Influence of anion structure on Li+ solvation and diffusion in ionic liquid-based electrolyte, and decomposition mechanism of anion

Ionic liquids (ILs) are promising electrolytes for lithium batteries. Particularly, the structure of anions plays a vital role in electrochemical performances. In this work, we chose two asymmetric anions ((fluorosulfonyl)(perfluoroethylsulfonyl)imide (FETI-) and 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide (TSAC-)) and compared with the symmetric one (bis(trifluoromethanesulfonyl)imide (TFSI-)) to investigate the microcosmic mechanism of the effect of structure on Li+ diffusion through classical molecular dynamics (MD) simulations. We investigated the Li+ solvation structure in electrolytes and found that the asymmetry of anions facilitates the formation of monodentate configuration, but they are not effective in accelerating the Li+ diffusion. On the other hand, we found that the faster Li+ diffusion comes from both the higher diffusion coefficient of anions and the smaller size of Li+ aggregates in electrolytes. In addition, the decomposition mechanism of these anions was investigated by ab initio molecular dynamics (AIMD) simulations. The F directly attached to sulfonyl is unstable, leading to the earlier breaking of the S-F bond compared to S-O and S-N bond, forming early protective components of LiF in solid electrolyte interphase (SEI). This work provides guidance for the design of anions, revealing that the influence of anion structure on Li+ solvation and diffusion in IL-based electrolytes, and decomposition mechanism of anions.