Synergistic N, S-Doping Modulates the Electronic Structure of Carbon Hosts for Highly Reversible and Rapid Iodine Conversion in Solid-State Batteries

All-solid-state lithium–iodine batteries (ASSLIBs) are constrained by low energy density and sluggish kinetics due to excessive solid-state electrolytes. Here, a synergistic N/S codoped Ketjen Black (NSKB) host is engineered to resolve the loading-kinetics trade-off. By modulating surface charge, NSKB achieves a remarkable I2/carbon mass ratio of 3.78, increasing the iodine cathode proportion to 53.6 wt %. Sulfur codoping significantly boosts nitrogen content and promotes graphitic nitrogen formation, providing abundant active sites for rapid redox conversion. Consequently, NSKB-based ASSLIBs demonstrate a high initial Coulombic efficiency of 92.8% and a 82.4% capacity retention over 1000 cycles. Even at a high areal loading of 3.26 mAh cm–2, excellent stability is maintained. Distribution of relaxation times (DRT) analysis confirms that NSKB reduces solid-state diffusion impedance and accelerates iodine conversion kinetics. This strategy offers insights for designing high-energy-density solid-state conversion chemistries.