Developing a high sulfur (S)-loading cathode with high capacity utilization and long term cyclability is a key challenge for commercial implementation of Li-S battery technology. To overcome this challenge, we propose a solid-phase conversion sulfur cathode by using an edible fungus slag-derived porous carbon (C_FS) as sulfur host to fabricate the S/C_FS composite and meanwhile, utilizing the vinyl carbonate (VC) as co-solvent of the ether-based electrolyte to in-situ form a protective layer on the S/C_FS composite surface through its nucleophilic reaction with the freshly generated lithium polysulfides (LiPSs) at the very beginning of initial discharge, thus isolating the interior sulfur from the outer electrolyte and inhibiting the further generation of soluble LiPSs. Benefitting from the ultrahigh specific surface area of > 3,000 m²·g⁻¹, ideal pore size of < 4 nm, and large pore volume of > 2.0 cm³·g⁻¹ of the C_FS host matrix, the S/C_FS cathode even with a high S-loading of 80 wt.% (based on the weight of S/C_FS composite) can still operate in a solid-phase conversion manner in the VC-ether co-solvent electrolyte to exhibit a high reversible capacity of 1,557 mAh·g⁻¹, a high rate capability with 50% remaining capacity at 2 A·g⁻¹ and a high cycling efficiency of 99.9% over 500 cycles. The results presented in this work suggest that a combined action of solid-phase conversion electrochemistry and nanoarchitectured host structure may provide a new path for the design and development of practical lithium-sulfur batteries.