Solid Polymer Electrolytes Designed for Lithium Metal Battery By Lithium Salt Induced Cationic Ring-Opening Polymerization

Abstract

Solid polymer electrolytes (SPEs) are envisioned as a dependable replacement for currently employed carbonate-based liquid electrolytes [1,2]. Nevertheless, many limitations [3] such as poor ionic conductivity, low Li+-ion transport properties and extreme processing conditions have impeded their intrusion into industrial production. Even though several possible industrial scale solvent-free processes such as in situ preparation [4] of polymer electrolytes using free radical and ionic polymerization technique have been proposed in the literature, industrial scale production of polymer electrolytes for lithium batteries (LiBs) is not yet achieved. Cationic ring-opening polymerization (CROP) is an alternate technique used to produce target-specific polymers for the development of commercial polymers such as polyethylene oxide, polysiloxane and polyphosphazene. It is an industrially well-established technique and the polymerisation reaction can be carried out using heat or ultraviolet irradiation. Generally, CROP is carried out using Brönsted acids (e.g. HCl, H2SO4, HClO4 and HOSO2CF3) or Lewis acids (BF3) as initiating species. Dreyfuss et. al. [5] in 1976 demonstrated that alkali metal salts could be used as an initiator generator for cationic ring-opening polymerization of heterocyclic molecules. In this context, we are proposing a solvent free synthesis of solid-state polymer electrolytes by a thermally induced and a lithium salt catalysed cationic ring-opening polymerization (CROP) process of glycidyl ether based oligomers as shown in the scheme. A synergistic approach using 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. The CROP process mechanism is elucidated using Molecular Dynamic simulation and Quantum Chemical calculations. The obtained SPEs are characterised using, real time FT-Raman spectroscopy, XPS, NMR, DSC, TGA-GC-MS analysis and electrochemical techniques. The SPEs produced exhibited high ionic conductivity (>0.1 mS/cm), low glass transition temperature (<-50°C), and wide electrochemical stability (>5 V vs. Li/Li+). The SPE also exhibited stable interface characteristics towards lithium metal. The precursor solution is directly deposited over the cathode and then polymerized to achieve an optimum interface. The polymer electrolyte coated C-LiFePO4 based composite cathode film is successfully cycled in lithium metal battery configuration at 40 and 60°C. The SPE is also galvanostatically cycled against LiNi1/3Mn1/3Co1/3O2 cathode, and the preliminary results have indicated encouraging results regarding specific capacity and Coulombic efficiency. The result achieved in our labs confirmed that thermally induced CROP is a upscalable and solvent free technique with enormous potential for the production of all solid-state polymer electrolytes.