High‐Capacity and Stable Sodium‐Sulfur Battery Enabled by Novel Molybdenum Carbide Electrocatalyst and Carbon Nanoporosity

Abstract

Room-temperature sodium - sulfur (RT Na-S) batteries are highly promising due to the favorable techno-economics and the greater availability of both sodium and sulfur. RT Na-S cells are held back by several primary challenges including dissolution of polysulfides species in liquid electrolytes, sluggish sulfur redox kinetics, as well as the large cathode volume expansion (170%) on cell discharge. Here, we develop a novel molybdenum carbide (MoC/Mo2C) electrocatalyst to promote Na-S reaction kinetics and extend batteries’ cycling lifetime. MoC/Mo2C nanoparticles is in-situ grown in conjunction with activation of nitrogen-doped hollow porous carbon nanotubes. Sulfur impregnation (50wt.% S) results in unique triphasic architecture termed MoC/Mo2C@PCNT-S. MoC/Mo2C electrocatalyst and carbon nanoporosity synergistically promote the sulfur utilization in carbonate electrolyte. In-situ time-resolved Raman, XPS and optical analysis demonstrate a quasi-solid-state phase transformation within MoC/Mo2C@PCNT-S, where minimal polysulfides are dissolved in electrolyte. As results, MoC/Mo2C@PCNT-S cathodes performed promising rate performance of 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 delivered a retained capacity of 650 mAh g-1 after 1000 cycles at 1.5 A g-1, which corresponds to only 0.028% capacity decay per cycle. Such promising cycling stability is also obtained for high mass loading cathodes (64wt.% S, 12.7 mg cm-2). Complementary Density Functional Theory (DFT) simulation provide fundamental insight regarding the electrocatalytic role of MoC/Mo2C nanoparticles. Favorable charge transfer between the sulfur and Mo sites on the surface of carbides contributes to a strong binding of Na2Sx (1 ≤ x ≤ 4) on MoC/Mo2C surfaces. Consequently, the formation energy of Na2Sx (1 ≤ x ≤ 4) on MoC/Mo2C is significantly lowered compared to the analogous redox in liquid, as well as the case of baseline ordered carbon.