Solid polymer electrolytes based on Li+/ionic liquid for lithium secondary batteries

Ionic liquid (IL)-based solid polymer electrolytes (SPEs) were synthesized by solution cast technique using polymer polyethylene oxide (PEO), lithium bis(trifluoromethane sulfonyl) imide (LiTFSI) salt, and IL 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl) imide (BMIMTFSI). The obtained polymer electrolytes (PEO + 20 wt.% LiTFSI) + x wt.% BMIMTFSI where x = 0, 5, 10, 15, and 20 were characterized by DSC, TGA, SEM, XRD, FTIR, electrochemical impedance spectroscopy (EIS), cyclic voltammetry, chronoamperometry, and chronocharge-discharge. Ionic conductivity of optimized composition SPE (PEO + 20 wt.% LiTFSI) + 20 wt.% BMIMTFSI is ∼1.5 × 10−4 S cm−1 at 30 °C and follows Arrhenius-type thermally activated behavior. The prepared SPEs are free-standing and flexible with excellent thermal and mechanical stabilities. SEM, XRD, and DSC studies show that the amorphicity of SPEs increases on increasing IL content due to the plasticization effect of IL. The assembled cell exhibits good electrochemical stability and cationic transference number tLi+∼0.27\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {t}_{{\mathrm{Li}}^{+}}\sim 0.27 $$\end{document}. The capacity of cell, Li | PEO + 20 wt.% LiTFSI + 20 wt.% BMIMTFSI | LiMn2O4, shows a stable cyclic performance and high Coulombic efficiency.