Efficiency, cost, and lifetime are the primary challenges for stationary energy storage with vanadium-redox flow and sodium-sulfur batteries as promising options. In particular, room temperature sodium-sulfur battery systems offer the potential for safe, simple, low-cost and high energy density storage, but the high reactivity or solubility of sodium polysulfides in common liquid electrolytes for carbonates or glycols, respectively, leads to rapid performance loss on cycling. Herein, we demonstrate a robust route to inhibit reactivity of the sulfides with carbonate electrolytes (and also inhibit the diffusion of polysulfides dissolved in TEGDME) and prevent performance loss on cycling using highly doped (≈40 atom%) nanoporous carbon from low-cost raw materials infused with sulfur as the cathode. This cathode design leads to an ultra-stable room temperature sodium-sulfur battery with less than 3% decay in the discharge capacity after 8000 cycles at a high current density of 4.6A/g. At 0.23A/g, the discharge capacity is approximately 400 mAh/g and stable over 350 cycles. This combination of low cost and excellent cycle stability is promising for stationary, grid-level energy storage.