Sodium-ion battery electrolyte, preparation method thereof and sodium-ion battery

Abstract

The invention relates to a sodium-ion battery electrolyte, a preparation method thereof and a sodium-ion battery, and belongs to the technical field of sodium-ion battery electrolytes. The electrolyte is a mixture of an electrolyte A and an electrolyte B, wherein the mass ratio of the electrolyte A to the electrolyte B is (0.5-5): (5-9.5); the electrolyte B comprises electrolyte salt, a film forming additive, an impedance reducing additive, a viscosity reducing additive, a functional additive and a solvent B; the functional additive is at least one of pyridine boron trifluoride salt, pyridine trifluoroacetamide salt, pyridine trifluoroacetate, 1-(2-pyridyl) trifluoroethylamine and pyridine; the electrolyte A comprises electrolyte salt, sodium tetrafluoroborate and a solvent A; when the electrolyte is used in the sodium ion battery, the charge-discharge efficiency and the good cycle performance of the battery in use at the normal temperature and the low temperature of-20 DEG C can be well considered, and the development of the fields of energy storage in cold regions and electric vehicles can be promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery electrolytes, and in particular to a sodium ion battery electrolyte, a preparation method thereof, and a sodium ion battery. Background Art

[0002] Under low temperature conditions, the viscosity of the electrolyte of sodium-ion batteries increases significantly, resulting in a decrease in the migration rate of sodium ions and a decrease in ionic conductivity. This phenomenon is closely related to the physical properties of the electrolyte itself - conventional organic electrolytes such as carbonate solvents tend to form crystals or viscous gels at low temperatures, hindering the ion transmission path. In addition, low temperatures will aggravate the deterioration of the interfacial compatibility between the electrolyte and electrode materials (such as hard carbon negative electrodes and layered oxide positive electrodes), triggering interfacial side reactions to form sodium dendrites, further inhibiting the utilization rate of electrode active materials.

[0003] The solid electrolyte interface film (SEI film) of sodium-ion batteries is prone to problems such as uneven film formation and insufficient mechanical strength under low-temperature conditions. Defects in the SEI film will lead to continuous decomposition of the electrolyte, consume active sodium ions and increase interfacial impedance, which will ultimately manifest as battery capacity decay and shortened cycle life. The capacity retention rate drops significantly after 100 low-temperature cycles. In addition, traditional liquid electrolytes often freeze below -20°C due to their high freezing point, resulting in complete blockage of ion transport channels. Even if they do not freeze, their increased viscosity will significantly reduce the solvation / desolvation efficiency of sodium ions, causing a sharp drop in battery charge and discharge efficiency.

[0004] At the same time, low temperatures also exacerbate the charge transfer impedance at the electrode / electrolyte interface, causing an increase in polarization voltage and even causing the battery to malfunction. The performance degradation of sodium-ion batteries in low-temperature environments is mainly caused by mechanisms such as increased electrolyte viscosity, SEI membrane failure, and a surge in interface impedance. Existing technologies mainly hope to improve the low-temperature adaptability of electrolytes through electrolyte system innovation (such as ionic liquids and solid-state technology) and interface engineering optimization.

[0005] Therefore, in order to promote the application of sodium-ion batteries in energy storage and electric vehicles in cold regions, it is urgent to develop an electrolyte that can take into account the charging and discharging efficiency and good cycle performance at room temperature and low temperature of -20°C, and adapt to a wide operating temperature range. Summary of the Invention

[0006] The present invention aims to overcome the shortcomings of the prior art by providing a sodium-ion battery electrolyte, a preparation method thereof, and a sodium-ion battery. The sodium-ion electrolyte provided by the present invention achieves both high charge-discharge efficiency and good cycle performance at both room temperature and low temperatures of -20°C, adapting to different battery operating environments.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] In a first aspect, the present invention provides a sodium ion battery electrolyte, which is a mixture of electrolyte A and electrolyte B, wherein the mass ratio of electrolyte A to electrolyte B is (0.5-5): (5-9.5); the electrolyte A is an ether electrolyte, and the electrolyte B is an ester electrolyte;

[0009] The components of the electrolyte B include: electrolyte salt, film-forming additive, impedance-reducing additive, viscosity-reducing additive, functional additive, and solvent B; the functional additive is at least one of pyridine boron trifluoride salt, pyridine trifluoroacetamide salt, pyridine trifluoroacetate, 1-(2-pyridyl)trifluoroethylamine, and pyridine;

[0010] The components of the electrolyte A include: electrolyte salt, sodium tetrafluoroborate, and solvent A.

[0011] The present invention provides a sodium ion battery electrolyte composed of two different components, electrolyte A and electrolyte B. Electrolyte A is an ether electrolyte, suitable for low temperatures and having low viscosity, which plays a regulatory role. Ester electrolyte B is the main component. The combined effect of the two ensures that the electrolyte has good cycling performance at both low and room temperatures.

[0012] The present invention utilizes a combination of electrolytes A and B, formulated according to a defined mass ratio of electrolyte A to electrolyte B. By allowing different electrolytes to predominantly act on the electrode surface, the interfacial impedance can be effectively controlled, thereby achieving battery performance over a wide temperature range. An inappropriate combination or the use of only a single electrolyte will result in inadequate performance across a wide temperature range, with significant reductions in charge and discharge efficiency at low temperatures.

[0013] In the present invention, the functional additives playing an important role in electrolyte B are selected from at least one of pyridinium boron trifluoride (PyBF3), pyridinium trifluoroacetamide (PyTFAA), pyridinium trifluoroacetate (PyTFA), 1-(2-pyridyl)trifluoroethylamine (PyTFEA), and pyridine (Py) (the corresponding abbreviations are in parentheses after the names). The functional additives can provide functional anions, which, on the one hand, form a fluorine-rich SEI interface film; on the other hand, the anions form an electrical double layer interface structure with the solvent and electrolyte, which can improve the stability of the solvent in the electrolyte.

[0014] In addition, electrolyte B also includes film-forming additives, impedance-reducing additives, and viscosity-reducing additives. Film-forming additives inhibit electrolyte decomposition, improve room temperature cycle life, and reversibility of electrode reactions at low temperatures by forming a stable and dense SEI film; impedance-reducing additives reduce impedance by optimizing ion transport kinetics at the electrode / electrolyte interface; viscosity-reducing additives can enhance ion migration rate and electrolyte wettability by reducing electrolyte viscosity, thereby improving ion conductivity and capacity retention in low-temperature cycles. Sodium tetrafluoroborate (NaBF4) in electrolyte A also plays a role in reducing viscosity. The combined effects of various additives regulate and change the desolvation ability and viscosity of the electrolyte, improve the kinetic properties of the electrolyte, etc.

[0015] The present invention selects specific electrolyte A and electrolyte B and blends them in an appropriate ratio, while optimizing the types of functional additives in the electrolyte. This allows the electrolyte to achieve charge and discharge performance and certain cycle performance at both room temperature and low temperature of -20°C when used in sodium ion batteries, thereby broadening the application scenarios of the battery.

[0016] Preferably, the mass ratio of the electrolyte A to the electrolyte B is: one or any two of 5:5, 4.5:5.5, 4:6, 3.5:6.5, 3:7, 2.5:7.5, 2:8, 1.5:8.5, 1:9, 0.5:9.5.

[0017] Preferably, the film-forming additive is at least one of fluoroethylene carbonate (FEC) and bis(2,2,2-trifluoroethyl) carbonate (DFDEC);

[0018] Preferably, the impedance-reducing additive is at least one of tris(trimethylsilyl)phosphite (TMSPI), phenyl vinyl sulfone (PVS), and tetravinylsilane (TVS);

[0019] Preferably, the viscosity reducing additive is sodium difluoro(oxalato)borate (NaDFOB).

[0020] Preferably, in the electrolyte B, by mass percentage, the content of the film-forming additive is 0.5-5%, the content of the impedance-reducing additive is 1-10%, the content of the viscosity-reducing additive is 0.5-2%, and the content of the functional additive is 0.5-5%.

[0021] Preferably, in the electrolyte B, the content of the film-forming additive is in a range of one or any two of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, and 5.0% by mass.

[0022] The content of the impedance reducing additive is one or two of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%, 9.0%, 9.5%, and 10.0%;

[0023] The content of the viscosity reducing additive is in the range of one or any two of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, and 2.0%;

[0024] The content of the functional additive is in the range of one or any two of 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, and 5.0%.

[0025] Preferably, the concentration of sodium tetrafluoroborate (NaBF4) in the electrolyte A is 0.01-0.1 mol / L.

[0026] Preferably, the electrolyte salt is sodium hexafluorophosphate;

[0027] Preferably, the concentration of the electrolyte salt in the electrolyte B is 0.5-5 mol / L;

[0028] Preferably, the concentration of the electrolyte salt in the electrolyte A is 0.5-2 mol / L.

[0029] As a preferred embodiment of the present invention, the concentration of the electrolyte salt NaPF6 in the electrolyte B is 1 mol / L; the concentration of the electrolyte salt NaPF6 in the electrolyte A is 0.7 mol / L.

[0030] Preferably, the solvent B is at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl acetate (EA), and propyl acetate (PA);

[0031] Preferably, the solvent A is at least one of diethylene glycol dimethyl ether (DEGDME), ethylene glycol dimethyl ether (DME), dioxolane (DOL), tetrahydrofuran (THF), fluorinated orthoformate (TFEO), fluorinated borate (TFEB), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), p-fluoroanisole, and fluorobenzene.

[0032] Further preferably, the solvent B is a mixture of propylene carbonate, diethyl carbonate and ethyl acetate, and the mass ratio of the propylene carbonate, diethyl carbonate and ethyl acetate is propylene carbonate: diethyl carbonate: ethyl acetate = (1-3): (5-8): (1-4);

[0033] Preferably, the solvent A is a mixture of diethylene glycol dimethyl ether, ethylene glycol dimethyl ether and fluorinated orthoformate, and the mass ratio of diethylene glycol dimethyl ether, ethylene glycol dimethyl ether and fluorinated orthoformate is diethylene glycol dimethyl ether: ethylene glycol dimethyl ether: fluorinated orthoformate = (1-3): (5-8): (1-5).

[0034] As a preferred embodiment of the present invention, the solvent B is a mixture of PC, DEC, and EA, with a mass ratio of PC:DEC:EA=2:7:1;

[0035] Preferably, the solvent A is a mixture of DEGDME, DME and TFEO, with a mass ratio of DEGDME:DME:TFEO=2:7:1.

[0036] Further preferably, the mass ratio of the electrolyte A to the electrolyte B is (0.5-3): (7-9.5);

[0037] Preferably, the film-forming additives are fluoroethylene carbonate and bis(2,2,2-trifluoroethyl) carbonate, and the mass ratio of the fluoroethylene carbonate to bis(2,2,2-trifluoroethyl) carbonate is 2:1;

[0038] Preferably, the impedance-reducing additive is tris(trimethylsilyl)phosphite and phenyl vinyl sulfone, and the mass ratio of tris(trimethylsilyl)phosphite to phenyl vinyl sulfone is 5:2;

[0039] Preferably, the functional additive is pyridinium trifluoroacetate (PyTFA);

[0040] Preferably, the content of the functional additive in the electrolyte B is 2-5% by mass.

[0041] The electrolyte with the optimal mixing ratio of electrolyte A and electrolyte B, as well as specific types and amounts of additives, can further improve the performance of the battery, achieving a capacity retention rate of up to 93% for 600 cycles at room temperature, and a capacity retention rate of 92% for 115 cycles at a low temperature of -20°C. The charge and discharge efficiency is higher than 99.7% at both room temperature and -20°C. It has wide temperature cycle characteristics and can adapt to a variety of usage environment scenarios.

[0042] In a second aspect, the present invention provides a method for preparing the above-mentioned sodium ion battery electrolyte, comprising the following steps:

[0043] Electrolyte A and electrolyte B are respectively prepared under an argon atmosphere, the electrolyte A and electrolyte B are mixed, and then zeolite molecular sieve is added to obtain the sodium ion battery electrolyte.

[0044] This preparation method can regulate the solvation structure required for sodium ion batteries, obtain better ionic conductivity, and control viscosity. It should be noted that the preparation method of the sodium ion battery electrolyte of the present invention is not limited to the steps listed. Those skilled in the art can adopt other commonly used preparation processes according to actual needs, as long as it does not affect the composition and use effect of the prepared product.

[0045] In a third aspect, the present invention provides a sodium ion battery comprising the above-mentioned sodium ion battery electrolyte, a separator, a positive electrode sheet and a negative electrode sheet.

[0046] Preferably, the positive electrode sheet contains a positive electrode active material, and the positive electrode active material includes at least one of a layered oxide and a polyanion material;

[0047] Preferably, the negative electrode sheet contains a negative electrode active material, and the negative electrode active material includes a hard carbon material.

[0048] The sodium ion battery prepared by using the sodium ion battery electrolyte provided by the present invention can achieve high charge and discharge efficiency and good cycle performance at room temperature and low temperature of -20°C, and has a wide temperature cycle effect.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] By blending specific electrolytes A and B and regulating the blending ratio, the present invention allows the different electrolytes to predominantly act on the electrode surface, regulating interfacial impedance. This, combined with the effects of various additive components, enhances SEI film stability. Furthermore, the present invention provides a sodium-ion battery utilizing the provided sodium-ion battery electrolyte, which achieves excellent charge-discharge efficiency and cycle performance at both room temperature and -20°C, potentially promoting the development of energy storage and electric vehicles in cold regions. DETAILED DESCRIPTION

[0051] To better illustrate the objectives, technical solutions, and advantages of the present invention, the present invention will be further described below with reference to specific examples. The experimental methods used in the following examples are conventional methods unless otherwise specified; the materials and reagents used are commercially available unless otherwise specified.

[0052] Example 1

[0053] An embodiment of the sodium ion battery electrolyte of the present invention, wherein the sodium ion battery electrolyte of this embodiment is obtained by mixing electrolyte A and electrolyte B in a mass ratio of 3:7;

[0054] The components of electrolyte B are as follows: 1 mol / L electrolyte salt NaPF6; by mass percentage, film-forming additives 1% DFDEC and 2% FEC, impedance-reducing additives 5% TMSPi and 2% PVS, viscosity-reducing additive 1% NaDFOB, and functional additive 2% PyTFA; solvent B is mixed at a mass ratio of PC:DEC:EA = 2:7:1;

[0055] The components of electrolyte A are as follows: electrolyte salt NaPF6 with a concentration of 0.7 mol / L, NaBF4 with a concentration of 0.07 mol / L; solvent A is mixed in a mass ratio of DEGDME:DME:TFEO=2:7:1.

[0056] The preparation method of the sodium ion battery electrolyte described in this embodiment is:

[0057] S1. Prepare electrolytes A and B in a glove box filled with high-purity argon gas. First, mix solvents A and B according to the ratio, then add electrolyte salt NaPF6 and other additives and mix well to obtain electrolyte A and electrolyte B respectively.

[0058] S2. Electrolyte A and electrolyte B are uniformly mixed in proportion, and then zeolite molecular sieve is added to obtain the sodium ion battery electrolyte.

[0059] Examples 2-4

[0060] Example 2-4 is an example of the sodium ion battery electrolyte of the present invention. The difference between Example 2-4 and Example 1 is only that the mixing mass ratio of electrolyte A and electrolyte B is different, as follows:

[0061] In Example 2, the mass ratio of electrolyte A to electrolyte B is 5:5;

[0062] In Example 3, the mass ratio of electrolyte A to electrolyte B is 1:9;

[0063] In Example 4, the mass ratio of electrolyte A to electrolyte B is 0.5:9.5.

[0064] Examples 5-8

[0065] Examples 5-8 are examples of sodium ion battery electrolytes of the present invention. The only difference between Examples 5-8 and Example 1 is that the functional additive pyridine trifluoroacetate PyTFA in electrolyte B is replaced with a functional additive with the same mass percentage, as follows:

[0066] In Example 5, the functional additive is replaced with pyridine Py;

[0067] In Example 6, the functional additive is replaced with pyridine boron trifluoride salt PyBF3;

[0068] In Example 7, the functional additive is replaced with pyridine trifluoroacetamide PyTFAA;

[0069] In Example 8, the functional additive is replaced by 1-(2-pyridyl)trifluoroethylamine PyTFEA.

[0070] Examples 9-11

[0071] Examples 9-11 are examples of sodium ion battery electrolytes of the present invention. The only difference between Examples 9-11 and Example 1 is that the amounts of the electrolyte salt and other additives are kept unchanged, and the mass percentage of the functional additive PyTFA in the electrolyte B is changed, as follows:

[0072] In Example 9, the mass percentage of PyTFA was adjusted to 1%;

[0073] In Example 10, the mass percentage of PyTFA was adjusted to 3%;

[0074] In Example 11, the mass percentage of PyTFA was adjusted to 5%.

[0075] Comparative Example 1-2

[0076] The only difference between Comparative Example 1-2 and Example 1 is that the mixing mass ratio of electrolyte A and electrolyte B is different:

[0077] In Comparative Example 1, the mass ratio of electrolyte A to electrolyte B is 9:1;

[0078] In Comparative Example 2, the mass ratio of electrolyte A to electrolyte B is 8:2.

[0079] Comparative Examples 3-4

[0080] Comparative Example 3 is the single electrolyte A in Example 1 without any blending;

[0081] Comparative Example 4 is the single electrolyte B in Example 1 without any blending.

[0082] Comparative Example 5

[0083] The only difference between Comparative Example 5 and Example 1 is that the amounts of the electrolyte salt and other additives are kept unchanged, and the mass percentage of the functional additive PyTFA in the electrolyte B is adjusted to 6%.

[0084] Effect Examples

[0085] In order to explore the performance of the sodium ion battery electrolyte provided by the present invention and the sodium ion battery using the sodium ion battery electrolyte of the present invention at room temperature and low temperature of -20°C, the charge and discharge efficiency and cycle test of the sodium ion battery were carried out, as follows:

[0086] The sodium ion electrolytes in the above embodiments and comparative examples are respectively used to prepare sodium ion batteries, which also include a separator, a positive electrode sheet and a negative electrode sheet, wherein the positive electrode active material is Na4Fe3(PO4)2P2O7 NFPP and the negative electrode active material is hard carbon.

[0087] (a) The charge, discharge and cycle test process at room temperature (25-30°C) is as follows:

[0088] 1. Charge and discharge for 2 cycles at room temperature (25°C, 0.2C);

[0089] 2. Charge at 0.33C constant current followed by constant voltage to 3.6V, let stand for 10 minutes, then discharge at 0.5C constant current to 1.5V, let stand for 10 minutes. Record the charge and discharge efficiency and battery capacity C1 (Ah) of the third cycle.

[0090] 3. Record the battery capacity C at the Nth cycle n (Ah), capacity retention rate CR (%) = C n / C1*100%;

[0091] (b) Low temperature charge and discharge and cycle test process is as follows:

[0092] 1. Charge and discharge for 2 cycles at 0.2C at room temperature (25°C), first charge at 0.2C constant current and then charge at constant voltage to 3.6V;

[0093] 2. Stand at -20℃ for 4 hours, discharge at 0.2C constant current to 1.5V, and stand for 10 minutes;

[0094] 3. Charge at -20°C at 0.1C constant current followed by constant voltage to 3.5V, let stand for 10 minutes, and discharge at 0.5C constant current to 1.5V. Record the charge and discharge efficiency and battery capacity C1 (Ah) of the third cycle.

[0095] 4. Record the battery capacity C at the Nth cycle n (Ah), capacity retention rate CR (%) = C n / C1*100%.

[0096] The test results are shown in Table 1 below.

[0097] Table 1 Performance test results of sodium ion batteries using electrolytes in Examples and Comparative Examples

[0098]

[0099]

[0100] From Table 1 we can see that:

[0101] The sodium ion electrolyte provided by the present invention in the embodiment has better overall application effect in the battery than the comparative example, and can take into account the use of the sodium ion battery at room temperature and low temperature to a certain extent.

[0102] Comparing Examples 1-4 and Comparative Examples 1-4, the electrolytes A and B are mixed in different proportions, which have different effects on the performance of the sodium ion battery at room temperature and low temperature. Among them, the mixing strategy of Example 3 has the best balance. At this ratio, the capacity retention rate of 600 cycles at room temperature is 93%, which is significantly better than other mixing strategies in the examples and comparative examples; and the capacity retention rate of 115 cycles at a low temperature of -20°C is 92%, which has the best comprehensive effect. At this time, under the comprehensive control of electrolytes A+B, the sodium ion battery can be cycled for a long time at both room temperature and low temperature, achieving long cycle of the battery in a wide temperature range. The electrolyte in Comparative Example 3 has a capacity retention rate of 80% after 150 cycles at room temperature, and its cycle performance at room temperature is average. Disassembly revealed that the electrolyte decomposed during the cycle temperature change process, but had an 80% retention rate for 300 cycles at a low temperature of -20°C, and had better cycle performance at low temperatures. The viscosity of the electrolyte was low, and it can be seen that electrolyte A can only be used in the low temperature range. Although Comparative Example 4 can achieve a capacity retention rate of 95% for 500 cycles at room temperature, the capacity quickly decays to below 80% after 10 cycles at a low temperature of -20°C, and can only be used within the room temperature range, with limited application scenarios.

[0103] By comparing Examples 1 and 5-8, it can be seen that the type of functional additive in electrolyte B also has a significant impact on the room temperature and low temperature performance of the balanced sodium ion battery. When the functional additive selected is pyridine trifluoroacetate PyTFA, the effect is optimal. This may be because the trifluoroacetate (TFA-) anion and the ether solvent and electrolyte in electrolyte A can form a double-layer interface structure, further improving the stability.

[0104] By comparing Examples 1, 9-11 and Comparative Example 5, it can be seen that the amount of the specific functional additives defined in the present invention in the examples can better take into account the long cycle performance of the sodium ion battery at low temperatures.

[0105] In summary, the present invention blends specific electrolyte A and electrolyte B, regulates the blending mass ratio, the type and amount of functional additives in electrolyte B, and combines the effects of the remaining additive components to improve the stability of the SEI film. When used in sodium ion batteries, it can well balance the charge and discharge efficiency and long cycle performance when used at room temperature and low temperature of -20°C, can promote the development of energy storage and electric vehicles in cold regions, and has high practical value.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A sodium ion battery electrolyte, characterized in that It is a mixture of electrolyte A and electrolyte B, wherein the mass ratio of electrolyte A to electrolyte B is (0.5-5): (5-9.5); The electrolyte A is an ether electrolyte, and the electrolyte B is an ester electrolyte; The components of the electrolyte B include: electrolyte salt, film-forming additive, impedance-reducing additive, viscosity-reducing additive, functional additive, and solvent B; the functional additive is at least one of pyridine boron trifluoride salt, pyridine trifluoroacetamide salt, pyridine trifluoroacetate, 1-(2-pyridyl)trifluoroethylamine, and pyridine; The components of the electrolyte A include: electrolyte salt, sodium tetrafluoroborate, and solvent A.

2. The sodium ion battery electrolyte according to claim 1, wherein The film-forming additive is at least one of fluoroethylene carbonate and bis(2,2,2-trifluoroethyl) carbonate; and / or, the impedance-reducing additive is at least one of tris(trimethylsilyl)phosphite, phenyl vinyl sulfone, and tetravinylsilane; And / or, the viscosity reducing additive is sodium difluoro(oxalato)borate.

3. The sodium ion battery electrolyte according to claim 1, wherein In the electrolyte B, by mass percentage, the content of the film-forming additive is 0.5-5%, the content of the impedance-reducing additive is 1-10%, the content of the viscosity-reducing additive is 0.5-2%, and the content of the functional additive is 0.5-5%.

4. The sodium ion battery electrolyte according to claim 1, wherein In the electrolyte A, the concentration of sodium tetrafluoroborate is 0.01-0.1 mol / L.

5. The sodium ion battery electrolyte according to claim 1, wherein The electrolyte salt is sodium hexafluorophosphate; and / or, in the electrolyte B, the concentration of the electrolyte salt is 0.5-5 mol / L; And / or, in the electrolyte solution A, the concentration of the electrolyte salt is 0.5-2 mol / L.

6. The sodium ion battery electrolyte according to claim 1, wherein The solvent B is at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate, and propyl acetate; And / or, the solvent A is at least one of diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, dioxolane, tetrahydrofuran, fluorinated orthoformate, fluorinated borate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, p-fluoroanisole, and fluorobenzene.

7. The sodium ion battery electrolyte according to claim 6, wherein The solvent B is a mixture of propylene carbonate, diethyl carbonate and ethyl acetate, and the mass ratio of the propylene carbonate, diethyl carbonate and ethyl acetate is propylene carbonate: diethyl carbonate: ethyl acetate = (1-3): (5-8): (1-4); And / or, the solvent A is a mixture of diethylene glycol dimethyl ether, ethylene glycol dimethyl ether and fluorinated orthoformate, and the mass ratio of diethylene glycol dimethyl ether, ethylene glycol dimethyl ether and fluorinated orthoformate is diethylene glycol dimethyl ether:ethylene glycol dimethyl ether:fluorinated orthoformate = (1-3): (5-8): (1-5).

8. The sodium ion battery electrolyte according to any one of claims 1 to 3, wherein The mass ratio of the electrolyte A to the electrolyte B is (0.5-3): (7-9.5); And / or, the film-forming additives are fluoroethylene carbonate and bis(2,2,2-trifluoroethyl) carbonate, and the mass ratio of the fluoroethylene carbonate to the bis(2,2,2-trifluoroethyl) carbonate is 2:1; And / or, the impedance reducing additive is tris(trimethylsilyl)phosphite and phenyl vinyl sulfone, and the mass ratio of tris(trimethylsilyl)phosphite to phenyl vinyl sulfone is 5:2; And / or, the functional additive is pyridine trifluoroacetate; And / or, in the electrolyte B, the content of the functional additive is 2-5% by mass.

9. The method for preparing a sodium ion battery electrolyte according to any one of claims 1 to 8, wherein: The following steps are involved: Electrolyte A and electrolyte B are respectively prepared under an argon atmosphere, the electrolyte A and electrolyte B are mixed, and then zeolite molecular sieve is added to obtain the sodium ion battery electrolyte.

10. A sodium ion battery, characterized in that: The invention comprises the sodium ion battery electrolyte, the separator, the positive electrode sheet and the negative electrode sheet according to any one of claims 1 to 8.

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