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Lithium hexafluorophosphate (LiPF 6 ) has been the dominant conducting salt in lithium-ion battery (LIB) electrolytes for decades; however, it is extremely unstable in even trace water (ppm level). Interestingly, in pure water, PF 6 – does not undergo hydrolysis. Hereby, we present a fresh understanding of the mechanism involved in PF 6 – hydrolysis through theoretical and experimental explorations. In water, PF 6 – is found to be solvated by water, and this solvation greatly improved its hydrolytic stability; while in the electrolyte, it is forced to “float” due to the dissociation of its counterbalance ions. Its hydrolytic susceptibility arises from insufficient solvation-induced charge accumulation and high activity in electrophilic reactions with acidic species. Tuning the solvation environment, even by counterintuitively adding more water, could suppress PF 6 – hydrolysis. The undesired solvation of PF 6 – anions was attributed to the perennial LIB electrolyte system, and our findings are expected to inspire new thoughts regarding its design.
Sheng et al. (Mon,) studied this question.