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.

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The solvent‐free in situ polymerization technique has the potential to tailor‐make conformal interfaces that are essential for developing durable and safe lithium metal polymer batteries (LMPBs). Hence, much attention has been given to the eco‐friendly and rapid ultraviolet (UV)‐induced in situ photopolymerization process to prepare solid‐state polymer electrolytes. In this respect, an innovative method is proposed here to overcome the challenges of UV‐induced photopolymerization (UV‐curing) in the zones where UV‐light cannot penetrate, especially in LMPBs where thick electrodes are used. The proposed frontal‐inspired photopolymerization (FIPP) process is a diverged frontal‐based technique that uses two classes (dual) of initiators to improve the slow reaction kinetics of allyl‐based monomers/oligomers by at least 50% compared with the conventional UV‐curing process. The possible reaction mechanism occurring in FIPP is demonstrated using density functional theory calculations and spectroscopic investigations. Indeed, the initiation mechanism identified for the FIPP relies on a photochemical pathway rather than an exothermic propagating front forms during the UV‐irradiation step as the case with the classical frontal photopolymerization technique. Besides, the FIPP‐based in situ cell fabrication using dual initiators is advantageous over both the sandwich cell assembly and conventional in situ photopolymerization in overcoming the limitations of mass transport and active material utilization in high energy and high power LMPBs that use thick electrodes. Furthermore, the LMPB cells fabricated using the in situ‐FIPP process with high mass loading LiFePO4 electrodes (5.2 mg cm‐2) demonstrate higher rate capability, and a 50% increase in specific capacity against a sandwich cell encouraging the use of this innovative process in large‐scale solid‐state battery production.