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

Abstract

abundance of sulfur. The lower cost and greater availability of sodium as compared to lithium precursors is spurring the incremental focus on Na-S batteries. The traditional high temperature Na-S batteries operating at 300-350 °C comprise the molten electrodes and the solid inorganic β-alumina electrolyte. This mature design is known to have safety issues and a relatively low theoretical energy 760 W h kg −1 (2Na + 3S → Na 2 S 3 ). [2] Instead, there is a strong incentive to develop room-temperature (RT) Na-S batteries which in principle allow the two-electron reduction of sulfur to Na 2 S, with a higher theoretical energy of 1273 Wh kg −1 and less of a safety concern. [3] In practice, RT Na-S cells are likewise held back by several primary challenges including polysulfide (Na 2 S x , 4 ≤ x ≤ 8) dissolution and crossover in liquid electrolytes. Other concerns are the insulating nature of sulfur (σ e = 5 × 10 −30 S cm −1 ) and associated sluggish sulfur redox kinetics, as well as the large volume expansion (170%) of the cathode on cell discharge. [4] Sodium ions have lower solid-state diffusivity and reactivity with solid S than lithium ions. Consequently the electrochemical redox processes in Na-S are more sluggish than in the Li-S counterpart. [5] For Na-S cells, the galvanostatic plateaus are more sloping and less well-defined, while the charge-discharge voltage hysteresis is This is the first report of molybdenum carbide-based electrocatalyst for sulfur-based sodium-metal batteries. MoC/Mo 2 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/Mo 2 C@PCNT-S). Quasi-solid-state phase transformation to Na 2 S is promoted in carbonate electrolyte, with in situ time-resolved Raman, X-ray photoelectron spectroscopy, and optical analyses demonstrating minimal soluble polysulfides. MoC/Mo 2 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 Na 2 S x (1 ≤ x ≤ 4) on surface of MoC/Mo 2 C is significantly lowered compared to analogous redox in liquid. Strong binding of Na 2 S x (1 ≤ x ≤ 4) on MoC/Mo 2 C surfaces results from charge transfer between the sulfur and Mo sites on carbides' surface.

Single-Atom Yttrium Engineering Janus Electrode for Rechargeable Na–S Batteries

“…Room-temperature sodium–sulfur (Na–S) batteries are arousing tremendous interest owing to their high theoretical energy density (1274 Wh kg –1 ) and merits of low cost, resource abundance, and environmental friendliness. − Their practical application yet faces the following fundamental challenges: (1) the low electrical conductivity of both the sulfur cathode and its discharge products, sluggish reaction kinetics, and the notorious shuttling effect of sodium polysulfides (NaPS) and (2) uncontrollable dendrite formation of the Na metal anode. − To address these issues, polar nanostructured materials including metal nanoparticles, sulfides, oxides, nitrides, and carbide-modified hosts consisting of well-designed active centers have been proven to effectively bind polar NaPS and can be used as electrocatalysts to accelerate the conversion of NaPS to Na 2 S. − However, the low-density catalytic active sites and high gravimetric density of bulk metal-containing species would counteract the superiority of the high energy density of Na–S batteries. On the other hand, in Na metal anodes, lightweight porous carbon materials have turned out to be promising host materials for suppressing the Na dendrite formation due to their excellent electrical conductivity and chemical stability. − Unfortunately, most carbon skeletons are generally Na-phobic, which requires controlled doping chemistry in a heteroatom-containing carbon matrix to achieve a low energy barrier for Na nucleation and stable Na deposition. , Therefore, it is of great significance to develop a synergistic strategy that can simultaneously regulate both cationic and anionic migration behavior for high-energy-density Na–S battery systems.…”

“…1−5 Their practical application yet faces the following fundamental challenges: (1) the low electrical conductivity of both the sulfur cathode and its discharge products, sluggish reaction kinetics, and the notorious shuttling effect of sodium polysulfides (NaPS) and (2) uncontrollable dendrite formation of the Na metal anode. 6−9 To address these issues, polar nanostructured materials including metal nanoparticles, 10 sulfides, 11 oxides, 12 nitrides, 13 and carbide 14 -modified hosts consisting of well-designed active centers have been proven to effectively bind polar NaPS and can be used as electrocatalysts to accelerate the conversion of NaPS to Na 2 S. 15−17 However, the low-density catalytic active sites and high gravimetric density of bulk metal-containing species would counteract the superiority of the high energy density of Na−S batteries. On the other hand, in Na metal anodes, lightweight porous carbon materials have turned out to be promising host materials for suppressing the Na dendrite formation due to their excellent electrical conductivity and chemical stability.…”

In situ implanting MnO nanoparticles into carbon nanorod-assembled microspheres enables performance-enhanced room-temperature Na–S batteries

“…22 Such a case has been widely discovered in other studies of S cathode materials for Na-ion storage. 23,24…”

“…22 Such a case has been widely discovered in other studies of S cathode materials for Na-ion storage. 23,24 Raman spectroscopy was utilized to further analyze the micro/nanostructures of the samples. For three samples (MnO-C, MnO@NACM, and NACM), two stable and identical characteristic peaks signify the D band (disordered structure) and G bands (graphitized structure) of carbons, respectively, as illustrated in Fig.…”

The Future for Room‐Temperature Sodium–Sulfur Batteries: From Persisting Issues to Promising Solutions and Practical Applications

“…[192][193][194] Remarkably, Hao et al reported a composite cathode with a high sulfur loading of 12.7 mg cm −2 (64 wt.%) and achieved stable cycling for more than 150 cycles. [195] According to the facts, in order to promote the sulfur loads from below 2 mg cm -2 to above 6 mg cm -2 , the focus for RT-Na/S system should be set on the "teamwork" perspective that integrates the abilities of both cathode, anode, electrolyte, separator, and binder rather than developing industrial technology alone.…”