Preparation Of LiBF₄ And Effect Of Water On Its Electrochemical Performance

Lithium tetrafluoroborate（LiBF₄）as a lithium-ion battery electrolyte salt has higher research value and market prospect.It has good thermal stability,less sensitive to moisture,but also has excellent low temperature performance and safety performance.Due to these characteristics,the proportion of LiBF ₄ used in lithium-ion battery electrolyte is gradually increasing.However,due to the limited preparation conditions,the content of obtained LiBF₄ impurity is relatively high,which easily reacts with the active material of lithium-ion battery and affects the normal use of the battery.Therefore,it must be purified before use.This,in turn,will significantly increase its manufacturing costs.This paper proposes a new method for the preparation of LiBF ₄.It describes the advantages of this method over conventional methods,which has high purity and can be produced industrially.The dosage of lithium source,boron tri fluoride diethyl ether and organic solvents are optimized by the aspects of yield and physical properties.The optimized process of LiBF₄ is summarized,the optimum molar ratio of the reaction materials in synthesis process is:Li ₂SO₃:（C₂H₅）₂O?BF₃=1:3.3;Compared to ether,dimethyl carbonate as the organic solvent can better dissolve the products during the reaction,and it has little toxicity,good safety performance and environmental protection.The thermal stability and moisture sensitivity of LiBF ₄ are important factors affecting battery performance.we investigates the influences of trace water on the electrochemical performance with LiBF₄ as electrolyte salt for lithium ion batteries.Fourier transform infrared spectroscopy measurements are used to investigate thermal stability for LiBF₄.The obtained results reveal that LiBF₄ hydrates are LiBF₄?H₂O,whose adsorbed water and crystal water is removed at 65°C and 170°C,respectively.LiBF₄ starts to decompose and generate Li F and BF ₃ at 160°C.The scanning electron microscopy and electrochemical results show that extra H ₂O can deteriorate the surface of SEI film and increase interface reaction,ultimately resulting in the inferior cycling capabilities with LiBF₄ of water absorption electrolyte systems.The results of X-ray photoelectron spectroscopy also confirm that the SEI film eroded by H ₂O possesses more Li F and B₂O₃ at the MCMB surface of LiBF₄ of water absorption electrolyte systems,increasing interface resistance.In addition,LiBF ₄ dried at 155°C for 2h can balance the contradictory of crystal water content and decomposition point,which leads to the least crystal water and the optimal performance.Thus,the effect of trace water on the electrochemical performance of LiBF ₄-based electrolyte is vital and the drying process after preparation is significant for LiBF ₄ salt.