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 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 Li(2)Se(n)on graphene and surface-functionalized Ti(3)C(2)MXenes. We used graphene as a reference material to assess Li(2)Se(n)binding strengths on functionalized Ti3C2X2(where X = S, O, F, and Cl). We observed that Ti(3)C(2)S(2)and Ti(3)C(2)O(2)exhibit superior anchoring behavior compared to graphene, Ti3C2F2, and Ti3C2Cl2. The calculated Li(2)Se(n)adsorption strengths, provided by S- and O-terminated Ti3C2, are greater than those of the commonly used ether-based electrolyte, which is a requisite for effective suppression of Li(2)Se(n)shuttling. Ti(3)C(2)X(2)and graphene with adsorbed Li(2)Se(n)retain their structural integrity without chemical decomposition. Density of states (DOS) analysis demonstrates that the conductive behavior of Ti(3)C(2)X(2)is preserved even after Li(2)Se(n)adsorption, which can provide electronic pathways to stimulate the redox electrochemistry of Li2Sen. Overall, our unprecedented simulation results reveal superior anchoring behavior of Ti(3)C(2)S(2)and Ti(3)C(2)O(2)for Li(2)Se(n)adsorption, and this developed understanding can be leveraged for designing carbon-free Ti(3)C(2)MXene-based selenium cathode materials to boost the electrochemical performance of Li-Se batteries.