On the use of LiPF₃(CF₂CF₃)₃ (LiFAP) solutions for Li-ion batteries. Electrochemical and thermal studies

Electrolyte solutions comprising a mixture of LiPF₆ and LiPF₃(CF₂CF₃)₃ (LiFAP) in alkyl carbonates (ethylene, dimethyl and diethyl carbonate) were found to be superior to single salt LiFAP or LiPF₆ solutions for lithium-graphite anodes at elevated temperatures. Graphite electrodes could be cycled (Li insertion-deinsertion) more than hundred times at 80 °C with high and stable capacity in the two-salt solutions, while in the single-salt solutions this was impossible. Preliminary studies by voltammetry and impedance spectroscopy indicate that the combination of the two salts in solution has a unique influence on the electrodes surface (not yet defined). Thermal studies by accelerating rate and differential scanning calorimetry show that thermal decomposition of LiFAP solutions has a higher onset, but very high heat and pressure developing rates, compared to LiPF₆ solutions. The presence of LiPF₆ in LiFAP solutions decreased their self-heating and pressure-developing rates pronouncedly. From product analysis of the thermal reactions by NMR, FTIR and MS, we can suggest possible unique bulk reactions that occur in LiPF₆-LiFAP solutions. One of these is a nucleophilic reaction between F^- and PF₃(CF₂CF ₃)₃^-, which may neutralize the effect of trace HF in solutions (thus forming new P-F bonds and HCF₂CF₃). Such a reaction should have a positive effect on both the performance of the Li-graphite electrodes and the thermal behavior of the solutions.