Self-Healing Commodity Polymer Electrolytes for Reliable Li Metal Batteries

Abstract

As countries worldwide pledge to decarbonize their economy, the global demand for batteries is rapidly increasing, urging the development of high-specific-energy (> 500 Wh/kg at cell level) and low-cost (< 100$/kWh at pack level) batteries.1 The safety and durability of these devices are of primary importance. Polymer electrolytes are safer than traditional liquid electrolytes, and further benefit from highly tunable structures.2 Thus, polymers can be designed to achieve desired properties, including the ability of self-healing from damages, which can greatly extend battery lifetime. While the synthesis of self-healing polymers often requires specifically designed monomers or other polymerization components, it was recently shown that commodity copolymers, based on methyl methacrylate (MMA) and n-butyl acrylate (nBA) or their derivatives, display self-healing behavior.3 The autonomous self-repair of these copolymers with certain compositions is governed by interchain van der Waals forces.Herein, this finding was exploited to prepare self-healing polymer electrolytes that ensured long-term performance of high-energy Li metal batteries. The electrolytes were based on polyethylene oxide (PEO), P(MMA-co-nBA) copolymers and Li bis(trifluoromethanesulfonyl)imide (TFSI) salt. Various structures and compositions were explored, including polymer blends and block copolymers made by atom transfer radical polymerization (ATRP) using a functionalized PEO-macroinitiator (Figure 1). The effect of polymer morphology, architecture, weight fraction of PEO and LiTFSI on electrolyte conductivity, thermal and electrochemical stability, Li transference number, mechanical strength and self-healing ability was analyzed.By recovering ionic conductivity and mechanical strength upon cracks, self-healing electrolytes can suppress the growth of dendrites that hinders the utilization of Li metal anodes.4 Moreover, the viscoelastic nature of these polymers contribute to the creation of conformal interfaces on Li metal.2 Therefore, the optimized commodity self-healing polymer electrolytes were tested in Li metal batteries, demonstrating the ability of the electrolytes to enable long-term stable cycling. In addition, tests were conducted to prove the self-repair of the electrolyte upon damaging the device.