Exploring nonlinear ion dynamics in polymer electrolytes from the perspective of hopping models

Abstract

Nonlinear ion transport in polymer electrolytes provides key information about the underlying energy landscape and transport mechanisms. Molecular dynamics simulations are employed to investigate the field-dependent ion dynamics in poly(ethylene oxide)/LiTFSI mixtures over a range of temperatures and salt concentrations. The electric-field dependence of the current and the parallel and orthogonal diffusivities is analyzed in detail. In the weak-field regime, the nonlinear response reflects the degree and character of energetic disorder, while in the high-field regime, effective hopping distances and barrier heights can be extracted. The resulting hopping lengths agree with the typical nearest-neighbor separations from structural analysis and show little dependence on salt concentration. The apparent linear decrease in the effective activation barriers with increasing field accounts for the onset of unbounded ion motion at high fields. Comparison with analytically tractable hopping models in disordered energy landscapes provides a consistent physical interpretation of both the low- and high-field regimes. Overall, the study demonstrates how hopping models can be employed to quantitatively and conceptually rationalize nonlinear ion dynamics in polymer electrolytes.