Preparation method of sodium difluorooxalate borate

Abstract

The invention discloses a preparation method of sodium difluorooxalate borate, and relates to the technical field of sodium ion batteries. The preparation method is: adding a boron trifluoride ether solution to a sodium oxalate aqueous solution, and performing vacuum distillation to obtain a mixture of sodium difluorooxalate borate and sodium fluoride; or first adding sodium fluoride to a sodium oxalate aqueous solution, and then dropping trimethyl borate, and performing vacuum distillation to obtain a mixture of sodium difluorooxalate borate and sodium hydroxide; then adding a mixture of sodium difluorooxalate borate and sodium fluoride or a mixture of sodium difluorooxalate borate and sodium hydroxide to acetonitrile or ether, stirring, filtering, vacuum distillation, and vacuum drying to obtain sodium difluorooxalate borate crystals. The one-step method of the present invention can synthesize sodium difluorooxalate borate, and the reaction is simple, environmentally friendly, and the yield is high. The prepared sodium difluorooxalate borate has high purity, low impurity content, and excellent performance as a sodium battery electrolyte additive.

Description

Technical Field

[0001] The invention relates to the technical field of sodium ion batteries, and in particular to a method for preparing sodium difluorooxalatoborate. Background Art

[0002] The working principles of sodium-ion batteries and lithium-ion batteries are very similar, but there are certain differences in battery materials. Sodium-ion batteries use the fact that sodium ions repeatedly embed and detach in the positive and negative electrode materials during charging and discharging, thereby causing a reversible redox reaction, so their working principle is similar to that of lithium-ion batteries.

[0003] The electrolyte of sodium-ion batteries is similar to that of lithium-ion batteries in formula, and both are composed of electrolytes, solvents, and additives. Carbonate solvents such as ethylene carbonate and polycarbonate have the advantages of wide electrochemical window, large dielectric constant, and good chemical stability, and are excellent organic solvents for sodium batteries. Ether solvents can form a thin and stable SEI film and high first coulomb efficiency on the negative electrode surface because of their better antioxidant reduction ability in sodium battery systems, but they are rarely used in lithium batteries. The solute is changed from lithium hexafluorophosphate (LiPF6) of lithium batteries to sodium hexafluorophosphate (NaPF6) of sodium batteries. The principles and technical routes of the synthesis of the two are similar, and sodium hexafluorophosphate is easy to mass produce. In terms of additives, sodium batteries mainly have film-forming additives, flame retardant additives, overcharge protection additives, etc.

[0004] Among the sodium battery additives, vinyl carbonate (VC) is the most widely used. However, more than 1.5% VC will increase the battery impedance, affecting the battery's rate performance and low-temperature performance. Moreover, the interface film of VC is unstable under high temperature and high voltage conditions. In actual applications, additives such as fluoroethylene carbonate (FEC) and propane sultone (1,3PS) are added for compound use. Compared with VC, the newly synthesized sodium difluorooxalatoborate in recent years has better performance than VC and VC compound systems, especially in terms of high temperature, low temperature and rate performance of sodium batteries.

[0005] There are some reports about sodium difluoro oxalate borate at present.Publication number is the Chinese patent document of CN105541890A, discloses the synthetic method of sodium difluoro oxalate borate, the synthetic method adds sodium fluoride, oxalic acid and hydrogen fluoride in the solution of boric acid, forms a uniform mixed aqueous solution, evaporates to dryness to obtain a white precipitate, then thoroughly dries to obtain a crude product, the obtained crude product is extracted in acetonitrile or ether solvent, and finally the extract is evaporated and crystallized to obtain the final product sodium difluoro oxalate borate.The method is not only complex in synthesis and purification process, and yield is not high, and the sodium difluoro oxalate borate obtained has low purity and high impurity content. The Chinese patent document with publication number CN111592559A discloses a one-step method for synthesizing sodium difluorooxalate borate, wherein sodium oxalate, BF3 compounds and bis(trimethylsilyl) oxalate are first dissolved in an organic solvent in a certain molar ratio to form a uniform solution; then the solution is continuously reacted at 50-90°C and 0.1-1MPa for 7-14 hours under the protection of an inert gas; finally, the sodium difluorooxalate borate product can be obtained by recrystallizing the reacted solution. This method is costly and not suitable for large-scale production. The Chinese patent document with publication number CN115304629A discloses a method for reacting lithium difluorooxalate borate with a sodium ion salt in an organic solvent to generate sodium difluorooxalate borate, which not only uses expensive lithium difluorooxalate borate, but also contains more impurities in the resulting sodium difluorooxalate borate. Summary of the invention

[0006] The purpose of the present invention is to provide a preparation method of sodium difluorooxalate borate, which has a simple process route, is environmentally friendly, and has a high yield and purity of the product. The present invention also evaluates sodium difluorooxalate borate as a sodium electrochemical additive, and the results show that the sodium difluorooxalate borate synthesized by the present invention has better performance than the currently commonly used VC and VC compound system.

[0007] To achieve the purpose of the present invention, the present invention provides a technical solution for the preparation of sodium difluorooxalatoborate:

[0008] Sodium oxalate is reacted with a fluorine source and a boric acid source, and after reduced pressure distillation, a mixture containing sodium difluorooxalatoborate is obtained, acetonitrile or ether is added to the mixture, stirred, filtered, and the filtrate is further subjected to reduced pressure distillation, and the obtained solid is dried to obtain sodium difluorooxalatoborate; the fluorine source and the boric acid source are boron trifluoride or trimethyl borate and sodium fluoride.

[0009] Specifically, the preparation method comprises the following steps (1) or (2):

[0010] (1) Add boron trifluoride ether solution dropwise to sodium oxalate aqueous solution under heating and stirring conditions, and distill the reaction solution under reduced pressure to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride; the reaction equation is as follows:

[0011]

[0012] Add a mixture of sodium difluorooxalatoborate and sodium fluoride into acetonitrile or ether, stir, filter, distill the filtrate under reduced pressure, and then vacuum dry the obtained solid to obtain sodium difluorooxalatoborate crystals;

[0013] (2) Sodium fluoride is first added to the sodium oxalate aqueous solution, and trimethyl borate is added dropwise under heating and stirring conditions. The reaction solution is distilled under reduced pressure to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide; the reaction equation is as follows:

[0014]

[0015] A mixture of sodium difluorooxalatoborate and sodium hydroxide is added to acetonitrile or ether, stirred, filtered, the filtrate is distilled under reduced pressure, and the obtained solid is vacuum dried to obtain sodium difluorooxalatoborate crystals.

[0016] Preferably, in step (1), the molar ratio of sodium oxalate to boron trifluoride ether is 1:0.8-1.2, the heating and stirring temperature is 60-90°C, and the time is 16-24h; the vacuum distillation temperature of the finished liquid is 50-70°C, and the time is 16-24h.

[0017] More preferably, in step (1), the heating and stirring temperature is 70-80° C., and the reaction time is 18-22 h.

[0018] Preferably, in step (1), the ratio of the mixture of sodium difluorooxalatoborate and sodium fluoride to acetonitrile or diethyl ether is 1 g: 5-15 ml, the filtrate is distilled under reduced pressure at a temperature of 40-60° C. for 12-18 h, and the vacuum drying temperature is 70-90° C. for 12-20 h.

[0019] Preferably, in step (2), the ratio of sodium oxalate to trimethyl borate and sodium fluoride is 1 mol: 0.8-1.2 mol: 1.8-2.2 mol; the heating and stirring temperature is 60-90°C for 12-20 h; the vacuum distillation temperature of the finished liquid is 50-70°C for 15-21 h.

[0020] More preferably, in step (2), the heating and stirring temperature is 70-80° C., and the reaction time is 14-18 h.

[0021] Preferably, in step (2), the ratio of the mixture of sodium difluorooxalatoborate and sodium hydroxide to acetonitrile or diethyl ether is 1 g: 6-12 ml, the filtrate is distilled under reduced pressure at a temperature of 60-80°C for 15-19 h, and the vacuum drying temperature is 60-90°C for 14-18 h.

[0022] The beneficial effects of the present invention are:

[0023] The present invention can synthesize sodium difluorooxalate borate in one step, and the solvent is water, the reaction is simple and environmentally friendly; the obtained sodium difluorooxalate borate has high purity (>99.9%), high yield, total yield of sodium difluorooxalate borate>92%, low impurity content (free acid <35ppm, water <40ppm, chloride ion <5ppm, sulfate ion <3ppm), and the prepared sodium difluorooxalate borate has better performance than the commonly used VC and VC compound system as a sodium battery electrolyte additive, and has better high temperature performance, rate performance and low temperature performance. The battery added with the sodium difluorooxalate borate prepared by the present invention has a capacity retention rate of 84.3%-93.4% after 300 cycles at 50°C, a 5C discharge battery capacity of 181.6mAh / g-196.2mAh / g, and a 0.5C-20°C discharge capacity retention rate of 85.0%-87.3%. This indicates that the sodium difluorooxalatoborate prepared by the present invention has better performance as a sodium electrolyte additive than the commonly used additives currently on the market and has broad industrial prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is the B-NMR spectrum of sodium difluorooxalatoborate of Example 1 of the present invention;

[0025] Figure 2 It is the F-NMR spectrum of sodium difluorooxalatoborate of Example 1 of the present invention. DETAILED DESCRIPTION

[0026] The technical scheme of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and embodiments. However, those skilled in the art will understand that the embodiments described below are part of the embodiments of the present invention, rather than all of the embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Example 1

[0027] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 1 mol of boron trifluoride ether solution (47 wt%) dropwise to the sodium oxalate aqueous solution under heating and stirring at 60°C, react for 20 hours, and after the reaction is completed, distill at 60°C under reduced pressure for 16 hours to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride.

[0028] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 100 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 50°C for 12 h, and dry the obtained solid under vacuum at 80°C for 16 h to obtain sodium difluorooxalatoborate crystals with a yield of 93%.

[0029] Sodium difluorooxalatoborate was heated at 3.3 V Na0.83 Li 0.25 Mn 0.75 The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added in Example 1 after 300 cycles at 50°C is 90.6%, while that of Comparative Example 1, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 80.2%, 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 186.5 mAh / g, while that of Comparative Example 1, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 176.5 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 186.5 mAh / g, while that of Comparative Example 1, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 176.5 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 85.2%, while that of Comparative Example 1, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 78.8%, 65.2%, 75.4%, and 74.3%, respectively. Example 2

[0030] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 1 mol of boron trifluoride ether solution (47 wt%) dropwise to the sodium oxalate aqueous solution under heating and stirring at 70°C, react for 18 hours, and distill under reduced pressure at 60°C for 16 hours after the reaction to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride. The yield is 96%.

[0031] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 100 ml of acetonitrile, filter, and distill the filtrate under reduced pressure at 50°C for 12 h. Dry the obtained solid under vacuum at 80°C for 16 h to obtain sodium difluorooxalatoborate crystals.

[0032] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added in Example 2 after 300 cycles at 50°C is 88.5%, while that of Comparative Example 2, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 79.6%, 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 189.7 mAh / g, while that of Comparative Example 2, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 175.2 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 189.7 mAh / g, while that of Comparative Example 2, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 175.2 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 86.5%, while that of Comparative Example 2, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 77.9%, 65.2%, 75.4%, and 74.3%, respectively. Example 3

[0033] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 1.2 mol of boron trifluoride ether solution (47 wt%) dropwise into the sodium oxalate aqueous solution under heating and stirring at 90°C, react for 24 hours, and after the reaction is completed, distill at 70°C under reduced pressure for 20 hours to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride.

[0034] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 150 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 60°C for 15 h, and dry the obtained solid under vacuum at 90°C for 20 h to obtain sodium difluorooxalatoborate crystals with a yield of 95%.

[0035] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added at 50°C for 300 cycles is 85.3%, while that of Comparative Example 3, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 79.8%, 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 185.9 mAh / g, while that of Comparative Example 3, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 177.3 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 185.9 mAh / g, while that of Comparative Example 3, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 177.3 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 85.6%, while that of Comparative Example 3, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 79.6%, 65.2%, 75.4%, and 74.3%, respectively. Example 4

[0036] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 0.9 mol of boron trifluoride ether solution (47 wt%) dropwise to the sodium oxalate aqueous solution under heating and stirring at 70°C, react for 18 hours, and after the reaction is completed, distill at 55°C under reduced pressure for 18 hours to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride.

[0037] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 80 ml of ether, filter, distill the filtrate under reduced pressure at 45°C for 14 h, and dry the obtained solid under vacuum at 75°C for 18 h to obtain sodium difluorooxalatoborate crystals with a yield of 98%.

[0038] The capacity retention rate of the battery with sodium difluorooxalate borate added at 50°C for 300 cycles is 92.6%, while that of Comparative Example 4, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 78.7%, 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 195.8 mAh / g, while that of Comparative Example 4, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 78.7%, 73.4%, 77.5%, and 78.2%, respectively. The discharge capacity retention rate of the battery with the addition of sodium difluorooxalatoborate at 0.5C-20℃ is 87.2%, while that of Comparative Example 4, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) is only 76.3%, 65.2%, 75.4%, and 74.3%, respectively. Example 5

[0039] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 1.1 mol of boron trifluoride ether solution (47 wt%) dropwise to the sodium oxalate aqueous solution under heating and stirring at 80°C, react for 22 hours, and after the reaction is completed, distill at 65°C under reduced pressure for 18 hours to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride.

[0040] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 120 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 55°C for 17 h, and dry the obtained solid under vacuum at 85°C for 16 h to obtain sodium difluorooxalatoborate crystals with a yield of 97%.

[0041] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added at 50°C for 300 cycles is 93.4%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 196.2 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 196.2 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 87.3%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 65.2%, 75.4%, and 74.3%, respectively. Example 6

[0042] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 0.8 mol of boron trifluoride ether solution (47 wt%) dropwise to the sodium oxalate aqueous solution under heating and stirring at 75°C, react for 16 hours, and after the reaction is completed, distill at 60°C under reduced pressure for 22 hours to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride.

[0043] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 50 ml of ether, filter, distill the filtrate under reduced pressure at 40°C for 16 h, and dry the obtained solid under vacuum at 70°C for 12 h to obtain sodium difluorooxalatoborate crystals with a yield of 94%.

[0044] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added at 50°C for 300 cycles is 86.1%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 184.3 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 184.3 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 85.5%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 65.2%, 75.4%, and 74.3%, respectively. Example 7

[0045] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 2 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 1 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 60°C, react for 16 hours, and after the reaction is completed, perform reduced pressure distillation at 60°C for 15 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0046] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 150 ml of ether, filter, distill the filtrate under reduced pressure at 70°C for 15 h, and dry under vacuum at 75°C for 16 h to obtain sodium difluorooxalatoborate crystals. The yield is 93%.

[0047] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added at 50°C for 300 cycles is 86.5%, while that of Comparative Example 5, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 81.5%, 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 184.4 mAh / g, while that of Comparative Example 5, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 175.3 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 184.4 mAh / g, while that of Comparative Example 5, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 175.3 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 85.1%, while that of Comparative Example 5, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 79.5%, 65.2%, 75.4%, and 74.3%, respectively. Example 8

[0048] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 2 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 1 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 70°C, react for 22 hours, and after the reaction is completed, perform reduced pressure distillation at 50°C for 21 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0049] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 150 ml of ether, filter, distill the filtrate under reduced pressure at 70°C for 15 h, and dry under vacuum at 75°C for 16 h to obtain sodium difluorooxalatoborate crystals. The yield is 96%.

[0050] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added in Example 8 after 300 cycles at 50°C is 89.5%, while that of Comparative Example 6, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 81.1%, 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 186.2 mAh / g, while that of Comparative Example 6, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 172.2 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 186.2 mAh / g, while that of Comparative Example 6, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 172.2 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 86.1%, while that of Comparative Example 6, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 78.2%, 65.2%, 75.4%, and 74.3%, respectively. Example 9

[0051] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 2.2 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 1.2 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 90°C, react for 20 hours, and after the reaction is completed, perform reduced pressure distillation at 70°C for 18 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0052] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 120 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 80°C for 17 h, and dry under vacuum at 90°C for 18 h to obtain sodium difluorooxalatoborate crystals. The yield is 95%.

[0053] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). In Example 9, the battery with sodium difluorooxalate borate added has a capacity retention rate of 84.3% after 300 cycles at 50°C, while that of Comparative Example 7, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 81.9%, 73.4%, 77.5%, and 78.2%, respectively; the battery with sodium difluorooxalate borate added has a 5C discharge battery capacity of 181.6 mAh / g, while that of Comparative Example 7, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 174.7 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the battery with sodium difluorooxalate borate added has a 0.5C discharge battery capacity of 181.6 mAh / g, while that of Comparative Example 7, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 174.7 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 86.2%, while that of Comparative Example 7, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 79.2%, 65.2%, 75.4%, and 74.3%, respectively. Example 10

[0054] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 2.1 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 1.1 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 70°C, react for 18 hours, and after the reaction is completed, perform reduced pressure distillation at 65°C for 16 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0055] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 100 ml of ether, filter, distill the filtrate under reduced pressure at 65°C for 18 hours, and dry under vacuum at 70°C for 15 hours to obtain sodium difluorooxalatoborate crystals. The yield is 97%.

[0056] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added at 50°C for 300 cycles is 92.2%, while that of Comparative Example 8, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 81.2%, 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 194.2 mAh / g, while that of Comparative Example 8, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 174.6 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 194.2 mAh / g, while that of Comparative Example 8, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 174.6 mAh / g, 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 87.1%, while that of Comparative Example 8, Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 77.5%, 65.2%, 75.4%, and 74.3%, respectively. Embodiment 11

[0057] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 1.9 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 0.9 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 80°C, react for 14 hours, and after the reaction is completed, perform reduced pressure distillation at 55°C for 20 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0058] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 80 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 75°C for 16 h, and dry under vacuum at 80°C for 17 h to obtain sodium difluorooxalatoborate crystals. The yield is 97%.

[0059] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added at 50°C for 300 cycles is 92.4%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 195.1 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 195.1 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 87.0%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 65.2%, 75.4%, and 74.3%, respectively. Example 12

[0060] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 1.8 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 1.1 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 75°C, react for 16 hours, and after the reaction is completed, perform reduced pressure distillation at 65°C for 19 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0061] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 110 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 80°C for 17 h, and dry under vacuum at 75°C for 18 h to obtain sodium difluorooxalatoborate crystals. The yield is 94%.

[0062] Sodium difluorooxalatoborate was heated at 3.3 V Na 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC+25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the content of the additive sodium difluorooxalate borate is 1.5%. And compared with the additives available on the market (1.5% VC, 1.5% VC+1.5% FEC, 1.5% VC+2% 1,3PS). The capacity retention rate of the battery with sodium difluorooxalate borate added at 50°C for 300 cycles is 88.1%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 73.4%, 77.5%, and 78.2%, respectively; the 5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 185.5 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively; the 0.5C discharge battery capacity of the battery with sodium difluorooxalate borate added is 185.5 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) is only 152.1 mAh / g, 173.3 mAh / g, and 174.5 mAh / g, respectively. The -20℃ discharge capacity retention rate is 85.0%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC), and Comparative Example 11 (1.5% VC+2% 1,3PS) are only 65.2%, 75.4%, and 74.3%, respectively. Comparative Example 1

[0063] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 1 mol of boron trifluoride ether solution (47 wt%) dropwise to the sodium oxalate aqueous solution under heating and stirring at 40°C, react for 12 hours, and after the reaction is completed, distill under reduced pressure at 40°C for 12 hours to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride.

[0064] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 80 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 35°C for 20 h, and dry the obtained solid under vacuum at 80°C for 16 h to obtain sodium difluorooxalatoborate crystals with a yield of 93%. Comparative Example 2

[0065] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 1 mol of boron trifluoride ether solution (47 wt%) dropwise to the sodium oxalate aqueous solution under heating and stirring at 40°C, react for 12 hours, and distill at 60°C under reduced pressure for 16 hours after the reaction to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride. The yield is 96%.

[0066] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 100 ml of acetonitrile, filter, and distill the filtrate under reduced pressure at 50°C for 12 h. Dry the obtained solid under vacuum at 80°C for 16 h to obtain sodium difluorooxalatoborate crystals. Comparative Example 3

[0067] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 1.1 mol of boron trifluoride ether solution (47 wt%) dropwise into the sodium oxalate aqueous solution under heating and stirring at 100°C, react for 26 hours, and after the reaction is completed, distill at 65°C under reduced pressure for 18 hours to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride.

[0068] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 120 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 55°C for 17 h, and dry the obtained solid under vacuum at 85°C for 16 h to obtain sodium difluorooxalatoborate crystals with a yield of 97%. Comparative Example 4

[0069] Dissolve 1 mol of sodium oxalate in water to prepare a 1 mol / L sodium oxalate aqueous solution; then add 0.8 mol of boron trifluoride ether solution (47 wt%) dropwise into the sodium oxalate aqueous solution under heating and stirring at 110°C, react for 28 hours, and after the reaction is completed, distill at 60°C under reduced pressure for 22 hours to obtain a mixture of sodium difluorooxalatoborate and sodium fluoride.

[0070] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium fluoride in 50 ml of ether, filter, distill the filtrate under reduced pressure at 40°C for 16 h, and dry the obtained solid under vacuum at 70°C for 12 h to obtain sodium difluorooxalatoborate crystals with a yield of 94%. Comparative Example 5

[0071] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 2 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 1 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 50°C, react for 10 hours, and after the reaction is completed, perform reduced pressure distillation at 60°C for 15 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0072] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 150 ml of ether, filter, distill the filtrate under reduced pressure at 70°C for 15 h, and dry under vacuum at 75°C for 16 h to obtain sodium difluorooxalatoborate crystals. The yield is 93%. Comparative Example 6

[0073] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 2 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 1 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 40°C, react for 8 hours, and after the reaction is completed, perform reduced pressure distillation at 50°C for 21 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0074] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 150 ml of ether, filter, distill the filtrate under reduced pressure at 70°C for 15 h, and dry under vacuum at 75°C for 16 h to obtain sodium difluorooxalatoborate crystals. The yield is 96%. Comparative Example 7

[0075] 1 mol of sodium oxalate is dissolved in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then 2.2 mol of sodium fluoride is added to the sodium oxalate aqueous solution, and then 1.2 mol of trimethyl borate is added dropwise to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 100° C., and the reaction is carried out for 22 hours. After the reaction is completed, the mixture of sodium difluorooxalatoborate and sodium hydroxide is obtained by vacuum distillation at 70° C. for 18 hours;

[0076] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 120 ml of acetonitrile, filter, distill the filtrate under reduced pressure at 80°C for 17 h, and dry under vacuum at 90°C for 18 h to obtain sodium difluorooxalatoborate crystals. The yield is 95%. Comparative Example 8

[0077] Dissolve 1 mol of sodium oxalate in water to prepare a 0.5 mol / L sodium oxalate aqueous solution; then add 2.1 mol of sodium fluoride to the sodium oxalate aqueous solution, and then dropwise add 1.1 mol of trimethyl borate to the mixed aqueous solution of sodium oxalate and sodium fluoride under heating and stirring at 110°C, react for 24 hours, and after the reaction is completed, perform reduced pressure distillation at 65°C for 16 hours to obtain a mixture of sodium difluorooxalatoborate and sodium hydroxide;

[0078] Dissolve 10 g of a mixture of sodium difluorooxalatoborate and sodium hydroxide in 100 ml of ether, filter, distill the filtrate under reduced pressure at 65°C for 18 hours, and dry under vacuum at 70°C for 15 hours to obtain sodium difluorooxalatoborate crystals. The yield is 97%. Comparative Example 9

[0079] The VC in this comparative example was purchased from the market, the manufacturer is McLean, the purity is 98%, and the brand is CAS872-36-6. 0.83 Li 0.25 Mn 0.75The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. Among them, the sodium electrolyte solvent is 30wt% DMC + 25wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the additive VC content is 1.5%. Comparative Example 10

[0080] This comparative example uses a combination of 1.5% VC + 1.5% FEC (fluoroethylene carbonate). FEC is purchased from the market. The manufacturer is Shangfu Technology, with a purity of 99% and a brand name of 114435-02-8. 0.83 Li 0.25 Mn 0.75 The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. The sodium electrolyte solvent is 30wt% DMC + 255wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the additive content is 1.5% VC + 1.5% FEC. Comparative Example 11

[0081] This comparative example uses a combination of 1.5% VC + 2% 1,3PS (1,3 propane sultone). 1,3PS is purchased from the market. The manufacturer is Jihechang, with a purity of 99.9% and a brand name of 1120-71-4. 0.83 Li 0.25 Mn 0.75 The O2 / hard carbon soft pack battery system was tested for high temperature performance, rate performance, and low temperature performance. The sodium electrolyte solvent is 30wt% DMC + 255wt% DEC, the electrolyte is sodium hexafluorophosphate, the content is 30wt%-40wt%, and the additive content is 1.5% VC + 2% 1,3PS.

[0082]

[0083] It can be seen from Table 1 that the sodium difluorooxalatoborate obtained in each embodiment has high purity (>99.9%) and low impurity content (free acid <35ppm, water <40ppm, chloride ion <5ppm, sulfate ion <3ppm). The impurity content of the comparative example (the synthesis reaction temperature and time are different from those of the embodiment) is higher than that of the embodiment. This shows that the reaction temperature and time of the present invention have better effects.

[0084] The purity of sodium difluorooxalate borate was tested by the nuclear magnetic resonance internal standard method. The internal standard was boron trifluoride in ether and the solvent was deuterated dimethyl carbonate. A small amount of sodium difluorooxalate borate and boron trifluoride in ether were dissolved in deuterated dimethyl carbonate to prepare the test solution. Then the test was performed to obtain 11 B NMR spectrum, the purity of sodium difluorooxalatoborate was obtained by internal standard method calculation formula.

[0085]

[0086] As can be seen from Table 2, the high temperature performance, rate performance and low temperature performance of the sodium difluorooxalate borate obtained in the embodiment are significantly higher than those of the comparative example. The capacity retention rate of the battery in Example 5 with sodium difluorooxalate borate added at 50°C for 300 cycles reached 93.4%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1.5% FEC) and Comparative Example 11 (1.5% VC+2% 1,3PS) were only 73.4%, 79.5% and 80.2%, respectively; the 5C discharge battery capacity of the battery in Example 5 with sodium difluorooxalate borate added reached 195.2 mAh / g, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC+1 .5% FEC), Comparative Example 11 (1.5% VC + 2% 1,3PS) are only 152.1mAh / g, 173.3mAh / g, and 174.5mAh / g respectively; the battery 0.5C-20℃ discharge capacity retention rate of Example 5 with the addition of sodium difluorooxalate borate reached 87.3%, while that of Comparative Example 9 (1.5% VC), Comparative Example 10 (1.5% VC + 1.5% FEC), and Comparative Example 11 (1.5% VC + 2% 1,3PS) were only 65.2%, 75.4%, and 74.3% respectively. Moreover, the battery performance of the embodiment with the addition of sodium difluorooxalate borate is better than that of Comparative Examples 1-8 (the synthesis reaction temperature and time are different from those of the embodiment), indicating that the reaction temperature and time of the present invention are more effective. Moreover, the sodium difluorooxalate borate of the present invention has better performance as a sodium battery additive than the additives currently commonly used on the market.

[0087] Figure 1 This is the B-NMR spectrum of sodium difluorooxalatoborate of Example 1 of the present invention. The 1.3 ppm absorption peak belongs to sodium difluorooxalatoborate.

[0088] Figure 2 This is the F-NMR spectrum of sodium difluorooxalatoborate of Example 1 of the present invention. The absorption peaks of 164.8 ppm and 165.9 ppm belong to sodium difluorooxalatoborate.

Claims

1. A method for preparing sodium difluorooxalatoborate, characterized in that: Sodium fluoride is first added to a sodium oxalate aqueous solution, trimethyl borate is added dropwise under heating and stirring conditions, and the reaction solution is distilled under reduced pressure to obtain a mixture of difluoro sodium oxalate borate and sodium hydroxide; the mixture of difluoro sodium oxalate borate and sodium hydroxide is added to acetonitrile or ether and stirred, filtered, and the filtrate is distilled under reduced pressure, and the obtained solid is vacuum dried to obtain difluoro sodium oxalate borate crystals; the ratio of sodium oxalate to trimethyl borate and sodium fluoride is 1 mol: 0.8-1.2mol:1.8-2.2mol; heating and stirring temperature is 60-90℃, time is 12-20h; the temperature of vacuum distillation of the finished liquid is 50-70℃, time is 15-21h.

2. The method for preparing sodium difluorooxalatoborate according to claim 1, characterized in that: The heating and stirring temperature is 70-80°C, and the reaction time is 14-18h.

3. The method for preparing sodium difluorooxalatoborate according to claim 1, characterized in that: The ratio of the mixture of sodium difluorooxalatoborate and sodium hydroxide to acetonitrile or ether is 1g:6-12ml, the filtrate is distilled under reduced pressure at a temperature of 60-80°C for 15-19h, and the vacuum drying temperature is 60-90°C for 14-18h.

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