First-Principle Study of Li-Ion Storage of Functionalized Ti2C Monolayer with Vacancies.

Two-dimensional transition metal carbides are notable as promising anode materials for Li-ion batteries (LIBs). Using first-principle calculations, we investigate the effect of vacancies on the Li adsorption and diffusion on Ti2C and Ti2CT2 (where T denotes surface terminations, F or OH) monolayers. Interestingly, we find that the carbon vacancies (VC) tend to enhance the adsorption of Li in Ti2C monolayer, whereas the titanium vacancies (VTi) play a similar role in Ti2CT2 when functional groups present. The presence of vacancies further leads to a change in the diffusion behavior of Li atoms. In this context, we propose an idea to mitigate the adverse effects on Li diffusion performance by regulating the functional groups. In the presence of VC, the surface of Ti2C monolayer is suggested to be modified with OH- groups due to its relatively low diffusion barrier in the range of 0.025-0.037 eV when Li diffuses around VC, whereas in the presence of VTi, the surface is suggested to remove the functional groups, resulting in a decrease of energy barrier by about 1 eV when Li atom diffuses around VTi. The present study may provide a guideline to improve the Li-ion storage performance of Ti2C monolayers as electrode materials in LIBs, with atomic vacancies being taken into consideration.