Humid-Air Stable and High-conductivity Fluoride Solid Electrolytes Induced by Liquid Metal Activation and Ga2O3 in situ Catalysis

Abstract

Poor humid air stability and bad compatibility with lithium metal anode are two critical challenges currently encountered with halide solid-state electrolytes (SSEs). Fluoride SSEs are expected to solve these problems owe to their superior chemical and electrochemical stability, but they are now plagued by inadequate room-temperature ionic conductivity. Herein, a novel fluoride SSE is reported with Li₃GaF_5.3Cl_0.7 as the main phase, which is synthesized via in situ oxidation of liquid metal gallium and in situ chlorination by LiCl. The in situ generated Ga₂O₃ not only function as a catalyst to solve the kinetic retardation of solid-phase synthesis by promoting the dissociation of LiF, but also serves as a soft template to regulate the growth of Li₃GaF_5.3Cl_0.7 nanoparticles. The optimized SSE exhibits an ionic conductivity close to 10⁻⁴ S cm⁻¹ at room-temperature and outstanding humidity tolerance (without conductivity degradation after exposure to a relative humidity up to 35%). A biphenyl complexed Li anode (BP-Li) is introduced to solve the problem of bad compatibility between anode and halide SSE. The BP-Li symmetric cell exhibits a long lifespan over 1800 h at 0.1 mA cm⁻². The stabilization of cycling is derived from the intrinsically homogenous electric field induced by the unpaired electrons delocalized in aromatic rings of BP.