Ionic liquid reinforced NaSICON-type oxide electrolyte films enabling solid state conversion metal fluoride-lithium batteries

Solid-state batteries (SSBs) with metal fluoride cathodes are an exciting combination due to their high theoretical energy density and good safety. However, owing to the high chemical compatibility requirements between metal fluoride cathode and the solid-state electrolyte (SSE), developing an adaptable SSE film with comprehensive properties is particularly challenging. Herein, a class of ionic liquid (IL) reinforced oxide SSE films was proposed. Among them, the combination of Li1.5Al0.5Ge1.5(PO4)3 (LAGP) oxide SSE and EMIM-based IL (LAGP-EMIM) exhibits good flame-retardant properties, exceptional air stability, high ionic conductivity and excellent chemical stability. The introduction of ILs can increase the ion transport channels in SSEs and render the usage of dry-process, well addressing the contradiction between the flexibility and high ionic conductivity of oxide films. The thickness of LAGP-EMIM films is 59 μm and they show the highest ionic conductivity of 1.05 mS cm−1 among the NaSICON-type oxide-based films till now. In addition, it can be stably matched with the conversion metal fluoride cathode such as FeF3 and CuF2 to construct semi-solid-state batteries, delivering high initial discharge capacity of 820.2 and 539.6 mAh g−1, respectively. The present work shed new insight into designing the composite electrolytes toward the next-generation solid state conversion batteries.