Insights into the Electrolyte Hydrolysis and Its Impacts on the Interfacial Chemistry of a Li⁺-Intercalated Anode during High-Temperature Calendar Aging

Calendar aging occurring during high-temperature storage has long plagued practical realization of long-life, high-safety lithium-ion batteries (LIBs). Generally, the aging process is ascribed to the hydrolysis reaction of fluorine-containing electrolyte salt that generates hydrofluoric acid and chemically corrodes the anode surface. Nevertheless, the underlying mechanism about electrolyte degradation, HF generation and surface corrosion remains concealed for various electrolytes. In this work, we employed in situ liquid time-of-flight secondary ion mass spectroscopy to resolve the chemical evolution during high-temperature calendar aging in the bulk of the electrolyte and at the anode/electrolyte interface. Two conventional salts, LiPF₆ and Li bis(fluorosulfonyl)imide (LiFSI), were employed for comparison. We identify that the high-temperature hydrolysis of LiPF₆ preferentially occurs when the anion aggregates ([PF₆+LiPF₆]⁻) are attacked by trace H₂O. HPO₂F₂, HF and LiF are generated and assist formation of an inorganics-rich solid electrolyte interphase (SEI), improving anode stability against parasitic reactions. The LiFSI-based electrolyte does not involve hydrolysis, which facilities the formation of an organics-rich SEI. Nevertheless, the SEI does not passivate the anode surface and could induce severe corrosions via electron tunneling at a high temperature. Our work offers original insights into rational design of electrolyte and interface for high-energy, long-calendar-life LIBs.