Sodium-ion battery flame-retardant electrolyte adaptive to carbon negative electrode material as well as preparation method and application of sodium-ion battery flame-retardant electrolyte

Abstract

The invention relates to a sodium-ion battery flame-retardant electrolyte adaptive to a carbon negative electrode material as well as a preparation method and application of the sodium-ion battery flame-retardant electrolyte. The sodium-ion battery flame-retardant electrolyte adaptive to the carbon negative electrode material comprises sodium salt, a carbonic ester organic solvent, a phosphate flame-retardant additive and a nitrile three-function additive, in the flame-retardant electrolyte, based on the total mass of the carbonic ester organic solvent, the phosphate flame-retardant additive and the nitrile three-function additive being 100%, the mass fraction of the phosphate flame-retardant additive is 1-40%, and the mass fraction of the nitrile three-function additive is 1-50%.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical energy storage batteries, and particularly relates to a flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials, a preparation method thereof, and an application thereof. Background Art

[0002] The shortage and uneven distribution of lithium resources limit the sustainable development of lithium-ion batteries. Sodium-ion batteries have properties similar to those of lithium-ion batteries, are inexpensive, and have excellent low-temperature performance, showing great development potential in large-scale energy storage applications. However, sodium-ion batteries still have safety problems related to flammable electrolytes (electrolyte combustion and battery explosion), and thermal runaway caused by impacts, punctures, overcharging, external short circuits, overheating, etc. brings great safety hazards to the use of sodium-ion batteries.

[0003] To solve the above safety hazards, researchers have carried out a lot of research work on flame-retardant electrolytes, including ionic liquids, hydrofluoroethers, and phosphate-based electrolytes, etc. Introducing flame retardants into the electrolyte can conveniently and effectively reduce the flammability of the electrolyte. Phosphate-based flame retardant additives have been widely used as flame retardant additives and co-solvents in flame-retardant electrolytes for sodium-ion batteries due to their excellent flame retardant performance, physical and chemical properties, low cost, easy solubility in metal salts, and high conductivity. Chinese Patent CN 115663287A discloses a flame retardant dimethyl (3,3-difluoro-2-oxoheptyl) phosphate, and the flame-retardant electrolyte prepared by using it only smokes and does not catch fire in the combustion test. Such electrolytes can essentially eliminate fire hazards and improve battery safety.

[0004] However, the flame retardant additives in such electrolytes often have poor compatibility with electrode materials (especially carbon negative electrode materials), and strong decomposition reactions are likely to occur on the surface of the carbon negative electrode, reducing the stability of the solid electrolyte interface (SEI) of the electrode material, and ultimately resulting in a decrease in the cycle stability of electrode materials such as carbon negative electrodes, leading to deterioration of battery performance. Summary of the Invention

[0005] Aiming at the safety problems caused by the flammable electrolyte of the above sodium-ion battery and the insufficient cycle life of the carbon negative electrode, the purpose of the present invention is to provide a flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials, a preparation method thereof, and an application thereof.

[0006] In a first aspect, the present invention provides a flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials, and the flame-retardant electrolyte includes: sodium salt, carbonate organic solvents, phosphate-based flame retardant additives, and nitrile-based trifunctional additives; In the flame-retardant electrolyte, based on the total mass of the carbonate organic solvent, the phosphate flame-retardant additive, and the nitrile trifunctional additive being 100%, the mass fraction of the phosphate flame-retardant additive is 1-40%, and the mass fraction of the nitrile trifunctional additive is 1-50%.

[0007] Preferably, the sodium salt includes one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, bis(oxalato)borate sodium, sodium trifluoromethanesulfonate, sodium bis(trifluoromethanesulfonyl)imide, and sodium bis(fluorosulfonyl)imide.

[0008] Preferably, the molar concentration of the sodium salt in the flame-retardant electrolyte is 0.1-10 M, preferably 0.5-3.0 M, and more preferably 1.0 M.

[0009] Preferably, the carbonate organic solvent includes one or more of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and fluoroethylene carbonate.

[0010] Preferably, the phosphate flame-retardant additive includes one or more of trimethyl phosphate, triethyl phosphate, tripropyl phosphate, trimethyl phosphite, trifluoroethyl phosphate, diphenyl phosphate, tributyl phosphate, and triphenyl phosphate.

[0011] Preferably, the nitrile trifunctional additive includes one or more of adiponitrile, succinonitrile, propionitrile, 2-methylglutaronitrile, and 2-chloropropionitrile.

[0012] In a second aspect, the present invention provides a method for preparing the above-mentioned flame-retardant electrolyte for a sodium-ion battery adapted to a carbon negative electrode material. The preparation method includes the following steps: First, add the sodium salt to the carbonate organic solvent to form a mixed solution; then, add the phosphate flame-retardant additive and the nitrile trifunctional additive to the mixed solution, and stir evenly to obtain the flame-retardant electrolyte for a sodium-ion battery adapted to a carbon negative electrode material.

[0013] In a third aspect, the present invention provides an application of the above-mentioned flame-retardant electrolyte for a sodium-ion battery adapted to a carbon negative electrode material in a sodium-ion battery with a carbon material as the negative electrode.

[0014] Beneficial effects The electrolyte provided by the present invention is simple to prepare and the raw materials are easily obtainable. A phosphate ester flame retardant additive and a nitrile trifunctional additive are added to the traditional carbonate-based electrolyte. Among them: (1) The phosphate ester flame retardant additive significantly improves the flame retardant properties of the carbonate-based electrolyte; (2) The nitrile trifunctional additive can adjust the sodium ion solvation structure in the electrolyte, accelerate the sodium ion transport, and improve the rate performance of the carbon negative electrode; alleviate the adverse effect of the phosphate ester flame retardant additive on the decomposition of the carbonate-based organic solvent on the surface of the carbon negative electrode, and ensure the formation of a dense and uniform SEI on the surface of the carbon negative electrode; nitriles can form sodium nitride during the electrochemical reaction process, helping to form a thin and dense SEI with a high inorganic component content on the surface of the carbon negative electrode material, greatly improving the stability of the SEI, and thus being beneficial to effectively improving the cycle stability of the carbon negative electrode. Description of the Drawings

[0015] Figure 1 It is a transmission electron microscope photograph of the hard carbon negative electrode material in the electrode liquid of Example 1 after cycling; Figure 2 It is a transmission electron microscope photograph of the hard carbon negative electrode material in the electrolyte of Example 2 after cycling; Figure 3 It is a transmission electron microscope photograph of the hard carbon negative electrode material in the electrolyte of Comparative Example 1 after cycling; Figure 4 It is a transmission electron microscope photograph of the hard carbon negative electrode material in the electrolyte of Comparative Example 2 after cycling; Figure 5 It is a transmission electron microscope photograph of the hard carbon negative electrode material in the electrolyte of Comparative Example 3 after cycling; Figure 6 It is a transmission electron microscope photograph of the hard carbon negative electrode material in the electrolyte of Comparative Example 4 after cycling. Detailed Embodiments

[0016] The present invention will be further described through the embodiments. It should be understood that the following embodiments are only used to illustrate the present invention and do not limit the present invention.

[0017] The electrolyte of the sodium-ion battery is a key factor affecting the safety performance and electrochemical performance of the battery. The combustion characteristics, ionic conductivity, sodium ion solvation structure, and electrode-electrolyte interface stability of the electrolyte all play a crucial role in the safety performance, rate performance, and cycle stability of the sodium-ion battery. For the carbon negative electrode material of the sodium-ion battery, the formation of a uniform, dense, and stable solid electrolyte interface (SEI) on its surface plays a decisive role in improving the cycle stability of the battery.

[0018] First, the present invention provides a flame retardant electrolyte for a sodium-ion battery adapted to a carbon negative electrode material. The flame retardant electrolyte may include: a sodium salt, a carbonate-based organic solvent, a phosphate ester flame retardant additive, and a nitrile trifunctional additive.

[0019] In some embodiments, the sodium salt may include one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, bis(oxalato)borate sodium, sodium trifluoromethanesulfonate, sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI).

[0020] In some embodiments, the molar concentration of the sodium salt in the flame-retardant electrolyte can be controlled to be 0.1 - 10 M, preferably 0.5 - 3.0 M, and more preferably 1.0 M. Too high or too low molar concentration of the sodium salt will affect the ionic conductivity of the electrolyte and the rate performance of the battery.

[0021] In some embodiments, the carbonate organic solvents may include one or more of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and fluorinated ethylene carbonate.

[0022] In some embodiments, the phosphate flame-retardant additives may include one or more of trimethyl phosphate, triethyl phosphate, tripropyl phosphate, trimethyl phosphite, trifluoroethyl phosphate, diphenyl phosphate, tributyl phosphate, and triphenyl phosphate. The phosphate flame-retardant additives can significantly improve the flame-retardant properties of carbonate electrolytes.

[0023] In some embodiments, the nitrile trifunctional additives may include one or more of adiponitrile, succinonitrile, propionitrile, 2-methylglutaronitrile, and 2-chloropropionitrile.

[0024] The nitrile trifunctional additives used in the flame-retardant electrolyte in the present invention have the following technical advantages: First, it adjusts the sodium ion solvation structure in the electrolyte, accelerates the sodium ion transport, and improves the rate performance of the carbon negative electrode; Second, it alleviates the adverse effects of the decomposition of phosphate flame-retardant additives on the surface of the carbon negative electrode, avoids the attenuation of electrochemical performance caused by the repeated formation of SEI, and ensures the formation of a dense and uniform SEI on the surface of the carbon negative electrode; Third, sodium nitride is formed during the electrochemical reaction process, which helps to form a thin and dense SEI with a high inorganic component content on the surface of the carbon negative electrode material, improves the stability of SEI and the cycle stability of the carbon negative electrode, and provides the possibility for the practical development of the sodium ion battery system.

[0025] In some embodiments, in the flame-retardant electrolyte, based on the total mass of the carbonate organic solvents, phosphate flame-retardant additives, and nitrile trifunctional additives being 100%, the mass fraction of the phosphate flame-retardant additives can be 1 - 40%, and the mass fraction of the nitrile trifunctional additives can be 1 - 50%.

[0026] Among them, when the amount of the flame retardant additive is too small, it cannot play a flame retardant role that meets the requirements. However, when the amount is too large, due to the poor compatibility of this component with the carbon negative electrode, the flame retardant additive is prone to decompose on the surface of the carbon negative electrode, and the SEI stability is poor. At the same time, one of the main functions of the trifunctional additive is to regulate the sodium ion solvation structure in the carbonate electrolyte. When the amount is too small, it is difficult to play the corresponding regulatory role. However, when the amount is too large, due to its own physical properties, the parameters such as the ionic conductivity and viscosity of the electrolyte will be poor, which is not suitable for the sodium ion electrolyte of the carbon negative electrode. In addition, its excessive decomposition on the surface of the carbon negative electrode will also lead to a low Coulomb efficiency, which has an adverse impact on the electrochemical performance of the battery.

[0027] The sodium ion battery flame retardant electrolyte provided by the present invention and adapted to the carbon negative electrode material shows a self-extinguishing electrolyte through a flame combustion test. And at a rate of 1C, the capacity retention rate of the hard carbon negative electrode in the sodium ion battery using the flame retardant electrolyte can reach more than 92% after 2000 cycles.

[0028] The following is an exemplary description of the preparation method of the sodium ion battery flame retardant electrolyte provided by the present invention and adapted to the carbon negative electrode material. The preparation method includes the following steps: First, add a sodium salt to a carbonate organic solvent to form a mixed solution; then, add a phosphate flame retardant additive and a nitrile trifunctional additive to the mixed solution, and stir evenly to obtain the sodium ion battery flame retardant electrolyte adapted to the carbon negative electrode material.

[0029] The preparation process of the sodium ion battery flame retardant electrolyte provided by the present invention is simple, and the raw material resources are rich. It can be used in sodium ion batteries with carbon materials as the negative electrode, and can effectively improve the stability of the solid electrolyte interface (SEI) on the surface of the carbon negative electrode, effectively extend the cycle life of the carbon negative electrode in the phosphate-based flame retardant electrolyte, and greatly improve the safety and cycle stability of sodium ion batteries using carbon negative electrodes during actual application.

[0030] The following further lists examples to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be construed as limiting the protection scope of the present invention. Those skilled in the art's non-essential improvements and adjustments based on the above content of the present invention all belong to the protection scope of the present invention. The specific process parameters and the like in the following examples are also only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description in this article, rather than being limited to the specific values in the following examples.

[0031] Example 1

[0032] The sodium ion battery flame retardant electrolyte provided in this Example 1 includes: (1) The sodium salt is sodium perchlorate, and the concentration is 1M; (2) The organic solvent is ethylene carbonate (EC) and propylene carbonate (PC), and the volume ratio of EC to PC is 1:1; (3) The flame retardant additive is trimethyl phosphate, and its mass fraction in the organic solvent, flame retardant additive and trifunctional additive is 15%; the trifunctional additive is adiponitrile, and its mass fraction in the organic solvent, flame retardant additive and trifunctional additive is 9%.

[0033] Example 2

[0034] The flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials provided in this Example 2 includes: (1) The sodium salt is sodium perchlorate, and the concentration is 1 M; (2) The organic solvent is ethylene carbonate (EC) and propylene carbonate (PC), and the volume ratio of EC to PC is 1:1; (3) The flame retardant additive is trimethyl phosphate, and its mass fraction in the organic solvent, flame retardant additive and trifunctional additive is 15%; the trifunctional additive is adiponitrile, and its mass fraction in the organic solvent, flame retardant additive and trifunctional additive is 4%.

[0035] Comparative Example 1

[0036] The electrolyte provided in this Comparative Example 1 includes: (1) The sodium salt is sodium perchlorate, and the concentration is 1 M; (2) The organic solvent is ethylene carbonate (EC) and propylene carbonate (PC), and the volume ratio of EC to PC is 1:1; (3) There is no flame retardant additive or trifunctional additive.

[0037] Comparative Example 2

[0038] The electrolyte provided in this Comparative Example 2 includes: (1) The sodium salt is sodium perchlorate, and the concentration is 1 M; (2) The organic solvent is ethylene carbonate (EC) and propylene carbonate (PC), and the volume ratio of EC to PC is 1:1; (3) The flame retardant additive is trimethyl phosphate, and its mass fraction in the organic solvent, flame retardant additive and trifunctional additive is 15%; the amount of the trifunctional additive is 0.

[0039] Comparative Example 3

[0040] The electrolyte provided in this Comparative Example 3 includes: (1) The sodium salt is sodium perchlorate, and the concentration is 1 M; (2) The organic solvent is ethylene carbonate (EC) and propylene carbonate (PC), and the volume ratio of EC to PC is 1:1; (3) The flame retardant additive is trimethyl phosphate, and its mass fraction in the organic solvent, the flame retardant additive, and the trifunctional additive is 60%; the trifunctional additive is adiponitrile, and its mass fraction in the organic solvent, the flame retardant additive, and the trifunctional additive is 9%.

[0041] Comparative Example 4

[0042] The electrolyte provided by this Comparative Example 4 includes: (1) The sodium salt is sodium perchlorate, and its concentration is 1 M; (2) The organic solvent is ethylene carbonate (EC) and propylene carbonate (PC), and the volume ratio of EC to PC is 1:1; (3) The flame retardant additive is trimethyl phosphate, and its mass fraction in the organic solvent, the flame retardant additive, and the trifunctional additive is 15%; the trifunctional additive is adiponitrile, and its mass fraction in the organic solvent, the flame retardant additive, and the trifunctional additive is 55%.

[0043] Pole piece coating: A slurry containing 80% hard carbon, 10% super P, and 10% carboxymethyl cellulose was coated on an aluminum foil to prepare a hard carbon negative electrode. After drying at 100 °C for 10 h, the electrode film was punched into a pole piece with a diameter of 14 mm. The mass loading of the active material was about 2.5 mg cm -2 .

[0044] Assembling a sodium-ion battery: In a glove box filled with argon, a battery was assembled using a PE separator with a thickness of 9 μm, the electrolytes prepared in the examples and comparative examples, the negative pole piece prepared above, and a sodium metal sheet.

[0045] Battery activation: The assembled battery was placed in an incubator at 45 °C, and the battery was activated using a Blue Power charge-discharge instrument. The activation voltage range was 0.05 - 1.5 V, the charge-discharge rate was 0.05 C for both charge and discharge, and the charge-discharge process was carried out for 3 weeks.

[0046] Battery cycle stability test: The activated battery was subjected to a 1C, 2000-cycle stability test, and the charge-discharge voltage range was 0.05 - 1.5 V. During the test, the instrument automatically recorded the charge-discharge data and related curves. After the long-term cycling was completed, the battery was disassembled, the hard carbon negative pole piece was taken out, the negative electrode material was scraped off, dispersed in ethanol, ultrasonically dispersed evenly, and then the solution was dropped onto a copper mesh to take a transmission electron microscope photo of the negative electrode material.

[0047] Battery rate performance test: The temperature was set at 25 °C, the charge-discharge rates were set at 0.05 C, 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C, and the charge-discharge was carried out 10 times under each rate condition. The charge-discharge voltage range was 0.05 - 1.5 V. During the test, the instrument automatically recorded the charge-discharge data.

[0048] Ignite the prepared electrolyte with a butane torch for 5 seconds, and calculate the self-extinguishing time (SET) of different electrolytes. Next, Table 1 lists the self-extinguishing times and flame retardant efficiencies of the electrolytes in Examples 1 and 2 and Comparative Examples 1-4: Electrolyte Flame retardant effect <![CDATA[Self-extinguishing time (sg -1 )]]> Flame retardant efficiency (η) Example 1 Cannot be ignited by open flame 0 1 Example 2 Cannot be ignited by open flame 0 1 Comparative Example 1 Can be ignited by open flame 265 0 Comparative Example 2 Cannot be ignited by open flame 0 1 Comparative Example 3 Cannot be ignited by open flame 0 1 Comparative Example 4 Cannot be ignited by open flame 0 1

[0049] The results in Table 1 show that the addition of a 15% mass fraction of trimethyl phosphate flame retardant additive makes the self-extinguishing time of the electrolyte 0 and the flame retardant efficiency 1. Compared with the carbonate electrolyte in Comparative Example 1, the safety performance of the electrolytes in Examples 1 and 2 and Comparative Examples 2, 3, and 4 has been significantly improved, which benefits from the excellent flame retardant effect of the trimethyl phosphate flame retardant additive.

[0050] The following Table 2 shows the capacity retention results of hard carbon batteries assembled with the electrolytes based on Examples 1 and 2 and Comparative Examples 1-4 at a charge-discharge rate of 1C at 25°C: Electrolyte Capacity retention rate after 2000 cycles Example 1 92.2％ Example 2 85.5％ Comparative Example 1 67.6％ Comparative Example 2 48.9％ Comparative Example 3 43.6％ Comparative Example 4 62.1％

[0051] The results in Table 2 show that compared with the electrolyte test results in Comparative Examples 1-4, the cycle stabilities of the electrolytes in Examples 1 and 2 for testing the hard carbon negative electrode have been improved. The cycle stability of the hard carbon negative electrode tested with the electrolyte in Example 1 is the best, and it is relatively better than the electrolyte with a relatively low adiponitrile addition amount in Example 2, further verifying the role of the nitrile trifunctional additive in improving the cycle stability of the carbon negative electrode material. The test results of the electrolyte in Comparative Example 3 show that the cycle stability of the hard carbon negative electrode is inferior to that of Comparative Examples 1 and 2, indicating that the excessive use of the flame retardant additive will lead to a further decline in the electrochemical stability of the hard carbon negative electrode; the test results of the electrolyte in Comparative Example 4 show that the cycle stability of the hard carbon negative electrode tested is close to that of Comparative Example 1 and significantly better than that of Comparative Example 2, indicating that the application of the nitrile trifunctional additive can significantly alleviate the adverse effect of the phosphate ester flame retardant additive on the stability of the hard carbon negative electrode. Thus, the cycle stabilities of the hard carbon negative electrodes tested with the electrolytes in Examples 1 and 2 have been significantly improved.

[0052] Disassemble the hard carbon batteries prepared with the electrolytes in Examples 1 and 2 and Comparative Examples 1-4 after long cycling, and observe the cycled hard carbon negative electrode material by transmission electron microscopy. The obtained photos are as Figures 1 - 6 shown.

[0053] As Figure 1 shown, after long cycling, the SEI formed on the surface of the hard carbon in Example 1 still exists and is dense.

[0054] As Figure 2 shown, after long cycling, the SEI formed on the surface of the hard carbon in Example 2 still exists.

[0055] As Figure 3 shown, after long cycling, the SEI formed on the surface of the hard carbon in Comparative Example 1 still exists.

[0056] As Figure 4 shown, after long cycling, there is no SEI on the surface of the hard carbon in Comparative Example 2. This is consistent with the poor cycling stability of the hard carbon in Comparative Example 2 because the flame retardant trimethyl phosphate in the electrolyte in Comparative Example 2 has poor compatibility with the hard carbon negative electrode material and will strongly decompose on the surface of the hard carbon, resulting in an unstable solid electrolyte interface.

[0057] As Figure 5 shown, after long cycling, there is no SEI on the surface of the hard carbon in Comparative Example 3. This is consistent with the worst cycling stability of the hard carbon in Comparative Example 3 because the excessive amount of the flame retardant trimethyl phosphate in the electrolyte in Comparative Example 3 and its poor compatibility with the hard carbon negative electrode material will strongly decompose on the surface of the hard carbon, resulting in an unstable solid electrolyte interface.

[0058] As Figure 6 shown, after long cycling, there is still SEI on the surface of the hard carbon in Comparative Example 4. This is consistent with the relatively better cycling stability of the hard carbon in Comparative Example 4 because of the role of the nitrile-based trifunctional additive, which makes the solid electrolyte interface more stable.

[0059] The above results show that the nitrile-based trifunctional additive can effectively improve the stability of the solid electrolyte interface (SEI) on the surface of the carbon negative electrode.

[0060] Table 3 below shows the rate performance test results of the hard carbon batteries assembled with the electrolytes based on Examples 1 and 2 and Comparative Examples 1-4 at a test temperature of 25°C:

[0061] The results in Table 3 show that compared with the electrolytes in Comparative Examples 1-4, the rate performance of the hard carbon negative electrode tested with the electrolytes in Examples 1 and 2 has been significantly improved. Thanks to the nitrile-based trifunctional additive such as adiponitrile, the sodium ion transport is accelerated, and the formed SEI film rich in inorganic components has higher sodium ion conductivity, reflecting the technical advantages of the novel sodium ion battery electrolyte adapted to the carbon negative electrode proposed by the present invention. The rate performance of the hard carbon negative electrode tested with the electrolyte in Comparative Example 4 is inferior to that in Examples 1 and 2 because the excessive amount of the nitrile-based trifunctional additive in Comparative Example 4 makes the SEI formed on the surface of the hard carbon negative electrode too thick, hindering the rapid transport of sodium ions.

[0062] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be recognized that the above description should not be considered as a limitation of the present invention. After those skilled in the art have read the above content, various modifications and alternatives to the present invention will be obvious. Therefore, the protection scope of the present invention should be defined by the appended claims.

Claims

1. A flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials, Characterized in that, The flame-retardant electrolyte includes: Sodium salts, carbonate organic solvents, phosphate flame-retardant additives and nitrile trifunctional additives; In the flame-retardant electrolyte, based on the total mass of carbonate organic solvents, phosphate flame-retardant additives and nitrile trifunctional additives being 100%, the mass fraction of the phosphate flame-retardant additives is 1-40%, and the mass fraction of the nitrile trifunctional additives is 1-50%.

2. The flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials according to claim 1, Characterized in that, The sodium salts include one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, bis(oxalato)borate sodium, sodium trifluoromethanesulfonate, sodium bis(trifluoromethanesulfonyl)imide and sodium bis(fluorosulfonyl)imide.

3. The flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials according to claim 1 or 2, Characterized in that, The molar concentration of the sodium salt in the flame-retardant electrolyte is 0.1-10 M, preferably 0.5-3.0 M, and more preferably 1.0 M.

4. The flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials according to any one of claims 1-3, Characterized in that, The carbonate organic solvents include one or more of propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, and fluoroethylene carbonate.

5. The flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials according to any one of claims 1-4, Characterized in that, The phosphate flame-retardant additives include one or more of trimethyl phosphate, triethyl phosphate, tripropyl phosphate, trimethyl phosphite, trifluoroethyl phosphate, diphenyl phosphate, tributyl phosphate, and triphenyl phosphate.

6. The flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials according to any one of claims 1-5, Characterized in that, The nitrile trifunctional additives include one or more of adiponitrile, succinonitrile, propionitrile, 2-methylglutaronitrile, and 2-chloropropene nitrile.

7. A preparation method of the flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials according to any one of claims 1-6, Characterized in that, The preparation method includes the following steps: First, add the sodium salt to the carbonate organic solvent to form a mixed solution; then, add the phosphate flame-retardant additive and the nitrile trifunctional additive to the mixed solution, and stir evenly to obtain the flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials.

8. An application of the flame-retardant electrolyte for sodium-ion batteries adapted to carbon negative electrode materials according to any one of claims 1-6 in a sodium-ion battery with a carbon material as the negative electrode.

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