Toward High‐Performance All‐Solid‐State Thin Film FeO x S y /LiPON/Li Microbatteries via Dual‐Interface Modification

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 x S y film is prepared, followed by the fabrication and investigation of FeO x S y /LiPON/Li TFBs. It is found that the structural degradation at both the LiPON/Li and FeO x S 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 Al 2 O 3 as an interlayer is proposed. During the TFB cycling, the Al 2 O 3 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 x S 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.