All-solid-state lithium-sulfur batteries (ASSLSBs) hold great promise for next-generation electrochemical energy storage due to sulfur’s high theoretical specific capacity and low cost. However, sluggish sulfur conversion kinetics and severe volume variations during cycling, as well as poor ionic percolation in composite cathodes, limit their practical viability. To overcome these challenges, we herein introduce solid electrolytes of nominal composition Li$_{10.5−x}$Si$_{1.5}$P$_{1.5}$S$_{12−x}$I$_x$ (with x = 0, 0.2, 0.4), possessing high ionic conductivities of ≥ 7 mS cm$^{−1}$ at room temperature. We show that increasing iodine content alters the phase composition and triggers reversible redox activity in these materials. If implemented as catholytes, this enables very fast sulfur conversion kinetics, ultimately leading to ASSLSBs with exceptional performance. The cells achieve 86% sulfur utilization at a rate of C/2 and at 45°C and offer high-rate capability by delivering 1175 mAh g$_{sulfur}$$^{−1}$ at 5C and 590 mAh g$_{sulfur}$$^{−1}$ at 15C. Furthermore, the synergistic effects of ionic percolation and redox-activity enable record areal capacities up to 14 mAh cm$^{−2}$ with a sulfur loading of 10 mg cm$^{−2}$. Taken together, our findings provide new strategies for designing redox-active catholytes for application in advanced ASSLSBs and further strengthen the redox-mediating role of iodine therein.