Enhanced rate and cycle performance of all-solid-state batteries with an ionic and electronic conductive composite strategy

All-solid-state batteries (ASSBs), which utilize nonflammable solid electrolytes, are being increasingly recognized as promising next-generation technologies. ASSBs are expected to offer superior safety compared to conventional lithium-ion batteries, while enabling high energy and power density performances. In this study, a synthesis process involving the mixing and subsequent heat treatment of sulfide-based solid electrolytes and carbon conductive additives was investigated to improve the performance of ASSBs. A significant focus has been directed toward the synthesis of sulfide-based solid electrolytes and carbon conductive additives aimed at concurrently enhancing ionic and electronic conductivities. The selection of the carbon-conductive additive is crucial for optimizing the electronic conductivity of cathode composites. Spherical carbon (SC) is widely used as a conductive carbon additive in cathode composites. However, the high volumetric occupancy of SCs frequently leads to undesirable oxidation reactions with sulfide-based solid electrolytes. To mitigate the decrease in cell performance caused by oxidation reactions, carbon nanofibers (CNFs) were used. CNFs offer a smaller specific surface area, a higher electronic conductivity, and ease of structural modification. However, the simple mixing of CNFs with cathode composites leads to agglomeration, resulting in inhomogeneous ion and electron transport networks. A novel approach was developed to resolve the agglomeration issues of CNFs. A sulfide-based solid electrolyte-CNF composite was synthesized by incorporating CNFs during the sulfide-based solid electrolyte synthesis processes of milling and heat treatment. The resulting structure featured CNFs that were distributed homogeneously between the sulfide-based solid electrolytes. Consequently, a cell using sulfide-based solid electrolyte-CNF showed a remarkable capacity (204.0 mA h g⁻¹ at 1C) and cycle performance (82% capacity retention after 100 cycles at 0.5C).