Multi-Electron Transfer Halide Cathode Materials Based on Intercalation-Conversion Reaction Towards All-Solid-State Lithium Batteries.

All-solid-state lithium batteries (ASSLBs) with non-flammable solid-state electrolytes offer high energy density and enhanced safety. However, their energy densities are greatly limited by low-capacity and low-ionic-conductivity oxide cathode materials, typically relying on the intercalation-deintercalation mechanism with a catholyte content of 15-30 wt %. Here we introduce the LiFeX (X=Cl, Br) families as high-capacity and high-ionic-conductivity alternatives, operating via a 3 mol e transfer intercalation-conversion coupling reaction. Notably, the catholyte-free ASSLBs using 95 wt % LiFeCl active material delivers a remarkable capacity of 446 mAh g and a high energy density of 912 Wh kg, which surpasses most oxide cathode materials. Of particular interest is the formation of amorphous Fe during the conversion process. The amorphous Fe exhibits high activity, catalyzing the conversion of LiX back to LiFeX, which proves instrumental in realizing reversible intercalation-conversion reactions. These halide cathode materials represent a significant advancement towards high-energy-density ASSLBs.