Elementary Decomposition Mechanisms of Lithium Hexafluorophosphate in Battery Electrolytes and Interphases

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 identify the decomposition of lithium hexafluorophosphate (LiPF6) salt under SEI formation conditions. Our results suggest that LiPF6 forms POF3 primarily through rapid chemical reactions with Li2CO3, while hydrolysis should be kinetically limited at moderate temperatures. We further find that the proposed autocatalysis of POF3 is selective, and that POF3 preferentially reacts with highly charged oxyanions. These results indicate a means of interphase design in LIBs, suggesting that LiPF6 reactivity may be controlled by varying the abundance or distribution of inorganic carbonate species or by limiting the transport of PF6- through the SEI.