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.