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/Mo2 C 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/Mo2 C@PCNT-S). Quasi-solid-state phase transformation to Na2 S is promoted in carbonate electrolyte, with in situ time-resolved Raman, X-ray photoelectron spectroscopy, and optical analyses demonstrating minimal soluble polysulfides. MoC/Mo2 C@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 Na2 Sx (1 ≤ x ≤ 4) on surface of MoC/Mo2 C is significantly lowered compared to analogous redox in liquid. Strong binding of Na2 Sx (1 ≤ x ≤ 4) on MoC/Mo2 C surfaces results from charge transfer between the sulfur and Mo sites on carbides' surface.