Vacancy-mediated lithium adsorption and diffusion on MXene

Two-dimensional (2D) transition metal carbides and/or nitrides (MXenes), promising electrode materials for Li-ion batteries (LIBs), have recently been confirmed the formation of vacancies during etching process. However, the specific influence of vacancies on the performance of MXene-based LIBs and the corresponding mechanism still remain open issues. Herein, we highlighted that compared with other well-studied 2D materials (such as graphene, 2D MoS₂), MXene are more likely to contain vacancies with defect formation energies as low as 0.96 eV. The difference of vacancy formation energy for MXenes mainly originates from the variation of M-X bonding strengths. Furthermore, the vacancy defect will enhance the adsorption of Li in the region near the defect through acting as a potential trap, which may cause the first cycle irreversibility for lithiation/delithiation process in Mo₂C MXene LIBs. Nevertheless, Li tends to migrate on defect-free surface of Mo₂C MXene with energy barrier of 0.096–0.115 eV and thus this potential trap has a minor influence on the charge-discharge rate of Mo₂C MXene LIBs. We believe that our findings provide not only some insights into the recent experimental results but also guidance for the preparation and practical application of MXene in LIBs.