Decomposition of Ionic Liquids at Lithium Interfaces. 2. Gas Phase Computations

Abstract

This is Part 2 of a two part series of papers on decomposition of two ionic liquids at lithium metal interfaces. In Part 1 of this series, ab initio molecular dynamics (AIMD) simulations were used to examine the stability and decomposition of two ionic liquids (ILs), [pyr14][TFSI] and [EMIM][BF4], on Li metal anodes. Here in Part 2, density functional calculations of ions and ion pairs in the gas phase are coupled with model electrode surface effects to provide an in-depth analysis of the results obtained from more computationally expensive AIMD simulations of electrolytes on the Li surface in Part 1. The gas phase approach is used to examine the cathodic and anodic stability, the electrochemical decomposition thermodynamics, and the kinetic barriers to the electrochemical decomposition of the ions on a Li surface. The states of the ILs are shown to mix with those of the Li surface, which leads to the reduction of the cations by one electron and a partial reduction of the anions. Upon reduction, many ion decomposition reactions are found to be thermodynamically favorable and to have small or moderate kinetic barriers. An examination of reaction transition states for reduced ions and ions in the presence of Li atoms suggests that the reductive decomposition of anions is mediated by chemical association with Li surface atoms, while reductive decomposition of the cations need not involve such chemical interactions. Overall, the gas phase results obtained here corroborate and extend understanding of the stability and decomposition behavior of ILs on Li metal anodes noted from the AIMD simulations in Part 1.