Highly Concentrated Polycarbonate-Based Solid Polymer Electrolytes Having Extraordinary Electrochemical Stability

Solid polymer electrolytes (SPEs) are compounds of great interest as safe and flexible alternative ionics materials, particularly suitable for energy storage devices. We study an unusual dependence on the salt concentration of the ionic conductivity in an SPE system based on poly(ethylene carbonate) (PEC). Dielectric relaxation spectroscopy reveals that the ionic conductivity of PEC/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte continues to increase with increasing salt concentration because the segmental motion of the polymer chains is enhanced by the plasticizing effect of the imide anion. Fourier transfer-infrared (FTIR) spectroscopy suggests that this unusual phenomenon arises because of a relatively loose coordination structure having moderately aggregated ions, in contrast to polyether-based systems. Comparative FTIR study against PEC/lithium perchlorate (LiClO4) electrolytes suggests that weak ionic interaction between Li and TFSI ions is also important. Highly concentrated electrolytes with both reasonable conductivity and high lithium transference number (t(+)) can be obtained in the PEC/LiTFSI system as a result of the unusual salt concentration dependence of the conductivity and the ionic solvation structure. The resulting concentrated PEC/LiTFSI electrolytes have extraordinary oxidation stability and prevent any Al corrosion reaction in a cyclic voltammetry. These are inherent effects of the highly concentrated salt. (C) 2016 Wiley Periodicals, Inc.