Cost-Effective and Humid Air-Stable Fluoride Solid Electrolyte with High Ionic Conductivity Induced by Microstructural Modulation

Among halide solid-state electrolytes (SSEs), fluorides show distinct advantages in chemical and electrochemical stability but are plagued by inadequate room-temperature (RT) ionic conductivity. Herein, we propose a cost-effective and humid air-stable fluoride SSE Li2TiF6 synthesized by a simple hydrothermal method, exhibiting a high ionic conductivity of 9.69 × 10–5 S/cm at RT and outstanding humidity tolerance. The enhanced ionic conductivity is demonstrated to originate from microstrain-induced crystal lattice expansion. Furthermore, a stable Li+-conductive and intimate interface between halide electrolyte and lithium metal is constructed by introducing a poly(ethylene oxide) protective layer, resulting in the symmetric cell exhibiting a long lifespan with stable cycling over 1000 h at 0.1 mA/cm2 as well as the solid-state lithium metal batteries based on the LiFePO4 cathode reversibly operated for at least 250 cycles at 0.5 C. This work provides fresh perspectives on resolving the cost and various stability issues encountered with halide SSEs.