Study of LiBF[sub 4] as an Electrolyte Salt for a Li-Ion Battery

Using 3:7 (wt) ethylene carbonate/ethylmethyl carbonate mixed solvent, we comparitively studied LiBF 4 and LiPF 6 as solutes for the electrolyte of a Li-ion battery. Results showed that the LiBF 4 -based electrolyte passivates Al (used as a current collector of the cathode) better than the LiPF 6 -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 LiBF 4 or LiPF 6 -based electrolyte at ambient temperature is negligible. Compared with the one using LiPF 6 -based electrolyte, the Li-ion cell using LiBF 4 -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 60°C with the electrolyte containing 80 ppm of water. At higher temperatures such as 80°C, the Li-ion cell suffered from a severe capacity fading, which is believed to he associated with the irreversible reactions between electrolyte and electrodes.