Toward High‐Performance All‐Solid‐State Thin Film FeOxSy/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 FeOxSy film is prepared, followed by the fabrication and investigation of FeOxSy/LiPON/Li TFBs. It is found that the structural degradation at both the LiPON/Li and FeOxSy/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 FeOxSy 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. Interfacial failure mechanism of the FeOxSy/LiPON/Li TFB during electrochemical cycling is investigated, and an Al2O3 interlayer dual‐interface modification strategy is developed to achieve high‐performance TFB. It is disclosed that the Al2O3 interlayer not only enables robust LiPON/Li contact, but also blocks the diffusion of Fe element from FeOxSy to LiPON, resulting in greatly improved interfacial stability for the TFB.