Sodium-ion battery electrolyte and preparation method thereof

Abstract

The invention discloses a sodium-ion battery electrolyte and a preparation method thereof, and relates to the technical field of sodium-ion batteries, the electrolyte comprises a sodium salt, a composite solvent, an additive and an auxiliary agent, the sodium salt is formed by compounding sodium bis (fluorosulfonyl) imide, sodium bis (trifluoromethanesulfonyl) imide and modified sodium bis (oxalato) borate, and the composite solvent is formed by compounding sulfolane, 1, 3-butanediol, 1, 3-butanediol, 1, 3-butanediol, 1, 3-butanediol, 1, 3-butanediol, 1, 3-butanediol, 1, 3-butanediol, 1, 3-butanediol and 1, 3-butanediol. The auxiliary agent is nano-scale ZIF-8, the additive is 1, 3-propylene glycol dimethyl ether, dimethyl carbonate, fluoroethylene carbonate and ethyl acetate, the additive is modified 1, 3-propane sultone, trimethyl phosphate and hydroquinone dimethyl ether, and the auxiliary agent is nano-scale ZIF-8. According to the electrolyte, sodium bis (oxalato) borate is modified after being treated by hydrogen peroxide and has efficient hydrofluoric acid resistance, meanwhile, hydrofluoric acid generated by the reaction of fluorine-containing sodium salt and water in the electrolyte can be specifically removed by utilizing an auxiliary agent of the nano ZIF-8 and a synergistic effect formed by the sodium salt and the auxiliary agent, and damage of the hydrofluoric acid to a negative electrode SEI film is avoided; and the cycle life of the sodium ion battery is prolonged.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a sodium-ion battery electrolyte and its preparation method. Background Technology

[0002] Sodium-ion batteries are secondary batteries that use sodium ions as charge carriers and achieve charging and discharging through their migration between the positive and negative electrodes. They are characterized by low cost, strong low-temperature performance, and outstanding fast charging capability. Their core structure is similar to that of lithium-ion batteries, achieving energy storage and release through the insertion and extraction of sodium ions between the positive and negative electrodes. However, sodium salt electrolytes and aluminum foil current collectors are used instead of copper foil. The positive electrode commonly uses layered oxides or Prussian blue compounds, while the negative electrode often uses hard carbon materials. They are widely used in energy storage power stations and two-wheeled vehicles.

[0003] Currently, sodium-ion batteries commonly use carbonate electrolytes. However, these electrolytes typically contain sodium fluoride salts, which readily react with trace amounts of water to form hydrofluoric acid (HF). The sodium salts in the electrolyte have insufficient resistance to hydrofluoric acid, making it difficult to effectively remove HF from the electrolyte. Consequently, the negative electrode SEI film is easily damaged by hydrofluoric acid (HF), and the positive electrode CEI film is prone to decomposition under high-voltage cycling. Ultimately, this results in a shortened battery cycle life and significant performance degradation at low and high temperatures, failing to meet the practical requirements of sodium-ion batteries for long-cycle and wide-temperature-range stability in the energy storage field. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a sodium-ion battery electrolyte and its preparation method, thus solving the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a sodium-ion battery electrolyte, wherein the electrolyte comprises: sodium salt, composite solvent, additives, and auxiliary agents;

[0006] The sodium salt is a compound of sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, and modified sodium dioxolaneborate. The compound solvent is sulfolane, 1,3-propanediol dimethyl ether, dimethyl carbonate, fluoroethylene carbonate, and ethyl acetate. The additives are modified 1,3-propanesulfonyl lactone, trimethyl phosphate, and hydroquinone dimethyl ether. The auxiliary agent is nano ZIF-8.

[0007] The copolymer of sulfolane and 1,3-propanediol dimethyl ether is prepared by compounding the sulfolane and 1,3-propanediol dimethyl ether. The sulfolane-1,3-propanediol dimethyl ether is prepared by reacting the sulfolane and 1,3-propanediol dimethyl ether at a volume ratio of 45:5 with azobisisobutyronitrile as an initiator at a temperature of 80°C for 2 hours.

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

[0009] The modified 1,3-propane sulfolane is prepared from 1,3-propane sulfolane, triethyl boron and tetraethoxysilane in a molar ratio of 1:0.2:0.1, with boron trifluoride ether as a catalyst, under the condition of 60℃ temperature 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 60℃ for 2h, and then vacuum drying at 80℃ for 3h.

[0011] Preferably, the surface hydroxyl groups of the modified sodium bisoxalate borate are distributed in a gradient after modification by the hydrogen peroxide, and the density of the hydroxyl groups near the surface layer of the particles of the modified sodium bisoxalate borate is 1.2 to 1.5 / nm 2 , and the density of the hydroxyl groups inside the particles of the modified sodium bisoxalate borate is 0.3 to 0.5 / 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 bisfluorosulfonylimide, the sodium bistrifluoromethylsulfonylimide, and the modified sodium bisoxalate 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 tetraethoxysilane and the triethyl boron grafted on the molecular chain of the modified 1,3-propane sultone are alternately distributed with corresponding silicon and boron groups, the interval of the silicon group is 1.2-1.5 nm, and the interval 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.

[0017] The number average molecular weight of the modified 1,3-propane sultone is 350-450, and the total proportion of the grafted silicon group and the boron group of the modified 1,3-propane sultone is 12% to 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% to 4.5% of the total mass of the electrolyte.

[0019] Preferably, the addition amount of the boron trifluoride etherate is 0.08% to 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%;

[0020] The addition amount of the azobis isobutyronitrile in the preparation of the sulfolane-1,3-propanediol dimethyl ether is 0.4% to 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 to obtain the composite solvent;

[0024] The sodium difluorosulfonimide, the sodium bistrifluoromethylsulfonimide and the modified sodium borate dioxalate are added into the composite solvent, and stirred at a temperature of 45-60 DEG C for 1-2h until completely dissolved;

[0025] The modified 1,3-propane sulfolane, the trimethyl phosphate, the hydroquinone dimethyl ether and the nano ZIF-8 are added again, and after the mixture is ultrasonically dispersed in an ultrasonic disperser for 30-45min, filtered through a 0.22mu polytetrafluoroethylene filter membrane to obtain the electrolyte of the sodium ion battery.

[0026] The present application provides a kind of sodium ion battery electrolyte and preparation method thereof.There are the following beneficial effects:

[0027] (1) the electrolyte is through sodium difluorosulfonimide, sodium bistrifluoromethylsulfonimide and modified sodium borate dioxalate complex composite sodium salt, wherein sodium borate dioxalate is modified after hydrogen peroxide treatment and has high hydrogen fluoride resistance, and the auxiliary agent of nano ZIF-8 is used, and the sodium salt and the auxiliary agent form a synergistic effect to remove hydrogen fluoride generated by the reaction of sodium salt containing fluorine and water in the electrolyte, avoid the damage of hydrogen fluoride to the negative electrode SEI film, and then improve the cycle life of sodium ion battery.

[0028] (2) 1,3-propane sulfolane is modified by reaction with triethyl boron and tetraethoxysilane, which enhances its film forming performance on the positive electrode surface, forms a dense and stable CEI film, reduces the oxidative decomposition of the electrolyte under high voltage cycling, avoids the problems of active material dissolution and poor ion conduction caused by electrode interfacial film damage, keeps the battery efficient energy conversion efficiency during long-term charging and discharging process, and improves the overall performance stability and reliability of sodium ion battery.

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

[0030] Figure 1 XPS spectrum of the modified sodium borate dioxalate of the embodiment of the present application;

[0031] Figure 2 Microscope image of the modified sodium borate dioxalate of the embodiment of the present application;

[0032] Figure 3A GPC spectrum of the sulfolane-1,3-propanediol dimethyl ether of the embodiment of the present application is shown in the figure;

[0033] Figure 4 A block ratio graph of the sulfolane-1,3-propanediol dimethyl ether of the embodiment of the present application is shown in the figure;

[0034] Figure 5 A microscope image of the modified 1,3-propane sulfolane of the embodiment of the present application is shown in the figure;

[0035] Figure 6 A molecular weight spectrum of the modified 1,3-propane sulfolane of the embodiment of the present application is shown in the figure;

[0036] Figure 7 A CEI film AFM characterization graph of the modified 1,3-propane sulfolane of the embodiment of the present application is shown in the figure;

[0037] Figure 8 A BET adsorption and desorption curve graph of the nano ZIF-8 of the embodiment of the present application is shown in the figure. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described below, obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection 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-trifluoromethanesulfonylimide and modified sodium dioxalate borate compound, 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] Wherein sulfolane and 1,3-propanediol dimethyl ether are compounded to prepare the copolymer of sulfolane-1,3-propanediol dimethyl ether, sulfolane-1,3-propanediol dimethyl ether is prepared by sulfolane and 1,3-propanediol dimethyl ether according to the volume ratio 45:5, adding azobisdimethyl isobutyronitrile as initiator, under the temperature condition of 80 DEG C, reaction 2h;

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

[0043] The modified 1,3-propane sultone is prepared by reacting 1,3-propane sultone with triethyl boron and tetraethoxysilane in 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;

[0044] The nano ZIF-8 is prepared by modifying ZIF-8 with a silane coupling agent, specifically by dispersing ZIF-8 in an ethanol aqueous solution, adding a silane coupling agent in an amount of 3% to 5% of the mass of ZIF-8, stirring at a temperature of 60°C for 2h, and then vacuum drying at a temperature of 80°C for 3h.

[0045] After the modified disodium borate oxalate is modified by hydrogen peroxide, the surface hydroxyl groups are distributed in a gradient. The density of the hydroxyl groups on the surface layer of the modified disodium borate oxalate particles is 1.2 to 1.5 per nm 2 , and the density of the hydroxyl groups inside the modified disodium borate oxalate particles is 0.3 to 0.5 per nm 2 .

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

[0047] The volume ratio of sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluoroethylene 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. 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% to 65%, and the non-block ratio is 35% to 40%.

[0050] The corresponding silicon and boron groups on the molecular chain of modified 1,3-propane sultone are alternately distributed. The spacing of the silicon group is 1.2-1.5nm, and the spacing of the boron group is 0.8-1.0nm, which makes the additive form a cross-linked CEI film. The thickness of the CEI film is controlled at 11-12nm, and the surface roughness of the CEI film is ≤1.2nm.

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

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

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

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

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

[0056] The electrolyte preparation includes the following steps:

[0057] Mix sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluorinated ethylene carbonate and ethyl acetate, stir at a temperature of 40-50℃ for 30-50min until uniform, to obtain a composite solvent;

[0058] Add sodium bisfluorosulfonylimide, sodium bis(trifluoromethylsulfonyl)imide and modified sodium bisoxalate borate into the composite solvent, stir at a temperature of 45-60℃ for 1-2h until completely dissolved;

[0059] Add modified 1,3-propane sultone, trimethyl phosphate, hydroquinone dimethyl ether and nano ZIF-8, and then ultrasonic dispersion in an ultrasonic dispersion machine for 30-45min, and then filter through a 0.22μm polytetrafluoroethylene filter membrane to obtain the electrolyte of sodium ion battery.

[0060] Example 1

[0061] Sodium salt: sodium bisfluorosulfonimide: sodium bis(trifluoromethanesulfonimide: modified sodium bisoxalate borate mixed in a molar ratio of 6:3:1, the total concentration of sodium salt is 1.0 mol / L, and the modified sodium bisoxalate borate is obtained by taking sodium bisoxalate borate (purity ≥ 99.9%), adding 3% hydrogen peroxide aqueous solution, stirring in a 50°C constant temperature water bath for 1h, then transferring to a vacuum drying oven, vacuum drying at 80°C for 4h, and cooling to room temperature to obtain modified sodium bisoxalate borate;

[0062] Composite solvent: copolymer of sulfolane-1,3-propanediol dimethyl ether: dimethyl carbonate: fluoroethylene carbonate: ethyl acetate in a volume ratio of 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%, wherein the copolymer of sulfolane-1,3-propanediol dimethyl ether is obtained by taking sulfolane and 1,3-propanediol dimethyl ether in a volume ratio of 45:5, mixing, then adding a total mass of 0.5% azobisisobutyronitrile as an initiator, heating to 80°C under nitrogen protection, and constant temperature reaction for 2h, and then cooling to obtain the copolymer of sulfolane-1,3-propanediol dimethyl ether;

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

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

[0065] Mix sulfolane-1,3-propanediol dimethyl ether, dimethyl carbonate, fluoroethylene carbonate and ethyl acetate, stir at 45°C for 40min to uniform, to obtain a composite solvent;

[0066] The sodium difluorosulfonimide, sodium bistrifluoromethylsulfonimide and modified sodium bisoxalate borate were added into the composite solvent and stirred at a temperature of 50°C for 1.5h until completely dissolved;

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

[0068] Example 2

[0069] The difference from Example 1 is that the total concentration of sodium salt is 0.8mol / 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 1.2mol / 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 bisoxalate borate in the sodium salt is replaced by unmodified sodium bisoxalate borate, and other components and preparation steps are consistent with Example 1.

[0082] Comparative Example 2

[0083] The difference from Example 1 is that the physical mixture of sulfolane and 1,3-propanediol dimethyl ether in the composite solvent is replaced by sulfolane-1,3-propanediol dimethyl ether, the volume ratio of which 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 sulfolane in the additive is replaced by unmodified 1,3-propane sulfolane, 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] Test Battery Assembly

[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 as a glass fiber membrane, and 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 the 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 thickness and surface roughness (Ra) of the CEI film 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 first time discharge capacity at 0.2C rate, 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 interval 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 content 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 amount of initiator 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 optimal and as low as 0.003 ppm, which proves that the synergistic effect of the double-stage anti-HF of the modified sodium borate bisoxalate 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 Example 1 (11.5 nm, Ra=1.0 nm) and Example 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-20℃ capacity retention rate of Examples 1-7 is all ≥82.8% (Example 1 reaches 87.5%), and the capacity retention rate after 60℃ cycle for 50 times is all ≥88.4% (Example 1 reaches 92.3%), combined with the 25℃ ion conductivity (1.08×10 -3 -1.35×10 -3 S / cm) can know 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 was added with unmodified sodium bis (oxalato) borate instead of modified sodium bis (oxalato) borate, the content of electrolyte HF increased to 0.065 ppm, which was 21.7 times of 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℃ decreased to 62.5%. The main reason was that the unmodified sodium bis (oxalato) borate had no gradient distribution of hydroxyl groups, which could not efficiently neutralize HF, resulting in continuous destruction of the SEI / CEI film by HF, which verified that the modified sodium bis (oxalato) borate was the basic core component of HF resistance;

[0104] Comparative Example 2 was replaced with a physical mixture of sulfolane and 1,3-propanediol dimethyl ether instead of the copolymer of sulfolane and 1,3-propanediol dimethyl ether, resulting in an electrolyte with a capacity retention rate at -20℃ of only 64.3%, which was 23.2% lower than that of Example 1, and the ionic conductivity at 25℃ 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, and ion transport was blocked, which verified that the copolymer of sulfolane and 1,3-propanediol dimethyl ether could guarantee the improvement of the ionic conductivity of the electrolyte in a wide temperature range;

[0105] Comparative Example 3 was added with unmodified 1,3-propane sulfone lactone instead of 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℃ 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, which verified that the modified 1,3-propane sulfone lactone was a high-efficiency additive for maintaining the high-temperature stability of the CEI film of the electrolyte;

[0106] Comparative Example 4 did not add nano ZIF-8, the content of HF in the electrolyte increased to 0.033 ppm, which was 11 times of 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, resulting in a decrease in the performance of the electrolyte, which verified that nano ZIF-8 was an optimized auxiliary agent for strengthening the anti-HF effect and supplementing the stability of the interface.

[0107] Although the 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 variations can be made to these embodiments 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 and the 1,3-propanediol dimethyl ether are compounded to prepare a copolymer of sulfolane-1,3-propanediol dimethyl ether, and the copolymer is prepared by adding azobisisobutyronitrile as an initiator under the condition of a volume ratio of 45:5 of the sulfolane to the 1,3-propanediol dimethyl ether and a temperature of 80℃ for 2h. 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℃ for 1h and then vacuum drying at 80℃ for 4h. 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 under the condition of a temperature of 60℃ for 3h, 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 the silane coupling agent in an amount of 3% to 5% of the mass of the ZIF-8, stirring at a temperature of 60℃ for 2h, and then vacuum drying at a temperature of 80℃ for 3h. 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 ≤10ppm. 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 the triethyl boron grafted on the molecular chain of the modified 1,3-propane sulfolane are alternately distributed, the spacing of the silicon groups is 1.2-1.5nm, 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. 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 grafting of the modified 1,3-propane sultone is 12%-15%. 7.The sodium-ion battery electrolyte of claim 1, wherein: 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. 8.The sodium-ion battery electrolyte of claim 1, wherein: 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%; The addition amount of the azobisdimethyl nitrile 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. 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 bistrifluoromethylsulfonylimide, 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 trimethyl phosphate, 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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