Ion Correlations and Their Impact on Transport in Polymer-Based Electrolytes

The development of next-generation polymer-based electrolytes for energy storage applications would greatly benefit from a deeper understanding of transport phenomena in these systems. In this Perspective, we argue that the Onsager transport equations provide an intuitive but underutilized framework for analyzing transport in polymer-based electrolytes. Unlike the ubiquitous Stefan–Maxwell equations, the Onsager framework generates transport coefficients with clear physical interpretation at the atomistic level and can be computed easily from molecular simulations using Green–Kubo relations. Herein we present an overview of the Onsager transport theory as it applies to polymer-based electrolytes and discuss its relation to experimentally measurable transport properties and the Stefan–Maxwell equations. Using case studies from recent computational work, we demonstrate how this framework can clarify nonintuitive phenomena such as negative cation transference number, anticorrelated cation–anion motion, and the dramatic failure of the Nernst–Einstein approximation. We discuss how insights from such analysis can inform design rules for improved systems.