Molecular Insights into the Effect of Asymmetric Anions on Lithium Coordination and Transport Properties in Salt-Doped Poly(ionic liquid) Electrolytes

Poly(ionic liquid) (PIL) electrolytes are under investigations as solid-state electrolytes for all-solid-state lithium batteries, yet the limited understanding of the ion transport mechanism in this class of electrolytes hinders their further applications. In this work, we performed molecular dynamics simulations to study the effect of asymmetric anions on the coordination features and transport mechanisms of Li+ ions in lithium salt-doped PIL electrolytes, containing the poly(diallyldimethylammonium) polycation and different imide-based anions of bis[(trifluoromethyl)sulfonyl]imide ([TFSI]), (fluorosulfonyl)-N-(trifluoromethylsulfonyl)imide ([FTFSI]), and 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide ([TFSAM]). Our simulation results showed that the Li+ ions relative to polycations have stronger interactions with anions via monodentate coordination and the Li+ ions prefer to form such coordination with fluorosulfonyl or cyano groups in the system with asymmetric anions. Meanwhile, it is found that there is a higher probability for asymmetric anions to co-coordinate with both Li+ ions and polycations. Besides, we observed that the asymmetric TFSAM anions enable the Li+ ions to achieve a relatively high diffusion coefficient and in turn a large transference number, which mainly results from a weaker association between the Li+ ions and the TFSAM anions. A detailed analysis disclosed a correlation between the diffusion rate of Li+ ions and the structural relaxation time of Li+–anion ion pairs. Furthermore, the diffusion of Li+ ions with neighboring anions is found to result from a combined contribution of vehicular and structural mechanisms, but an increasing contribution of structural mechanism is seen for the PIL electrolyte with asymmetric anions.