Single-Atom Electrocatalyst for Engineered Cathode Interfaces in Sodium-Sulfur Batteries
Authors: Islam Mahbub , Jayan Rahul 
Published: July 2022

The demand for portable rechargeable energy storage devices is ever increasing, especially because of the advent of electric vehicles and widespread usage of portable electronics. The lithium-ion batteries are currently leading the battery market; however, the high-cost and potentially depleting storage of lithium metals are stimulating the search for alternative technologies. Metal-sulfur batteries are deemed to be promising candidates to supplant the ubiquitously used lithium-ion batteries owing to their high energy density, specific capacity, low cost of sulfur, and environmental benignity. Room temperature sodium sulfur batteries (RT Na-S) is a technologically viable alternative candidate which possesses astounding advantages such as low cost (both sodium and sulfur), non–toxicity, natural abundance, and high theoretical energy density (1274 Whkg-1). However, the inevitable problems such as the solubility of higher order polysulfides to the electrolyte, known as shuttle effect, and the slow kinetics of electrochemical conversion reactions of intermediate sodium polysulfides (Na2Sn) significantly impede the practical realization of Na-S batteries. The conventionally used various forms of carbonaceous nanomaterials for cathode design have floundered to overcome the challenges because their nonpolar nature cannot produce adequate anchoring and enhanced polysulfides reaction kinetics. The polar anchoring materials (AM) have exhibited promising performance to improve sulfur chemistry. It is generally understood that catalytic performance is directly connected to the surface area of catalytic particles, and the single-atomic level provides the maximum surface area, resulting in the highest catalytic efficacy. Herein, we use first principles-based density functional theory (DFT) simulations to investigate the interfacial interactions between Na2Sn and novel transition metal (TM) single-atom catalysts (SACs) embedded on nitrogen doped graphene and various lattice sites of transition metal chalcogenides (TMDC) (chalcogenides- and Metal-substitution, Metal-top sites). For example, the pristine and Mo-sub sites of MoS2 are found to be ineffective for efficient confinement of the polysulfides within the cathode material. We demonstrate that SACs on both S-site and Mo-top sites of MoS2 and on nitrogen doped graphene possess strong adsorption strength with the Na2Sn which are superior to the commonly used ether electrolyte solvents, a requisite to prevent shuttle effect. We illustrate the influence of d-band center of SACs as an important descriptor in describing Na2Sn interactions with them. The underlying anchoring mechanism of polysulfide adsorption over AM is examined through Bader charge, charge density difference and projected density of states (PDOS) analysis. We also investigate the effect of SACs in improving the kinetics of sulfur reduction reactions (SRRs) and catalytic decomposition of short-chain polysulfides which are crucial for achieving excellent rate capability and longer cycle life. Overall, the unprecedented insights obtained on the role of SACs in tailoring the polysulfides redox chemistry at the interfaces and their relation to their TM’s d-band center is an important step towards rational design cathode materials for high-performance Na-S batteries, in particular, but metal-chalcogenide batteries, in general.