Flame-retardant electrolyte for sodium-ion battery and sodium-ion secondary battery

Abstract

The application provides a kind of sodium ion battery with flame-retardant electrolyte, it includes the following components: base electrolyte and functional additives;Wherein, the base electrolyte includes solvent and sodium salt;The solvent includes main solvent and optional secondary solvent;The sodium salt includes main sodium salt and optional secondary sodium salt;The main solvent is phosphorus-containing flame-retardant solvent, the secondary solvent is selected from at least one of carbonate solvent, carboxylic acid ester solvent and ether solvent of non-fluorinated ether;The molar ratio of the phosphorus-containing flame-retardant solvent and sodium salt is 4.2-17:1;The mass fraction of the main sodium salt in sodium salt is 70%-100%;The volume fraction of the main solvent in the solvent is 80%-100%;The mass fraction of the functional additives in the electrolyte is greater than 0 and less than or equal to 5%.The application also provides a kind of sodium ion secondary battery, which includes positive electrode material, negative electrode material and the sodium ion battery with flame-retardant electrolyte of the application.The electrolyte of the application is low in cost, high in safety, and can be compatible with carbon negative electrode.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of battery materials. Specifically, the present application relates to a flame-retardant electrolyte for sodium ion batteries and a sodium ion secondary battery. BACKGROUND

[0002] High-performance secondary batteries are the most efficient and most convenient energy storage and conversion devices, which are essential for establishing a clean energy system and realizing large-scale energy storage. Sodium ion batteries are considered to be a beneficial supplement to lithium ion batteries due to their abundant resources, wide distribution and low cost, and are one of the ideal devices for application in large-scale energy storage fields. In recent years, the research and development of sodium ion battery technology have attracted widespread attention from research groups around the world.

[0003] However, the safety hazards of sodium ion batteries have always been the primary concern of people. Because the electrolyte used in sodium ion batteries is a flammable carbonate and / or ether solvent system, when under the conditions of overcharge, short circuit or heating, sodium ion batteries may catch fire, burn or explode, thereby causing safety accidents.

[0004] CN108736010A discloses the use of pure phosphate ester as the electrolyte of sodium ion batteries, and the use of phosphate compound as the negative electrode material. The disadvantage of this patent application is that the disclosed electrolyte is not compatible with carbon negative electrodes.

[0005] CN110518287A discloses the use of phosphate ester and fluoroether as flame-retardant solvents in a volume ratio of 1:1-2:1, and the use of sodium perchlorate as a sodium salt. The disadvantage of this patent application is that due to the low content of phosphate ester as a flame retardant (about 33.3%-66.6% by volume of the solvent), the battery of this patent application is not flammable, but still cannot pass the puncture test. Using a large amount of fluoroether as a solvent, on the one hand, will lead to an increase in the cost of the electrolyte; on the other hand, the vapor pressure of the fluoroether solvent is usually high, and the boiling point is low, which is not conducive to the packaging of the battery. In addition, fluoroethers have anesthetic effects, for example, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (Cas: 16627-68-2) commonly used in the industry has a strong acute anesthetic effect. In the production process, the volatile gas and residual liquid of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether must be controlled to prevent unnecessary harm to personnel. In addition, fluoroethers have low dielectric constants and almost no dissociation of sodium salts, resulting in low conductivity of the electrolyte with fluoroether as the main solvent and poor rate performance of the battery. In addition, the use of sodium perchlorate as a sodium salt poses a certain safety hazard. Sodium perchlorate is a strong oxidizing product and an explosive product, and its use is subject to control.

[0006] CN108028429A discloses an electrolyte containing a high concentration of alkali metal salt. In the electrolyte, the amount of solvent is 4 mol or less relative to 1 mol of alkali metal salt. In this patent application, the anion of the alkali metal salt is selected from one or more of fluorosulfonyl, trifluoromethylsulfonyl and perfluoroethylsulfonyl. The disadvantage of this patent application is that, on the one hand, the salts disclosed are all organic salts, which are relatively expensive. In addition, in this patent application, the cost of the salt is about ten times that of the solvent. High salt concentration electrolyte results in high overall electrolyte price. High salt concentration electrolyte can increase the viscosity of the electrolyte and reduce the conductivity. Therefore, under low temperature or high current charge and discharge conditions, the battery performance is very poor. In addition, this patent application only gives the performance of sodium metal and hard carbon negative electrode, and does not give the performance of hard carbon negative electrode and layered oxide full battery. On the other hand, high salt concentration flame-retardant electrolyte is not safe (see Hou J, Lu L, Wang L, et al. Thermal runaway of lithium-ion batteries employing LiN(SO2F)2-based concentrated electrolytes [J]. Nature communications, 2020, 11(1): 1-11).

[0007] The prior art discloses an electrolyte containing NaBOB (sodium bis(oxalato)borate) as a sodium salt (see Mogensen R, Colbin S, Menon A S, et al. Sodium bis(oxalato)borate in trimethyl phosphate: a fire-extinguishing, fluorine-free, and low-cost electrolyte for full-cell sodium-ion batteries [J]. ACS Applied Energy Materials, 2020, 3(5): 4974-4982; and Mogensen R, Buckel A, Colbin S, et al. A Wide-Temperature-Range, Low-Cost, Fluorine-Free Battery Electrolyte Based On Sodium Bis(Oxalate)Borate [J]. Chemistry of Materials, 2021, 33(4): 1130-1139). Due to the easy reduction of the salt, the impedance thereof is large. The battery has a low coulombic efficiency in the first cycle, and the coulombic efficiency is less than or equal to 99.85% during the cycle process. This means that there is a significant side reaction in the battery, resulting in poor cycle stability of the battery.

[0008] There is an urgent need for an electrolyte with low cost, high safety, and excellent electrochemical performance such as cycle performance, which is compatible with carbon negative electrodes. SUMMARY

[0009] The purpose of the present application is to provide a flame-retardant electrolyte with low cost, high safety, and excellent electrochemical performance such as cycle performance, which is compatible with carbon negative electrodes.

[0010] The above-mentioned purpose of the present application is achieved by the following technical solutions.

[0011] In a first aspect, the present application provides a flame-retardant electrolyte for sodium-ion batteries, which comprises the following components:

[0012] a base electrolyte and a functional additive; wherein,

[0013] The base electrolyte comprises a solvent and a sodium salt; the solvent comprises a primary solvent and an optional secondary solvent; the sodium salt comprises a primary sodium salt and an optional secondary sodium salt; the primary solvent is a phosphorus-containing flame-retardant solvent, the secondary solvent is selected from at least one of a carbonate solvent, a carboxylate solvent and an ether solvent other than a fluorinated ether, the primary sodium salt is sodium hexafluorophosphate and sodium tetrafluoroborate, and the secondary sodium salt is selected from one or more of sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonate), sodium trifluoromethylsulfonate and sodium perchlorate;

[0014] The molar ratio of the phosphorus-containing flame-retardant solvent to the sodium salt is 4.2-17:1, preferably 5-10:1, and more preferably 7-9:1; the mass fraction of the primary sodium salt in the sodium salt is 70%-100%;

[0015] The volume fraction of the primary solvent in the solvent is 80%-100%, and more preferably 100%;

[0016] The mass fraction of the functional additive in the electrolyte is greater than 0 and less than or equal to 5%, and preferably 1%-3%.

[0017] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the phosphorus-containing flame-retardant solvent is a phosphate ester with a chemical formula of (RO)3P=O or a phosphonate ester with a chemical formula of R(RO2)P=O, wherein R is a phenyl group or a low-carbon alkyl group with a carbon chain length of 1-6 or a halogenated alkyl group.

[0018] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the phosphate ester or phosphonate ester is selected from one or more of triphenyl phosphate (TPP), trimethyl phosphate (TMP), triethyl phosphate (TEP), dimethyl methylphosphonate (DMMP) and bis(2,2,2-trifluoroethyl)methylphosphonate (TFMP).

[0019] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 1:50-50:1, and more preferably 1:11-11:1.

[0020] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the carbonate solvent is a cyclic carbonate solvent and / or a chain carbonate solvent.

[0021] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the cyclic carbonate is ethylene carbonate and / or propylene carbonate.

[0022] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the chain carbonate is a carbonate synthesized from a straight-chain or branched-chain aliphatic monoalcohol with a carbon number of 3-8 and carbonic acid.

[0023] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, and methyl ethyl carbonate.

[0024] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the carboxylic acid ester solvent is a cyclic carboxylic acid ester solvent and / or a chain carboxylic acid ester solvent.

[0025] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the cyclic carboxylic acid ester is γ-butyrolactone, and the chain carbonate is a chain carboxylic acid ester having a carbon number of 3-8.

[0026] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the chain carboxylic acid ester is selected from one or more of methyl acetate, ethyl acetate, propyl acetate, propyl propionate, and ethyl propionate.

[0027] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the ether of the non-fluorinated ether is selected from one or more of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, dimethoxymethane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0028] Preferably, in the flame-retardant electrolyte for sodium ion batteries according to the present application, the functional additive is selected from one or more of vinylene carbonate, fluorinated vinylene carbonate, 1,3-propane sultone (PS), 1,4-butane sultone, propenyl-1,3-sultone (PST), cyclohexylbenzene, tert-butylbenzene, tert-amylbenzene, ethylene sulfate, dimethyl sulfite, diethyl sulfite, and succinonitrile.

[0029] In a second aspect, the present application provides a sodium ion secondary battery comprising a positive electrode material, a negative electrode material, and the flame-retardant electrolyte for sodium ion batteries according to the present application.

[0030] Preferably, in the sodium ion secondary battery according to the present application, the positive electrode material is a layered metal oxide, a polyanion-based positive electrode material, or a Prussian blue-based material, and the negative electrode material is a carbon-based material.

[0031] Preferably, in the sodium ion secondary battery according to the present application, the positive electrode material is a layered metal oxide, and the negative electrode material is hard carbon.

[0032] The inventors of the present application surprisingly found that when the sodium salt is the sodium hexafluorophosphate and sodium tetrafluoroborate claimed by the present application and optionally a small amount of a sodium sub-salt, and when the solvent is the kind and amount of the primary solvent and the secondary solvent claimed by the present application, the electrolyte with low cost, high safety and excellent electrochemical performance such as cycle performance compatible with carbon negative electrode can be achieved by using a lower concentration of sodium salt without using a higher concentration of sodium salt.

[0033] The present application has the following beneficial effects:

[0034] The sodium-ion secondary battery of the present application has high safety and excellent electrochemical performance such as cycle performance. The average coulombic efficiency of the sodium-ion secondary battery of the present application during room temperature cycling is greater than 99.9%. In the case of battery abuse or accidental damage (such as puncture), the sodium-ion secondary battery of the present application can reduce or avoid the risk of fire.

[0035] Under the conventional concentration of sodium salt, the electrolyte of the present application achieves the compatibility of flame-retardant electrolyte with carbon negative electrode. The sodium-ion secondary battery of the present application has high industrial value.

[0036] The electrolyte of the present application has low cost, high safety and is compatible with carbon negative electrode. BRIEF DESCRIPTION OF DRAWINGS

[0037] Hereinafter, the embodiments of the present application will be described in detail with reference to the accompanying drawings, in which:

[0038] Figure 1 is the charge-discharge curve of the first two cycles of the battery of Example 1 at room temperature;

[0039] Figure 2 is the cycle performance comparison chart of the batteries of Example 1 and Comparative Example 1 at room temperature;

[0040] Figure 3 is the coulombic efficiency comparison chart of the batteries of Example 1 and Comparative Example 1 during the cycle at room temperature;

[0041] Figure 4 is the coulombic efficiency comparison chart of the batteries of Example 1 and Comparative Example 1 during the cycle at high temperature;

[0042] Figure 5 is the charge-discharge curve of the first two cycles of the battery of Example 2 at room temperature;

[0043] Figure 6 is the charge-discharge curve of the first two cycles of the battery of Example 3 at room temperature;

[0044] Figure 7 is the charge-discharge curve of the first two cycles of the battery of Example 14 at room temperature;

[0045] Figure 8 is a plot of the charge-discharge profile of the first two cycles at room temperature for the battery of Example 17;

[0046] Figure 9 is a plot of the charge-discharge profile of the first two cycles at room temperature for the battery of Example 19;

[0047] Figure 10 is a plot of the charge-discharge profile of the first two cycles at room temperature for the battery of Example 23;

[0048] Figure 11 is a plot of the charge-discharge profile of the first two cycles at room temperature for the battery of Example 27. DETAILED DESCRIPTION

[0049] The application will be further described in conjunction with the specific embodiments, which are given only by way of illustration and not to limit the scope of the application.

[0050] O3-Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 ]O2as the positive active material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, and mixed uniformly in a mass ratio of 90:6:4. Then, N-methylpyrrolidone was added and a slurry was prepared. The slurry was coated on an aluminum foil. Vacuum drying was performed at 120°C overnight. After rolling, the positive electrode sheet was obtained. Hard carbon was used as the negative active material, acetylene black as the conductive agent, and polytetrafluoroethylene as the binder, and mixed uniformly in a mass ratio of 90:5:5. Then, N-methylpyrrolidone was added and a slurry was prepared. The slurry was coated on an aluminum foil, vacuum drying was performed at 120°C overnight. After rolling, the negative electrode sheet was obtained.

[0051] Preparation of electrolyte: In an argon-filled glove box, the primary sodium salt and optional secondary sodium salt were weighed according to the data in Table 1. Then, a certain volume of solvent was added. After stirring thoroughly, the prepared electrolyte was obtained. Note that most of the salts can be completely dissolved. In the comparative examples, some of the sodium salts were not completely dissolved. When some of the sodium salts in the comparative examples were not completely dissolved, the upper layer saturated solution was taken as the electrolyte.

[0052] The English abbreviations in Table 1 are as follows:

[0053] TMP: trimethyl phosphate; TEP: triethyl phosphate; DEC: diethyl carbonate; EC: ethylene carbonate; EP: propyl acetate; DEGDME: diethylene glycol dimethyl ether; HFE: 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether; VC: vinylene carbonate; PS: 1,3-propanesulfone; PC: propylene carbonate; FEC: fluoroethylene carbonate; DTD: ethylene sulfate; VEC: vinyl ethylene carbonate.

[0054]

[0055]

[0056]

[0057] Figure 1 is the first charge-discharge curve of the battery of Example 1 at room temperature. Figure 1 shows that in the hard carbon / O3-Na[Cu 1 / 9Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 ]O2full battery, the first charge-discharge curve at 0.1C rate (1C = 100 mA / g). Figure 1 、 Figures 5-11 shows that based on the electrolyte of the application, the discharge specific capacity in the full battery is greater than 120 mAh / g (based on the positive electrode), and the first coulombic efficiency is high (about 85%).

[0058] Figure 2 is a comparison chart of the cycle performance of the batteries of Example 1 and Comparative Example 1 at room temperature. Figure 2 shows that the first discharge specific capacity of the battery of Example 1 is 129 mAh / g, the first coulombic efficiency is 84.7%, and the capacity retention rate after 180 cycles is 92.7%. The capacity retention rate of the battery of Comparative Example 1 after 150 cycles is 71.7%.

[0059] Figure 3 is a comparison chart of the coulombic efficiency of the batteries of Example 1 and Comparative Example 1 during the cycle at room temperature. The average coulombic efficiency of the conventional electrolyte of Comparative Example 1 from the fourth cycle is 99.75%. In comparison, the average coulombic efficiency of the flame-retardant electrolyte of Example 1 from the fourth cycle is 99.995%. The cycle coulombic efficiency of the battery of Comparative Example 1 is lower compared to the cycle coulombic efficiency of the battery of Example 1. This indicates that the flame-retardant electrolyte of Example 1 has fewer side reactions.

[0060] The electrolyte of Examples 1-29 and the electrolyte of Comparative Examples 1 and 2 were injected into cylindrical cells (2.5 Ah capacity). The cells were cycled for 2 cycles, and the needle-penetration test was performed on the full cells. During the penetration process, the cells of Examples 1-29 did not emit smoke, did not catch fire, and did not explode. During the penetration process, the cells of Comparative Examples 1 and 2 emitted thick smoke, and had safety hazards. This comparison shows that a higher content of flame retardant (volume fraction of solvent is more than 80%) has higher battery safety performance, and a lower content (volume fraction of solvent is less than or equal to 68%) has lower battery safety performance. An actual electrolyte containing more than 30% of flame retardant can make the electrolyte non-flammable (Wang X, Yasukawa E, Kasuya S. Nonflammable trimethylphosphate solvent-containing electrolytes for lithium-ion batteries: I. Fundamental properties [J]. Journal of The Electrochemical Society, 2001, 148(10): A1058). The actual electrolyte is non-flammable, but does not mean that the battery will not have thermal runaway.

[0061] Examples 1-13 show the performance of electrolytes with different molar ratios of NaBF4 and NaPF6. When the molar ratio of NaBF4 to NaPF6 is 50:1 to 1:50, the battery has a very high capacity retention rate (84.4%-95%) after 100 cycles. When the molar ratio of NaBF4 to NaPF6 is 11:1 to 1:11, the battery has a higher capacity retention rate (88.6%-95%) after 100 cycles.

[0062] Comparative Example 3 shows that the electrolyte using only NaPF6 salt has poor cycle stability, and the 100-cycle capacity retention rate is only 35%.

[0063] Comparative Example 4 shows that the electrolyte using only NaBF4 salt (the electrolyte will have a small amount of NaBF4 precipitate, so the supernatant is used as the electrolyte) has poor cycle stability, and the 100-cycle capacity retention rate is only 60%.

[0064] Comparative Example 5 shows that when the molar ratio of NaBF4 to NaPF6 is 1:59, the 100-cycle capacity retention rate of the battery is only 65%.

[0065] Comparative Example 6 shows that when the molar ratio of NaBF4 to NaPF6 is 54.6:1, the 100-cycle capacity retention rate of the battery is only 66%.

[0066] Examples 14-18 show that when using a suitable mixed sodium salt, and using pure flame retardant TEP as the solvent, the capacity retention of the battery is still greater than 85% after 100 cycles.

[0067] Examples 19-24 show that when other solvents are carbonates (such as EC, DEC), carboxylates (such as EP), non-fluorinated ethers (DEGDME), the battery cycle performance is stable and the capacity retention is high.

[0068] Comparative Example 9 shows that when the other solvent is a fluorinated ether, for example 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether accounts for 10% of the total solvent by volume, the capacity retention of the battery is only 74% after 100 cycles. The battery of Comparative Example 9 has poor performance. Without wishing to be bound by theory, the reason for the poor performance of Comparative Example 9 can be that the fluorinated ether can also act as a film-forming additive, and when the fluorinated ether is used in combination with NaPF6 and NaBF4, a stable interfacial film cannot be formed.

[0069] Examples 25-27 show that when NaPF6 and NaBF4 are used as the main sodium salt, and other small amounts of sodium salts such as NaFSI, NaClO4, NaCF3SO3 are used in combination, the battery still maintains good cycle stability.

[0070] Comparative Examples 6-8 show that when other salts are selected for combination, such as NaPF6 + NaClO4 or NaBF4 + NaClO4, the battery cycle stability is poor. This fully demonstrates the superiority of using NaPF6 and NaBF4 as the main sodium salt.

[0071] Examples 28-29 show that when NaPF6 and NaBF4 are used as the main sodium salt, even if the concentration of the sodium salt is low, the battery still maintains good cycle stability.

[0072] Examples 30-35 show that when NaPF6 and NaBF4 are used as the main sodium salt, and common additives in the art (such as VC, FEC, VEC, DTD, PS) are selected, the battery still maintains good cycle stability.

[0073] Figure 4 is a comparison chart of the coulombic efficiency of the batteries of Example 1 and Comparative Example 1 during the high-temperature cycle process. The electrolyte in Example 1 was cycled at a high temperature of 60°C for 100 times, and the capacity retention of the battery was 84%, and the average coulombic efficiency was 99.87%. The electrolyte in Comparative Example 1 was cycled at a high temperature of 60°C for 100 times, and the capacity retention of the battery was 75%, and the average coulombic efficiency was 98.79%. It can be seen that the electrolyte of the present application has good high-temperature performance.

Claims

1. A sodium-ion secondary battery comprising a positive electrode material, a negative electrode material, and a flame-retardant electrolyte for sodium-ion batteries, wherein the positive electrode material is a layered metal oxide, a polyanion-based positive electrode material, or a Prussian blue-based material, and the negative electrode material is hard carbon; the flame-retardant electrolyte for sodium-ion batteries comprises the following components: a base electrolyte and a functional additive; wherein the base electrolyte comprises a solvent and a sodium salt; the solvent comprises a primary solvent and an optional secondary solvent; the sodium salt comprises a primary sodium salt and an optional secondary sodium salt; the primary solvent is a phosphorus-containing flame-retardant solvent, the secondary solvent is selected from at least one of a carbonate solvent, a carboxylate solvent, and an ether solvent other than a fluorinated ether, the primary sodium salt is sodium hexafluorophosphate and sodium tetrafluoroborate, and the secondary sodium salt is selected from one or more of sodium bis(trifluoromethylsulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium trifluoromethylsulfonate, and sodium perchlorate; the molar ratio of the phosphorus-containing flame-retardant solvent to the sodium salt is 4.2-17:1; the mass fraction of the primary sodium salt in the sodium salt is 70%-100%; the volume fraction of the primary solvent in the solvent is 80%-100%; the mass fraction of the functional additive in the electrolyte is greater than 0 and less than or equal to 5%; the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 1:50-50:1; the functional additive is selected from one or more of vinylene carbonate, fluorinated vinyl carbonate, 1,3-propane sultone, 1,4-butane sultone, propenyl-1,3-sultone, cyclohexylbenzene, tert-butylbenzene, tert-amylbenzene, ethylene sulfate, dimethyl sulfite, diethyl sulfite, and succindionitrile.

2. The sodium-ion secondary battery according to claim 1, wherein, the phosphorus-containing flame-retardant solvent is a phosphate ester of the formula (RO) 3 P=O or a phosphonate ester of the formula R(RO 2)P=O, wherein R is a phenyl group or a low-carbon alkyl group or a halogenated alkyl group with a carbon chain length of 1-6.

3. The sodium-ion secondary battery of claim 2, wherein, the phosphate ester or phosphonate ester is selected from one or more of triphenyl phosphate, trimethyl phosphate, triethyl phosphate, tripropyl phosphate, tributyl phosphate, dimethyl methylphosphonate, and bis(2,2,2-trifluoroethyl)methylphosphonate.

4. The sodium-ion secondary battery of claim 1, wherein, the molar ratio of sodium hexafluorophosphate to sodium tetrafluoroborate is 1:11-11:

1.

5. The sodium-ion secondary battery of claim 1, wherein, the carbonate solvent is a cyclic carbonate solvent and / or a chain carbonate solvent.

6. The sodium-ion secondary battery according to claim 5, wherein, the cyclic carbonate is vinylene carbonate and / or propylene carbonate.

7. The sodium-ion secondary battery of claim 5, wherein, the chain carbonate is a carbonate synthesized from a straight-chain or branched-chain aliphatic monool with a carbon number of 3-8 and carbonic acid.

8. The sodium-ion secondary battery of claim 5, wherein, the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, and methyl ethyl carbonate.

9. The sodium-ion secondary battery of claim 1, wherein, the carboxylate solvent is a cyclic carboxylate solvent and / or a chain carboxylate solvent.

10. The sodium-ion secondary battery of claim 9, wherein, the cyclic carboxylate is γ-butyrolactone, and the chain carboxylate is a chain carboxylate with a carbon number of 3-8.

11. The sodium-ion secondary battery of claim 9, wherein, the chain carboxylate is selected from one or more of methyl acetate, ethyl acetate, propyl acetate, propyl propionate, and ethyl propionate.

12. The sodium-ion secondary battery of claim 1, wherein, The ether of the non-fluorinated ether is selected from one or more of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, dimethoxymethane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The ether of the non-fluorinated ether is selected from one or more of tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, dimethoxymethane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. The ether of the non-fluorinated ether is selected from one or more of tetrahydrofuran, 2-methyltetrahydro

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