A Simulation Study of the Lithium Ion Transport Mechanism in Ternary Polymer Electrolytes - The Critical Role of the Segmental Mobility

TL;DR

This study uses molecular dynamics simulations and an extended transport model to analyze lithium ion movement in ternary polymer electrolytes, revealing the critical influence of segmental mobility and new diffusion mechanisms.

Contribution

It introduces an extended analytical model for lithium transport that accounts for TFSI-supported diffusion and compares two strategies for electrolyte design.

Findings

Segmental mobility enhances lithium diffusion in PEO-based electrolytes.

Adding ionic liquid plasticizes the polymer, increasing ion mobility.

Substituting PEO with ionic liquid reduces free ether oxygens, offsetting mobility gains.

Abstract

We present an extensive molecular dynamics (MD) simulation study of the lithium ion transport in ternary polymer electrolytes consisting of poly(ethylene oxide) (PEO), lithium-bis(trifluoromethane)sulfonimide (LiTFSI) and the ionic liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonimide (PYR13TFSI). In particular, we focus on two different strategies by which the ternary electrolytes can be devised, namely by (a) adding the ionic liquid to PEO20LiTFSI, and (b) substituting the PEO chains in PEO20LiTFSI by the ionic liquid. In order to grasp the changes of the overall lithium transport mechanism, we employ an analytical, Rouse-based cation transport model (Maitra et al., Phys. Rev. Lett., 2007, 98, 227802), which has originally been devised for binary PEO-based electrolytes. This model distinguishes three different microscopic transport mechanisms, each quantified by an individual time scale. In the course of our analysis, we extend this mathematical description to account for an entirely new transport mechanism, namely the TFSI-supported diffusion of lithium ions decoupled from the PEO chains, which emerges for certain stoichiometries. We find that the segmental mobility plays a decisive role in PEO-based polymer electrolytes. That is, whereas the addition of the ionic liquid to PEO20LiTFSI plasticizes the polymer network and thus also increases the lithium diffusion, the amount of free, mobile ether oxygens reduces when substituting the PEO chains by the ionic liquid, which compensates the plasticizing effect. In total, our observations allow us to formulate some general principles about the lithium ion transport mechanism in ternary polymer electrolytes. Moreover, our insights also shed light on recent experimental observations (Joost et al., Electrochim. Acta, 2012, 86, 330).