Toward High‐Performance All‐Solid‐State Thin Film FeO_xS_y/LiPON/Li Microbatteries via Dual‐Interface Modification

Abstract Featuring high theoretical capacity, low cost, and low preparation temperature, Li‐free cathodes are considered promising for all‐solid‐state thin‐film lithium microbatteries (TFBs). In this work, a Li‐free cathode of amorphous FeO_xS_y film is prepared, followed by the fabrication and investigation of FeO_xS_y/LiPON/Li TFBs. It is found that the structural degradation at both the LiPON/Li and FeO_xS_y/LiPON interfaces results in a rapid capacity loss for the TFB during electrochemical cycling. To achieve both high rate capability and long cycle life for the TFB, a dual‐interface modification approach using amorphous Al₂O₃ as an interlayer is proposed. During the TFB cycling, the Al₂O₃ interlayer not only enables robust LiPON/Li contact by preventing the formation of Li voids but also blocks the diffusion of Fe element from FeO_xS_y to LiPON electrolyte, effectively suppressing the interfacial resistance growth. Consequently, the dual‐interface modified TFB achieves greatly improved rate capability (194.3 mAh g⁻¹ at 5 A g⁻¹) and cycle performance (71% capacity retention after 800 cycles), which are superior to the pristine TFB and single‐interface modified TFB. This work advances the fundamental understanding of the failure mechanisms at the solid‐solid electrode/electrolyte interfaces, offering a feasible interfacial modulation strategy to develop advanced all‐solid‐state lithium batteries.