Investigating the Physical State of Polymer Electrolyte: Influence of Temperature and LiTFSI Concentration on the Phase of the Different States of the Polymer Electrolyte PEO-LiTFSI

Abstract

A critical analysis of the physical state {solid or liquid state} of the PEO-LiTFSI system was investigated in this study. The findings show one crystallite type in PEO and four in LiTFSI. The physical state of the binary mixture PEO-LiTFSI is predominate by the semi-crystalline properties of pure PEO when w _e is lower than 33 wt%, and the crystallization of the mixture is only induced by PEO. Nevertheless, LiTFSI reduces the degree of crystallinity of PEO due to its solvation by a part of PEO crystallites. Besides, as the solubility limit of LiTFSI in PEO is achieved, salt crystallites appear within the resulting electrolyte. These crystallites in the high w _e domain were identified as LiTFSI crystallites complexed with PEO. However, rising temperature promotes their dissolution. The functional groups implicated in the crystallization of PEO-LiTFSI have been highlighted using the IR technique. Besides, the experimental result shows that the glass transition temperature (T _g ) and the melting point (T _m ) of the binary mixture exhibit a non-linear trend with w _e and M _w . A simple mathematical treatment is proposed to predict glass transition temperature as a function of w _e and M _w . Our model considers the additive effect of lithium salt on the T _g variation.