Electrolytes ageing in lithium-ion batteries: A mechanistic study from picosecond to long timescales

The ageing phenomena occurring in various diethyl carbonate/LiPF₆ solutions are studied using gamma and pulse radiolysis as a tool to generate similar species as the ones occurring in electrolysis of Li-ion batteries (LIBs). According to picosecond pulse radiolysis experiments, the reaction of the electron with (Li⁺, PF₆^-) is ultrafast, leading to the formation of fluoride anions that can then precipitate into LiF(s). Moreover, direct radiation-matter interaction with the salt produces reactive fluorine atoms forming HF(g) and C₂H₅F(g). The strong Lewis acid PF₅ is also formed. This species then forms various R¹R²R³P=O molecules, where R is mainly -F, -OH, and -OC₂H₅. Substitution reactions take place and oligomers are slowly formed. Similar results were obtained in the ageing of an electrochemical cell filled with the same model solution. This study demonstrates that radiolysis enables a description of the reactivity in LIBs from the picosecond timescale until a few days. (authors)