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.