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

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 we 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 we 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 (Tg) and the melting point (Tm) of the binary mixture exhibit a non-linear trend with we and Mw. A simple mathematical treatment is proposed to predict glass transition temperature as a function of we and Mw. Our model considers the additive effect of lithium salt on the Tg variation.