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.