Revealing the binding mechanism of redox intermediates in sodium–sulfur batteries by two-dimensional Janus monolayers

Abstract

Ultrahigh theoretical energy density and naturally abundant electrode materials (i.e., sodium and sulfur) have rendered room-temperature sodium-sulfur batteries (Na-SBs) to be the emerging alternative for the large-scale applications. Nevertheless, rapid capacity decay caused by the shuttle effect, poor electrical conductivity of sulfur, and sluggish electrochemistry pose major obstacles to achieving commercial viability. Herein, we have proposed a new functionality of Janus-type transition metal dichalcogenides (TMDs) as cathode hosting materials to resolve the mentioned hindrances. Based on density functional theory (DFT), this work reports the interfacial interactions of sodium polysulfides (Na2Sn) and a series of Janus MSX (M = Mo, W and X = Se, Te), electronic properties, and the crucial parameters of the electrochemical reaction. Among MSX, we find that MoSTe binds with Na2Snvia chemical Na–S bonds causing the strongest binding energies (−1.18 to −2.48 eV) which are greater than do the electrolytes (−0.20 to −0.98 eV), thus effectively alleviating the shuttle effect. This immense binding is attributed to the magnified polar nature of MoSTe which intensifies the Na2Sn–MoSTe interaction. Moreover, the charge accumulation in MoSTe as donated by Na2Sn maximizes the electronic conductivity of MoSTe to improve the charge transport during the redox process. Importantly, this material facilitates the overall reversible electrochemical reactions by lowering the energy barriers of conversions among the redox intermediates in the sulfur reduction reaction (SRR), the decomposition barrier of final discharge product Na2S, and diffusion barrier of Na+ ions. Hence, Janus MoSTe offers the manifold benefits for boosting the efficiency of Na-SBs.