Controlling Li+ transport in ionic liquid electrolytes through salt content and anion asymmetry: a mechanistic understanding gained from molecular dynamics simulations

In this work, we report the results from molecular dynamics simulations of lithium salt-ionic liquid electrolytes (ILEs) based either on the symmetric bis[(trifluoromethyl)sulfonyl]imide (TFSI-) anion or its asymmetric analogue 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide (TFSAM(-)). Relating lithium's coordination environment to anion mean residence times and diffusion constants confirms the remarkable transport behaviour of the TFSAM(-)-based ILEs that has been observed in recent experiments: for increased salt doping, the lithium ions must compete for the more attractive cyano over oxygen coordination and a fragmented landscape of solvation geometries emerges, in which lithium appears to be less strongly bound. We present a novel, yet statistically straightforward methodology to quantify the extent to which lithium and its solvation shell are dynamically coupled. By means of a Lithium Coupling Factor (LCF) we demonstrate that the shell anions do not constitute a stable lithium vehicle, which suggests for this electrolyte material the commonly termed vehicular lithium transport mechanism could be more aptly pictured as a correlated, flow-like motion of lithium and its neighbourhood. Our analysis elucidates two separate causes why lithium and shell dynamics progressively decouple with higher salt content: on the one hand, an increased sharing of anions between lithium limits the achievable LCF of individual lithium-anion pairs. On the other hand, weaker binding configurations naturally entail a lower dynamic stability of the lithium-anion complex, which is particularly relevant for the TFSAM(-)-containing ILEs.

Automated summary

In this study, molecular dynamics simulations were used to investigate lithium salt-ionic liquid electrolytes based on different anions, revealing the unique transport behavior of TFSAM(-)-based ILEs. A new methodology, the Lithium Coupling Factor (LCF), was introduced to quantify the dynamic coupling between lithium ions and their solvation shells. The analysis showed that lithium and shell dynamics decouple progressively with higher salt content due to increased sharing of anions between lithium ions and weaker binding configurations.