Unanticipated Mechanism of the Trimethylsilyl Motif in Electrolyte Additives on Nickel-Rich Cathodes in Lithium-Ion Batteries

Abstract

The introduction of a trimethylsilyl (TMS) motif in electrolyte additives is regarded as an effective way to remove corrosive hydrofluoric (HF) acid that structurally and compositionally damages the electrode-electrolyte interface and gives rise to transition metal dissolution from the cathode in lithium-ion batteries (LIBs). Here, we report that electrolyte additives with TMS moieties lead to continued capacity loss of polycrystalline (PC)-LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes coupled with graphite anodes compared to additives without TMS as the cycle progresses. Through a comparative study using electrolyte additives with and without TMS moieties, it is revealed that the TMS group is prone to react with residual lithium compounds, in particular, lithium hydroxide (LiOH) on the surface of the PC-NCM811 cathode, and the resulting TMS-OH triggers the decomposition of strong Lewis-acid PF5 formed by the equilibrium decomposition of LiPF6 in the electrolyte that generates reactive species, namely, HF and POF3. This work aims to offer a way to build favorable interface structures for Ni-rich cathodes covered with residual lithium compounds through a fundamental understanding of the roles of TMS moieties of electrolyte additives.