Controlling $\text{Li}^+$ transport in ionic liquid electrolytes through salt content and anion asymmetry: A mechanistic understanding gained from molecular dynamics simulations

TL;DR

This study uses molecular dynamics simulations to understand how salt content and anion asymmetry influence lithium ion transport in ionic liquid electrolytes, revealing a flow-like, correlated motion rather than vehicular transport.

Contribution

It introduces a novel Lithium Coupling Factor to quantify lithium-solvation shell dynamics and clarifies the mechanisms behind decoupling of lithium and shell motions at higher salt concentrations.

Findings

TFSAM- based electrolytes show less strong lithium binding with increased salt.

Lithium transport is more flow-like than vehicular, involving correlated motion.

Decoupling of lithium and shell dynamics increases with salt content.

Abstract

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 ($\text{TFSI}^-$) anion or its asymmetric analog 2,2,2-(trifluoromethyl)sulfonyl-N-cyanoamide ($\text{TFSAM}^-$). Relating lithium's coordination environment to anion mean residence times and diffusion constants confirms the remarkable transport behaviour of the $\text{TFSAM}^-$-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 $\text{TFSAM}^-$-containing ILEs.