Elucidating the Reactivity of Tris(trimethylsilyl)phosphite and Tris(trimethylsilyl)phosphate Additives in Carbonate Electrolytes-A Comparative Online Electrochemical Mass Spectrometry Study

The electrolyte additives tris(trimethylsilyl)phosphite (TMPSi) and tris(trimethylsilyl)phosphate (TMSPa) have shown their potential to improve lifetime and decrease impedance build-up in lithium ion batteries (LIBs) in several studies. Online electrochemical mass spectrometry (OEMS) was applied to study the effect of TMSPi and TMSPa on a gas releasing parasitic reaction during cycling of HE-NCM/graphite cells in different electrolytes to elucidate their working mechanisms, for which so far contradictive claims can be found in the literature. The results underline the chemical instability of the two additives in solution and show that they have, with one exception, little or no effect on the release of O-2 and CO2 which would be expected in the case of a passivating surface layer suppressing electrolyte decomposition. The decreased POF3 evolution and the formation of Me3SiF indicates that the main role of both additives is that of a chemical scavenger, e.g., for HF, which leads to decreased acidification of the electrolyte and thus a longer lifetime of LIBs containing these electrolytes.