Delineating the Intricate Impact of Carbon in All‐solid‐state Lithium‐Sulfur Batteries

All‐solid‐state lithium‐sulfur batteries (ASSLSBs) offer enhanced safety, energy density, and cost efficiency. However, the insulating nature of sulfur necessitates excessive conductive carbon to support sulfur redox. Common sulfide solid‐state electrolytes (SSEs) like Li 5.5 PS 4.5 Cl 1.5 decompose electrochemically within the operating voltage range of sulfur, a process enhanced by intimate contact with carbon. While this decomposition increases cell capacity, it reduces ionic conductivity, and its link to carbon characteristics in sulfur cathodes remains underexplored. Herein, the relationship between carbon specific surface area (SSA) and SSE decomposition in sulfur cathodes is examined. It is established that carbon blending can form a robust electronic conducting network that enhances both SSE and sulfur redox independently of carbon SSA. With a second derivative analysis (SDA), electrochemical impedance spectroscopy (EIS), and a galvanostatic intermittent titration technique (GITT), sulfur redox is separated from SSE redox, and active material loss is established as the dominant failure mechanism. Carbon blending with nanofibers is shown to mitigate cell fading despite increasing SSE decomposition. Finally, optimized cathodes are synthesized with high SSE and sulfur capacity capable of high‐rate cycling and cycling at high S‐loading against Li‐metal. The study highlights how tailoring carbon blends can significantly improve sulfur utilization, cyclability, and rate performance in ASSLSBs.