Adsorption mechanism of sodium polysulfide clusters on selenium-doped Ti2CO2 MXenes for application in sodium-sulfur batteries

Abstract

Room-temperature sodium-sulfur batteries show great potential for future energy storage systems; however, challenges such as the shuttle effect and poor conductivity hinder their practical application. The shuttle effect not only leads to energy loss but also diminishes the electrochemical performance of the batteries. The movement of these polysulfides can result in a gradual decrease in battery capacity, making it difficult to maintain consistent performance over time. Additionally, low conductivity can hinder charge transfer, slowing reaction kinetics and further reducing overall battery efficiency. One effective approach to address these issues is to use two-dimensional (2D) MXenes as electrode anchoring materials, which can help suppress the shuttle effect and enhance the electronic conductivity of sodium-sulfur batteries. This study investigates the effects of doping selenide atoms into 2D MXenes using first-principles methods to improve the stability and electronic properties of sodium-sulfur batteries. The selenide atoms are introduced into the termination layer to capture sodium polysulfide clusters. Our findings indicate that by doping with selenide atoms, the interaction between the Se-4p and S-3p orbitals enhances the ability of Ti₂CO₂ MXenes to adsorb Na₂S and Na₂S₂ clusters compared to the pristine systems. We provide a detailed discussion of the bonding mechanism between the Na₂S_x clusters and the selenide-doped MXenes. Furthermore, we highlight the differences in adsorption mechanisms between low-sulfur content (Na₂S, Na₂S₂, and Na₂S₄) and high-sulfur content (Na₂S₆ and Na₂S₈) clusters, focusing on charge transfers and electronic properties. The distinctive structure of MXenes allows them to interact effectively with polysulfides, which can suppress the shuttle effect, thereby preventing polysulfide migration and reducing energy loss. Moreover, the enhanced conductivity provided by MXenes facilitates improved charge transfer, leading to superior overall performance in sodium-sulfur batteries. Our results emphasize the critical role of selenide atoms in 2D MXene electrode materials, enhancing the adsorption mechanism of sodium polysulfides for their application in sodium-sulfur rechargeable batteries.