Rubber-Derived Sulfur Composite Cathodes for Realizing High-Energy-Density All-Solid-State Li-S Batteries (Supporting Information)

All-solid-state lithium-sulfur batteries (ASSLSBs) are promising next-generation high-energy-density storage systems. However, their practical application is hindered by sulfur’s insulating nature and the difficulty of establishing continuous ion-conducting pathways at solid-solid interfaces. In this study, we maximize the performance of rubber-derived sulfur composite cathodes, in which sulfur is effectively confined within a polymer network, by integrating them with a polyether-based solid polymer electrolyte (SPE) using a polymer-electrolyte impregnation technique. This method establishes a robust three-dimensional lithium-ion conducting network within the electrode and forms an adhesive SPE layer on the cathode surface, thereby ensuring intimate interfacial contact. The reduction of cathode particle size (D50 < 1.5 µm) was found to be a governing factor for enhancing charge-discharge performance by increasing the effective contact area with the SPE. The optimized cell exhibited high discharge capacities of 600 mAh g−1 at 60 °C, even with a high areal capacity of ca. 2 mAh cm−2. Furthermore, a five-layer bipolar cell achieved 180 mAh with an average discharge voltage of 8.5 V. Notably, it showed a specific capacity of 800 mAh g−1 (normalized by single-layer active mass), thereby overcoming the low-voltage limitation of Li-S systems. This work provides key design principles for high-energy-density polymer-based ASSLSBs.