Effects of electrolyte interlayers on suppressing reactions with LiFePO4 and argyrodite-based sulfides in solid-state batteries†

Although LiFePO₄ (LFP) is considered a promising cathode active material (CAM) because of its excellent cost-effectiveness and thermal stability, its interfacial compatibility with sulfide-based solid electrolytes in all-solid-state batteries (ASSBs) remains a major technical hurdle. Herein, we investigated the effects of electrolyte interlayers to achieve high-efficiency LFP-based CAMs by applying interlayers of an argyrodite-based sulfide [Li_5.4PS_4.4Cl_0.8Br_0.8 (LPSCB)] or a halide with high Li-ion conductivity [Li₃InCl₆ (LIC) or Li_2.5Y_0.5Zr_0.5Cl₆ (LYZC)]. As a result, the ASSB cell with an LIC or LYZC interlayer exhibited better electrochemical performance than that with LPSCB, with initial discharge capacities of 113.5 and 110.3 mAh g⁻¹ at 0.1C, respectively. In particular, the sulfide interlayer suffered from serious Fe²⁺ oxidation because it interacted with other anions. Regarding the halides, LYZC was stable in contact with the sulfide SE but showed low reactivity with LFP, whereas LIC was stable with LFP but quite unstable with the sulfide SE. This strategy provides valuable insights for achieving superior interfacial structures in LFP-based ASSBs.