Functionalized MXenes as effective polyselenide immobilizers for lithium-selenium batteries: a density functional theory (DFT) study.

The practical applications of lithium selenium (Li-Se) batteries are impeded primarily due to the dissolution and migration of higher-order polyselenides (LiSe) into the electrolyte (known as the shuttle effect) and inactive deposition of lower-order polyselenides. The high electrical conductivity and mechanical strength of MXenes make them a suitable candidate to provide adequate anchoring to prevent polyselenide dissolution and improved electrochemical performance. Herein, we used density functional theory (DFT) calculations to understand the binding mechanism of LiSe on graphene and surface-functionalized TiC MXenes. We used graphene as a reference material to assess LiSe binding strengths on functionalized TiCX (where X = S, O, F, and Cl). We observed that TiCS and TiCO exhibit superior anchoring behavior compared to graphene, TiCF, and TiCCl. The calculated LiSe adsorption strengths, provided by S- and O-terminated TiC, are greater than those of the commonly used ether-based electrolyte, which is a requisite for effective suppression of LiSe shuttling. TiCX and graphene with adsorbed LiSe retain their structural integrity without chemical decomposition. Density of states (DOS) analysis demonstrates that the conductive behavior of TiCX is preserved even after LiSe adsorption, which can provide electronic pathways to stimulate the redox electrochemistry of LiSe. Overall, our unprecedented simulation results reveal superior anchoring behavior of TiCS and TiCO for LiSe adsorption, and this developed understanding can be leveraged for designing carbon-free TiC MXene-based selenium cathode materials to boost the electrochemical performance of Li-Se batteries.