Functionalized Hf₃C₂ and Zr₃C₂ MXenes for suppression of shuttle effect to enhance the performance of sodium–sulfur batteries

Sodium-sulfur batteries show great potential for storing large amounts of energy due to their ability to undergo a double electron-redox process, as well as the plentiful abundance of sodium and sulfur resources. However, the shuttle effect caused by intermediate sodium polysulfides (Na₂S_n) limits their performance and lifespan. To address this issue, here we propose using Hf₃C₂T₂ and Zr₃C₂T₂ (T = F, O), two functionalized MXenes, as cathode additives to suppress the shuttle effect. By using density-functional theory calculations, we investigate nature of the interactions between Na₂S_n and MXene, such as the strength of adsorption energy, the electronic density of states, the charge exchange, and the dissociation energy of the Na₂S molecule. Our findings show that both Hf₃C₂T₂ and Zr₃C₂T₂ systems inhibit the shuttle effect by binding to Na₂S_n with a binding energy stronger than the commonly used electrolyte solvents. These MXenes retain their metallicity during this process and the decomposition barrier for Na₂S_n on the oxygen-functionalized MXenes gets reduced which enhances the electrochemical process. Among the MXene systems studied, Zr₃C₂O₂ shows the best performance in suppressing the shuttle effect and catalyzing the electrochemistry process and, thus, increasing the battery's reversible capacity and lifespan.