Sodium ion battery

Abstract

The invention relates to a sodium ion battery, and belongs to the technical field of sodium ion batteries. The battery cell of the sodium-ion battery comprises a positive plate, a negative plate and an electrolyte, the active material of the negative plate is carbon-coated nitrogen-doped metal carbide; the chemical formula of the carbon-coated nitrogen-doped metal carbide is M2C1-xNx (at) C, x is more than or equal to 0.05 and less than or equal to 0.15, and M is selected from Mo and / or Ti; the particle size of the metal carbide is lt; 50 nm; the thickness of an amorphous carbon layer in the carbon-coated nitrogen-doped metal carbide is 3 nm to 5 nm; the electrolyte consists of fluoride-free electrolyte salt, an additive and an organic solvent; the additive is selected from one or more of polythiocarbonate, sulfonated polyether, poly (ethylene disulfide) and poly (trifluoro ethyoxyl) phosphazene, and the problem of volume expansion of the sodium ion battery negative electrode is systematically solved.

Description

Technical Field

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

[0002] With the ever-growing global demand for sustainable energy storage, the development of efficient, low-cost, and environmentally friendly large-scale energy storage systems has become an urgent priority. Sodium-ion batteries, with their significant advantages of abundant, widely distributed, and inexpensive sodium resources, have demonstrated enormous application potential in the field of large-scale energy storage and have received widespread attention from the scientific and industrial communities in recent years.

[0003] Sodium-ion batteries primarily rely on the reversible insertion and extraction of sodium ions between the positive and negative electrodes to store and release charge. In this process, the negative electrode material, as a core component affecting battery energy density, cycle life, and safety, has always been a hot topic and key area of ​​research in sodium-ion batteries. Currently, researchers have developed various types of sodium-ion battery negative electrode materials, mainly including carbon-based materials, titanium-based compounds, metal oxides, and metal carbides. Different types of negative electrode materials have their own characteristics, among which metal carbides (such as TiC and Mo2C) have become highly anticipated candidate materials due to their unique sodium storage mechanism and performance characteristics.

[0004] Metal carbides possess high theoretical specific capacity, good conductivity, and excellent chemical stability. During sodium storage, their unique crystal structure provides abundant storage sites for sodium ions, while their chemical bonding characteristics help stabilize the insertion and extraction of sodium ions, thus exhibiting good rate performance and cycle stability. However, like other sodium-ion battery anode materials, metal carbides also face many challenges in practical applications. Sodium ions have a larger radius than lithium ions, an inherent characteristic that causes significant volume changes during insertion / extraction from the anode material. This volume change generates substantial structural stress within the electrode material. With increasing charge-discharge cycles, the accumulated structural stress leads to cracking and pulverization of the electrode material, ultimately causing the active material to peel off from the electrode surface. These problems severely affect the battery's cycle life, accelerate capacity decay, and become a major bottleneck restricting the performance improvement and commercial application of sodium-ion batteries.

[0005] Furthermore, the stability of the solid electrolyte interphase (SEI) membrane is also a key factor affecting the cycle performance of sodium-ion batteries. Traditional SEI membranes (such as those with Na₂CO₃ / NaF as the main component) are brittle and prone to cracking during battery cycling. This cracking triggers a series of problems: on the one hand, it leads to the continuous decomposition of the electrolyte, consuming a large amount of active sodium through the "SEI self-healing" process; on the other hand, it causes electrode particle pulverization and continuous accumulation of interfacial impedance. These problems work together to ultimately lead to increased battery polarization and a significant decrease in cycle performance.

[0006] Therefore, developing sodium-ion batteries that can suppress volume expansion, stabilize the SEI film structure, and achieve high capacity, high conductivity, and excellent cycle performance remains a key technical challenge that urgently needs to be solved, and is of great significance for promoting the practical application of sodium-ion batteries. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a sodium-ion battery that systematically solves the volume expansion problem of the negative electrode in sodium-ion batteries.

[0008] The purpose of this invention is to provide a sodium-ion battery, wherein the cell of the sodium-ion battery includes a positive electrode, a negative electrode, and an electrolyte;

[0009] The active material of the negative electrode is a carbon-coated nitrogen-doped metal carbide; the chemical formula of the carbon-coated nitrogen-doped metal carbide is M2C. 1-x N x @C, where 0.05≤x≤0.15, M is selected from Mo and / or Ti; the particle size of the metal carbide is <50nm; the thickness of the amorphous carbon layer in the carbon-coated nitrogen-doped metal carbide is 3nm-5nm;

[0010] The electrolyte is composed of a fluorine-free electrolyte salt, additives, and an organic solvent; the additives are selected from one or more of polythiocarbonate, sulfonated polyether, poly(ethylene disulfide), and poly(trifluoroethoxy)phosphazene.

[0011] In one embodiment of the present invention, the method for preparing the carbon-coated nitrogen-doped metal carbide includes the following steps:

[0012] S1. Dissolve the metal source, carbon / nitrogen source and chelating agent in a solvent, and obtain nitrogen-doped metal carbide by freeze drying and sintering.

[0013] S2. The nitrogen-doped metal carbide described in S1 is dispersed in an organic carbon source solution, and then heated, stirred, and annealed to obtain the carbon-coated nitrogen-doped metal carbide.

[0014] In one embodiment of the present invention, in S1, the metal source is selected from a titanium source and / or a molybdenum source; the titanium source is selected from titanium tetrachloride (TiCl4) and / or tetrabutyl titanate (C 16 H 36 The molybdenum source is selected from molybdenum pentachloride (MoCl5) and / or ammonium molybdate ((NH4)2MoO4);

[0015] The carbon / nitrogen source is selected from one or more of urea (CO(NH2)2), melamine (C3H6N6), and dicyandiamide (C2H4N4);

[0016] The chelating agent is selected from one or more of ethylenediaminetetraacetic acid (EDTA), tartaric acid, acetic acid, oxalic acid, gluconic acid, polyacrylic acid (PAA), and citric acid.

[0017] The solvent is selected from one or more of ethylene glycol, ethanol, and water;

[0018] The amounts of the metal source and the carbon / nitrogen source are used according to the stoichiometric ratio, and the amount of carbon / nitrogen source is 5%-10% higher than the stoichiometric ratio to prevent incomplete reaction.

[0019] In one embodiment of the present invention, in S1, the drying is performed by rapid freezing at -55°C to -45°C followed by vacuum drying for 16-48 hours;

[0020] The sintering process is carried out in an H2 / Ar atmosphere, first heating to 400℃-800℃ at a rate of 3℃ / min-5℃ / min and holding for 1h-2h, then holding at 900℃-1200℃ for 2h-3h; the volume ratio of H2 in the H2 / Ar atmosphere is 5%-10%; in the low-temperature stage, the metal source decomposes; in the high-temperature stage, carbothermic reduction occurs, and nitrogen atoms enter the metal carbide lattice through chemical adsorption or substitution of carbon sites to form doping.

[0021] In one embodiment of the present invention, in S2, the organic carbon source is selected from one or more of citric acid, ethanol, sucrose, glucose and polyacrylic acid;

[0022] The concentration of the organic carbon source solution is 4wt%-6wt%, for example, it can be 4wt%, 5wt%, 6wt%, etc.; the solvent is one or more of ethanol, ethylene glycol, isopropanol, N-methylpyrrolidone methanol and water.

[0023] In one embodiment of the present invention, in S2, the heating and stirring temperature is 75°C-85°C, for example, it can be 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, etc.

[0024] The annealing is performed under a protective atmosphere at 450℃-550℃ for 1-2 hours.

[0025] In one embodiment of the present invention, the fluorine-free electrolyte salt is selected from one or more of sodium perchlorate, sodium bis(oxalate)borate, sodium bis(salicylic acid)borate, and sodium tetraphenylborate;

[0026] The organic solvent is selected from one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, and diethylene glycol dimethyl ether.

[0027] In one embodiment of the present invention, the conductive agent of the negative electrode is obtained by mixing carbon nanotubes (CNTs) and reduced graphene oxide (rGO) in a mass ratio of (0.9-1.1):0.2; when CNTs and rGO are used as composite conductive agents, the conductive network they construct can buffer volume changes through a slip-rearrangement mechanism.

[0028] In one embodiment of the present invention, the binder of the negative electrode sheet is obtained by mixing polyacrylic acid (PAA) and sodium alginate (SA) in a mass ratio of (2.8-3.2):1; the carboxyl groups of SA can form hydrogen bonds with the hydroxyl groups on the surface of the metal carbide, while the elastic segments of PAA (with an elastic modulus of about 0.1 GPa) can play a role in relieving strain.

[0029] In one embodiment of the present invention, the concentration of the fluoride-free electrolyte salt in the electrolyte is 0.1 mol / L-2 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L. The concentrations of additives are 0.1 wt% to 2 wt%, for example, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2 wt%, etc.

[0030] In one embodiment of the present invention, the active material of the positive electrode is a polyanionic compound.

[0031] In one embodiment of the present invention, the polyanionic compound is selected from Na+ phosphate. x M y (PO4) z Na pyrophosphate x M y (P2O7) z Mixed phosphate Na x M y (PO4) z (P2O7) z Na fluorophosphate x M y (PO4) z F, sulfate Na x M y (SO4) z and silicate Na x M y One or more of (SiO4); wherein M is selected from one or more of Fe, Mn, Ti, V, Ni, Co, Zr and Cr.

[0032] Further, the phosphate is Na3V(PO4)2 phosphate; the mixed phosphate Na x M y (PO4) z (P2O7) z Selected from Na4Fe3(PO4)2(P2O7) and / or Na4Fe 3-x Mn x (PO4)2(P2O7); the sulfate is selected from Na2Fe2(SO4)3 and / or NaFe(SO4)2.

[0033] The technical solution of the present invention has the following advantages compared with the prior art:

[0034] (1) The sodium-ion battery of the present invention uses a nitrogen-doped metal carbide coated with carbon as the negative electrode active material. The introduction of nitrogen will form a local lattice compression region (such as N-Mo2C). This structure can control the lattice parameters of the metal carbide through doping or defect engineering, thereby reducing the lattice distortion rate during sodium ion insertion and ultimately effectively reducing the volume expansion rate caused by sodium ion insertion. In addition, by constructing a nanocrystalline-amorphous composite structure, the high elastic modulus of the amorphous phase (amorphous carbon layer) can be used to absorb grain boundary stress.

[0035] (2) The sodium-ion battery of the present invention incorporates S or P-containing polymers into the electrolyte. These polymers preferentially polymerize on the surface of metal carbides to form an elastic SEI film. The specific mechanism is as follows: by designing the functional groups on the polymer surface, an SEI film with a flexible-rigid composite structure is induced to form. The phosphorus-carbon (PC) bond or carbon-sulfur (CS) bond has low bond energy, enabling the SEI film formed on the negative electrode surface to possess molecular chain flexibility and dynamic breaking and recombination capabilities, similar to a "molecular spring." This characteristic allows the SEI film to effectively mitigate the anisotropic volume changes generated in the metal carbides during sodium ion insertion and extraction, thereby ensuring the battery's cycle performance. Detailed Implementation

[0036] The present invention will be further described below with reference to specific embodiments, so that those skilled in the art can better understand and implement the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. It should be understood that the specific embodiments are only used to explain the present invention, but the embodiments are not intended to limit the present invention.

[0037] In this invention, unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0038] In this invention, unless otherwise stated, the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0039] In this invention, unless otherwise specified, the experimental methods used in the embodiments of this invention are conventional methods, and the materials and reagents used are commercially available unless otherwise specified.

[0040] In this invention, unless otherwise stated, the Mw of the polyacrylic acid used in the embodiments of this invention is 450,000.

[0041] In this invention, unless otherwise stated, the polythiocarbonate used in the embodiments of this invention has a Mw of approximately 1000.

[0042] Example 1

[0043] The sodium-ion battery and its preparation method in this embodiment specifically include the following steps:

[0044] S1, Preparation of the negative electrode

[0045] S11. Preparation of nitrogen-doped metal carbides: Tetrabutyl titanate and urea were dissolved in ethylene glycol at a molar ratio of 1:4. Citric acid (0.1 mol / L in solution) was added as a chelating agent and stirred at 60 °C for 12 h. Then, the mixture was rapidly frozen at -50 °C and vacuum dried for 48 h. Finally, the mixture was placed in a tube furnace and heated to 400 °C at a rate of 5 °C / min in an H2 / Ar atmosphere (H2 volume percentage of 5%) and held for 1 h. Then, it was held at 1200 °C for 3 h to obtain nitrogen-doped metal carbides.

[0046] S12. Preparation of carbon-coated nitrogen-doped metal carbides: Nitrogen-doped metal carbides were dispersed in a 5 wt% citric acid / ethanol solution and evaporated to a gel state under stirring at 80°C. Then, the solution was annealed at 500°C for 1 h under an argon atmosphere to form an amorphous carbon layer with a thickness of approximately 4 nm on the surface of the nitrogen-doped metal carbides, thus obtaining carbon-coated nitrogen-doped metal carbides TiC. 0.92 N 0.08 @C, the particle size of the nitrogen-doped metal carbide was tested to be 40 nm.

[0047] S13, Preparation of the negative electrode:

[0048] The carbon-coated nitrogen-doped metal carbide was dried in a vacuum oven at 100°C for 12 hours, and then mixed with zirconium oxide balls (3 mm in diameter) at a mass ratio of 1:10. The mixture was then wet-milled in anhydrous ethanol at a speed of 300 rpm for 6 hours to obtain pretreated carbon-coated nitrogen-doped metal carbon.

[0049] Carbon nanotubes (CNTs) and reduced graphene oxide (rGO) were dissolved in a 1 wt% sodium dodecylbenzenesulfonate (SDBS) solution at a mass ratio of 1:0.2 and ultrasonically treated with a power of 500 W and a frequency of 40 kHz for 2 h to obtain a CNT / rGO conductive agent suspension with a total concentration of 5 mg / mL.

[0050] Polyacrylic acid (PAA) and sodium alginate (SA) were dissolved in deionized water at a mass ratio of 3:1 and magnetically stirred at 60°C for 6 hours to form a PAA-SA binder solution (solid content 5wt%).

[0051] Pretreated carbon-coated nitrogen-doped metallic carbon was added to a CNT / rGO conductive agent suspension and magnetically stirred at 500 rpm for 30 min. Then, a PAA-SA binder solution was added and stirred at 2000 rpm for 1 h. After adjusting the viscosity to approximately 4000 mPa·s, the mixture was transferred to a vacuum degassing machine and stirred at 50 rpm for 10 min under a vacuum of -0.095 MPa to remove air bubbles, resulting in a negative electrode slurry (the mass ratio of carbon-coated nitrogen-doped metallic carbon, CNT / rGO conductive agent, and PAA-SA binder was 80:10:10). Finally, the negative electrode slurry was transferred to a coating machine and uniformly coated onto the surface of a negative electrode current collector (6 μm thick aluminum foil) using a 200 μm doctor blade. After drying and cold pressing, a negative electrode sheet was obtained, with a double-sided dry film surface density of 16 mg / cm². 2 .

[0052] S2. Preparation of the positive electrode sheet: Na4Fe3(PO4)2(P2O7) was used as the active material, conductive carbon black as the conductive agent, and polyvinylidene fluoride as the binder. These components were mixed at a mass ratio of 94.5:3.5:2.0, and N-methylpyrrolidone was added and stirred to form a uniform and stable positive electrode slurry. The slurry was then transferred to a coating machine and coated with a 200μm blade to uniformly coat the surface of the positive electrode current collector (6μm thick aluminum foil). After drying and cold pressing, the positive electrode sheet was obtained. The surface density of the double-sided dry film was 16 mg / cm³. 2 .

[0053] S3. Preparation of electrolyte: Sodium perchlorate, polythiocarbonate additive, and methyl ethyl carbonate are mixed evenly to obtain an electrolyte; wherein the concentration of sodium perchlorate in the electrolyte is 1 mol / L and the concentration of polythiocarbonate additive is 1 wt%.

[0054] S4. Separator: A polyethylene film with a thickness of 9μm.

[0055] S5. Assembly of sodium-ion batteries: Arrange the negative electrode, separator, positive electrode and separator in a Z-shape, wind them into a cell, inject electrolyte and assemble them into a sodium-ion soft pack battery.

[0056] Example 2

[0057] Basically the same as Example 1, except that the nitrogen-doped metal carbide is coated with carbon, specifically including the following steps:

[0058] S11. Preparation of nitrogen-doped metal carbides: Ammonium molybdate and dicyandiamide were dissolved in deionized water at a molar ratio of 1:3. Citric acid (0.1 mol / L in solution) was added as a chelating agent and ultrasonically dispersed for 30 min. Then, the mixture was rapidly frozen at -50℃ and vacuum dried for 24 h. Finally, the mixture was placed in a tube furnace and heated to 700℃ at a rate of 3℃ / min in an H2 / Ar atmosphere (H2 volume percentage of 5%) and held for 1.5 h. Then, it was held at 950℃ for 2.5 h to obtain nitrogen-doped metal carbides.

[0059] S12. Preparation of carbon-coated nitrogen-doped metal carbides: Nitrogen-doped metal carbides were dispersed in a 5 wt% citric acid / ethanol solution and evaporated to a gel state under stirring at 80°C. Then, the solution was annealed at 500°C for 1 h under an argon atmosphere to form an amorphous carbon layer with a thickness of approximately 5 nm on the surface of the nitrogen-doped metal carbides, thus obtaining carbon-coated nitrogen-doped metal carbides Mo2C. 0.9 N 0.1 @C, the particle size of the nitrogen-doped metal carbide was tested to be 35 nm.

[0060] Comparative Example 1

[0061] It is basically the same as Example 1, except that: no amorphous carbon layer is applied.

[0062] Comparative Example 2

[0063] It is basically the same as Example 1, except that nitrogen doping is not performed.

[0064] Comparative Example 3

[0065] The basic structure is the same as in Example 1, except that PAA is replaced with SBR (styrene-butadiene rubber) and SA is replaced with CMC (sodium carboxymethyl cellulose).

[0066] Comparative Example 4

[0067] The basic structure is the same as in Example 1, except that CNT and rGO are replaced with acetylene black.

[0068] Comparative Example 5

[0069] The basic principle is the same as in Example 1, except that polythiocarbonate was not added to the electrolyte.

[0070] Comparative Example 6

[0071] The basic structure is the same as in Example 1, except that polythiocarbonate is replaced with fluoroethylene carbonate (FEC).

[0072] Test Example 1

[0073] The expansion rate of the sodium-ion pouch cells prepared in Examples 1-2 and Comparative Examples 1-6 was tested (by measuring electrode thickness change via laser reflection). The specific steps included: first, fixing the sodium-ion pouch cell on an adjustable fixture and attaching reflective markers (approximately 10 mm apart) to its surface; then, using a sensor array composed of multi-channel laser probes, synchronously and in real-time acquiring the thickness changes at multiple key points on the battery surface during charge-discharge cycles (e.g., 0.5C cycle), with a measurement accuracy of ±1 μm; after the charge-discharge process was completed, the thickness D at the point of discharge completion was recorded. D Thickness D when charging is complete C Then, through data fusion algorithms, based on the formula expansion rate R = (D C -D D ) / D D The overall thickness expansion rate R of the battery was calculated, and the results are shown in Table 1:

[0074] Table 1

[0075] Sample Expansion rate (%) Example 1 14.2 Example 2 15.8 Comparative Example 1 25.4 Comparative Example 2 28.8 Comparative Example 3 30.1 Comparative Example 4 29.4 Comparative Example 5 20.2 Comparative Example 6 21.6

[0076] As shown in Table 1, the sodium-ion pouch cell of the embodiment exhibits a low expansion rate. This is because the introduction of nitrogen creates a localized lattice compression region. This structure can be modified by doping or defect engineering to control the lattice parameters of the metal carbide, thereby reducing the lattice distortion rate during sodium ion insertion and ultimately effectively reducing the resulting volume expansion rate. Simultaneously, the good elasticity of the amorphous carbon layer and the flexible SEI film formed by FEC further delay lattice expansion.

[0077] Comparing Example 1 and Comparative Example 1, it can be seen that the expansion rate is 25.4% without the amorphous carbon layer coating. This is because the high elastic modulus of the amorphous carbon layer can absorb grain boundary stress, thereby reducing the expansion rate during charging and discharging.

[0078] Comparing Example 1 and Comparative Example 2, it can be seen that the expansion rate is 28.8% without nitrogen doping. This is because without the introduction of nitrogen, a local lattice compression region cannot be formed, which increases the volume expansion rate caused by sodium ion insertion.

[0079] Comparing Example 1 and Comparative Example 3, it can be seen that when PAA is replaced with SBR (styrene-butadiene rubber) and SA is replaced with CMC (sodium carboxymethyl cellulose), the expansion rate increases to 30.1%. This is because the carboxyl groups of SA can form hydrogen bonds with the hydroxyl groups on the surface of the metal carbide, and the elastic segments of PAA (with an elastic modulus of approximately 0.1 GPa) can alleviate strain; however, when replaced with SBR and CMC, this effect is not achieved.

[0080] Comparing Example 1 and Comparative Example 4, it can be seen that when CNT and rGO are replaced with acetylene black, the expansion rate increases to 29.4%. This is because when CNT and rGO are used as composite conductive agents, the conductive network they construct can buffer volume changes through a slip-rearrangement mechanism; while conventional acetylene black is a dot-like conductive agent and does not have this buffering effect.

[0081] Comparing Example 1 and Comparative Example 5, it can be seen that the expansion rate increased to 20.2% when polythiocarbonate was not added to the electrolyte. This is because the absence of polythiocarbonate prevents the formation of a flexible SEI film, which would normally help alleviate the expansion force.

[0082] Comparing Example 1 and Comparative Example 6, it can be seen that when polythiocarbonate was replaced with fluoroethylene carbonate (FEC), the expansion rate increased to 21.6%. This is because FEC does not have the function of forming a flexible SEI film, while a flexible SEI film can play a role in mitigating expansion forces.

[0083] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A sodium-ion battery, characterized in that, The cell of the sodium-ion battery includes a positive electrode, a negative electrode, and an electrolyte; The active material of the negative electrode is a carbon-coated nitrogen-doped metal carbide; the chemical formula of the carbon-coated nitrogen-doped metal carbide is M2C. 1-x N x @C, where 0.05≤x≤0.15, M is selected from Mo and / or Ti; the particle size of the metal carbide is <50nm; the thickness of the amorphous carbon layer in the carbon-coated nitrogen-doped metal carbide is 3nm-5nm; The electrolyte is composed of a fluorine-free electrolyte salt, additives, and an organic solvent; the additives are selected from one or more of polythiocarbonate, sulfonated polyether, poly(ethylene disulfide), and poly(trifluoroethoxy)phosphazene.

2. The sodium-ion battery according to claim 1, characterized in that, The method for preparing the carbon-coated nitrogen-doped metal carbide includes the following steps: S1. Dissolve the metal source, carbon / nitrogen source and chelating agent in a solvent, and obtain nitrogen-doped metal carbide by freeze drying and sintering. S2. The nitrogen-doped metal carbide described in S1 is dispersed in an organic carbon source solution, and then heated, stirred, and annealed to obtain the carbon-coated nitrogen-doped metal carbide.

3. The sodium-ion battery according to claim 2, characterized in that, In S1, the metal source is selected from titanium and / or molybdenum; the titanium source is selected from titanium tetrachloride and / or tetrabutyl titanate; the molybdenum source is selected from molybdenum pentachloride and / or ammonium molybdate. The carbon / nitrogen source is selected from one or more of urea, melamine, and dicyandiamide; The chelating agent is selected from one or more of ethylenediaminetetraacetic acid, tartaric acid, acetic acid, oxalic acid, gluconic acid, polyacrylic acid, and citric acid. The solvent is selected from one or more of ethylene glycol, ethanol, and water; The amounts of the metal source and the carbon / nitrogen source are used according to the stoichiometric ratio, and the amount of carbon / nitrogen source is 5%-10% more than the stoichiometric ratio.

4. The sodium-ion battery according to claim 2, characterized in that, In S1, the drying is performed by rapid freezing at -55°C to -45°C followed by vacuum drying for 16-48 hours; The sintering is carried out in an H2 / Ar atmosphere, first heating to 400℃-800℃ at a rate of 3℃ / min-5℃ / min and holding for 1h-2h, and then holding at 900℃-1200℃ for 2h-3h; the volume ratio of H2 in the H2 / Ar atmosphere is 5%-10%.

5. The sodium-ion battery according to claim 2, characterized in that, In S2, the organic carbon source is selected from one or more of citric acid, ethanol, sucrose, glucose, and polyacrylic acid; The concentration of the organic carbon source solution is 4wt%-6wt%, and the solvent is one or more of ethanol, ethylene glycol, isopropanol, N-methylpyrrolidone methanol, and water.

6. The sodium-ion battery according to claim 2, characterized in that, In S2, the temperature of the heating and stirring is 75℃-85℃; The annealing is performed under a protective atmosphere at 450℃-550℃ for 1-2 hours.

7. The sodium-ion battery according to claim 1, characterized in that, The fluorine-free electrolyte salt is selected from one or more of sodium perchlorate, sodium bis(oxalate)borate, sodium bis(salicylic acid)borate, and sodium tetraphenylborate; The organic solvent is selected from one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, and diethylene glycol dimethyl ether.

8. The sodium-ion battery according to claim 1, characterized in that, The concentration of the fluorine-free electrolyte salt in the electrolyte is 0.1 mol / L-2 mol / L, and the concentration of the additive is 0.1 wt%-2 wt%.

9. The sodium-ion battery according to claim 1, characterized in that, The active material of the positive electrode is a polyanionic compound.

10. The sodium-ion battery according to claim 9, characterized in that, The polyanionic compound is selected from Na+ phosphate. x M y (PO4) z Na pyrophosphate x M y (P2O7) z Mixed phosphate Na x M y (PO4) z (P2O7) z Na fluorophosphate x M y (PO4) z F, sulfate Na x M y (SO4) z and silicate Na x M y One or more of (SiO4); wherein M is selected from one or more of Fe, Mn, Ti, V, Ni, Co, Zr and Cr.