Study of LiBF4 as an electrolyte salt for a Li-ion battery

Using 3:7 (wt) ethylene carbonate/ethylmethyl carbonate mixed solvent, we comparatively studied LiBF4 and LiPF6 as solutes for the electrolyte of a Li-ion battery. Results showed that the LiBF4-based electrolyte passivates Al (used as a current collector of the cathode) better than the LiPF6-based one does, while the latter has higher ionic conductivity. It was found that the graphite electrode and the lithium nickel-based mixed oxide cathode have similar lithiation and delithiation cycling behaviors in these two electrolytes. While the difference in the cycling performance of the Li-ion cells using either the LiBF4 or LiPF6-based electrolyte at ambient temperature is negligible. Compared with the one using LiPF6-based electrolyte, the Li-ion cell using LiBF4-based electrolyte is less moisture sensitive and shows much better cycling performance at elevated temperatures. Such a cell could perform even if the electrolyte contained up to 620 ppm of water. The cell also could perform at 60degreesC with the electrolyte containing 80 ppm of water. At higher temperatures such as 80degreesC, the Li-ion cell suffered from a severe capacity fading, which is believed to be associated with the irreversible reactions between electrolyte and electrodes. (C) 2002 The Electrochemical Society.