Lithium Ion Transport Mechanism in Ternary Polymer Electrolyte-Ionic Liquid Mixtures - A Molecular Dynamics Simulation Study

TL;DR

This study uses molecular dynamics simulations and a Rouse-based model to understand lithium ion transport in ternary polymer electrolytes with ionic liquids, revealing enhanced mobility due to polymer dynamics rather than ionic liquid coordination.

Contribution

It introduces a combined MD simulation and analytical modeling approach to elucidate lithium transport mechanisms in ternary electrolytes, highlighting the role of polymer dynamics.

Findings

Lithium ions are mainly coordinated by PEO chains, not ionic liquids.

Ionic liquids enhance lithium mobility by increasing PEO chain dynamics.

Simulation diffusion coefficients agree with experimental data.

Abstract

The lithium transport mechanism in ternary polymer electrolytes, consisting of PEO/LiTFSI and various fractions of the ionic liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonimide, are investigated by means of MD simulations. This is motivated by recent experimental findings [Passerini et al., Electrochim. Acta 2012, 86, 330-338], which demonstrated that these materials display an enhanced lithium mobility relative to their binary counterpart PEO/LiTFSI. In order to grasp the underlying microscopic scenario giving rise to these observations, we employ an analytical, Rouse-based cation transport model [Maitra at al., PRL 2007, 98, 227802], which has originally been devised for conventional polymer electrolytes. This model describes the cation transport via three different mechanisms, each characterized by an individual time scale. It turns out that also in the ternary electrolytes essentially all lithium ions are coordinated by PEO chains, thus ruling out a transport mechanism enhanced by the presence of ionic-liquid molecules. Rather, the plasticizing effect of the ionic liquid contributes to the increased lithium mobility by enhancing the dynamics of the PEO chains and consequently also the motion of the attached ions. Additional focus is laid on the prediction of lithium diffusion coefficients from the simulation data for various chain lengths and the comparison with experimental data, thus demonstrating the broad applicability of our approach.