Solid Polymer Electrolytes for Lithium Metal Battery via Thermally Induced Cationic Ring-Opening Polymerization (CROP) with an Insight into the Reaction Mechanism

Abstract

We report the synthesis of solid polymer electrolytes (SPEs) using a thermally induced and a lithium salt catalyzed cationic ring-opening polymerization (CROP) technique. A synergistic approach using two salts such as lithium tetrafluoroborate-LiBF4 and lithium bis(trifluoromethane sulfonyl)imide-LiTFSI has assured a complete monomer to polymer conversion and fast reaction kinetics during the CROP process. The initiation mechanism of lithium salt-induced CROP is elucidated using molecular dynamic simulation, quantum chemical calculation, real-time FT-Raman spectroscopy, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetry–mass spectrometry analysis techniques. The cross-linked 3D network of ethylene oxide based SPE is prepared without the use of any solvents or external catalysts. In particular, a mixture of poly(ethylene glycol) diglycidyl ether, LiBF4, and LiTFSI in appropriate proportions after a baking process produced a freestanding, flexible, and nontacky film. The synthesized SPEs exhibit low glass transition temperature (< −50 °C), high ionic conductivity (>0.1 mS cm–1), and excellent oxidation stability (>5.5 V vs Li/Li+). The SPE is polymerized directly onto a carbon-coated LiFePO4 cathode film and successfully cycled in a lithium metal battery configuration at 40 and 60 °C. As evidence, the SPE is galvanostatically cycled against a high-voltage LiNi1/3Mn1/3Co1/3O2 cathode, and the preliminary results indicated exciting characteristics in terms of specific capacity and Coulombic efficiency.