A composite electrolyte containing LiF and a tris(pentafluorophenyl)borane (TPFPB) anion complexing agent displays superior thermal and electrochemical stability compared to commercial LiPF6-based electrolytes. Severe capacity fading of a Li/LiMn2O4 cell. with a commercial LiPF6 electrolyte, was observed when cycled at 55 degreesC. The capacity fading was attributed to acidic products, from salt hydrolysis and decomposition of the solvent, which degraded LiMn2O4 electrode. The composite electrolyte containing LiF and TPFPB is not moisture sensitive and does not produce acidic species. At room temperature, the composite electrolyte shows an improvement in capacity maintenance with cycling. However, at 55 degreesC the improvement is much greater.