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 (Li2Sen) into the electrolyte (known as shuttle effect) and inactive deposition of lower order polyselenides. The high electrical conductivity and mechanical strengths of MXenes make them a suitable candidate to provide adequate anchoring to prevent polyselenides dissolution and improved electrochemical performance. Herein, we used density functional theory (DFT) calculations to understand the binding mechanism of Li2Sen on graphene and surface functionalized Ti3C2 MXenes. We used graphene as reference material to assess Li2Sen binding strengths on functionalized Ti3C2X2 (where X = S, O, F, and Cl). We observed that Ti3C2S2 and Ti3C2O2 exhibit superior anchoring behavior compared to graphene, Ti3C2F2, and Ti3C2Cl2. The calculated Li2Sen adsorption strength provided by S and O terminated Ti3C2 are stronger than the commonly used ether-based electrolyte, which is a requisite for effective suppression of the Li2Sen shuttling. The adsorbed Li2Sen on Ti3C2X2 and graphene retains their structural integrity without a chemical decomposition. The density of states (DOS) analysis exhibits that the conductive behavior of the Ti3C2X2 is preserved even after Li2Sen adsorption, which can stimulate the electrochemical activity of involved Li2Sen chemistry. Based on our unprecedented results, Ti3C2S2 and Ti3C2O2 are found to exhibit superior anchoring behavior for Li2Sen adsorption, which can be leveraged for designing effective selenium-based cathode materials to boost the electrochemical performance of the Li-Se battery system