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 degreesC 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(CF2CF3)(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. (C) 2003 Elsevier B.V. All rights reserved.