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 Na2Sx (1 ≤ x ≤ 4) on surface of MoC/Mo2C is significantly lowered compared to analogous redox in liquid. Strong binding of Na2Sx (1 ≤ x ≤ 4) on MoC/Mo2C surfaces results from charge transfer between the sulfur and Mo sites on carbides’ surface. The charge‐storage kinetics of a sulfur‐based sodium‐metal battery are accelerated by incorporating highly redox‐active MoC/Mo2C nanoparticles at the cathode. First‐principle calculations shed fundamental insight on the source of this electrocatalytic enhancement. The resultant Na–S cells deliver among the most promising rate performance and cycling stabilities in the literature.