Silyl-Functionalized Electrolyte Additives and Their Reactivity toward Lewis Bases in Li-Ion Cells

Abstract

Silyl groups are included in a wide range of electrolyteadditives to enhance the performance of state-of-the-art Li-ion batteries. Arecognized representative thereof is tris-(trimethylsilyl)phosphate(TMSPa) which, along with the similarly structured phosphite, has beenat the center of numerous electrolyte studies. Even though the silyl grouphas already been widely reported to be specifically reactive towardsfluorides, herein, a reactivity towards several Lewis bases typically found inLi-ion cells is postulated and investigated with the aim to establish a moresimplified and generally applicable reaction mechanism thereof. Bothgaseous and electrolyte soluble reactants and products are monitored bycombining nuclear magnetic resonance and injection cell-coupled massspectrometry. Experimental observations are supported by computationalmodels. The results clearly demonstrate that the silyl groups react withwater, hydroxide, and methoxide and thereby detach in a stepwise fashion from the central phosphate in TMSPa. Intermolecularinteraction between TMSPa and the reactants likely facilitates dissolution and lowers the free energy of reaction. Lewis bases are wellknown to trigger side reactions involving both the Li-ion electrode and electrolyte. By effectively scavenging these, the silyl group canbe explained to lower cell impedance and prolong the lifetime of modern Li-ion batteries.