Structural Transformation of MXene (V2C, Cr2C, and Ta2C) with O Groups during Lithiation: A First-Principles Investigation.

For high capacities and extremely fast charging rates, two-dimensional (2D) crystals exhibit a significant promising application on lithium-ion batteries. With density functional calculations, this paper systematically investigated the Li storage properties of eight 2D M2CO2 (M = V, Cr, Ta, Sc, Ti, Zr, Nb, and Hf), which are the recently synthesized transition-metal carbides (called MXenes) with O groups. According to whether the structural transformation occurs or not during the adsorption of the first Li layer, the adsorption of Li can be grouped into two types: V-type (V2CO2, Cr2CO2, and Ta2CO2) and Sc-type (Sc2CO2, Ti2CO2, Zr2CO2, Nb2CO2, and Hf2CO2). The structural transformation behaviors of V-type are reversible during lithiation/delithiation and are confirmed by ab initio molecular dynamic simulations. Except for Nb-MXene, the V-type prefers the sandwich H2H1T-M2CO2Li4 structure and the Sc-type prefers the TH1H2-M2CO2Li4 structure during the adsorption of the second Li layer. The H2H1T-M2CO2Li4 structure of O layer sandwiched by two Li layers preferred by V-type can prevent forming Li dendrite and therefore stabilize the lithiated system. The tendency of O bonding to Li rather than M in V-type is bigger than that in Sc-type, which causes that the sandwich structure of H2H1T-M2CO2Li4 is more suitable for V-type than Sc-type.