Molecular Mechanisms Underlying Fast Alkali Metal Ions Transport in Poly(ionic liquid)-in-Salt Electrolytes by Using Asymmetric Trifluoromethanesulfonyl-Cyano Anion

Polymer ionic liquid (polyIL) electrolytes possess both the solid-state stability of polymers and the electrochemical stability of ionic liquids, exhibiting good compatibility with reactive alkali metal anodes in solid-state alkali metal batteries. To screen and design high-performance polyIL electrolyte materials, endeavors require in-depth insight into the transport mechanisms of alkali metal (M+) ions in polyILs. Herein, we conducted molecular dynamics simulations to understand salt-dependent transport behaviors and coordination characteristics of Na+ and K+ ions in polyIL-in-salt electrolytes with an asymmetric trifluoromethanesulfonyl-N-cyanoamide (TFSAM) anion. Our simulation results predicted that both M+ ions exhibit rapid diffusion dynamics in salts with asymmetric anions and the ion transference number of Na+ ions is as high as 0.61. M+ ions and asymmetric anions are monodentate coordinated by the cyano group and form stable clusters dominated by [Na([TFSAM])5]4– and [K([TFSAM])6]5–. Although M+ ions preferentially form cocoordination structures with polycations by bridging anions, this cocoordination configuration decreases with increasing salt concentration due to the increase of M+–anion in local environment. The association strength of M+–anion directly affects the effective diffusion of M+ ions and the decreased structural relaxation times corresponds to the increased self-diffusion coefficients of M+ ions (DM+). Besides, the van Hove space-time correlation function indicates that M+ ions diffuse farther in a shorter time and the contribution of ion hopping to M+ ions transport increases under high salt concentrations.