Low-temperature compatible flame-retardant sodium-ion battery electrolyte

Abstract

The application discloses a low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility, which comprises a sodium salt, an electrolyte solvent and an electrolyte additive, the electrolyte solvent is a composite solvent system, the composite solvent system comprises a first solvent, a second solvent and a third solvent, the first solvent is one or more than two of (2,2,2)-trifluoroethyl diethyl phosphate, di-(2,2,2)-trifluoroethyl phosphate and tri-(2,2,2)-trifluoroethyl phosphate, the second solvent is one or more than two of methyl isopropyl ketone, 2-methyl tetrahydrofuran and methyl propionate, and the third solvent is diethylene glycol dimethyl ether. The low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility has the characteristics of excellent low-temperature performance, high cycle stability and good safety.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility. Background Technology

[0002] Sodium-ion batteries, with their similar working principle to lithium-ion batteries, abundant sodium resources, and low cost, have become a strong contender for next-generation energy storage technology. Currently, the industrialization of sodium-ion batteries is progressing rapidly. Similar to lithium-ion batteries, sodium-ion batteries need to be developed with different functionalities to adapt to various application scenarios. For example, the military and aerospace fields often face extreme low-temperature conditions, requiring sodium-ion batteries to operate stably at extremely low temperatures. Moreover, sodium-ion batteries are also a strong competitor in large-scale energy storage technology. Due to the severe impact of safety accidents, large-scale energy storage technologies have even more stringent requirements for battery safety. Therefore, simultaneously improving the low-temperature performance and enhancing the safety of sodium-ion batteries is of great significance for their commercial application.

[0003] Currently, the electrolyte used in sodium-ion batteries is formulated with ethylene carbonate (EC) as the main solvent and linear carbonate as a co-solvent. This is because EC has good electrochemical compatibility with hard carbon anode materials, making it an essential solvent for sodium-ion battery electrolytes. However, EC has a high melting point (36°C) and is highly flammable. This can cause electrolyte solidification in sodium-ion batteries at low temperatures, resulting in poor low-temperature performance and, under thermal runaway conditions, fire or even explosion, leading to serious safety accidents. Therefore, developing an electrolyte with high electrochemical compatibility that combines low-temperature performance and flame retardancy is of great significance to the development of sodium-ion batteries. Summary of the Invention

[0004] The purpose of this invention is to provide a low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility, featuring excellent low-temperature performance, high cycle stability, and good safety.

[0005] This invention can be achieved through the following technical solutions: This invention discloses a low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility, comprising a sodium salt, an electrolyte solvent, and an electrolyte additive. The electrolyte solvent is a composite solvent system, which includes a first solvent, a second solvent, and a third solvent. The first solvent is one or more of (2,2,2)-trifluoroethyl phosphate, di-(2,2,2)-trifluoroethyl phosphate, and tri-(2,2,2)-trifluoroethyl phosphate. The second solvent is one or more of methyl isopropyl ketone, 2-methyltetrahydrofuran, and methyl propionate. The third solvent is diethylene glycol dimethyl ether.

[0006] In this invention, the first solvent is a flame-retardant solvent. The higher the phosphorus and fluorine content of the solvent, the higher its flame-retardant efficiency. Using fluorophosphate as a co-solvent can achieve better flame-retardant performance with a lower flame-retardant solvent content. The lower flame-retardant solvent content has less impact on the ionic conductivity of the electrolyte. The second solvent is a low-viscosity, low-melting-point solvent, preferably an ultra-low viscosity and melting-point co-solvent. This solvent provides high solubility for sodium salts and ensures ion conduction performance at low temperatures. The third solvent is a highly compatible solvent, preferably a solvent with good compatibility with negative electrode materials. This can greatly improve the electrochemical compatibility of the electrolyte and enhance the cycle stability of sodium-ion batteries. The first, second, and third solvents interact with each other as follows: The first solvent is preferably a fluorinated phosphate solvent, which not only has good flame retardant properties, but also allows anions to participate in the solvation structure of sodium ions, increasing the LUMO energy level of the electrolyte solvation structure and promoting the preferential decomposition of anions to generate an inorganic-rich SEI film. However, the fluorinated phosphate electrolyte may still be unstable during long-term battery cycling. The addition of the third solvent can further compensate for this problem. The third solvent itself can be reduced and decomposed to generate a thin polymer SEI film, and the inorganic matter generated by the decomposition of anions can fill the gaps in the polymer SEI film. The presence of this composite inorganic-polymer SEI film can effectively improve the electrochemical compatibility of the electrolyte. Therefore, although the reduction stability of the second solvent itself is also relatively poor, due to the combined modification effect of the first and third solvents on the electrolyte / electrode interface, the addition of a large amount of low-viscosity, low-melting-point second solvent to improve the low-temperature performance of the electrolyte can still ensure good electrochemical compatibility of the electrolyte, which can guarantee the long-term cycling stability of sodium-ion batteries at room temperature. Furthermore, the addition of electrolyte additives can further enhance the compatibility between the electrolyte and electrode materials. Sodium-ion batteries using this electrolyte exhibit excellent safety performance, good low-temperature performance, and a long cycle life, meeting the application requirements of numerous extreme temperature scenarios and large-scale energy storage technologies.

[0007] Furthermore, the volume ratio of the first solvent, the second solvent, and the third solvent is 3:4:3. In this ratio, the changes in their respective contents affect the performance of the electrolyte system as follows: When the combined volume of the first and third solvents is less than 60%, it is impossible to guarantee the formation of a stable and dense inorganic-polymer composite SEI film on the hard carbon anode surface, resulting in poor electrochemical compatibility and reduced long-cycle stability of the sodium-ion battery. However, more of the first and third solvents will reduce the ionic conductivity of the electrolyte at low temperatures. Therefore, the proportion of the first solvent should not be less than 30%, as less of the first solvent will reduce the flame retardancy of the electrolyte, leading to poor safety of the sodium-ion battery. The addition of 60% of the first and third solvents is also to compensate for the poor reduction stability of the second solvent itself, while 40% of the second solvent is sufficient to improve the low-temperature conductivity of the electrolyte, thus significantly improving the low-temperature performance of the electrolyte.

[0008] Further, the volume content of the first solvent is 2-90%, the volume content of the second solvent is 2-90%, and the volume content of the third solvent is 2-90%. Specifically, a lower volume content of the first solvent results in poor flame retardant properties of the electrolyte, while a higher volume content results in lower conductivity; preferably, it is 30%. A lower volume content of the second solvent results in poor low-temperature performance of the electrolyte, while a higher volume content results in poor electrochemical compatibility; preferably, it is 40%. A lower volume content of the third solvent results in poor electrochemical compatibility of the electrolyte, while a higher volume content results in poor low-temperature performance; preferably, it is 30%.

[0009] Furthermore, the electrolyte additive is one or more of the following: fluoroethylene carbonate (FEC), ethylene sulfate (DTD), propylene sulfate (TS), sodium bis(oxalate)borate (NaBOB), and sodium difluorooxalateborate (NaDFOB).

[0010] Furthermore, the mass content of the electrolyte additive is 0.01-10%wt, preferably 2%wt.

[0011] Furthermore, the sodium salt is one or more of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluorosulfonyl)imide (NaTFSI), preferably NaPF6 or NaClO4.

[0012] Furthermore, the concentration of the sodium salt is 0.1-3.0 mol / L, preferably 0.7 mol / L.

[0013] Furthermore, the negative electrode material of the sodium-ion battery is one or more of hard carbon, tin alloy, and phosphorus alloy.

[0014] Furthermore, the cathode material of sodium-ion batteries is a polyanionic cathode material, Prussian blue material, or layered oxide material.

[0015] Furthermore, the polyanionic cathode material is an iron-based polyanionic material, a cobalt-based polyanionic material, a nickel-based polyanionic material, or a vanadium-based polyanionic material.

[0016] This invention provides a low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility, which has the following beneficial effects: In the electrolyte system of this invention, the first flame-retardant solvent generates a large number of phosphorus and fluorine free radicals during combustion, quenching the free radicals produced by the combustion reaction and achieving a highly efficient flame-retardant effect, thereby improving the safety of the sodium-ion battery. The second low-viscosity, low-melting-point solvent, with its ultra-low viscosity and melting point, co-solvents such as methyl isopropyl ketone and 2-methyltetrahydrofuran, ensures that the electrolyte remains in a liquid state and has high ionic conductivity at low temperatures, thus improving the low-temperature performance of the sodium-ion battery. More importantly, the first and third solvents work together to modify the electrolyte / electrode interface, generating an inorganic-polymer SEI film that greatly improves the electrochemical compatibility of the electrolyte and compensates for the poor reduction stability of the second solvent. Therefore, the sodium-ion battery using the above electrolyte exhibits excellent low-temperature performance and superior safety performance while also achieving stable long-life cycling. Attached Figure Description

[0017] Figure 1 The discharge capacity curves of the sodium iron pyrophosphate / hard carbon pouch cell in Example 1 at room temperature and low temperature; Figure 2 The graph shows the cycle capacity and efficiency of the sodium iron pyrophosphate / hard carbon pouch full cell in Example 2. Figure 3 The discharge capacity curves of sodium iron pyrophosphate / hard carbon pouch batteries in Example 3 and Comparative Example 3 at low temperatures are shown. Detailed Implementation

[0018] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments.

[0019] This invention discloses a low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility, comprising a sodium salt, an electrolyte solvent, and an electrolyte additive. The electrolyte solvent is a composite solvent system, which includes a first solvent, a second solvent, and a third solvent. The first solvent is one or more of (2,2,2)-trifluoroethyl phosphate, di-(2,2,2)-trifluoroethyl phosphate, and tri-(2,2,2)-trifluoroethyl phosphate. The second solvent is one or more of methyl isopropyl ketone, 2-methyltetrahydrofuran, and methyl propionate. The third solvent is diethylene glycol dimethyl ether.

[0020] Further, in this invention, the first solvent is preferably (2,2,2)-trifluoroethyl phosphate diethyl ester, the second solvent is preferably methyl isopropyl ketone and 2-methyltetrahydrofuran, and the third solvent is preferably diethylene glycol dimethyl ether. Specifically, in the second solvent, the specific parameters of different solvents are: methyl isopropyl ketone (melting point: -92℃, viscosity: 0.47 mPa·s), isobutyronitrile (melting point: -72℃, viscosity: 0.456 mPa·s), 2-methyltetrahydrofuran (melting point: -136℃, viscosity: 0.47 mPa·s), and methyl propionate (melting point: -87.5℃, viscosity: 0.43 mPa·s).

[0021] Furthermore, the volume ratio of the first solvent, the second solvent, and the third solvent is 3:4:3.

[0022] Furthermore, the volume content of the first solvent is 2-90%, the volume content of the second solvent is 2-90%, and the volume content of the third solvent is 2-90%.

[0023] Furthermore, the electrolyte additive is one or more of fluoroethylene carbonate (FEC), ethylene sulfate (DTD), propylene sulfate (TS), sodium bis(oxalato)borate (NaBOB), and sodium difluorooxalato)borate (NaDFOB), preferably fluoroethylene carbonate.

[0024] Furthermore, the mass content of the electrolyte additive is 0.01-10%wt, preferably 2%wt.

[0025] Furthermore, the sodium salt is one or more of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluorosulfonyl)imide (NaTFSI), preferably NaPF6 or NaClO4.

[0026] Furthermore, the concentration of the sodium salt is 0.1-3.0 mol / L, preferably 0.7 mol / L.

[0027] Furthermore, the negative electrode material of the sodium-ion battery is one or more of hard carbon, tin alloy, and phosphorus alloy.

[0028] Furthermore, the cathode material of sodium-ion batteries is a polyanionic cathode material, Prussian blue material, or layered oxide material.

[0029] Furthermore, the polyanionic cathode material is an iron-based polyanionic material, a cobalt-based polyanionic material, a nickel-based polyanionic material, or a vanadium-based polyanionic material.

[0030] Furthermore, the polyanion of the polyanionic cathode material is one or more of phosphate, pyrophosphate, sulfate, silicate, and borate.

[0031] Furthermore, sodium-ion batteries are hard carbon / sodium half-cells, sodium vanadium phosphate / sodium half-cells, sodium iron phosphate pyrophosphate / sodium half-cells, sodium vanadium fluorophosphate / sodium half-cells, sodium / sodium half-cells, sodium vanadium phosphate / hard carbon full-cells, sodium vanadium fluorophosphate / hard carbon full-cells, and sodium iron phosphate pyrophosphate / hard carbon full-cells.

[0032] Example 1 In this embodiment, the electrolyte used contains sodium hexafluorophosphate as the sodium salt, (2,2,2)-trifluoroethyl phosphate as the first flame retardant solvent, methyl isopropyl ketone as the second low-viscosity, low-melting-point solvent, diethylene glycol dimethyl ether as the third high-compatibility solvent, and sodium difluorooxalate borate as the electrolyte additive. The sodium salt concentration is 0.7M, the volume ratio of the first flame retardant solvent to the second low-viscosity, low-melting-point solvent to the third high-compatibility solvent is 3:5:3, the electrolyte additive content is 2%wt, and the battery is a sodium iron pyrophosphate / hard carbon soft-pack full battery with a nominal capacity of 1Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0033] As shown in Table 1, the electrolyte in Example 1 is completely non-flammable, demonstrating its excellent safety performance. The sodium iron pyrophosphate / hard carbon pouch battery using this electrolyte exhibits a high first-cycle coulombic efficiency of 84.3%, and retains 97.5% capacity after 500 cycles, with a coulombic efficiency exceeding 99.9% during cycling, indicating good compatibility between the electrolyte and the positive and negative electrode materials in Example 1. Furthermore, the ionic conductivity of the electrolyte described in Example 1 remains at 0.71 mS / cm even under low-temperature testing conditions of -60°C. Figure 1 As shown, the low-temperature discharge capacity retention rate was 47.2%. In contrast, the electrolyte in Comparative Example 1 without the addition of a third highly compatible solvent exhibited rapid capacity decay during cycling. This demonstrates that adding a third highly compatible solvent to the electrolyte can significantly improve the electrochemical compatibility of the electrolyte and the cycle life of the sodium-ion battery.

[0034] Example 2 In this embodiment, the electrolyte used contains sodium bis(fluorosulfonyl)imide, a first flame retardant solvent is ethyl di-(2,2,2)-trifluoroethyl phosphate, a second low-viscosity, low-melting-point solvent is 2-methyltetrahydrofuran, a third highly compatible solvent is diethylene glycol dimethyl ether, and an electrolyte additive is fluoroethylene carbonate. The sodium salt concentration is 0.7M, the volume ratio of the first flame retardant solvent to the second low-viscosity, low-melting-point solvent to the third highly compatible solvent is 3:5:3, the electrolyte additive content is 2%wt, and the battery is a sodium iron pyrophosphate / hard carbon pouch full battery with a nominal capacity of 1Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0035] As shown in Table 1, the electrolyte in Example 2 is completely non-flammable, demonstrating its excellent safety performance. The sodium iron pyrophosphate / hard carbon pouch battery using the above electrolyte exhibits a high first-cycle coulombic efficiency of 87.9%. Figure 2 As shown, the battery retains 99.4% of its capacity after 800 cycles, with a coulombic efficiency exceeding 99.9% during cycling, indicating good compatibility between the electrolyte and the positive and negative electrode materials in Example 2. Furthermore, under low-temperature testing conditions of -60°C, the ionic conductivity of the electrolyte described in Example 2 remains at 0.86 mS / cm, and the discharge capacity retention is 56.4%. In contrast, the electrolyte in Comparative Example 2 without the first flame-retardant solvent is extremely flammable, demonstrating that adding the first flame-retardant solvent to the electrolyte can significantly improve the safety performance of sodium-ion batteries.

[0036] Example 3 In this embodiment, the electrolyte used contains sodium perchlorate as the sodium salt, (2,2,2)-trifluoroethyl phosphate as the first flame retardant solvent, isobutyronitrile as the second low-viscosity, low-melting-point solvent, diethylene glycol dimethyl ether as the third high-compatibility solvent, and sodium difluorooxalate borate as the electrolyte additive. The sodium salt concentration is 0.7M, the volume ratio of the first flame retardant solvent to the second low-viscosity, low-melting-point solvent to the third high-compatibility solvent is 3:5:3, the electrolyte additive content is 2%wt, and the battery is a sodium iron pyrophosphate / hard carbon soft-pack full battery with a nominal capacity of 1Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0037] As shown in Table 1, the electrolyte in Example 3 is completely non-flammable, demonstrating its excellent safety performance. The sodium iron pyrophosphate / hard carbon pouch battery using this electrolyte exhibits a high first-cycle coulombic efficiency of 84.6%, and retains 93.5% capacity after 500 cycles. The coulombic efficiency during cycling exceeds 99.9%, indicating good compatibility between the electrolyte and the positive and negative electrode materials in Example 3. Figure 3 As shown, under the low-temperature test conditions of -50℃, the ionic conductivity of the electrolyte described in Example 3 can still be maintained at 1.54 mS / cm, and the discharge capacity retention rate is 67.4%. In contrast, the electrolyte in Comparative Example 3 without the addition of a second low-viscosity, low-melting-point solvent has an ionic conductivity of only 0.35 mS / cm at -60℃, and a low-temperature discharge capacity retention rate of only 24.6%. This indicates that adding a second low-viscosity, low-melting-point solvent to the electrolyte can significantly improve the low-temperature performance of sodium-ion batteries.

[0038] Example 4 In this embodiment, the electrolyte used contains sodium hexafluorophosphate as the sodium salt, tri-(2,2,2)-trifluoroethyl phosphate as the first flame retardant solvent, 2-methyltetrahydrofuran as the second low-viscosity, low-melting-point solvent, diethylene glycol dimethyl ether as the third high-compatibility solvent, and vinyl sulfate as the electrolyte additive. The sodium salt concentration is 0.7M, the volume ratio of the first flame retardant solvent to the second low-viscosity, low-melting-point solvent to the third high-compatibility solvent is 3:5:3, the electrolyte additive content is 2%wt, and the battery is a sodium iron pyrophosphate / hard carbon pouch full battery with a nominal capacity of 1Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0039] As shown in Table 1, the electrolyte in Example 4 is completely non-flammable, demonstrating its excellent safety performance. The sodium iron pyrophosphate / hard carbon pouch battery using the above electrolyte exhibits a high first-cycle coulombic efficiency of 83.5%, and still retains 96.4% of its capacity after 800 cycles. The coulombic efficiency during the cycle exceeds 99.9%, indicating good compatibility between the electrolyte and the positive and negative electrode materials in Example 4. In addition, under the low-temperature test condition of -50°C, the ionic conductivity of the electrolyte described in Example 4 can still be maintained at 1.63 mS / cm, and the discharge capacity retention rate is 61.3%. In contrast, the electrolyte in Comparative Example 4 without the addition of a second low-viscosity, low-melting-point solvent solidified at 50°C and could not be charged and discharged normally. This shows that adding a ketone co-solvent to the electrolyte can significantly improve the low-temperature performance of sodium-ion batteries.

[0040] Example 5 In this embodiment, the electrolyte used contains sodium hexafluorophosphate as the sodium salt, (2,2,2)-trifluoroethyl phosphate as the first flame retardant solvent, methyl propionate as the second low-viscosity, low-melting-point solvent, diethylene glycol dimethyl ether as the third high-compatibility solvent, and vinyl sulfate as the electrolyte additive. The sodium salt concentration is 0.7M, the volume ratio of the first flame retardant solvent to the second low-viscosity, low-melting-point solvent to the third high-compatibility solvent is 3:5:3, the electrolyte additive content is 2%wt, and the battery is a sodium iron pyrophosphate / hard carbon soft-pack full battery with a nominal capacity of 1Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0041] As shown in Table 1, the electrolyte in Example 5 is completely non-flammable, demonstrating its excellent safety performance. The sodium iron pyrophosphate / hard carbon pouch battery using the above electrolyte exhibits a high first-cycle coulombic efficiency of 86.4%, and retains 93.4% of its capacity after 500 cycles, with a coulombic efficiency exceeding 99.9% during cycling, indicating good compatibility between the electrolyte and the positive and negative electrode materials in Example 5. Furthermore, under low-temperature testing conditions of -50°C, the ionic conductivity of the electrolyte described in Example 5 remains at 0.96 mS / cm, and the discharge capacity retention rate is 53.4%. In contrast, the electrolyte in Comparative Example 5 without the first flame-retardant solvent is extremely flammable, demonstrating that adding or not adding the first flame-retardant solvent to the electrolyte can significantly improve the safety performance of sodium-ion batteries.

[0042] Comparative Example 1 In this comparative example, the electrolyte used contains sodium hexafluorophosphate as the sodium salt, (2,2,2)-trifluoroethyl phosphate as the first flame retardant solvent, methyl isopropyl ketone as the second low-viscosity, low-melting-point solvent, and no third highly compatible solvent or electrolyte additives are added. The sodium salt concentration is 0.7M, the volume ratio of the first flame retardant solvent to the second low-viscosity, low-melting-point solvent is 3:5, and the battery is a sodium iron pyrophosphate / hard carbon pouch full battery with a nominal capacity of 1Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0043] Comparative Example 2 In this comparative example, the electrolyte used contained sodium difluorosulfonamide, 2-methyltetrahydrofuran as the second low-viscosity, low-melting-point solvent, diethylene glycol dimethyl ether as the third highly compatible solvent, no first flame retardant solvent was added, sodium difluorooxalate borate as the electrolyte additive, wherein the sodium salt concentration was 0.7M, the volume ratio of the second low-viscosity, low-melting-point solvent to the third highly compatible solvent was 5:3, the electrolyte additive content was 2%wt, the battery was a sodium iron pyrophosphate / hard carbon soft-pack full battery, and the nominal capacity of the soft-pack battery was 1Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0044] Comparative Example 3 In this comparative example, the electrolyte used contained sodium perchlorate as the sodium salt, (2,2,2)-trifluoroethyl phosphate as the first flame retardant solvent, diethylene glycol dimethyl ether as the third highly compatible solvent, and no second low-viscosity, low-melting-point solvent was added. The electrolyte additive was sodium difluorooxalate borate, with a sodium salt concentration of 0.7 M. The volume ratio of the first flame retardant solvent to the third highly compatible solvent was 1:1. The electrolyte additive content was 2% wt. The battery was a sodium iron pyrophosphate / hard carbon pouch full battery with a nominal capacity of 1 Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0045] Comparative Example 4 In this comparative example, the electrolyte used contained sodium hexafluorophosphate as the sodium salt, tri-(2,2,2)-trifluoroethyl phosphate as the first flame retardant solvent, diethylene glycol dimethyl ether as the third high-compatibility solvent, and no second low-viscosity, low-melting-point solvent was added. The electrolyte additive was vinyl sulfate, with a sodium salt concentration of 0.7 M. The volume ratio of the first flame retardant solvent to the third high-compatibility solvent was 1:1. The electrolyte additive content was 2% wt. The battery was a sodium iron pyrophosphate / hard carbon pouch full battery with a nominal capacity of 1 Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0046] Comparative Example 5 In this comparative example, the electrolyte used contains sodium hexafluorophosphate as the sodium salt, methyl propionate as the second low-viscosity, low-melting-point solvent, diethylene glycol dimethyl ester as the third high-compatibility solvent, no first flame retardant solvent is added, and the electrolyte additive is vinyl sulfate. The sodium salt concentration is 0.7M, the volume ratio of the second low-viscosity, low-melting-point solvent to the third high-compatibility solvent is 5:3, the electrolyte additive content is 2%wt, and the battery is a sodium iron pyrophosphate / hard carbon soft-pack full battery with a nominal capacity of 1Ah. The ionic conductivity of the electrolyte at room temperature and ultra-low temperature was tested using a conductivity meter. The testing method for sodium iron pyrophosphate / hard carbon pouch cells involved activating the cells at a current density of 0.1A between 1.5V and 3.65V for one week, followed by continuous cycling at a current density of 1A between 1.5V and 3.6V, comparing the capacity retention during cycling. The low-temperature performance of the cells was tested by charging them to 3.65V at room temperature with a current density of 0.1A, followed by discharging them at a current density of 0.1A at low temperature, comparing the capacity retention during discharge at low temperatures.

[0047] Table 1 Performance Test Results The above embodiments are merely specific examples of the present invention, and their descriptions are quite specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these obvious substitutions all fall within the protection scope of the present invention.

Claims

1. A low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility, comprising sodium salt, electrolyte solvent, and electrolyte additives, characterized in that: The electrolyte solvent is a composite solvent system, comprising a first solvent, a second solvent, and a third solvent. The first solvent is one or more of (2,2,2)-trifluoroethyl phosphate, di-(2,2,2)-trifluoroethyl phosphate, and tri-(2,2,2)-trifluoroethyl phosphate. The second solvent is one or more of methyl isopropyl ketone, 2-methyltetrahydrofuran, and methyl propionate. The third solvent is diethylene glycol dimethyl ether. The volume ratio of the first solvent, the second solvent, and the third solvent is 3:4:

3. The volume content of the first solvent is 30%, the volume content of the second solvent is 40%, and the volume content of the third solvent is 30%. The electrolyte additive is one or more of the following: fluoroethylene carbonate, ethylene sulfate, propylene sulfate, sodium bis(oxalate)borate, and sodium difluorooxalateborate.

2. The low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility according to claim 1, characterized in that: The electrolyte additive has a mass content of 0.01-10%wt.

3. The low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility according to claim 1, characterized in that: The sodium salt is one or more of sodium hexafluorophosphate, sodium perchlorate, sodium difluorosulfonamide, and sodium ditrifluorosulfonamide.

4. The low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility according to claim 1, characterized in that: The concentration of the sodium salt is 0.1-3.0 mol / L.

5. The low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility according to claim 1, characterized in that: The negative electrode material of the sodium-ion battery is one or more of hard carbon, tin alloy, and phosphorus alloy.

6. The low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility according to claim 1, characterized in that: The positive electrode material of the sodium-ion battery is a polyanionic positive electrode material, Prussian blue material, or layered oxide material.

7. The low-temperature flame-retardant sodium-ion battery electrolyte with good compatibility according to claim 6, characterized in that: The polyanionic cathode material is an iron-based polyanionic material, a cobalt-based polyanionic material, a nickel-based polyanionic material, or a vanadium-based polyanionic material.

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