Population-Weighted First-Shell Coordination States Rationalize Li+ and Na+ Mobility in PEO and PEC Polymer Electrolytes

Abstract

Understanding the molecular origin of the distinct transport behavior of Li⁺ and Na⁺ in polymer electrolytes remains a central challenge for next-generation energy-storage materials. Here, cation transport in poly(ethylene oxide) (PEO) and poly(ethylene carbonate) (PEC) electrolytes is investigated using atomistic molecular dynamics simulations combined with quantum mechanical calculations of representative first-shell coordination motifs. Across 14 PEO-based formulations, Na⁺ exhibits higher mobility than Li⁺, associated with a more diffuse and dynamically labile coordination environment, whereas Li⁺ preferentially occupies compact, long-lived, high-coordination states. Four PEC-based systems assess how matrix rigidity modulates the relationship between local coordination energetics and long-range transport. We introduce a population-weighted energetic descriptor, ⟨E_int⟩, constructed from Boltzmann-averaged interaction energies and MD-derived coordination-state populations. The descriptor captures variations in the cationic contribution to conductivity, σt⁺, within PEO, while PEC reveals the additional influence of polymer dynamics. Within the predominantly polymer-mediated regime examined here, anion participation can stabilize persistent coordination states associated with reduced cation mobility. A reduced representation based on dominant coordination states retains the principal PEO transport relationship, highlighting the importance of state-specific populations and interaction energies rather than average coordination properties while emphasizing limits to cross-family descriptor transferability.