Molybdenum Carbide Electrocatalyst In Situ Embedded in Porous Nitrogen-Rich Carbon Nanotubes Promotes Rapid Kinetics in Sodium-Metal–Sulfur Batteries

This is the first report of molybdenum carbide-based electrocatalyst for sulfur-based sodium-metal batteries. MoC/Mo₂C is in situ grown on nitrogen-doped carbon nanotubes in parallel with formation of extensive nanoporosity. Sulfur impregnation (50 wt% S) results in unique triphasic architecture termed molybdenum carbide–porous carbon nanotubes host (MoC/Mo₂C@PCNT–S). Quasi-solid-state phase transformation to Na₂S is promoted in carbonate electrolyte, with in situ time-resolved Raman, X-ray photoelectron spectroscopy, and optical analyses demonstrating minimal soluble polysulfides. MoC/Mo₂C@PCNT–S cathodes deliver among the most promising rate performance characteristics in the literature, achieving 987 mAh g⁻¹ at 1 A g⁻¹, 818 mAh g⁻¹ at 3 A g⁻¹, and 621 mAh g⁻¹ at 5 A g⁻¹. The cells deliver superior cycling stability, retaining 650 mAh g⁻¹ after 1000 cycles at 1.5 A g⁻¹, corresponding to 0.028% capacity decay per cycle. High mass loading cathodes (64 wt% S, 12.7 mg cm⁻²) also show cycling stability. Density functional theory demonstrates that formation energy of Na₂S_x (1 ≤ x ≤ 4) on surface of MoC/Mo₂C is significantly lowered compared to analogous redox in liquid. Strong binding of Na₂S_x (1 ≤ x ≤ 4) on MoC/Mo₂C surfaces results from charge transfer between the sulfur and Mo sites on carbides’ surface.