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

Abstract

The practical applications of lithium selenium (Li–Se) batteries are impeded primarily due to the dissolution and migration of higher-order polyselenides (Li₂Se_n) 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 Li₂Se_n on graphene and surface-functionalized Ti₃C₂ MXenes. We used graphene as a reference material to assess Li₂Se_n 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 Li₂Se_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 Li₂Se_n shuttling. Ti₃C₂X₂ and graphene with adsorbed Li₂Se_n retain their structural integrity without chemical decomposition. Density of states (DOS) analysis demonstrates that the conductive behavior of Ti₃C₂X₂ is preserved even after Li₂Se_n adsorption, which can provide electronic pathways to stimulate the redox electrochemistry of Li₂Se_n. Overall, our unprecedented simulation results reveal superior anchoring behavior of Ti₃C₂S₂ and Ti₃C₂O₂ for Li₂Se_n adsorption, and this developed understanding can be leveraged for designing carbon-free Ti₃C₂ MXene-based selenium cathode materials to boost the electrochemical performance of Li–Se batteries.