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

Multiple unfavorable features, such as poor electronic conductivity of sulfur cathodes, the dissolution and shuttling of sodium polysulfides (NaS) in electrolytes, and the slower kinetics for the decomposition of solid NaS, make sodium-sulfur batteries (NaSBs) impractical. To overcome these obstacles, novel double-transition metal (DTM) MXenes, MoTiCT, (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 MoTiCS and MoTiCO as AMs by analyzing their interactions with S/NaS ( = 1, 2, 4, 6, and 8). MoTiCS provides moderate adsorption strength compared to MoTiCO, therefore, it is expected to effectively inhibit NaS dissolution and shuttling without causing decomposition of NaS. 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 MoTiCT during discharge. Additionally, both MoTiCS and MoTiCO demonstrated effective electrocatalytic activity for the decomposition of NaS, with a substantial reduction in the energy barrier to 1.59 eV for MoTiCS and 1.67 eV for MoTiCO. While MoTiCO had superior binding properties, structural distortion is observed in NaS, 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, MoTiCS can be considered as an effective AM for suppressing the shuttle effect and improving the performance of NaSBs.