A systematic computational investigation of lithiation-induced structural phase transitions of O-functionalized MXenes.

Along with Li-ion extraction/intercalation during charge and discharge processes, structural phase transitions often occur in the electrode materials of Li-ion batteries (LIBs). By determining atomic positions before and after Li adsorptions, structural phase transitions of two-dimensional MXenes were investigated systematically using first-principles density functional calculations. The lithiation-induced phase transitions of ten MC MXenes with oxygen groups can be divided into three types. No phase transitions occur for Ti-type MXenes including TiCO, ZrCO and HfCO. The oxygens in Ta-type MXenes (ScCO, YCO, NbCO and TaCO) move from one type of octahedral void to another type of octahedral void. However, for Mo-type MXenes including VCO, CrCO and MoCO, the oxygens move from octahedral voids to tetrahedral voids. The mechanisms whether phase transitions happen or not are dependent on the sizes of M ions. Furthermore, all the predicted phase transitions were confirmed by molecular dynamics simulations. The calculated results of electron localization functions and Bader charge illustrate that there exist strong Coulomb interactions (ionic bonds) between Li and MXene surfaces. The band structure, diffusion energy barrier, open circuit voltage and storage capacity were calculated to evaluate the lithium storage properties of different MXenes, which reveals that VCO and CrCO should be optimal candidates as electrode materials for LIBs.