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 (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.

Efficient Control of the Shuttle Effect in Sodium–Sulfur Batteries with Functionalized Nanoporous Graphenes

“…Furthermore, it is beneficial to compare the role of O-doped and N-doped NPG to other 2D materials in catalyzing Na 2 S decomposition in RT-NaSBs. The obtained barriers are 0.83 and 0.51 eV for O-doped and N-doped NPG, respectively, significantly lower than 1.61 eV for N-doped graphene, 1.59 and 1.67 eV for MXenes-type Mo 2 TiC 2 S 2 and Mo 2 TiC 2 O 2 , respectively, and 1.51 eV for Ti 2 CS 2 . The remarkable reduction in the doped NPG can be attributed to the enhanced polarity of the O and N dopants, which facilitates the weakening of Na–S bonds …”

Atomic-Scale Insights into Comparative Mechanisms and Kinetics of Na–S and Li–S Batteries

“…It is well known that PBE/GGA functional underestimates the band gap properties and based on the nature of the TM doping, DFT + U calculations were performed to consider the d electrons involved in the adsorption energy calculations. From our previous reports on 2D materials such as MoS 2 , VS 2 , and Mo 2 TiC 2 comprising TM atoms, we found a negligible change in the adsorption energies after the inclusion of DFT + U , and the binding energies calculated by GGA/PBE functional was adequate in explaining the adsorption characteristics. ,, Hence, we believe that the chosen PBE/GGA is well sufficient for elucidating both adsorption and electronic properties.…”

“…From our previous reports on 2D materials such as MoS 2 , VS 2 , and Mo 2 TiC 2 comprising TM atoms, we found a negligible change in the adsorption energies after the inclusion of DFT + U, and the binding energies calculated by GGA/PBE functional was adequate in explaining the adsorption characteristics. 52,91,92 Hence, we believe that the chosen PBE/GGA is well sufficient for elucidating both adsorption and electronic properties.…”

Progress in the development of solid-state electrolytes for reversible room-temperature sodium–sulfur batteries

“…73 The process is known as the shuttling effect. [74][75][76] Furthermore, the LE with a narrow electrochemical window is prone to produce an unstable SEI, as the electrolyte oxidizes at the anode, thereby reducing the capacity and Coulombic efficiency of the cell. [77][78][79] As solid electrolytes are free from organic liquids, the bottleneck issue of polysulfide dissolution and the shuttling effect is mitigated; this enhances solid-state batteries' Coulombic efficiency and maximizes capacity retention.…”