Elucidating Synergistic Mechanisms of Adsorption and Electrocatalysis of Polysulfides on Double-Transition Metal MXenes for Na–S Batteries

Abstract

Multiple unfavorable features, such as poor electronic conductivity of sulfur cathodes, the dissolution and shuttling of sodium polysulfides (Na₂S_<i>n</i>) in electrolytes, and the slower kinetics for the decomposition of solid Na₂S, make sodium-sulfur batteries (NaSBs) impractical. To overcome these obstacles, novel double-transition metal (DTM) MXenes, Mo₂TiC₂T₂, (T = O and S) are studied as an anchoring material (AM) to immobilize higher-order polysulfides and to expedite the otherwise slower kinetics of insoluble short-chain polysulfides. Density functional theory (DFT) calculations are carried out to justify and compare the effectiveness of Mo₂TiC₂S₂ and Mo₂TiC₂O₂ as AMs by analyzing their interactions with S₈/Na₂S_<i>n</i> (<i>n</i> = 1, 2, 4, 6, and 8). Mo₂TiC₂S₂ provides moderate adsorption strength compared to Mo₂TiC₂O₂, therefore, it is expected to effectively inhibit Na₂S_<i>n</i> dissolution and shuttling without causing decomposition of Na₂S_<i>n</i>. The calculated Gibbs free energies of the rate-determining step for sulfur reduction reactions (SRR) are found to be significantly lower (0.791 eV for S and 0.628 eV for O functionalization) than that in vacuum (1.442 eV), suggesting that the SRR is more thermodynamically favorable on Mo₂TiC₂T₂ during discharge. Additionally, both Mo₂TiC₂S₂ and Mo₂TiC₂O₂ demonstrated effective electrocatalytic activity for the decomposition of Na₂S, with a substantial reduction in the energy barrier to 1.59 eV for Mo₂TiC₂S₂ and 1.67 eV for Mo₂TiC₂O₂. While Mo₂TiC₂O₂ had superior binding properties, structural distortion is observed in Na₂S_<i>n</i>, which may adversely affect cyclability. On the other hand, because of its moderate binding energy, enhanced electronic conductivity, and significantly faster oxidative decomposition kinetics of polysulfides, Mo₂TiC₂S₂ can be considered as an effective AM for suppressing the shuttle effect and improving the performance of NaSBs.