Synthesis, Water‐Removing Method and Influences of Trace Water for LiBF 4

Recent works show the promise for lithium tetrafluoroborate (LiBF 4 ) as an electrolyte salt for lithium‐ion batteries. In this paper, LiBF 4 is synthesized by a novel reaction between the as‐prepared anhydrous Li 2 SO 3 and (C 2 H 5 ) 2 O⋅BF 3 in an aprotic solvent. The moisture sensitivity and water‐removing method of LiBF 4 materials which has been dried under vacuum for 2 h at different temperature, is studied by Fourier transform infrared spectroscopy measurements. Results show that almost all of the adsorbed water would be removed at above 65 °C. With the increase of drying temperature, crystal water will decrease slowly, and almost disappear completely at 170 °C. However, due to the thermal decomposition of LiBF 4 , LiF and BF 2 OH impurities are formed as the temperature reaches 165 °C. So, we determine that the proper drying temperature is 155 °C, though the trace water cannot be removed thoroughly. Besides, influences of trace water on the electrochemical performance of Li/MCMB half‐cell with LiBF 4 as electrolyte salt has been investigated. Results show that discharge plateaus at about 1.5 V vs. Li/Li + will be lengthened with the increase of water content. Subsequently, the so‐called solid‐electrolyte interphase (SEI) film will become thicker and more resistant, which leads to poor Li + ion migration and bad cycling performance.