Active Interphase Enables Stable Performance for an All‐Phosphate‐Based Composite Cathode in an All‐Solid‐State Battery

Abstract High interfacial resistance and unstable interphase between cathode active materials (CAMs) and solid‐state electrolytes (SSEs) in the composite cathode are two of the main challenges in current all‐solid‐state batteries (ASSBs). In this work, the all‐phosphate‐based LiFePO₄ (LFP) and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) composite cathode is obtained by a co‐firing technique. Benefiting from the densified structure and the formed redox‐active Li_3–xFe_2–x–yTi_xAl_y(PO₄)₃ (LFTAP) interphase, the mixed ion‐ and electron‐conductive LFP/LATP composite cathode facilitates the stable operation of bulk‐type ASSBs in different voltage ranges with almost no capacity degradation upon cycling. Particularly, both the LFTAP interphase and LATP electrolyte can be activated. The cell cycled between 4.1 and 2.2 V achieves a high reversible capacity of 2.8 mAh cm^–2 (36 µA cm^–2, 60 °C). Furthermore, it is demonstrated that the asymmetric charge/discharge behaviors of the cells are attributed to the existence of the electrochemically active LFTAP interphase, which results in more sluggish Li⁺ kinetics and more expansive LFTAP plateaus during discharge compared with that of charge. This work demonstrates a simple but effective strategy to stabilize the CAM/SSE interface in high mass loading ASSBs.