Sodium-ion battery electrolyte and preparation method thereof

Abstract

The application discloses a kind of sodium ion battery electrolyte and preparation method thereof, it is related to sodium ion battery technical field, the electrolyte includes: sodium salt, composite solvent, additive and auxiliary agent, the sodium salt is sodium difluorosulfurylimide, sodium bis-trifluoromethanesulfonimide and modified sodium borate dihydroxide dioxalate compound composition, composite solvent is sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluoroethylene carbonate and ethyl acetate, additive is modified 1,3-propane sulfone lactone, trimethyl phosphate and hydroquinone dimethyl ether, auxiliary agent is nano ZIF-8.This electrolyte sodium borate dihydroxide dioxalate is modified after hydrogen peroxide treatment, with high-efficiency anti-hydrofluoric acid capacity, while using the auxiliary agent of nano ZIF-8, sodium salt and auxiliary agent form synergistic effect can be targeted to remove hydrogen fluoride sodium salt and moisture reaction generated in electrolyte hydrofluoric acid, avoid the damage of hydrofluoric acid to negative electrode SEI film, and then improve the cycle life of sodium ion battery.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a sodium ion battery electrolyte and a preparation method thereof. BACKGROUND

[0002] The sodium ion battery is a secondary battery taking sodium ions as charge carriers and realizing charge and discharge through migration of the sodium ions between the positive and negative electrodes, has the characteristics of low cost, strong low-temperature performance and outstanding fast-charging capability, and the core structure is similar to that of a lithium ion battery, energy storage and release are realized through embedding and de-embedding of sodium ions between the positive and negative electrodes, but sodium salt electrolyte and aluminum foil current collector are used to replace copper foil, the positive electrode commonly uses layered oxides or Prussian blue compounds, and the negative electrode mainly uses hard carbon materials, and the sodium ion battery is widely applied to energy storage power stations and two-wheeled vehicles.

[0003] At present, the sodium ion battery commonly uses a carbonate electrolyte, the electrolyte of the electrolyte usually contains a sodium salt containing fluorine, is easy to react with trace moisture to generate hydrofluoric acid (HF), the sodium salt of the electrolyte is insufficient in hydrofluoric acid resistance, HF in the electrolyte is difficult to effectively remove, the negative electrode SEI film is easy to be damaged by hydrofluoric acid (HF), and the positive electrode CEI film is easy to decompose under high-voltage circulation, finally, the battery cycle life is shortened, and the low-temperature and high-temperature performance is significantly attenuated, so the practical demand of the sodium ion battery on long cycle, wide temperature range stability in the energy storage field cannot be met. SUMMARY

[0004] In view of the defects of the prior art, the application provides a sodium ion battery electrolyte and a preparation method thereof, and the problems mentioned in the background are solved.

[0005] To achieve the above object, the application is implemented by the following technical scheme: a sodium ion battery electrolyte, the electrolyte comprises: a sodium salt, a composite solvent, an additive and an auxiliary agent;

[0006] The sodium salt is composed of sodium bisfluorosulfonylimide, sodium bis-trifluoromethylsulfonylimide and modified sodium bisoxalate borate, the composite solvent is sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluorinated ethylene carbonate and ethyl acetate, the additive is modified 1,3-propane sulfolactone, trimethyl phosphate and hydroquinone dimethyl ether, and the auxiliary agent is nano ZIF-8.

[0007] The sulfolane-1,3-propanediol dimethyl ether is prepared by mixing sulfolane and 1,3-propanediol dimethyl ether at a volume ratio of 45:5, adding azobisdimethyl isobutyronitrile as an initiator and reacting at a temperature of 80 DEG C for 2h.

[0008] The modified sodium bisoxalate borate is prepared by stirring sodium bisoxalate borate in a 3% concentration hydrogen peroxide solution in a constant-temperature water bath at 50 DEG C for 1h, and then vacuum drying at 80 DEG C for 4h.

[0009] The modified 1,3-propane sultone is prepared by reacting 1,3-propane sultone with triethyl boron and tetraethoxysilane at a molar ratio of 1:0.2:0.1 and using boron trifluoride etherate as a catalyst at a temperature of 60°C for 3h, and inert gas is continuously introduced during the reaction;

[0010] The nano ZIF-8 is prepared by modifying ZIF-8 with a silane coupling agent, specifically by dispersing the ZIF-8 in an ethanol aqueous solution, adding 3% to 5% of the silane coupling agent by mass of the ZIF-8, stirring at a temperature of 60°C for 2h, and then vacuum drying at a temperature of 80°C for 3h.

[0011] Preferably, after the modified sodium dihydrogen bis(oxalato)borate is modified with the hydrogen peroxide, the surface hydroxyl groups are distributed in a gradient, and the density of the hydroxyl groups on the surface layer of the modified sodium dihydrogen bis(oxalato)borate is 1.2 to 1.5 per nm 2 , and the density of the hydroxyl groups inside the modified sodium dihydrogen bis(oxalato)borate particles is 0.3 to 0.5 per nm 2 .

[0012] Preferably, the total concentration of the sodium salt in the electrolyte is 0.8 to 1.2 mol / L, and the molar ratio of the sodium bis(fluorosulfonyl)imide, the sodium bis(trifluoromethylsulfonyl)imide, and the modified sodium dihydrogen bis(oxalato)borate is 6:3:1.

[0013] Preferably, the volume ratio of the sulfolane-1,3-propanediol dimethyl ether, the dimethyl carbonate, the fluoroethylene carbonate, and the ethyl acetate is 45:28:10:12.

[0014] The ethyl acetate is purified by molecular distillation, and the purity of the purified ethyl acetate is 99.99%, and the water content of the purified ethyl acetate is ≤10ppm.

[0015] Preferably, the number average molecular weight of the sulfolane-1,3-propanediol dimethyl ether is 1200-1500, and the sulfolane-1,3-propanediol dimethyl ether is specifically a multi-block copolymer, and the block ratio of the sulfolane in the sulfolane-1,3-propanediol dimethyl ether is 60% to 65%, and the block-free ratio is 35% to 40%.

[0016] Preferably, the silicon and boron groups corresponding to the grafted tetraethoxysilane and triethyl boron on the molecular chain of the modified 1,3-propane sultone are alternately distributed, the spacing of the silicon groups is 1.2-1.5nm, and the spacing of the boron groups is 0.8-1.0nm, so that the additive forms a cross-linked CEI film, the thickness of the CEI film is controlled to be 11-12nm, and the surface roughness of the CEI film is ≤1.2nm.

[0017] The number average molecular weight of the modified 1,3-propane sultone is 350-450, and the total proportion of the silicon group and the boron group in the graft of the modified 1,3-propane sultone is 12%-15%.

[0018] Preferably, the mass ratio of the modified 1,3-propane sultone, the trimethyl phosphate, and the hydroquinone dimethyl ether is 5:1:1, and the total mass of the modified 1,3-propane sultone, the trimethyl phosphate, and the hydroquinone dimethyl ether accounts for 2.5%-4.5% of the total mass of the electrolyte.

[0019] Preferably, the addition amount of the boron trifluoride etherate is 0.08%-0.12% of the total mass of the 1,3-propane sultone, triethyl boron, and tetraethoxysilane, and the inert gas introduced during the preparation of the modified 1,3-propane sultone is argon, the argon introduction rate is 0.4-0.6 L / min, and the gas purity of the argon is ≥99.999%;

[0020] The addition amount of the azobis isobutyronitrile in the preparation of the sulfolane-1,3-propanediol dimethyl ether is 0.4%-0.6% of the total mass of the sulfolane and 1,3-propanediol dimethyl ether.

[0021] Preferably, the silane coupling agent is KH-550, the nano ZIF-8 forms a core-shell structure after modification, the inner core of the nano ZIF-8 is a ZIF-8 porous framework, the pore size of the ZIF-8 porous framework is 0.9-1.1 nm, and the outer shell of the nano ZIF-8 is an aminosiloxane layer formed by hydrolysis of the KH-550, and the thickness is 5-8 nm.

[0022] A preparation method of a sodium ion battery electrolyte, comprising the following steps:

[0023] Mix the sulfolane-1,3-propanediol dimethyl ether, the dimethyl carbonate, the fluoroethylene carbonate, and the ethyl acetate, stir until uniform at a temperature of 40-50°C for 30-50 min, and obtain the composite solvent;

[0024] Add the sodium bisfluorosulfonylimide, the sodium bis(trifluoromethylsulfonyl)imide, and the modified sodium bisoxalate borate to the composite solvent, and stir until completely dissolved at a temperature of 45-60°C for 1-2 h;

[0025] Further add the modified 1,3-propane sultone, the trimethyl phosphate, the hydroquinone dimethyl ether, and the nano ZIF-8, ultrasonically disperse the mixture in an ultrasonic disperser for 30-45 min, filter through a 0.22 μm polytetrafluoroethylene filter membrane, and obtain the electrolyte of the sodium ion battery.

[0026] The application provides a sodium ion battery electrolyte and a preparation method thereof.

[0027] (1) The electrolyte is a composite sodium salt of sodium bisfluorosulfonylimide, sodium bis(trifluoromethylsulfonyl)imide and modified sodium bis(oxalato)borate, wherein the sodium bis(oxalato)borate is modified by hydrogen peroxide treatment and has high hydrogen fluoride resistance, and the sodium salt and the auxiliary agent ZIF-8 form a synergistic effect to remove hydrogen fluoride generated by the reaction of the fluorine-containing sodium salt and water in the electrolyte, thereby avoiding the damage of hydrogen fluoride to the negative electrode SEI film and improving the cycle life of the sodium ion battery.

[0028] (2) The 1,3-propane sulfolane is modified by reaction with triethyl boron and tetraethoxysilane to enhance the film forming performance on the positive electrode surface, form a dense and stable CEI film, reduce the oxidative decomposition of the electrolyte under high voltage cycling, avoid the problems of active material dissolution and poor ion conduction caused by the damage of the electrode interface film, keep the battery with high energy conversion efficiency during long-term charging and discharging, and improve the overall performance stability and reliability of the sodium ion battery.

[0029] (3) The composite sodium salt and the optimized composite solvent form a good ion transmission environment, and the auxiliary agent ZIF-8 with stable dispersion is matched, which avoids the problems of ion conduction blockage and component stratification of the electrolyte under low temperature or high temperature environment, alleviates the performance decay of the battery under extreme temperature, makes the sodium ion battery work stably in a wider temperature range, and improves the adaptability of the sodium ion battery in different application environments. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 XPS spectrum of the modified sodium bis(oxalato)borate of the embodiment of the application;

[0031] Figure 2 Microscope image of the modified sodium bis(oxalato)borate of the embodiment of the application;

[0032] Figure 3 Chromatogram GPC spectrum of the sulfolane-1,3-propanediol dimethyl ether of the embodiment of the application;

[0033] Figure 4 Block ratio graph of the sulfolane-1,3-propanediol dimethyl ether of the embodiment of the application;

[0034] Figure 5 Microscope image of the modified 1,3-propane sulfolane of the embodiment of the application;

[0035] Figure 6 Molecular weight spectrum of the modified 1,3-propane sulfolane of the embodiment of the application;

[0036] Figure 7 CEI membrane AFM characterization chart of modified 1,3-propane sulfolane of the embodiment of the present application;

[0037] Figure 8 BET adsorption and desorption curve chart of nano ZIF-8 of the embodiment of the present application. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0039] The present application provides a kind of sodium ion battery electrolyte, to realize above-mentioned purpose, the present application is realized by the following technical scheme: electrolyte includes: sodium salt, composite solvent, additive and auxiliary agent;

[0040] Sodium salt is sodium bisfluorosulfonylimide, sodium bis-trifluoromethanesulfonimide and modified sodium dioxalate borate are compounded to form, the composite solvent is sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluoroethylene carbonate and ethyl acetate, additive is modified 1,3-propane sulfolane, trimethyl phosphate and hydroquinone dimethyl ether, auxiliary agent is nano ZIF-8;

[0041] Sulfolane-1,3-propanediol dimethyl ether is prepared by mixing sulfolane and 1,3-propanediol dimethyl ether at a volume ratio of 45:5, adding azobisdimethyl isobutyronitrile as initiator and reacting at a temperature of 80 DEG C for 2 hours.

[0042] Modified sodium dioxalate borate is prepared by stirring sodium dioxalate borate in a 3% concentration hydrogen peroxide solution in a constant temperature water bath at 50 DEG C for 1 hour, and then vacuum drying at 80 DEG C for 4 hours.

[0043] Modified 1,3-propane sulfolane is prepared by reacting 1,3-propane sulfolane with triethyl boron and tetraethoxysilane at a molar ratio of 1:0.2:0.1, and using boron trifluoride ether as a catalyst at a temperature of 60 DEG C for 3 hours, and continuously introducing inert gas during the reaction.

[0044] Nano ZIF-8 is prepared by modifying ZIF-8 with a silane coupling agent. Specifically, ZIF-8 is dispersed in an ethanol aqueous solution, 3% to 5% of the mass of ZIF-8 of a silane coupling agent is added, and then stirred at a temperature of 60 DEG C for 2 hours, and finally vacuum dried at a temperature of 80 DEG C for 3 hours.

[0045] The surface hydroxyl group of the modified sodium borate bishydroxynaphthalene-2, 3-dicarboxylate is distributed in a gradient, and the density of the hydroxyl group of the modified sodium borate bishydroxynaphthalene-2, 3-dicarboxylate is 1.2-1.5 per nm 2 , the density of the hydroxyl group of the modified sodium borate bishydroxynaphthalene-2, 3-dicarboxylate is 0.3-0.5 per nm 2 .

[0046] The total concentration of sodium salt in the electrolyte is 0.8-1.2 mol / L, and the molar ratio of sodium bisfluorosulfonylimide, sodium bis(trifluoromethylsulfonyl)imide and modified sodium borate bishydroxynaphthalene-2, 3-dicarboxylate is 6:3:1.

[0047] The volume ratio of sulfolane-1, 3-propanediol dimethyl ether, dimethyl carbonate, fluorinated ethylene carbonate and ethyl acetate is 45:28:10:12;

[0048] The ethyl acetate is purified by molecular distillation, the purity of the purified ethyl acetate is 99.99%, and the water content of the purified ethyl acetate is ≤10ppm.

[0049] The number average molecular weight of sulfolane-1, 3-propanediol dimethyl ether is 1200-1500, and the sulfolane-1, 3-propanediol dimethyl ether is a multi-block copolymer, the block ratio of sulfolane in sulfolane-1, 3-propanediol dimethyl ether is 60%-65%, and the block-free ratio is 35%-40%.

[0050] The silicon and boron groups corresponding to the grafted tetraethoxysilane and triethyl boron on the molecular chain of modified 1, 3-propane sulfolane are alternately distributed, the spacing of the silicon group is 1.2-1.5 nm, and the spacing of the boron group is 0.8-1.0 nm, so that the additive forms a cross-linked CEI film, the thickness of the CEI film is controlled to be 11-12 nm, and the surface roughness of the CEI film is ≤1.2 nm;

[0051] The number average molecular weight of modified 1, 3-propane sulfolane is 350-450, and the total proportion of grafted silicon and boron groups of modified 1, 3-propane sulfolane is 12%-15%.

[0052] The mass ratio of modified 1, 3-propane sulfolane, trimethyl phosphate and hydroquinone dimethyl ether is 5:1:1, and the total mass of modified 1, 3-propane sulfolane, trimethyl phosphate and hydroquinone dimethyl ether accounts for 2.5%-4.5% of the total mass of the electrolyte.

[0053] The addition amount of boron trifluoride ether is 0.08%-0.12% of the total mass of 1, 3-propane sulfolane, triethyl boron and tetraethoxysilane, and the inert gas introduced during the preparation of modified 1, 3-propane sulfolane is argon, the argon introduction rate is 0.4-0.6 L / min, and the gas purity of argon is ≥99.999%;

[0054] The amount of the added azobisisobutyronitrile in the preparation of the sulfolane-1,3-propanediol dimethyl ether is 0.4%-0.6% of the total mass of the sulfolane and 1,3-propanediol dimethyl ether.

[0055] The silane coupling agent is KH-550, and the nano ZIF-8 forms a core-shell structure after modification, the inner core of the nano ZIF-8 is a ZIF-8 porous framework, the pore size of the ZIF-8 porous framework is 0.9-1.1 nm, and the outer shell of the nano ZIF-8 is an amino siloxane layer formed by hydrolysis of KH-550, and the thickness is 5-8 nm.

[0056] The preparation of the electrolyte solution comprises the following steps:

[0057] The sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluoroethylene carbonate and ethyl acetate are mixed and stirred at a temperature of 40-50℃ for 30-50 min until uniform to obtain a composite solvent;

[0058] The sodium bisfluorosulfonylimide, sodium bistrifluoromethylsulfonylimide and modified sodium bisoxalate borate are added to the composite solvent and stirred at a temperature of 45-60℃ for 1-2 h until completely dissolved;

[0059] The modified 1,3-propane sulfolane, trimethyl phosphate, hydroquinone dimethyl ether and nano ZIF-8 are further added, and the mixture is ultrasonically dispersed in an ultrasonic disperser for 30-45 min, and then filtered through a 0.22 μm polytetrafluoroethylene filter membrane to obtain the electrolyte solution for sodium ion batteries.

[0060] Example 1

[0061] The sodium salts: sodium bisfluorosulfonylimide, sodium bistrifluoromethylsulfonylimide and modified sodium bisoxalate borate are mixed in a molar ratio of 6:3:1, and the total concentration of the sodium salts is 1.0 mol / L, wherein the modified sodium bisoxalate borate is prepared by taking sodium bisoxalate borate (purity ≥ 99.9%), adding 3% concentration hydrogen peroxide aqueous solution, stirring in a 50℃ constant temperature water bath for 1 h, and then transferring to a vacuum drying oven for vacuum drying at 80℃ for 4 h, and cooling to room temperature to obtain the modified sodium bisoxalate borate;

[0062] The volume ratio of the composite solvent: copolymer of sulfolane-1,3-propanediol dimethyl ether: dimethyl carbonate: fluoroethylene carbonate: ethyl acetate is 45:28:10:12, the number average molecular weight of the copolymer of sulfolane-1,3-propanediol dimethyl ether is 1300, the sulfolane block accounts for 62%, and the addition amount of azobisisobutyronitrile is 0.5%. The copolymer of sulfolane-1,3-propanediol dimethyl ether is taken by volume ratio 45:5 of sulfolane and 1,3-propanediol dimethyl ether, mixed, and then 0.5% of azobisisobutyronitrile as an initiator is added, the temperature is raised to 80°C under nitrogen protection, and constant temperature reaction is carried out for 2h. After cooling, the copolymer of sulfolane-1,3-propanediol dimethyl ether is obtained.

[0063] The mass ratio of the additive: modified 1,3-propane sulfolane: trimethyl phosphate: hydroquinone dimethyl ether is 5:1:1, and the total mass accounts for 4.0% of the electrolyte. The modified 1,3-propane sulfolane is obtained by taking 1,3-propane sulfolane, triethyl boron and tetraethoxysilane by a molar ratio of 1:0.2:0.1, adding 0.1% of boron trifluoride ether as a catalyst, and constant temperature reaction is carried out for 3h at 60°C.

[0064] The auxiliary agent is nano ZIF-8, which accounts for 0.5% of the electrolyte. The core-shell structure is modified by KH-550, the inner core diameter is 1.0nm, the outer shell aminosiloxane layer thickness is 6nm, the initial raw material of nano ZIF-8 (unmodified ZIF-8) is produced by Jiangsu Xianfeng Nanometer Material Technology Co., Ltd., the product brand is XF-ZIF-8-01, and the parameters are: particle size 50-100nm, purity ≥99.5%, specific surface area 1450-1550m² / g. The specific parameters are shown in the product specification provided by the manufacturer. The nano ZIF-8 is obtained by taking ZIF-8 powder, dispersing in an ethanol aqueous solution, adding 4% of silane coupling agent KH-550 based on the mass of ZIF-8, stirring for 2h at 60°C, and then transferring to a vacuum drying oven for vacuum drying at 80°C for 3h.

[0065] The sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluoroethylene carbonate and ethyl acetate are mixed and stirred at a temperature of 45°C for 40min to be uniform to obtain a composite solvent;

[0066] The sodium bisfluorosulfonylimide, sodium bis(trifluoromethylsulfonyl)imide and modified sodium bisoxalate borate are added to the composite solvent, and stirred at a temperature of 50°C for 1.5h to be completely dissolved;

[0067] Then, modified 1,3-propane sultone, trimethyl phosphate, hydroquinone dimethyl ether and nano-ZIF-8 were added, and the mixture was ultrasonically dispersed in an ultrasonic disperser for 35 min at a power of 300 W, and then filtered through a 0.22 μm polytetrafluoroethylene filter membrane to obtain the electrolyte for sodium ion batteries.

[0068] Example 2

[0069] The difference from Example 1 is that the total concentration of sodium salt is 0.8 mol / L, and other components and preparation steps are consistent with Example 1.

[0070] Example 3

[0071] The difference from Example 1 is that the total concentration of sodium salt is 1.2 mol / L, and other components and preparation steps are consistent with Example 1.

[0072] Example 4

[0073] The difference from Example 1 is that the addition amount of azobisisobutyronitrile in the preparation of sulfolane-1,3-propanediol dimethyl ether is 0.4%, and other components and preparation steps are consistent with Example 1.

[0074] Example 5

[0075] The difference from Example 1 is that the addition amount of azobisisobutyronitrile in the preparation of sulfolane-1,3-propanediol dimethyl ether is 0.6%, and other components and preparation steps are consistent with Example 1.

[0076] Example 6

[0077] The difference from Example 1 is that the total mass of the additive accounts for 2.5% of the electrolyte, and other components and preparation steps are consistent with Example 1.

[0078] Example 7

[0079] The difference from Example 1 is that the total mass of the additive accounts for 4.5% of the electrolyte, and other components and preparation steps are consistent with Example 1.

[0080] Comparative Example 1

[0081] The difference from Example 1 is that the modified sodium dihydrogen bis(oxalato)borate is replaced by unmodified sodium dihydrogen bis(oxalato)borate in the sodium salt, and other components and preparation steps are consistent with Example 1.

[0082] Comparative Example 2

[0083] The difference from Example 1 is that the sulfolane-1,3-propanediol dimethyl ether in the composite solvent is replaced by a physical mixture of sulfolane and 1,3-propanediol dimethyl ether, and the volume ratio of the two is 45:5, and sulfolane and 1,3-propanediol dimethyl ether are not copolymerized, and other components and preparation steps are consistent with Example 1.

[0084] Comparative Example 3

[0085] The difference from Example 1 is that the modified 1,3-propane sultone in the additive is replaced by unmodified 1,3-propane sultone, and other components and preparation steps are consistent with Example 1.

[0086] Comparative Example 4

[0087] The difference from Example 1 is that no nano-ZIF-8 is added, and other components and preparation steps are consistent with Example 1.

[0088] Test Example

[0089] Battery assembly for testing

[0090] The electrolytes of Examples 1-8 and Comparative Examples 1-4 are all used to assemble CR2032 type button sodium ion batteries: the positive electrode is made of Na3V2(PO4)3, acetylene black, and polyvinylidene fluoride mixed and coated on aluminum foil, the negative electrode is made of hard carbon, acetylene black, and sodium carboxymethyl cellulose mixed and coated on copper foil, and the separator is selected from glass fiber membrane. The whole process is operated in an argon glove box to prepare the test sodium ion battery.

[0091] Test method

[0092] Cycle performance: using a blue CT2001A test system (produced by Beijing Zongshi Technology Co., Ltd., model CT2001A / CT2001C), the sodium ion battery is cycled 100 times at a voltage of 3.0-3.8V and a rate of 0.5C, and the initial discharge capacity and capacity retention rate after 100 cycles are recorded;

[0093] HF content test: the concentration of free hydrofluoric acid (HF) in the electrolyte of the sodium ion battery is detected by an ion chromatograph (produced by Thermo Fisher Scientific (China) Co., Ltd., model ICS-2100);

[0094] CEI film characterization: the surface morphology of the positive electrode of the sodium ion battery is observed by a scanning electron microscope (produced by Carl Zeiss Optical (China) Co., Ltd., model Sigma300), and the CEI film thickness and surface roughness (Ra) are tested by an atomic force microscope (produced by Hitachi Science Instruments (Beijing) Co., Ltd., model AFM100);

[0095] High and low temperature performance: at-20℃ (low temperature) and 60℃ (high temperature) environments, the sodium ion battery is tested for the initial discharge capacity at a rate of 0.2C, and the retention rate relative to the capacity at 25℃ (normal temperature) is calculated;

[0096] Ion conductivity: the ion conductivity of the electrolyte of the sodium ion battery is tested at 25℃ by alternating current impedance method.

[0097] Table 1 is the performance test results of the electrolyte of the sodium-ion battery:

[0098]

[0099] According to the test results in Table 1, the capacity retention rates of Examples 1-7 after 100 cycles are all in the range of 92.5% to 96.2%, among which Examples 1 (96.2%) and 7 (96.0%) are optimal, and Example 6 (92.5%) is still much higher than Comparative Examples 1-4 (75.3% to 84.5%) because the total mass percentage of the additive is reduced to 2.5% compared with Example 1, which indicates that the electrolyte can guarantee the long-term cycle stability of the battery when the total additive percentage is in the range of 2.5% to 4.5%, the sodium salt concentration is in the range of 0.8 to 1.2 mol / L, and the initiator dosage is in the range of 0.4% to 0.6%;

[0100] The free HF content in the electrolyte of Examples 1-7 is all ≤0.006 ppm, and Examples 1, 5, and 7 are optimally as low as 0.003 ppm, which proves that the synergistic effect of the two-stage anti-HF of the modified sodium borate dihydroxymalonate and nano-ZIF-8 can always efficiently capture HF in the electrolyte within the parameter adjustment range, avoiding the destruction of the electrode interface;

[0101] The CEI film thickness of Examples 1-7 is concentrated in the range of 11.3-12.2 nm, and the surface roughness (Ra) is ≤1.3 nm, among which the CEI film of Examples 1 (11.5 nm, Ra=1.0 nm) and 3 (11.3 nm, Ra=0.9 nm) is the thinnest and smoothest, which indicates that the modified 1,3-propane sulfone lactone can stably form a cross-linked dense CEI film, and even if the parameters are slightly adjusted, the film layer integrity will not be damaged, which proves that the cross-linked dense CEI film formed by the mutual interlacing of silicon and boron has a stable film formation morphology;

[0102] The capacity retention rate at -20℃ of Examples 1-7 is all ≥82.8% (Example 1 reaches 87.5%), and the capacity retention rate after 50 cycles at 60℃ is all ≥88.4% (Example 1 reaches 92.3%), combined with the ion conductivity at 25℃ (1.08×10 -3 -1.35×10 -3 S / cm), it can be known that the copolymer of sulfolane-1,3-propanediol dimethyl ether effectively balances the low-temperature fluidity and high-temperature stability of the electrolyte.

[0103] Comparative Example 1: The unmodified sodium bis(oxalato)borate was added instead of the modified sodium bis(oxalato)borate, the electrolyte HF content increased to 0.065 ppm, which was 21.7 times that of Example 1, the capacity retention rate after 100 cycles was only 75.3%, which was 20.9% lower than that of Example 1, the CEI film thickened to 16.2 nm, Ra = 3.6 nm, and the capacity retention rate at -20°C decreased to 62.5%. The main reason was that the unmodified sodium bis(oxalato)borate did not have a gradient distribution of hydroxyl groups, which could not efficiently neutralize HF, resulting in continuous destruction of the SEI / CEI film by HF, which confirmed that the modified sodium bis(oxalato)borate was the basic core component of HF resistance.

[0104] Comparative Example 2: The physical mixture of sulfolane and 1,3-propanediol dimethyl ether was used instead of the copolymer of sulfolane and 1,3-propanediol dimethyl ether, resulting in an electrolyte capacity retention rate at -20°C of only 64.3%, which was 23.2% lower than that of Example 1, and the ionic conductivity at 25°C decreased to 0.92 x 10 -3 S / cm. The reason was that the physical mixed solvent had high viscosity and unstable dielectric constant at low temperature, which hindered ion transport, which confirmed that the copolymer of sulfolane and 1,3-propanediol dimethyl ether could improve the ionic conductivity of the electrolyte in a wide temperature range.

[0105] Comparative Example 3: The unmodified 1,3-propane sulfone lactone was added instead of the modified 1,3-propane sulfone lactone, the CEI film thickness in the electrolyte increased to 17.1 nm, Ra = 4.3 nm, the capacity retention rate after 50 cycles at 60°C was only 76.2%, which was 16.1% lower than that of Example 1, and the retention rate after 100 cycles was 79.2%. The main reason was that the unmodified 1,3-propane sulfone lactone could not form a cross-linked dense CEI film, and the electrolyte was easily decomposed at high voltage, resulting in damage to the film layer. This confirmed that the modified 1,3-propane sulfone lactone was a highly efficient additive for maintaining the high-temperature stability of the electrolyte CEI film.

[0106] Comparative Example 4: No nano-ZIF-8 was added, the HF content of the electrolyte increased to 0.033 ppm, which was 11 times that of Example 1, the retention rate after 100 cycles was 84.5%, which was 11.7% lower than that of Example 1, and the CEI film of the electrolyte thickened to 13.8 nm, Ra = 2.3 nm. The reason was that the adsorption effect of nano-ZIF-8 on residual HF was lost, and a small amount of HF still eroded the electrode interface, causing the performance of the electrolyte to decrease. This confirmed that nano-ZIF-8 was an optimized auxiliary agent for strengthening the anti-HF effect and supplementing the stability of the interface.

[0107] Although embodiments of the present application have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and alterations can be made thereto without departing from the principles and spirit of the present application.

Claims

1. A sodium-ion battery electrolyte, characterized in that: The electrolyte comprises: sodium salt, composite solvent, additive and auxiliary agent; The sodium salt is sodium bisfluorosulfonimide, sodium bis(trifluoromethylsulfonyl)imide and modified sodium bis(oxalato)borate, the composite solvent is sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluorinated ethylene carbonate and ethyl acetate, the additive is modified 1,3-propane sulfolane, trimethyl phosphate and hydroquinone dimethyl ether, and the auxiliary agent is nano ZIF-8. The sulfolane-1,3-propanediol dimethyl ether is prepared by mixing sulfolane and 1,3-propanediol dimethyl ether at a volume ratio of 45:5, adding azobisisobutyronitrile as an initiator and reacting at a temperature of 80 DEG C for 2 hours. The modified sodium bis(oxalato)borate is prepared by stirring sodium bis(oxalato)borate in a 3% hydrogen peroxide solution at a constant temperature of 50 DEG C for 1 hour and then vacuum drying at 80 DEG C for 4 hours. The modified 1,3-propane sulfolane is prepared by reacting 1,3-propane sulfolane, triethyl boron and tetraethoxysilane at a molar ratio of 1:0.2:0.1 and using boron trifluoride ether as a catalyst at a temperature of 60 DEG C for 3 hours, and inert gas is continuously introduced during the reaction. The nano ZIF-8 is prepared by modifying ZIF-8 with a silane coupling agent, specifically by dispersing the ZIF-8 in an ethanol aqueous solution, adding 3% to 5% of the silane coupling agent based on the mass of the ZIF-8, stirring at a temperature of 60 DEG C for 2 hours and then vacuum drying at a temperature of 80 DEG C for 3 hours. 2.The sodium-ion battery electrolyte of claim 1, wherein: The surface hydroxyl group of the modified sodium boron dioxalate is distributed in a gradient manner, the density of the hydroxyl group of the surface layer of the modified sodium boron dioxalate is 1.2-1.5 per nm 2 , and the density of the internal hydroxyl group of the modified sodium boron dioxalate is 0.3-0.5 per nm 2 . 3.The sodium-ion battery electrolyte of claim 1, wherein: The total concentration of the sodium salt in the electrolyte is 0.8 to 1.2 mol / L, and the molar ratio of the sodium bisfluorosulfonimide, the sodium bis(trifluoromethylsulfonyl)imide and the modified sodium bis(oxalato)borate is 6:3:

1. 4.The sodium-ion battery electrolyte of claim 1, wherein: The volume ratio of the sulfolane-1,3-propanediol dimethyl ether, the dimethyl carbonate, the fluorinated ethylene carbonate and the ethyl acetate is 45:28:10:

12. The ethyl acetate is purified by molecular distillation, and the purity of the purified ethyl acetate is 99.99%, and the water content of the purified ethyl acetate is ≤10 ppm. 5.The sodium-ion battery electrolyte of claim 1, wherein: The number average molecular weight of the sulfolane-1,3-propanediol dimethyl ether is 1200-1500, and the sulfolane-1,3-propanediol dimethyl ether is specifically a multi-block copolymer, the block ratio of the sulfolane in the sulfolane-1,3-propanediol dimethyl ether is 60% to 65%, and the random segment ratio is 35% to 40%. 6.The sodium-ion battery electrolyte of claim 1, wherein: The silicon and boron groups corresponding to the tetraethoxysilane and triethyl boron grafted on the molecular chain of the modified 1,3-propane sulfolane are alternately distributed, the spacing of the silicon groups is 1.2-1.5 nm, and the spacing of the boron groups is 0.8-1.0 nm, so that the additive forms a cross-linked CEI film, the thickness of the CEI film is controlled at 11-12 nm, and the surface roughness of the CEI film is ≤1.2 nm. The number average molecular weight of the modified 1,3-propane sulfolane is 350-450, and the total ratio of the silicon and boron groups grafted on the modified 1,3-propane sulfolane is 12% to 15%. 7.The sodium-ion battery electrolyte of claim 1, wherein: The mass ratio of the modified 1,3-propane sultone, the phosphoric acid trimethyl ester and the hydroquinone dimethyl ether is 5:1:1, and the total mass of the modified 1,3-propane sultone, the phosphoric acid trimethyl ester and the hydroquinone dimethyl ether accounts for 2.5%-4.5% of the total mass of the electrolyte. 8.The sodium-ion battery electrolyte of claim 1, wherein: The addition amount of the boron trifluoride diethyl ether is 0.08%-0.12% of the total mass of the 1,3-propane sultone, triethyl boron and tetraethoxysilane, and the inert gas introduced during preparation of the modified 1,3-propane sultone is argon, the argon introduction rate is 0.4-0.6 L / min, and the gas purity of the argon is ≥99.999%; The addition amount of the azobisdimethyl isobutyronitrile in preparation of the sulfolane-1,3-propanediol dimethyl ether is 0.4%-0.6% of the total mass of the sulfolane and 1,3-propanediol dimethyl ether. 9.The sodium-ion battery electrolyte of claim 1, wherein: The silane coupling agent is KH-550, the nano ZIF-8 forms a core-shell structure after modification, the inner core of the nano ZIF-8 is a ZIF-8 porous framework, the pore size of the ZIF-8 porous framework is 0.9-1.1 nm, and the outer shell of the nano ZIF-8 is an aminosiloxane layer formed by hydrolysis of the KH-550, and the thickness is 5-8 nm.

10. A process for the preparation of the electrolyte for sodium-ion batteries according to any one of claims 1-9, characterized by: The method comprises the following steps: The sulfolane-1,3-propanediol dimethyl ether, the dimethyl carbonate, the fluoroethylene carbonate and the ethyl acetate are mixed and stirred at a temperature of 40-50°C for 30-50 min until uniform to obtain the composite solvent; The sodium bisfluorosulfonylimide, the sodium bis(trifluoromethylsulfonyl)imide and the modified sodium bisoxalate borate are added to the composite solvent, and stirred at a temperature of 45-60°C for 1-2 h until completely dissolved; The modified 1,3-propane sultone, the phosphoric acid trimethyl ester, the hydroquinone dimethyl ether and the nano ZIF-8 are further added, and after ultrasonic dispersion of the mixture in an ultrasonic dispersion machine for 30-45 min, the electrolyte for sodium ion batteries is obtained by filtering through a 0.22 μm polytetrafluoroethylene filter membrane.

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