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