The (In‐)Stability of the Ionic Liquids [(TMEDA)BH2][TFSI] and −[FSI] on the Li(001) Surface

Abstract

AbstractElectrolytes that can enable the use of a Li metal anode at a vast 3860 mAh/g, in place of currently used graphite anodes (372 mAh/g), are required for the advancement of next‐generation rechargeable Li batteries. Both quaternary ammonium and boronium (trimethylamine)(dimethylethylamine)dihydroborate [NNBH2]+ cation‐based ionic liquids (ILs) show high electrochemical stability windows and thermal stability for use in Li batteries. Cyclization of the former cation shows improved electrolyte stability compared to the open‐chain counterpart. However, it is not known whether this is the case for the cyclic derivative of [NNBH2]+, N,N,N’,N’‐tetramethylethylenediamine)dihydroborate [(TMEDA)BH2]+. Here, the details of the initial stages of solid−electrolyte interphase (SEI) layer formation on a lithium metal surface, Li(001), for the [(TMEDA)BH2]+ based ILs are revealed using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. These indicate that [(TMEDA)BH2]+ remains intact, displaying a similarly weak interaction with the Li metal surface as the open‐chain analogue. The chemical stability shown by the boronium cation indicates spontaneous and unwanted side reactions with the Li anode are unlikely to occur, which could help to facilitate long‐term cycling stability of the battery. Altogether, the findings suggest the [(TMEDA)BH2]+ ILs, like their [NNBH2]+ IL counterparts, are viable candidates for rechargeable Li metal batteries.