Prolonged cycle life of composite cathodes via ionically permeable Li3PO4 surface engineering on conductive agents to suppress degradation of sulfide solid electrolytes

Sulfide-based all-solid-state batteries (ASSBs) are promising candidates for next-generation energy storage systems owing to their notable ionic conductivity and stability against explosions. However, the low chemical stability of sulfide-based solid electrolytes (SSEs) causes problems at their interfaces with other electrode components. Among them, addressing the side reactions between conductive agents and SSEs is crucial for commercialization. Herein, a conductive agent surface-modified with Li₃PO₄ is employed to enhance the interfacial stability of SSEs. Density functional theory-based analysis reveals that Li₃PO₄, characterized by strong inter-element bonding, exhibits high ionic conductivity and stability at the interface with SSEs. Electrochemical measurements confirm that Li₃PO₄-coated conductive agents suppress the interfacial decomposition of SSEs, thereby securing the targeted ionic conductivity in the composite cathode. Consequently, ASSBs adopting surface-engineered conductive agents demonstrate remarkable rate capability (153.6 mAh g⁻¹ at 2 C) and cycle performance (88.8 % retention over 1000 cycles) with a high areal capacity (4 mAh cm⁻²). This study provides a novel concept for conductive agents that enhance charge transport characteristics and mitigate SSE degradation, paving the way for the development of long cycle life ASSBs.