A sodium-ion battery electrolyte, its preparation method, and its application.

Abstract

This invention relates to the field of secondary batteries, specifically, to providing an organic electrolyte for sodium-ion batteries, a preparation method thereof, and its application in sodium-ion batteries. The electrolyte for sodium-ion batteries comprises sodium salts, organic solvents, and additives, wherein the additives include nitrile compounds, sulfonates, and organic sodium salts; this electrolyte can generate a dense SEI layer rich in various inorganic elements, ensuring the sodium... + The rapid diffusion of ions, combined with a suitable reduction in the organic content of the SEI, improves the stability of the SEI during long-term cycling. Compared to sodium-ion batteries assembled using phosphate materials as the positive electrode active material, the capacity retention rate is greater than 80% after 1000 cycles at 1C rate.

Description

Technical Field

[0001] This invention relates to a sodium-ion battery electrolyte, its preparation method, and its application. Background Technology

[0002] As industries such as transportation and digital technology become increasingly reliant on lithium-ion batteries, the shortage of lithium resources is beginning to constrain their development. Sodium shares similar properties with lithium, and is abundant and inexpensive. Sodium-ion batteries offer advantages such as good safety performance, low price, and high specific energy, making them a promising energy storage technology. Compared to lithium-ion batteries, sodium-ion batteries offer the advantages of lower cost and higher safety.

[0003] In a battery system, the electrolyte, as the medium for electrochemical reactions, is an important component of the battery and a key factor determining the thermodynamic and kinetic processes of the interfacial reactions of electrode materials, thus affecting the cycle stability and rate performance of sodium-ion batteries.

[0004] In sodium-ion batteries, phosphate-based cathode materials possess advantages such as fast ion conduction speed and structural stability, making them an important raw material for sodium-ion batteries. Among them, sodium vanadium fluorophosphate and sodium manganese vanadium phosphate, for example, have charge / discharge voltage platforms above 4V, thus endowing the batteries with higher capacity. Their corresponding energy densities are expected to rival those of lithium iron phosphate batteries. For instance, a battery based on sodium vanadium fluorophosphate can achieve a capacity of 4.2V (relative to Na+). + / Na) has a high voltage platform, and the battery's specific energy exceeds 140Wh / kg. -1 However, when the positive electrode potential exceeds 4.2V, the commonly used traditional electrolytes are not stable enough under high voltage, resulting in poor cycle performance, which seriously restricts the commercial application of this type of positive electrode material.

[0005] To address the issue of low electrolyte stability under high voltage conditions, researchers have recently proposed strategies such as high-concentration electrolytes and locally high-concentration electrolytes. High-concentration electrolytes not only increase the bulk oxidative stability of the electrolyte but also enhance the proportion of coordination contact ion pairs (CIPs) and ion aggregates (AGGs) in the solvation structure, which is beneficial for forming inorganic-rich SEI and CEI. These advantages of high-concentration electrolytes significantly improve the cycle stability of sodium-ion batteries. However, in high-salt systems, as the concentration increases, the interaction forces between anions and cations also increase, leading to increased electrolyte viscosity and decreased ionic conductivity. This results in poorer wettability of the electrolyte to the electrodes, i.e., poorer interfacial compatibility. Simultaneously, the increased use of conductive sodium salts raises battery costs, hindering the commercial application of the electrolyte. Locally high-concentration electrolytes involve adding diluents such as fluorinated ethers to the high-concentration electrolyte to reduce the concentration while maintaining a similar locally solvation structure. The diluent itself has a similar or wider electrochemical window to the high-concentration electrolyte, is insoluble in salts but miscible with solvents, forming an (ion-solvent) diluent state. Chinese Patent CN 116231084 A discloses a high-voltage sodium-ion battery electrolyte that uses a low-polarity, highly fluorinated ether compound (such as TTE, HFE) solvent, weakening the interaction between sodium ions and solvent molecules, thereby accelerating the desolvation of sodium ions at the electrode interface. Furthermore, the diluent can increase the local concentration, forming an excellent CEI at the interface, thus achieving long battery cycle life. The electrolyte obtained by this invention has good compatibility with layered oxide cathodes and sodium metal anodes, and overcomes the problem of continuous decomposition at the interface in conventional sodium-ion battery electrolytes, which is beneficial for sodium-ion batteries to have good cycle performance and rate performance under high-voltage operating conditions. Summary of the Invention

[0006] This invention proposes an electrolyte suitable for sodium-ion batteries using phosphate materials as the positive electrode active material. This electrolyte exhibits high conductivity, low cost, a dense and stable interface layer, and good electrical insulation between the electrode and the electrolyte, making it particularly suitable for sodium-ion batteries with discharge platforms above 4V. Details are as follows:

[0007] A sodium-ion battery electrolyte is composed of a conductive sodium salt, an organic solvent, and a composite additive. The composite additive includes nitrile compounds, sulfonate esters, and organic sodium salts, and the total mass concentration of the additive in the electrolyte is 2 wt%-10 wt%.

[0008] The organic sodium salt is sodium bis(trifluoromethyl)sulfonamide, and its concentration in the electrolyte is 0.2-2%.

[0009] The sulfonate additive is one of the following: propenyl-1,3-sulfonyl lactone, 1,3-propanesulfonyl lactone (PS), propenyl-1-methyl-1,3-sulfonyl lactone, propenyl-1-ethyl-1,3-sulfonyl lactone, butenyl-1,4-sulfonyl lactone, butenyl-1-methyl-1,4-sulfonyl lactone, and butenyl-1-ethyl-1,4-sulfonyl lactone, with a concentration of 0.5% to 5%.

[0010] The nitrile additive is a straight-chain dinitrile with two C≡N functional groups located at both ends of the molecule, and its general molecular structure is C n H 2n-4 N2, where n is a positive integer, 3≤n≤6, has a concentration of 0.2~3% in the electrolyte.

[0011] The organic solvent is a carbonate compound, which is a mixture of cyclic carbonate and chain carbonate, wherein the chain carbonate has a volume percentage of 20%-80% in the organic solvent.

[0012] The cyclic carbonate is one or a mixture of ethylene carbonate and propylene carbonate.

[0013] The chain carbonate is one of diethyl carbonate, dimethyl carbonate, or ethyl methyl carbonate.

[0014] The conductive sodium salt is soluble in the organic solvent, including one of sodium hexafluorophosphate, sodium perchlorate, and sodium tetrafluoroborate, with a total sodium salt concentration of 0.5–1.5 M.

[0015] The preparation method of the sodium-ion battery electrolyte is as follows: In an argon-protected glove box, the additives and sodium salts are added to the organic solvent in sequence, stirred and mixed, and after all solid substances are completely dissolved and stirred evenly, molecular sieves are added to the electrolyte to remove water.

[0016] The water content in the electrolyte should be less than 10 ppm.

[0017] The electrolyte described herein is used as a high-voltage electrolyte in sodium-ion batteries. The high-voltage electrolyte refers to a battery with a charging upper limit cutoff voltage range of 4.3–4.6V.

[0018] The sodium-ion battery positive electrode active material is a phosphate-based sodium salt, including sodium vanadium phosphate (Na3V2(PO4)3), sodium vanadium fluorophosphate (Na3V2(PO4)2F3), sodium vanadium iron phosphate (Na3VFe(PO4)3), sodium vanadium manganese phosphate (Na3VMn(PO4)3), sodium vanadium chromium phosphate (Na3VCr(PO4)3), composite sodium manganese iron phosphate (Na4FeMn(PO4)P2O7), and sodium salts of the above materials that have been doped with heterovalent ions.

[0019] This electrolyte can generate a dense SEI layer rich in various inorganic components, which can ensure the Na+... + Rapid ion diffusion, combined with a suitable reduction in the organic content of the SEI, improves the stability of the SEI during long-term cycling. Compared to sodium-ion batteries assembled using phosphate materials as the positive electrode active material, the capacity retention rate is greater than 80% after 1000 cycles at 1C rate.

[0020] The present invention has the following beneficial effects:

[0021] 1. Adding organic sodium salt NaTFSI additive to the electrolyte can generate an SEI layer rich in various inorganic components, which can ensure the Na... + Ion diffusion, along with appropriately reducing the organic content in the SEI, improves the stability of the SEI during long-term cycling. Studies have found that the anions of the electrolyte salt control the overall properties of the SEI layer formed on the negative electrode, including the composition and depth distribution of substances within the SEI. The SEI on sodium carbon materials contains both organic and inorganic substances, obtained from the degradation of the electrolyte solvent and salt anions, respectively. The inorganic layer is dense and stable, usually in close contact with the negative electrode, while the organic layer is a gel-like outer layer of a polymer network, close to the electrolyte side, and has poor stability. Many components can dissolve in the electrolyte. For example, some articles have reported that the dissolution order of SEI components in the electrolyte is Li₂EDC > LiOCO₂CH₃ > LiOH > LiOCO₂C₂H₅ > LiOCH₃ > LiF > [LiCO₂]₂ > Li₂CO₃ > Li₂O. Therefore, an SEI rich in inorganic components is more conducive to inhibiting electrolyte decomposition and related parasitic reactions. In commonly used ester electrolytes, the main solvent (such as cyclic carbonates EC, PC, etc.) is highly polar and reacts with Na. + Strong coordination occurs, leading to the formation of the PF6 anion in the commonly used sodium salt (NaPF6). - ) and Na + The coordination between them is weak, Na + PF6 in the solvation layer - The share is very small, and the free PF6 - Due to the repulsive force of the electric field, it is difficult for NaF to penetrate into the electric double layer on the negative electrode surface, resulting in a very small amount of NaF generated by NaPF6 on the negative electrode surface. In the organic sodium salt NaTFSI, Na... + With TFSI - Stronger ionic coordination between anions, TFSI - Anions can enter Na+ in a coordinated manner. +In the solvation shell, an electrochemical reduction reaction occurs at the negative electrode to generate a dense layer rich in various inorganic components such as Na3N, NaF, Na2O, and Na2SO3. At the same time, the number of coordinating solvent molecules in the solvation layer is reduced, and the content of organic matter in the SEI is appropriately reduced.

[0022] 2. The combination of sulfonates with unsaturated double bonds and nitrile additives can form a protective interfacial layer on the positive and negative electrode surfaces of sodium-ion batteries. In the nitrile molecule, the N atom in the C≡N structure has a lone pair of electrons. In the discharged state, the surface of the positive electrode material is electron-deficient. Under the influence of an electric field, nitrile molecules are readily adsorbed and undergo oxidation reactions on the positive electrode surface (3.6V vs. Na). + The formation of CEI (acetic acid ester) from the electrolyte (Na₂O) electrically insulates the main solvent (carbonate) from the positive electrode surface, thus preventing electrochemical oxidation of the main solvent and improving battery stability. However, nitrile molecules are repelled by the negative electrode electric field and cannot form an effective interface layer on the negative electrode surface. In contrast, sulfonates with unsaturated double bonds have higher reduction potentials and can undergo reduction reactions at higher potentials to form a dense inorganic layer (Na₂O, Na₂SO₃, etc.), effectively blocking electrical contact between the electrolyte and the negative electrode surface, thereby improving the electrolyte's resistance to reduction. Attached Figure Description

[0023] Figure 1 Example 1: Cyclic stability of sodium-ion pouch cells assembled with electrolyte at 1C rate. Voltage window: 2-4.6V; Upper charge cutoff voltage: 4.6V; Charging mode: Constant current + Constant voltage.

[0024] Appendix Explanation

[0025] Appendix 1 lists the embodiments of the sodium-ion battery of the present invention, the electrolyte composition, and the corresponding positive electrode active materials.

[0026] Appendix 2: Correspondence between sodium-ion battery examples and capacity retention rates after 1000 cycles at 1C. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be further described below through examples.

[0028] Example 1

[0029] 1. The preparation process of sodium-ion battery electrolyte is as follows:

[0030] Electrolyte #1 was prepared in an argon-protected glove box. NaPF6 was used as the sodium salt at a concentration of 0.5 M. A mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the organic solvent in a 1:1 volume ratio. Additives consisted of NaTFSI, malononitrile (MN), and propenyl-1,3-sulfonyl lactone (PST). The mass concentrations of each component in the electrolyte are detailed in Table 1.

[0031] Add the additive and sodium salt to the organic solvent, stir and mix. After all solid substances are completely dissolved and stirred evenly, add molecular sieves to the electrolyte to remove water. The water content is tested to be 6.5 ppm.

[0032] 2. Assembly and Cycle Stability Testing of Sodium-ion Batteries

[0033] The positive electrode sheet was prepared as follows: Self-made sodium vanadium phosphate (NVP) was used as the active material, mixed in a mass ratio of active material (NVP): conductive agent (super P): binder (polyvinylidene fluoride (PVDF)) = 95:2:3. N-methylpyrrolidone (NMP) was used as a dispersant to prepare a slurry, which was then coated onto the surface of double-sided carbon-coated aluminum foil. After drying, the positive electrode sheet was formed. In this embodiment, the coating surface density of the positive electrode was 15 mg / cm³. -2 .

[0034] The negative electrode sheet was prepared as follows: Biomass hard carbon (HC) was used as the negative electrode active material. The active material (HC): conductive agent (super P): binder (sodium carboxymethyl cellulose CMC) were mixed in a mass ratio of 92:4:4, and deionized water was used as a dispersant to prepare a slurry. This slurry was coated onto the surface of aluminum foil and dried to form the negative electrode sheet. The coating density of the negative electrode was calculated based on a negative / positive capacity ratio (N / P ratio) of 1.02. Battery assembly and performance testing: In an environment with a dew point ≤-20°C, a 4-positive, 5-negative sodium-ion soft-pack battery was assembled using a wet-process polyethylene film (PE type, model SK12) as the battery separator. The effective area of ​​the battery was 78 cm². 2 Then, it is assembled with the electrolyte prepared in step 1 above, with an injection volume of 6g. After assembly, it undergoes processes such as encapsulation, resting, formation, and secondary encapsulation to obtain sodium-ion battery #1 based on NVP. After capacity testing at room temperature (0.2C), a 1C rate charge-discharge cycle test is performed, and the discharge capacity of the first cycle is recorded as C. dc-1 The test was performed for 1000 cycles, and the discharge capacity on the 1000th cycle was recorded as C. dc-1000 The capacity retention rate of 1C rate cycling is calculated according to formula (1). The electrochemical window for the cycle test is 2V to 4.6V, and the test mode is constant rate + constant voltage. The cutoff condition for the 4.6V constant voltage is that the rate is ≤0.05C.

[0035] 1C Capacity Retention Rate (%) = C dc-1000 ×100% / C dc-1 (1)

[0036] During the charging process of sodium-ion batteries using ester-based electrolytes, as the sodium storage at the negative electrode increases, the negative electrode potential gradually shifts negative. Components in the electrolyte with lower LUMO values ​​in contact with the negative electrode undergo electrochemical reduction reactions, generating inorganic substances such as sodium carbonate (Na₂CO₃) and organic solids such as sodium alkyl carbonate (RCO₃Na) and sodium alkyl alcohol (RONa). These solids deposit on the negative electrode surface to form an SEI (Sediment-Insulated Electrode), while simultaneously consuming Na₂O from the positive electrode. + SEI (Sediment-Insulated Electrode) electrically isolates the electrolyte from the negative electrode, preventing continuous electrochemical reactions and consumption of the electrolyte. During discharge, as Na... + As the electrolyte continuously desorbs from the negative electrode, the negative electrode potential gradually increases. When the negative electrode potential increases to above 0.7V, some organic components in the SEI will undergo electrochemical dissolution, such as RONa, which will cause the SEI to become rough and porous, and the "electrolytic isolation" effect on the electrolyte will also deteriorate.

[0037] At the positive electrode, as the charging process proceeds, Na... + As the active material in the positive electrode is extracted from the lattice, the positive electrode potential increases. Components with higher HOMO values ​​in the electrolyte that are in contact with the positive electrode will undergo electrochemical oxidation. The higher the upper cutoff voltage, the higher the potential of the positive electrode, and the more intense the oxidation reaction in the electrolyte that is in contact with the positive electrode.

[0038] In this embodiment, the Na of the organic sodium salt NaTFSI + With TFSI - Stronger ionic coordination between anions, TFSI - Anions can enter Na+ in a coordinated manner. + Within the solvated shell. During battery charging, TFSI - Anions undergo electrochemical reduction to generate a dense layer rich in various inorganic components, such as Na3N, NaF, Na2O, and Na2SO3. Simultaneously, this reduces the number of coordinating solvent molecules in the solvation layer, lowering the organic content in the SEI. During the subsequent discharge process, as the sodium removal process proceeds, these inorganic components do not undergo electrochemical oxidation and dissolution as the negative electrode potential gradually increases, thus ensuring the "electrical isolation" of the SEI from the electrolyte.

[0039] Furthermore, the combination of sulfonates with unsaturated double bonds and nitrile additives can generate a protective interfacial layer on the positive and negative electrode surfaces of sodium-ion batteries. In the nitrile molecule, the N atom in the C≡N structure possesses a lone pair of electrons. In the discharged state, the surface of the positive electrode material is electron-deficient. Under the influence of an electric field, nitrile molecules are readily adsorbed and undergo oxidation reactions on the positive electrode surface (3.6V vs. Na₂O).+ The formation of CEI (acetic acid ester) from the electrolyte (Na₂O) electrically insulates the main solvent (carbonate) from the positive electrode surface, thus preventing electrochemical oxidation of the main solvent and improving battery stability. However, nitrile molecules are repelled by the negative electrode electric field and cannot form an effective interface layer on the negative electrode surface. In contrast, sulfonates with unsaturated double bonds have higher reduction potentials and can undergo reduction reactions at higher potentials to form a dense inorganic layer (Na₂O, Na₂SO₃, etc.), effectively blocking electrical contact between the electrolyte and the negative electrode surface, thereby improving the electrolyte's resistance to reduction.

[0040] Based on the above analysis, it can be seen that the combination of three additives with unsaturated double bonds—sulfonates, nitriles, and organic sodium salt NaTFSI—can construct a high-density interfacial layer at both the positive and negative electrodes of sodium-ion batteries, thereby ensuring that the battery can operate at a higher upper limit charging cutoff voltage and a wider electrochemical window.

[0041] The calculation results in Table 2 show that the battery using the electrolyte of the present application has a high capacity retention rate. The capacity retention rate after 1000 cycles at 1C rate is greater than 80%, indicating that the corresponding electrolyte formulation provided by the present invention can provide stable operation for sodium ions assembled with phosphate-based cathode materials. In particular, it can suppress the electrochemical oxidation and electrochemical reduction reactions of the main solvent on the positive and negative electrode surfaces under the conditions of charging voltage ≥4.3V and discharging voltage ≥2V, thus ensuring the safety of the battery.

[0042] Examples 2-9

[0043] The electrolytes are numbered 2# to 9#. The composition and positive electrode active material of the assembled battery are detailed in Table 1. The negative electrode, battery assembly, and test mode are the same as in Example 1. The electrochemical window for cycle testing is shown in Table 2.

[0044] Appendix 1: Examples of Sodium-ion Batteries, Electrolyte Compositions, and Corresponding Positive Electrode Active Materials

[0045]

[0046] explain:

[0047] In the electrolyte composition expressions in Table 1, the letter "v" represents the volume ratio of each solvent component in the electrolyte; the symbol "%" represents the mass percentage content of the additive in the electrolyte; and the letter "M" represents the molar concentration of the sodium salt in the electrolyte.

[0048] For example, the electrolyte composition in Example 1-1# is expressed as EC:DEC = 1:1 (v) / / 2% PST / 0.5% MN / 0.5% NaTFSI / 0.5M NaPF6, indicating that the electrolyte composition is: the volume ratio of EC:DEC is 1:1, the mass percentage of additive PST (propenyl-1,3-sulfonyl lactone) is 2%, the mass percentage of MN (malononitrile) is 0.5%, the mass percentage of organic sodium salt NaTFSI is 0.5%, and the molar concentration of NaPF6 is 0.5M.

[0049] Appendix 2: Correspondence between sodium-ion battery examples and capacity retention rates after 1000 cycles at 1C.

[0050] Example number Electrochemical window CR / % after 1000 cycles of 1C Example 1 2V~4.6V 87 Example 2 2V~4.3V 90 Example 3 2V~4.3V 88 Example 4 2V~4.5V 86 Example 5 1.5V~4.3V 83 Example 6 2V~4.5V 85 Example 7 2V~4.6V 88 Example 8 2V~4.3V 82 Example 9 1.5V~4.3V 84 Comparative Example 1 2V~4.6V 71 Comparative Example 2 2V~4.3V 74 Comparative Example 3 1.5V~4.3V 62 Comparative Example 4 2V~4.5V 64 Comparative Example 5 2V~4.5V 54 Comparative Example 6 2V~4.5V 56 Comparative Example 7 2V~4.5V 69

[0051] The calculation results in Table 2 show that the battery using the electrolyte of the present application has a high capacity retention rate. The capacity retention rate after 1000 cycles at 1C rate is greater than 80%, indicating that the corresponding electrolyte formulation provided by the present invention can provide stable operation for sodium ions assembled with phosphate-based cathode materials. In particular, it can suppress the electrochemical oxidation and electrochemical reduction reactions of the main solvent on the positive and negative electrode surfaces under the conditions of charging voltage ≥4.2V and discharging voltage ≥2V, thus ensuring the safety of the battery.

[0052] In Comparative Example 1, the concentration of the sodium salt used for ion conduction was 1.8 M, which is close to the upper limit of solubility in ester electrolytes, resulting in excessively high electrolyte viscosity. + The migration rate is low, and in high-rate cycling at 1C, the low conductivity of the electrolyte is insufficient to provide enough Na to the electrochemical reaction interface. + This leads to high electrochemical polarization, resulting in a continuous decline in battery performance. In contrast, the sodium salt concentration in Comparative Example 5 was only 0.3 M, and the Na+ used for conduction in the electrolyte... + The quantity is too small and cannot provide sufficient Na to the electrochemical reaction interface during high-rate cycling. + This leads to high electrochemical polarization, resulting in a continuous deterioration of battery performance.

[0053] In Comparative Example 2, the concentration of unsaturated sulfonate was 8%. Due to the formation of a thicker interfacial layer on the negative electrode surface, Na... + The migration path is too long, and its higher interfacial resistance will also cause high electrochemical polarization, which is not conducive to the stable output of battery performance.

[0054] In the electrolytes of Comparative Examples 3, 4, and 6, the amounts of unsaturated sulfonate esters, NaTFSI, and nitrile additives were all lower than the concentration ranges given in this invention. Because the content of inorganic salts (such as NaF, Na2S, Na2SO3, Na3N, etc.) in the interfacial layer was low, it was insufficient to form an effective dense inner layer on the electrode surface. Consequently, the electrolyte and electrode could not be effectively electrically insulated, and the main solvent would continue to undergo electrochemical decomposition, resulting in low battery stability. The capacity retention rate after 1000 cycles at 1C was less than 70%, which did not meet practical requirements.

[0055] In Comparative Example 7, the high concentration of nitrile additives resulted in poor battery cycle stability. The main reason is that the additives in the electrolyte failed to form an effective SEI barrier layer at the negative electrode. The adsorption of high-concentration nitrile molecules on the negative electrode surface repelled unsaturated sulfonate molecules from approaching the surface, leading to a low number of reduced molecules on the electrode surface. This was insufficient to form an effective dense inorganic layer, failing to effectively insulate the electrolyte from the electrode. Consequently, the main solvent continued to undergo electrochemical decomposition, resulting in unstable charge and discharge of the battery.

[0056] As described above, these are merely embodiments of this application, and the scope of protection of this application is not limited to these specific embodiments, but is determined by the claims of this application.

Claims

1. A sodium-ion battery, characterized in that, The sodium-ion battery electrolyte comprises sodium salt, organic solvent, and additives, wherein the additives include malononitrile, unsaturated sulfonates, and sodium bis(trifluoromethyl)sulfonylimide, with a mass concentration of 0.2-2% in the electrolyte; The organic solvent is a carbonate compound, and the carbonate is a mixture of cyclic carbonate and chain carbonate, wherein the chain carbonate has a volume percentage of 20%-80% in the organic solvent. The unsaturated sulfonate additive is one or more of the following: propenyl-1,3-sulfonyl lactone, propenyl-1-methyl-1,3-sulfonyl lactone, propenyl-1-ethyl-1,3-sulfonyl lactone, butenyl-1,4-sulfonyl lactone, butenyl-1-methyl-1,4-sulfonyl lactone, and butenyl-1-ethyl-1,4-sulfonyl lactone, with a mass concentration of 0.5% to 5% in the electrolyte. The positive electrode active material of the sodium-ion battery is a phosphate-based sodium salt, including sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium vanadium iron phosphate, sodium vanadium manganese phosphate, sodium vanadium chromium phosphate, sodium manganese iron phosphate, and sodium salts of the above materials that have been doped with heterovalent ions.

2. The sodium-ion battery according to claim 1, characterized in that, The additives include malononitrile, unsaturated sulfonates, and sodium bis(trifluoromethyl)sulfonamide, with a mass concentration of 0.3-1.8% in the electrolyte; The chain carbonate has a volume percentage of 30-70% in the organic solvent. The mass concentration of unsaturated sulfonate additives in the electrolyte is 1-4%.

3. The sodium-ion battery according to claim 1, characterized in that, The total mass concentration of additives in the electrolyte is 2wt%-10wt%.

4. The sodium-ion battery according to claim 3, characterized in that, The total mass concentration of additives in the electrolyte is 3 wt%-9 wt%.

5. The sodium-ion battery according to claim 1, characterized in that, The cyclic carbonate is one or a mixture of ethylene carbonate and propylene carbonate; The chain carbonate is one or more of diethyl carbonate, dimethyl carbonate, or methyl ethyl carbonate.

6. The sodium-ion battery according to claim 1, characterized in that, The sodium salt is soluble in the organic solvent, including either sodium hexafluorophosphate or sodium perchlorate, and the total concentration of the sodium salt is 0.5-1.5M.

7. The sodium-ion battery according to claim 6, characterized in that, The total sodium salt concentration is 0.6~1.2M.

8. The sodium-ion battery according to claim 1, characterized in that, The electrolyte preparation method for sodium-ion batteries is as follows: In an argon-protected glove box, the additives and sodium salts are added to an organic solvent, stirred and mixed, and after all solid substances are completely dissolved and stirred evenly, molecular sieves are added to the electrolyte to remove water. The water content in the electrolyte is equal to or less than 10 ppm.

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