Ion Transport Mechanism in Polymer Electrolytes - Bridging the Scales Via Molecular Simulations and Theory
Authors: Diddens Diddo , Heuer Andreas 
Published: February 2020

Solid polymer electrolytes (SPEs) consisting of a salt in a polymer matrix are considered as safe alternatives to liquid electrolytes. However, at ambient temperatures, their conductivity is still too low for an efficient use in modern energy storages. To overcome this deficiency, several strategies have been proposed. For instance, it was demonstrated that the addition of a low-molecular-weight additive such as an ionic liquid (IL) results in improved transport properties [1].

To unravel the mechanisms how the ion transport is enhanced at the molecular level, molecular dynamics (MD) simulations are an extremely powerful tool. In case of classical SPEs based on poly(ethylene oxide) (PEO), three different microscopic transport mechanisms can generally be identified from MD simulations [2]: First, diffusion of coordinating lithium ions along the PEO backbone, second, cooperative motion of the ions with the PEO dynamics, and third, ion transfer processes between distinct PEO chains (see sketch). However, due to the limited size of the simulation system (in particular the length of the polymer chains), the comparison between experiments and the numerical data alone can be made only qualitatively. To bridge the gap between the different length and time scales characteristic for experiments and simulations, we devised an analytical framework based on polymer theory that allows us to extrapolate the numerical data to experimental scales.

To illustrate this concept, we demonstrate that the way in which an IL may enhance the lithium mobility can in principle be twofold: First, the IL can act as a plasticizer, enhancing the dynamics of the polymer segments and thus also the motion of the attached lithium ions [3,4], and second, for specifically designed IL molecules that directly coordinate to the lithium ions (such as pyrrolidinium ions with chemically bound oligoether chains [5]), the IL may serve as a molecular shuttle detaching the lithium ions from the slow polymer chains [6], resulting in a much larger enhancement of the lithium ion transport.

In a second step, we focus on SPEs based on diblock copolymers, consisting of a mechanically stable block and an ionically conducting PEO domain. Due to their increased overall rigidity, these electrolytes tend to suppress dendrite growth more efficiently, and thus are considered to be key candidates for lithium metal batteries. Special emphasis is again placed on the role of low-molecular-weight additives that recently have been studied experimentally [7], and on their impact on the molecular transport mechanism.

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[7] T. S. Dörr, A. Pelz, P. Zhang, T. Kraus, M. Winter, H.-D. Wiemhöfer, Chem. Eur. J.
2018, 24(32), 8061-8065

Figure 1