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 (V_C) tend to enhance the adsorption of Li in Ti₂C monolayer, whereas the titanium vacancies (V_Ti) 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 V_C, 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 V_C, whereas in the presence of V_Ti, 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 V_Ti. 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.