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

Abstract

Relevant information about the nature of the dynamics of ions in electrolytes can be obtained by studying the nonlinear dependence on an applied electric field. Here we use molecular dynamics (MD) simulations to study the field effects for a polymer electrolyte, i.e. a mixture of PEO with Li-TFSI salt, for a range of different temperatures and salt contents. Specifically, the effects of the electric field on the current and the diffusivities parallel and orthogonal to the electric fields are analyzed. It is argued that the nonlinear effects in the weak-field regime provide information about the nature of the disorder. In contrast, the nonlinear effects in the high-field regime allow one to extract effective hopping distances. They are in close agreement with typical nearest-neighbor length scales obtained from detailed structural analysis and are hardly dependent on the salt content. Furthermore, from the study of the temperature dependence in the high-field regime, effective barrier heights can be determined, which in this regime decrease linearly with increasing field. The disappearance of the effective barriers, estimated by linear extrapolation, occurs close to the fields where the MD simulations start to be numerically unstable. Finally, the interpretation is supported by a comparison with analytically known solutions of disordered hopping models.