Room temperature sodium−sulfur (Na−S) batteries, because of their high theoretical energy density and low cost, are considered as a promising candidate for next-generation energy storage devices. However, the practical utilization of the Na−S batteries is greatly hindered by various deleterious factors such as dissolution of sodium polysulfides (Na 2 S n ) into the electrolyte commonly termed as "shuttle effect," sluggish decomposition of solid Na 2 S, and poor electronic conductivity of sulfur. To overcome the challenges, we introduced single-layer vanadium disulfide (VS 2 ) as an anchoring material (AM) to immobilize higher-order polysulfides from the dissolution and also to accelerate the otherwise sluggish kinetics of insoluble short-chain polysulfides. We employ density functional theory (DFT) calculations to elucidate the Na 2 S n interactions at the VS 2 interfaces. We show that the adsorption strengths of various Na 2 S n species on the VS 2 basal plane are adequate (1.21−4.3 eV) to suppress the shuttle effect, and the structure of Na 2 S n are maintained without any decomposition, which is necessary to mitigate capacity fading. The calculated projected density of states (PDOS) reveals that the metallic character of the pristine VS 2 is retained even after Na 2 S n adsorption. The calculated Gibbs free energy of each elementary sulfur reduction reaction indicates a significant decrement in the free energy barrier due to the catalytic activity of the VS 2 surface. Furthermore, VS 2 is found to be an excellent catalyst to significantly reduce the oxidative decomposition barrier of Na 2 S, which facilitates accelerated electrode kinetics and higher utilization of sulfur. Overall, VS 2 with strong adsorption behavior, enhanced electronic conductivity, and improved oxidative decomposition kinetics of polysulfides can be considered as an effective AM to prevent the shuttle effect and to improve the performance of Na−S batteries.

The room-temperature sodium–sulfur (Na–S) batteries have attracted remarkable attention because of their promise to deliver high-capacity and low-cost earth-abundant sodium and sulfur. However, the practical development of Na–S batteries is hindered due to multiple challenges, including rapid capacity fading stems from the dissolution of intermediate sodium polysulfides (Na2S n ) and slow kinetics of electrochemical conversion reactions. In this study, we introduced novel transition-metal single-atom catalysts (SACs) on nitrogen-doped graphene (NG) to impede the dissolution of higher-order Na2S n and improve otherwise sluggish kinetics of short-chain polysulfides. The density functional theory (DFT) calculations are used to elucidate the detailed interactions of the polysulfides on the SACs. The pristine and nitrogen-doped graphenes are also considered and found to provide ineffective anchoring for trapping polysulfides. However, the SACs embedded via monodispersed transition-metal atoms in NG (TM-NG where TM = Cr, Fe, and Co) exhibit adequate binding strength toward Na2S n species. The calculated adsorption energies of soluble Na2S n on SACs are superior compared to that of commonly used ether-type electrolyte solvents; thus, the SACs are predicted to serve as effective immobilizers for soluble Na2S n to prevent shuttling. The enhanced binding strength for TM-doped substrates arises from the strong TM–S covalent interactions. The density of state (DOS) calculations reveal that both the pristine and polysulfide adsorbed TM-NG exhibits metallic behavior and illustrate the mechanisms of stronger polysulfide interactions originated from the hybridization of TM-3d and S-2p orbitals. Furthermore, the electron-deficient SACs are found to substantially reduce the Na2S decomposition barrier, which demonstrates effective electrocatalysis in favor of complete reversible conversion of polysulfides. Overall, the effectiveness of the SACs on preventing shuttle effect and improving the kinetics of electrocatalytic conversion of polysulfides unraveled in this study will lead to a paradigm shift in developing advanced Na–S batteries with highly efficient electrocatalysts for Na2S n conversion.

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.

Sulfur and oxygen functionalized MXenes are promising anchoring materials to inhibit polyselenides shuttling in Li–Se batteries.