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

The shuttling of soluble sodium polysulfides (Na2Sn) 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 Na2Sn 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 Na2Sn 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 Na2Sn. 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 Na2Sn-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.