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.

In situpolymerization process: an essential design tool for lithium polymer batteries

“…In a recent report, Nair et al have proposed a different mechanism for the CROP of the diglycidyl ether oligomer that can be used for the in situ processing of SPEbased LPB by a separator assisted or a separator-free approach. 505 The reported mechanism is shown in Figure 40c, which demonstrates that the anions, in combination with a trace amount of water molecules, can initiate the CROP polymerization without decomposing into BF 3 OH and HF. This mechanism can also explain the capability of other Li-salts such Figure 40d.…”

Materials Design for High‐Safety Sodium‐Ion Battery

“…Typical thermopolymerization monomers include organic metallic locenes, divinyl ether, oxitane, dimethacrylate‐based derivatives, etc. [ 159–163 ]…”

Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid‐State Lithium Batteries

“…It has been demonstrated that lithium salt species and its concentration are crucial for "salt-in-polymer" SPEs. [132,133] Traditional lithium salts are usually employed in SPE includes LiPF 6 , [134] LiClO 4 , [130] lithium borates (LiBF 4 , LiBOB, and LiODFB), [135,136] and lithium imides (LiTFSI, LiFSI). [137] However, the low thermal stability and high moisture sensitivity of LiPF 6 , the strong oxidability of LiClO 4 , and poor solubility of lithium borates restrain their application as lithium salt in SPE.…”