Stability of Boronium Cation-Based Ionic Liquid Electrolytes on the Li Metal Anode Surface

Augmented cycling performance in lithium metal batteries is often attributed to the formation of a stable solid electrolyte interphase (SEI) layer that forms on the Li surface. Boronium cation-based ionic liquid (trimethylamine)(dimethylethylamine)dihydroborate bis(trifluoromethanesulfonyl)imide, [NNBH2]-[TFSI], has shown stable cycling in a Li vertical bar LiFePO4 cell; however, it is not known why this ionic liquid performs well nor what species contribute to the SEI layer formation. To gain an understanding of the SEI layer that forms using boronium cation-based ILs, the chemical stability and reaction mechanisms occurring between [NNBH2]-[TFSI] and [NNBH2] bis(fluorosulfonyl)imide [FSI] with the Li(001) surface are investigated using a combination of density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations at 298, 358, and 428 K. These simulations showed that the surface interaction with the cation is weak, while both the [FSI](-) and [TFSI](-) anions readily dissociate, forming several chemical species (including LiF, Li2O, and Li2S), with the dissociation of [FSI](-) occurring at a lower temperature. The absence of cation dissociation at elevated temperatures implies that the initial stages of SEI formation are restricted to the reaction products from the anions studied. This work provides further evidence that the [NNBH2](+) cation is stable against a lithium metal surface, with the spontaneous decomposition of the cation unlikely to occur.