PNNL

Abstract

Lithium hexafluorophosphate (LiPF6)-based carbonate electrolytes are widely used in commercial lithium-ion batteries (LIBs), but their thermal instability limits the cycle life and safety of LIBs at elevated temperatures. Few studies have yielded insight into the PF6- decomposition reaction that promotes thermal instability of LiPF6-based electrolytes. Here, with nuclear magnetic resonance techniques and ab initio molecular dynamics simulations, we find that Li+ ion hopping assisted by the overall reorientational motion of propylene carbonate solvent molecules facilitates PF6- decomposition at elevated temperatures in 1 M LiPF6/PC electrolyte. Further, we demonstrate that urea additives, by preventing Li+ ion hopping, suppress the initial LiPF6 decomposition reaction and enhance the thermal stability of the electrolyte. LIB cell tests with LiNi0.6Mn0.2Co0.2O2 || Li4Ti5O12 (NMC || LTO) show improved LIB performance at elevated temperatures in the presence of urea additives. This study provides key insights into the design of thermally stable LiPF6-based carbonate electrolytes for improving the cycle life, calendar life, and safety of LIBs in elevated-temperature applications.