Interfacial Coupling with Halide Solid Electrolyte Enables High‐Energy‐Density and Reversible FeCl ₃ Cathode for All‐Solid‐State Lithium Batteries

Abstract FeCl ₃ , a high‐capacity conversion‐type cathode, holds great promise for high‐energy‐density lithium‐ion batteries. However, its practical application is hindered by the severe dissolution issues with poor achievable capacity and cycling stability. Herein, the challenges are addressed by coupling FeCl ₃ with a F‐containing halide solid‐state electrolyte (SSE) to construct a high‐performance all‐solid‐state lithium battery (ASSLB). This design effectively suppresses the dissolution of FeCl ₃ and fully leverages its high theoretical capacity, delivering a remarkable discharge capacity of 497 mAh g ⁻¹ at 0.05 A g ⁻¹ and a high energy density of 1201 Wh kg _cathode ⁻¹ . The favorable interfacial coupling between FeCl ₃ and SSE weakens Fe─Cl and Li ─ Cl bonds, promoting the reversible redox reactions and phase transition reversibility. Moreover, the in situ formation of the LiF‐rich cathode‐electrolyte interface enhances both interfacial stability and Li ⁺ transport kinetics by lowering charge transfer resistance and boosting Li⁺ diffusion. As a result, the ASSLBs demonstrate the excellent rate capability (314 mAh g ⁻¹ at 0.5 A g ⁻¹ ) and the outstanding cycling stability (a capacity retention of 84% over 500 cycles at 1 A g ⁻¹ ). This work highlights the critical role of cathode‐electrolyte interfacial engineering and provides a viable design strategy for advancing conversion‐type metal chloride cathodes for high‐energy ASSLBs.