Preparation of lithium tetrafluoroborate [Background and Overview]
The electrolyte solution is an important part of the lithium-ion battery. It plays the role of transporting ion conduction current between the positive and negative electrodes and is an indispensable part to complete the electrochemical reaction. Choosing the appropriate electrolyte is also the key to obtaining lithium-ion batteries with high energy density and power density, long cycle life and good safety performance. The performance of the electrolyte directly affects the optimization and improvement of lithium-ion battery performance. At present, the electrolyte of commercial lithium-ion batteries is mainly lithium hexafluorophosphate (LiBF6). Lithium hexafluorophosphate has the advantages of good conductivity, the ability to form a stable solid electrolyte membrane, small internal resistance, and fast charge and discharge speed. It is currently the preferred electrolyte for lithium-ion battery electrolytes. However, this kind of electrolyte has problems such as being too sensitive to moisture, unstable when heated, and easily releasing PF5 when heated. The resistance of the SEI film generated at low temperature is too large, and it is easy to decompose when the temperature of the electrolyte rises above 80°C, thereby causing solvent break down.
Lithium tetrafluoroborate, molecular formula LiBF4, molecular weight 93.74, is mainly used as an electrolyte lithium salt in lithium-ion battery electrolyte. With the rapid development of mobile communications and portable appliances in modern society, lithium-ion secondary batteries are widely used in people’s daily lives due to their many advantages such as high energy density, high working voltage, small memory effect, low self-discharge rate, lightness and convenience. application. The classic electrolyte system currently used in lithium-ion secondary batteries is that the electrolyte salt is dissolved in an organic aprotic solvent. As one of the basic raw materials of lithium-ion batteries, electrolyte directly affects the working performance of lithium-ion batteries.
In comparison, lithium tetrafluoroborate (LiBF4) has good chemical stability and thermal stability, is sensitive to environmental water distribution, and is expected to develop into an excellent electrolyte widely used in the field of energy storage and power lithium-ion batteries. Calcium magnesium carbonate system. At present, the preparation methods of lithium tetrafluoroborate include aqueous solution method, gas-solid reaction method and non-aqueous solution method. The aqueous solution method uses hydrofluoric acid, boric acid and lithium carbonate as raw materials. The process conditions are mild, no explosive chemicals are involved, and the synthesis is simple and safe. However, the product lithium tetrafluoroborate monohydrate obtained after the reaction of the aqueous solution method is easily soluble in lithium tetrafluoroborate monohydrate when heated when dry. It crystallizes water and becomes a molten state, making it difficult to dehydrate and the insoluble content is high.
Preparation of lithium tetrafluoroborate [Application]
At present, LiBF4 is mainly used as an additive in LiBF6-based electrolyte systems to improve cycle life and improve lithium-ion battery performance; as a film-forming additive, LiBF4 has been widely used When used in the current electrolyte, adding LiBF4 can broaden the operating temperature range of lithium-ion batteries and improve the high and low temperature discharge performance of the battery.
If there is research that provides an electrolyte for a lithium-ion battery that can discharge at ultra-low temperature, has stable performance, and has excellent cycle indicators, and its lithium-ion battery, the technical solution is: the electrolyte for a lithium-ion battery that can discharge at ultra-low temperature. The electrolyte is characterized in that the electrolyte is made of a mixture of lithium hexafluorophosphate, lithium tetrafluoroborate and a solvent. The solvent includes ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, dimethoxyethane, and the weight of lithium hexafluorophosphate and lithium tetrafluoroborate. The ratio is between 1:5 and 10:1. The concentration of the composite salt formed after mixing lithium hexafluorophosphate and lithium tetrafluoroborate is 0.7~1.2 mol/L. Between ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate In proportion, dimethoxyethane accounts for 0.5% to 1% of the total weight of the mixture of lithium hexafluorophosphate, lithium tetrafluoroborate, ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. According to the present invention, the ratio between ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate and dimethoxyethane is 5:5:5:1, and the ratio between lithium hexafluorophosphate and lithium tetrafluoroborate is 1: 4. The concentration of the complex salt formed after mixing lithium hexafluorophosphate and lithium tetrafluoroborate is 1.0 mol/L.
Preparation of lithium tetrafluoroborate [Preparation]
Method 1: A method for preparing lithium tetrafluoroborate, including the following steps:
1) According to the molar ratio of H3BO3 to HF, add boric acid to hydrofluoric acid for reaction to obtain a fluoroboric acid solution;
2) According to the molar ratio of Li to B of 1.05 to 1.5:1, add the lithium compound into the fluoroboric acid solution obtained in step 1) for reaction to obtain a lithium tetrafluoroborate solution;
3) Concentrate the lithium tetrafluoroborate solution obtained in step 2), cool and crystallize, and filter to obtain lithium tetrafluoroborate crystals;
4) Dry the lithium tetrafluoroborate crystals obtained in step 3) to obtain crude lithium tetrafluoroborate;
5) Place the crude lithium tetrafluoroborate obtained in step 4) into a reactor, introduce fluorine gas or a fluorine-nitrogen mixture to react, and obtain the lithium tetrafluoroborate.
Method 2: A method for preparing high-purity lithium tetrafluoroborate, including the following steps:
(1) Under the conditions of -10℃~10℃, slowly add boric acid to the hydrofluoric acid solution, and control the molar ratio of boric acid to anhydrous hydrofluoric acid to be: H3BO3:HF=1:(4~6) , then continue to stir and react for 0.5 to 5 hours to prepare a fluoroboric acid solution;
(2) Under the conditions of -10℃~15℃, add the lithium compound into the fluoroboric acid solution prepared in step (1) according to the molar ratio Li:B=1.05~1.5, and stir for 0.5-5h. Obtain lithium tetrafluoroborate solution;
(3) The lithium tetrafluoroborate solution is -0.05～-0.09MPa, 50℃～90℃.Concentrate under certain conditions to 1/3 to 1/8 of the original volume of the lithium tetrafluoroborate solution, then cool to room temperature for crystallization, and filter. The resulting solid is lithium tetrafluoroborate crystal;
(4) The lithium tetrafluoroborate crystal is preliminarily dried at 30-60°C for 2-8 hours, and then vacuum-dried at 50-100°C for 2-8 hours to obtain crude lithium tetrafluoroborate;
(5) Dissolve the crude lithium tetrafluoroborate obtained in step (4) into anhydrous hydrofluoric acid to obtain a reaction liquid, control the reaction temperature to 0°C to 15°C, and pass fluorine gas or fluorine-nitrogen mixture into Impurity removal is carried out in the reaction solution, and the amount of fluorine gas is: the number of moles of fluorine gas: the number of moles of crystal water in the crude lithium tetrafluoroborate = (1.5 ~ 15): 1. Experiments have proved that under this condition, tetrafluoroborate The impurity removal effect in the crude lithium fluoroborate is good, and the obtained product has high purity. After impurity removal by passing fluorine, the lithium tetrafluoroborate hydrofluoric acid mother liquor is obtained;
(6) Gradient cooling crystallization of the lithium tetrafluoroborate and hydrofluoric acid mother liquor, the crystallization temperature is -40~0°C, filter, and the crystallized product is dried under nitrogen protection at 30~80°C to obtain high-purity tetrafluoroboric acid Lithium products, filtrate recovery and recycling.
Preparation of lithium tetrafluoroborate [main reference materials]
[1] Li Yunfeng; Yang Huachun; Hou Hongjun; Zheng Jinxia; Ma Guanghui; Chen Hu; 10-20
[2] Li Yunfeng; Hou Hongjun; Yang Huachun; Yan Chunsheng; Ma Guanghui;
[3] Jin Minggang; Chen Gang; Shen Yao. Electrolyte for lithium-ion battery capable of ultra-low temperature discharge and its lithium-ion battery CN200710067426.9, application date 2007-03-06
