LiPF₃(CF₂CF₃)₃: A salt for rechargeable lithium ion batteries

LiPF₃(CF₂CF₃)₃ from Merck KGaA (LiFAP) was tested as a new electrolyte for Li-ion batteries that can replace the commonly used LiPF₆. The latter salt is known to be unstable, to decompose thermally to LiF and PF₅, and to readily undergo hydrolysis with protic species to form HF contamination in solutions. The latter contamination may have a detrimental impact on the performance of both anodes and cathodes for Li-ion batteries. Solutions comprising LiFAP, LiPF₆, and LiN(SO₂CF₂CF₃)₂ (LiBETI) in mixtures of ethylene, dimethyl, and diethyl carbonates were tested with composite graphite and LiMn₂O₄ electrodes. The tools for this study included voltammetry (fast and slow scan rates), chronopotentiometry, impedance spectroscopy, Fourier transform infrared, and X-ray and photoelectron spectroscopies. It was found that LiFAP is superior to LiPF₆ as an electrolyte for both graphite anodes and LiMn₂O₄ cathodes. This should be attributed to the different surface chemistry developed on these electrodes when LiPF₆ is replaced by LiFAP. An important impact of such a replacement is probably the absence of possible pronounced HF contamination in LiFAP solutions.