium-ion batteries is 8μm≤D50≤12μm. This is to allow for the formation of larger gaps when the particles are stacked, which promotes the porosity of the negative electrode surface to approach 40-50%, thus meeting the high porosity design requirements of the negative electrode surface. The particle size of the large-diameter particles in the upper layer of sodium-ion batteries ranges from 15μm to D90 to 20μm, accounting for a large proportion. This is used to further increase the pore size and number on the surface of the negative electrode, fully realize the high porosity of the negative electrode surface, facilitate rapid electrolyte penetration and ion diffusion, and buffer the volume change stress during charging and discharging. The specific surface area of ​​large-particle hard carbon products for sodium-ion batteries is 4m² / g to 6m² / g, and the tap density is 0.6g / cm³ to 0.8g / cm³.

[0021] According to the present invention, carboxymethyl chitosan accounts for 3-5 wt% of the total weight of the sodium-ion battery high-viscosity binder system. It is used to increase the viscosity of the sodium-ion battery high-viscosity binder system, enhance the interaction with the active material and the surface of the sodium-ion battery current collector, improve the bonding strength, ensure that the bonding strength between the negative electrode sheet and the sodium-ion battery current collector is ≥1.2 N / cm, and at the same time improve the flexibility and water resistance of the sodium-ion battery high-viscosity binder system, and improve the processing performance and stability of the negative electrode sheet. Nano-sized silica particles, with a particle size of 10-150 nm, account for 2-3 wt% of the total weight of the high-viscosity binder system for sodium-ion batteries. They are used to increase the tensile modulus of the high-viscosity binder system to 0.8-1.2 GPa, thereby enhancing the mechanical properties of the binder, strengthening the internal structure of the high-viscosity binder system, improving the overall bonding performance and stability, and enhancing the wear and tear resistance of the negative electrode sheet.

[0022] In this invention, polyacrylic acid (PAA) is used as the main component. The polyacrylic acid molecular chain contains a large number of carboxyl groups, which can form hydrogen bonds or ionic bonds with the active material of the electrode and the hydroxyl groups on the surface of the current collector, providing basic adhesion and ensuring the bonding between the active material and the current collector. At the same time, it has good water solubility, which can uniformly disperse the components during the preparation of the slurry. In addition, it has a certain viscosity, which helps to form a high-viscosity binder system sodium-ion battery, laying the foundation for the overall bonding performance.

[0023] In this invention, carboxymethyl chitosan (CMCS) has a unique macromolecular structure that can intertwine with polyacrylic acid molecular chains to form a more complex network structure, significantly increasing the viscosity of the binder. At the same time, the functional groups such as amino and carboxyl groups in the molecule can further enhance the interaction with the active material and the surface of the current collector, improve the bonding strength, ensure that the bonding strength between the electrode and the current collector is ≥1.2 N / cm, and improve the flexibility and water resistance of the binder system, thereby improving the processing performance and stability of the electrode.

[0024] In this invention, nano-silica (Si) Due to their nanoscale size (10-150 nm), the nanoparticles can be uniformly dispersed in the binder system, filling the gaps between molecular chains and acting as physical cross-linking points. This restricts the relative movement of molecular chains, thereby increasing the tensile modulus of the binder to 0.8~1.2 GPa and enhancing its mechanical properties. In addition, the nano-silica particles have a large number of silanol groups on their surface, which can form hydrogen bonds with other components, further strengthening the internal structure of the binder system, improving the overall bonding performance and stability, and helping to improve the wear and tear resistance of the electrodes in sodium-ion batteries.

[0025] According to the present invention, the single-walled carbon nanotube conductive network of the sodium-ion battery has a diameter of 1~2 nm, a length of 5~15 μm, and an electronic conductivity ≥1. S / m is used to build a more efficient electron transport network, reduce the internal resistance of the negative electrode, adapt to the volume change of the negative electrode during charging and discharging, reduce the risk of conductive network breakage, improve cycle stability, closely adhere to the surface of the active material, increase the contact area, and is suitable for the conductivity requirements of high-capacity active materials. The amount of single-walled carbon nanotube conductive network added to the positive electrode formulation of sodium-ion batteries is 0.05~1.0wt%, and the amount added to the negative electrode formulation is 0.01~1.5wt%.

[0026] In this invention, the single-walled carbon nanotube conductive network exhibits higher conductivity: the single-walled carbon nanotubes have a smaller diameter (typically < 2 nm), fewer structural defects, and a conductivity reaching (10⁻⁶ nm). 4 ~10 5 S / m), significantly better than multi-walled carbon nanotubes (10 3 ~10 4 S / m) can be used to build a more efficient electron transport network and reduce the internal resistance of the electrode. Greater flexibility and dispersibility: The single-walled carbon nanotubes have uniform length and smooth surface, making them easier to disperse in electrode slurry. Their flexible structure can better adapt to volume changes during electrode charging and discharging, reducing the risk of conductive network breakage and improving cycle stability. Superior interface compatibility: Nanoscale dimensions (e.g., 5~15μm length) can closely adhere to the surface of active materials, increasing the contact area and enhancing the interface electronic conduction efficiency, making it particularly suitable for the conductivity requirements of high-capacity active materials (e.g., layered positive electrodes, hard carbon negative electrodes).

[0027] According to the present invention, the thickness of the three-layer composite separator of sodium-ion battery is 5-8 μm; The porosity of the polyethylene nanofiber membrane for sodium-ion batteries is 60-70%, and the thickness is 3-5 μm. The thickness of the polypropylene / polyimide composite layer in sodium-ion batteries is 1.0-2.0 μm.

[0028] According to the present invention, the functional components of a sodium-ion battery include an ionic liquid, nano-magnesium oxide, and borate ester derivatives. The amount of ionic liquid added to sodium-ion batteries accounts for 5-10 wt% of the total weight of the polypropylene / polyimide composite layer, in order to improve ionic conductivity. The amount of nano-magnesium oxide added to sodium-ion batteries accounts for 2-5 wt% of the total weight of the polypropylene / polyimide composite layer, and is used to capture acidic substances. The borate ester derivatives of sodium-ion batteries account for 1-3 wt% of the total weight of the polypropylene / polyimide composite layer in sodium-ion batteries and are used to form a stable SEI film.

[0029] According to the present invention, the three-layer composite structure membrane of sodium-ion battery has a porosity of 50~60%, an air permeability of 120~180s / 100cc, a compressive strength of ≥30MPa, a compression ratio of ≤80%, and a resilience of ≥90%.

[0030] In this invention, the membrane structure adopts a three-layer composite structure with a total thickness of 7 μm. The middle layer is a polyethylene (PE) nanofiber membrane (porosity 60-70%) with a thickness of 4 μm. The two sides are polypropylene (PP) / polyimide (PI) composite layers with a thickness of 1.5 μm. The internal voids are filled with functional components. Among the functional components, 5-10 wt% ionic liquid (such as EMIM-TFSI) is used to improve ionic conductivity, 2-5 wt% nano magnesium oxide (MgO) is used to capture acidic substances such as HF, and 1-3 wt% borate ester derivative is used to form a stable SEI membrane.

[0031] In this invention, the middle of the separator is a high-porosity polyethylene nanofiber membrane, and the two sides are polypropylene sodium-ion battery / sodium-ion battery polyimide composite layers, filled with functional components such as ionic liquid and nano-magnesium oxide, which have the combined functions of high ion conductivity, capture of acidic substances and stable SEI film formation capability; its cell achieves an energy density of 178~188Wh / kg, significantly improving the overall performance of the battery.

[0032] The present invention will be described in more detail below through embodiments.

[0033] Example 1

[0034] This embodiment provides a sodium-ion battery, such as Figure 1 As shown, it includes: Gradient porosity negative electrode has a multi-level gradient structure with gradually increasing porosity from the current collector to the surface of the negative electrode, which is used to buffer the volume change stress during the charging and discharging process of sodium-ion batteries. The high-viscosity binder system is based on polyacrylic acid, including carboxymethyl chitosan and nano-silica particles to ensure high viscosity and strong adhesion performance, and also includes a single-walled carbon nanotube conductive network to enhance conductivity. The membrane has a three-layer composite structure, with a high-porosity polyethylene nanofiber membrane in the middle layer and polypropylene / polyimide composite layers on both sides. The polypropylene / polyimide composite layers are filled with functional components to improve ion conductivity and stabilize SEI membrane formation.

[0035] This embodiment provides a sodium-ion battery fabrication process and testing method as shown below: (1) Preparation of positive electrode: Weigh out 33 mol of nickel oxide (NiO) and 7.7 mol of iron oxide (Fe3O4) with a purity ≥ 99.5%. ), 18 mol manganese tetroxide (M 5 mol vanadium pentoxide ( ) and 55 mol sodium carbonate (N C The powder was used as the raw material for synthesis. It was thoroughly mixed with 2g of carbon nanotubes (CNTs, conductive additives) and placed in an alumina crucible. The mixture was heated to 950°C in a muffle furnace at a heating rate of 2°C / min and 5% C2O2 was introduced. After calcining with a mixed Ar gas for 12 hours and naturally cooling to room temperature, the sintered material was pulverized in a planetary ball mill at 400 rpm for 2 hours and then classified through a 300-mesh sieve (sieve aperture size of about 50 μm) to obtain a layered oxide sodium-ion battery cathode material with an average particle size of 8 μm. Preparation of positive electrode slurry: 96-98 wt% active material, 0.3-1 wt% conductive carbon black, 0.05-1 wt% single-walled carbon nanotubes, 0.9-1.5 wt% PVDF binder, and NMP solvent; the slurry is coated on 10 μm thick aluminum foil with an areal density of 300-450 g / m². 2 After drying, it is rolled to a compaction density of 3.2 g / cm³; (2) Anode preparation: 27.8 mol of glucose, 0.5 mol of sodium nitrate, and 0.03 mol of ferric citrate (iron dopant) were weighed and dissolved in deionized water. The mixture was stirred at 80°C to form a gel. It was pre-carbonized at 500°C under nitrogen atmosphere for 2 hours, then ground and carbonized at 1300°C under argon atmosphere for 5 hours. The product was then soaked in hydrochloric acid to remove impurities, washed with deionized water until neutral, and vacuum dried at 100°C for 12 hours. After drying, it was crushed by ball milling and sieved by air classifier to obtain two hard carbon anode materials with average particle sizes of 5 μm and 9 μm. Negative electrode slurries were prepared using two different hard carbon materials, with active material comprising 88-96 wt%, conductive agent (single-walled carbon nanotubes) comprising 0.01-1.5 wt%, and binder comprising PAA:CMCS:Si. =85:10:5) 3~10.5wt%, solvent is deionized water; coated on a planar aluminum current collector, of which 5µm paste is applied to the bottom and 9µm paste is applied to the top, with a total areal density of 150~250 g / m³. 2 After drying, it is rolled to a compaction density of 1.1 g / cm³; (3) Cell assembly and testing: Using a stacking process, the positive electrode, separator, and negative electrode are stacked sequentially to form a 7.5Ah battery cell; an electrolyte (EC:PC:EMC = 3:3:4, 1.2M NaP) is injected at a rate of 4g / Ah. (2% FEC + 1% VC + 0.5% TMSB); encapsulated in a vacuum environment, pre-charged and formed to form a soft-pack battery; the initial charge and discharge efficiency and cycle life of the cell are tested in the voltage range of 1.5-4.15V.

[0036] The specific control parameters and test results of Examples 2-6 and Comparative Examples 1-2 of this application are shown in Table 1.

[0037] Table 1. Specific control parameters and test results for Examples 2-6 and Comparative Examples 1-2

[0038] As can be seen from the comparison of the specific control parameters and test results of Examples 2-6 and Comparative Examples 1-2 in Table 1, the battery cell adopts a gradient porosity electrode structure, and the porosity gradient is formed inside the electrode by controlling the coating process; the binder system is mainly composed of polyacrylic acid, with the addition of carboxymethyl chitosan and nano silica particles, combined with a single-walled carbon nanotube conductive network, and the separator adopts a three-layer composite structure. The energy density, first charge and discharge efficiency and cycle capacity retention of the sodium-ion battery are significantly improved.

[0039] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A sodium-ion battery, characterized in that, include: Gradient porosity negative electrode sheet has a multi-level gradient structure with gradually increasing porosity from the current collector to the surface of the negative electrode sheet, which is used to buffer the volume change stress during the charging and discharging process of sodium-ion battery. The high-viscosity binder system is based on polyacrylic acid, including carboxymethyl chitosan and nano-silica particles to ensure high viscosity and strong adhesion performance, and also includes a single-walled carbon nanotube conductive network to enhance conductivity. The membrane has a three-layer composite structure, with a middle layer of high-porosity polyethylene nanofiber membrane and two sides of polypropylene / polyimide composite layers. The polypropylene / polyimide composite layers are filled with functional components to improve ion conductivity and stabilize SEI membrane formation.

2. The sodium-ion battery according to claim 1, characterized in that, The porosity near the current collector of the gradient porosity negative electrode is 20-30%, and the surface porosity is 40-50%.

3. The sodium-ion battery according to claim 1, characterized in that, The multi-level gradient structure includes: Small-diameter hard carbon products, near the current collector side, include bottom small-diameter particles, bottom medium-diameter particles, and bottom large-diameter particles. The large-diameter hard carbon product, near the surface of the negative electrode, includes upper layer small-diameter particles, upper layer medium-diameter particles, and upper layer large-diameter particles.

4. The sodium-ion battery according to claim 3, characterized in that, The particle size of the bottom small-diameter particles is in the range of 5μm≤D10≤2.5μm, which is used to ensure that a tightly packed structure is formed near the current collector, reduce the contact resistance between the current collector and the active material, and at the same time keep the porosity near the current collector in the target range of 20~30%. The particle size of the median particle in the bottom layer is in the range of 5μm≤D50≤7μm. It is used to cooperate with the small particle size in the bottom layer to build a dense bottom layer structure on the surface of the current collector, providing stable support for the large particle hard carbon product in the upper layer, and maintaining a low porosity in this area. The particle size of the bottom layer large-diameter particles ranges from 8μm to D90 to 10μm. Their proportion is small, which is used to avoid the formation of excessively large pores near the current collector, maintain the compactness and stability of the bottom layer structure, and meet the requirements of porosity and structural stability near the current collector. The specific surface area of ​​the small-diameter hard carbon product is 2m² / g to 4m² / g, and the tap density is 0.8g / cm³ to 1.0g / cm³.

5. The sodium-ion battery according to claim 3, characterized in that, The particle size of the upper layer small-diameter particles ranges from 3μm≤D10≤5μm, which is used to form larger initial pores near the surface of the negative electrode, providing more channels for electrolyte wetting and ion transport, and helping to improve the porosity of the negative electrode surface. The particle size of the upper median particle is in the range of 8μm≤D50≤12μm, which is used to form larger gaps when the particles are stacked, so as to promote the porosity of the negative electrode surface to approach 40-50%, and meet the high porosity design requirements of the negative electrode surface. The particle size of the upper layer large-diameter particles ranges from 15μm≤D90≤20μm, which accounts for a large proportion. This is used to further increase the pore size and number on the surface of the negative electrode, fully realize the high porosity of the negative electrode surface, facilitate rapid electrolyte penetration and rapid ion diffusion, and buffer the volume change stress during charging and discharging. The specific surface area of ​​the large-diameter hard carbon product is 4m² / g to 6m² / g, and the tap density is 0.6g / cm³ to 0.8g / cm³.

6. The sodium-ion battery according to claim 1, characterized in that, The carboxymethyl chitosan accounts for 3-5 wt% of the total weight of the high-viscosity binder system. It is used to increase the viscosity of the high-viscosity binder system, enhance the interaction with the active material and the surface of the current collector, improve the bonding strength, ensure that the bonding strength between the negative electrode sheet and the current collector is ≥1.2 N / cm, and at the same time improve the flexibility and water resistance of the high-viscosity binder system, and improve the processing performance and stability of the negative electrode sheet. The nano-silica particles account for 2-3 wt% of the total weight of the high-viscosity binder system, and have a particle size of 10-150 nm. They are used to increase the tensile modulus of the high-viscosity binder system to 0.8-1.2 GPa, thereby enhancing the mechanical properties of the binder, strengthening the internal structure of the high-viscosity binder system, improving the overall bonding performance and stability, and enhancing the wear resistance and tear resistance of the negative electrode sheet.

7. The sodium-ion battery according to claim 1, characterized in that, The single-walled carbon nanotube conductive network has a diameter of 1~2 nm, a length of 5~15 μm, and an electronic conductivity ≥1. S / m is used to build a more efficient electron transport network, reduce the internal resistance of the negative electrode, adapt to the volume change of the negative electrode during charging and discharging, reduce the risk of conductive network breakage, improve cycle stability, closely adhere to the surface of the active material, increase the contact area, and is suitable for the conductivity requirements of high-capacity active materials. The single-walled carbon nanotube conductive network is added in the sodium-ion battery positive electrode formulation at an amount of 0.05~1.0wt% and in the negative electrode formulation at an amount of 0.01~1.5wt%.

8. The sodium-ion battery according to claim 1, characterized in that, The thickness of the three-layer composite membrane is 5-8 μm; The polyethylene nanofiber membrane has a porosity of 60-70% and a thickness of 3-5 μm. The thickness of the polypropylene / polyimide composite layer is 1.0-2.0 μm.

9. The sodium-ion battery according to claim 1, characterized in that, The functional components include ionic liquids, nano-magnesium oxide, and borate ester derivatives. The amount of ionic liquid added accounts for 5-10 wt% of the total weight of the polypropylene / polyimide composite layer, and is used to improve ionic conductivity. The amount of nano-magnesium oxide added accounts for 2-5 wt% of the total weight of the polypropylene / polyimide composite layer, and is used to capture acidic substances; The borate ester derivative accounts for 1-3 wt% of the total weight of the polypropylene / polyimide composite layer and is used to form a stable SEI film.

10. The sodium-ion battery according to claim 1, characterized in that, The three-layer composite membrane has a porosity of 50-60%, an air permeability of 120-180s / 100cc, a compressive strength of ≥30MPa, a compression rate of ≤80%, and a resilience of ≥90%.

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