Decomposition of Ionic Liquids at Lithium Interfaces. 1. <i>Ab Initio</i> Molecular Dynamics Simulations

Abstract

This work is Part 1 in a two part series that investigates the interfacial decomposition chemistry of [pyr14][TFSI] and [EMIM][BF4] ionic liquids (IL) at Li metal interfaces. Here, the decomposition is probed primarily through ab initio molecular dynamics (AIMD) simulations. For single ion pairs adsorbed on a Li(100) surface, hybrid ion states are found to emerge about the Fermi level. Interestingly, these states have a significant contribution from both ions, which suggests that the cathodic (reductive) stability could in part be governed by the anions. Room temperature AIMD simulations reveal rapid decomposition of the TFSI anion initiated by C–S and/or S–N bond cleavage due to charge transfer from Li to the anion. The unusual phenomenon of reductive decomposition of the anion is supported by recent experimental reports. The reaction products observed included LiF, LiO, Li2F, Li2O, SO2, NSO2, NSO2CF3, etc., which are all in excellent agreement with the XPS results. Initial decomposition reactions for both cations and the BF4 anion were only observed in high temperature AIMD simulations. For bulk ILs interfaces with a Li(100) surface, interfacial decomposition reactions again result from charge transfer to the IL from the Li surface, in particular, to anions at the interfaces. The initial decomposition event at bulk interfaces is found to vary depending on the interface structure. The extensive computational analyses presented in this work provide valuable insights into the fundamental interfacial chemistry of ILs in contact with Li metal. In Part 21 of this series, we consider these results further by systematically examining ion reductive stability, the thermodynamics of decomposition, and kinetic limitations to decomposition using gas phase density functional theory (DFT) computations. Results from these studies can be used for further design of these, or perhaps other, ILs to obtain more stable solid electrolyte interface (SEI) layers to improve cycling in advanced battery chemistries.