Correlation between Solvation Structure and Ion-Conductive Behavior of Concentrated Poly(ethylene carbonate)-Based Electrolytes

Solid polymer electrolytes are important materials in realizing safe and flexible energy storage devices. The present study looks at correlation between solvation structure and the ion-conductive behavior of poly(ethylene carbonate) (PEC)/lithium bis(fluorosulfonyl)imide (LiFSI) electrolytes which have high Li transference number (t+) and show unusual salt-concentration dependence of conductivity. From FT-IR and Raman spectroscopy, we determined that Li ions interact with carbonyl (C═O) groups and also with FSI ions, which can be referred to as contact ion pair or aggregate. 7Li magic-angle-spinning NMR spectroscopy and density functional theory calculations for model species suggest that a loose coordination structure, in which Li ions interact with C═O groups and FSI ions with appropriate strength, allows the electrolytes to have both reasonable conductivity and high t+ with a flexible and transparent character. A high salt dissociation rate is generally considered essential in SPEs, but the presence of aggregated ions having the loose coordination structure gives rise to favorable performance in highly concentrated PEC-based solid polymer electrolytes.