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

Electrolytes 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.