Elementary Decomposition Mechanisms of Lithium Hexafluorophosphate in Battery Electrolytes and Interphases

Abstract

Electrolyte decomposition constitutes an outstanding challenge to long-life Li-ion batteries (LIBs) as well as emergent energy storage technologies, contributing to protection via solid electrolyte interphase (SEI) formation and irreversible capacity loss over a battery’s life. Major strides have been made to understand the breakdown of common LIB solvents; however, salt decomposition mechanisms remain elusive. In this work, we use density functional theory to explain the decomposition of lithium hexafluorophosphate (LiPF<sub>6</sub>) salt under SEI formation conditions. Our results suggest that LiPF<sub>6</sub> forms POF<sub>3</sub> primarily through rapid chemical reactions with Li<sub>2</sub>CO<sub>3</sub>, while hydrolysis should be kinetically limited at moderate temperatures. We further identify selectivity in the proposed autocatalysis of POF<sub>3</sub>, finding that POF<sub>3</sub> preferentially reacts with highly anionic oxygens. These results provide a means of interphase design in LIBs, indicating that LiPF<sub>6</sub> reactivity may be controlled by varying the abundance or distribution of inorganic carbonate species or by limiting the transport of PF<sub>6</sub>- through the SEI.