Effect of extreme hydrostatic pressure on ion diffusion in polymer electrolytes: Emergence of glass-transition pressure

Abstract

Polymer electrolytes in solid-state batteries are critical for applications demanding mechanical flexibility and tolerance. Recent research progress has underscored the potential significance of employing solid electrolytes in extreme environments, such as the high hydrostatic pressure encountered during deep-sea exploration. Consequently, understanding how extreme hydrostatic pressure affects ion diffusion in polymer electrolytes is of substantial importance. In this work, large-scale molecular dynamics simulations are utilized to investigate the diffusion of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a representative polymer electrolyte, specifically poly(ethylene oxide) (PEO). We reveal a previously unreported mechanism associated with the emergence of a glass-transition pressure, above which the polymer matrix exhibits glass-like characteristics. The transport behavior of Li⁺ ions shows a distinct contrast below and beyond this critical pressure. Through theoretical scaling analysis, we show that ionic diffusivity is proportional to material volume, and is therefore governed by this same phase-transition pressure, which rationalizes our simulation results. This work provides potential guidance for understanding and designing polymer electrolytes with tolerance to extreme pressures.