Nitride MXenes as sulfur hosts for thermodynamic and kinetic suppression of polysulfide shuttling: a computational study

Abstract

The practical applications of lithium–sulfur (Li–S) batteries are greatly hindered by the poor conductivity of sulfur, the shuttling of lithium polysulfides (LiPSs), and the sluggish kinetics in the charge–discharge process. In order to solve these problems, here we propose the surface-functionalized V₂N MXenes as the host materials to improve the electrochemical performance of Li–S batteries. Based on the density functional theory (DFT) calculations, we found that both the bare and functionalized V₂NT₂ (T = O, F, OH, and S) exhibit metallicity, and three of them (V₂NO₂, V₂NF₂, and V₂NS₂) possess moderate LiPS adsorption strength, which thermodynamically benefits the suppression of the dissolution and shuttling of LiPSs. Besides, V₂NS₂ shows the lowest Gibbs free energy barrier for the sulfur reduction reaction (0.49 eV) during discharge, which kinetically suppresses the dissolution and shuttling of LiPSs by expediting the decomposition process from soluble LiPSs to insoluble ones. Moreover, surface functionalized V₂NT₂ also exhibits outstanding catalytic ability for Li₂S decomposition during charge, which decreases the energy barrier from 3.64 eV (bare V₂N) to 1.55 (V₂NO₂) and 1.19 eV (V₂NS₂), and increases the charging kinetics. Based on these results, V₂NS₂ monolayers are suggested as promising host materials for S cathodes due to the fast charge/discharge kinetics and effective suppression of LiPS shuttling. This theoretical study provides further insight into the application of nitride MXenes and other two-dimensional materials as conductive anchoring materials for Li–S batteries.