Unraveling the Hydrolysis Mechanism of LiPF₆ in Electrolyte of Lithium Ion Batteries

Lithium hexafluorophosphate (LiPF₆) 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, PF₆^- does not undergo hydrolysis. Hereby, we present a fresh understanding of the mechanism involved in PF₆^- hydrolysis through theoretical and experimental explorations. In water, PF₆^- 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 PF₆^- hydrolysis. The undesired solvation of PF₆^- anions was attributed to the perennial LIB electrolyte system, and our findings are expected to inspire new thoughts regarding its design.