Design Principles of Bifunctional Electrocatalysts for Engineered Interfaces in Na–S Batteries

Abstract

The shuttling of soluble sodium polysulfides (Na2S n ) and sluggish conversion kinetics are major roadblocks toward the practical realization of sodium–sulfur (Na–S) batteries. To undertake the challenges, we use first-principles calculations to design bifunctional electrocatalysts to achieve engineered interfaces with sulfur-based cathode materials. We illustrate the detailed behavior of Na2S n adsorption, sulfur reduction reactions (SRRs), and catalytic decomposition on transition-metal (TM)-based single-atom catalysts (SACs) embedded on MoS2 substrates (SACs@MoS2). We observe that SACs doped on sulfur substitution and molybdenum top sites result in adequate binding energies to immobilize higher-order Na2S n species. We found the d-band center as an important “descriptor” in dictating polysulfide adsorption energies and catalytic activities on SACs@MoS2. We elucidate that the larger upward shift of the d-band center toward the Fermi level and the involved higher number of vacant antibonding states are directly correlated to the adsorption strength of the Na2S n . The V and Ni SACs are found to exhibit higher and lower binding energies, respectively, consistent with the d-band theory. Furthermore, the SACs that are electron-deficient sites demonstrate bifunctional electrocatalytic activity through reduced free energy for SRR and lower the barrier for Na2S decomposition in favor of accelerated electrode kinetics during discharge and charge processes, respectively. The electronic structure calculations reveal a significantly reduced band gap of the pristine and Na2S n -adsorbed SACs@MoS2 due to mid-gap states, majorly stemming from TM-d orbitals, thus expected to improve the electronic conductivity of the substrates. The insight developed on the role of SACs in tailoring the polysulfides’ chemistry at the interfaces in relation to their d-band center is an important step toward the rational design of cathode materials for high-performance Na–S batteries.

Atomic-Scale Insights into Comparative Mechanisms and Kinetics of Na–S and Li–S Batteries

“…It is well known that PBE/GGA functional underestimates the band gap properties and based on the nature of the TM doping, DFT + U calculations were performed to consider the d electrons involved in the adsorption energy calculations. From our previous reports on 2D materials such as MoS 2 , VS 2 , and Mo 2 TiC 2 comprising TM atoms, we found a negligible change in the adsorption energies after the inclusion of DFT + U , and the binding energies calculated by GGA/PBE functional was adequate in explaining the adsorption characteristics. ,, Hence, we believe that the chosen PBE/GGA is well sufficient for elucidating both adsorption and electronic properties.…”

“…The 3d block elements such as Ni, Fe, V, and Co are reported as the most commonly used SACs. ,− In particular, V SAC has been shown to possess superior catalytic activity and polysulfides trapping behavior . Additionally, we previously used DFT computations to investigate first-row TMs as SACs supported on MoS 2 to evaluate the binding behavior of Na 2 S n intermediates . V, Fe, and Ni were utilized as SACs, and the V@MoS 2 is found to have superior performance in terms of Na 2 S n adsorption strengths, SRR, and catalytic oxidative decomposition of Na 2 S among the investigated examples.…”

“…We insert a V atom in the (5 × 5) supercell of WSe 2 and perform DFT calculations to determine the adsorption energies of polysulfides. Previous studies suggest that the metal top site is the most energetically favorable site for the TM SACs, and therefore, in this study, we only considered the W top site for V insertion …”

“…Compared to Li 2 S, the decomposition barrier for Na 2 S is lower (2.44 eV), yet it is high enough to facilitate the decomposition reaction. There have already been a significant number of theoretical studies on Na–S and Li–S batteries, in which 2D materials such as graphene, h-BAs, TMDs, MXenes, and so on have been shown to reduce the decomposition barrier effectively. ,,,,− In this section, one of our purposes is to examine and compare the effectiveness of the studied AMs in lowering the decomposition barrier in Na–S and Li–S systems. Since V-doping has already been proven to be highly effective in reducing the decomposition barrier in other 2-D materials, it is anticipated that V@WSe 2 will be more effective than pristine WSe 2 for both systems …”

“…87 Additionally, we previously used DFT computations to investigate first-row TMs as SACs supported on MoS 2 to evaluate the binding behavior of Na 2 S n intermediates. 91 V, Fe, and Ni were utilized as SACs, and the V@MoS 2 is found to have superior performance in terms of Na 2 S n adsorption strengths, SRR, and catalytic oxidative decomposition of Na 2 S among the investigated examples. Considering WSe 2 and MoS 2 have similar structural and electrical configurations, we chose V as the SAC for this comparative study.…”