TiC₂: a new two-dimensional sheet beyond MXenes

MXenes are attracting attention due to their rich chemistry and intriguing properties. Here a new type of metal–carbon-based sheet composed of transition metal centers and C₂ dimers rather than individual C atom is designed. Taking the Ti system as a test case, density functional theory calculations combined with a thermodynamic analysis uncover the thermal and dynamic stability of the sheet, as well as a metallic band structure, anisotropic Young's modulus and Poisson's ratio, a high heat capacity, and a large Debye stiffness. Moreover, the TiC₂ sheet has an excellent Li storage capacity with a small migration barrier, a lower mass density compared with standard MXenes, and better chemical stability as compared to the MXene Ti₂C sheet. When Ti is replaced with other transition metal centers, diverse new MC₂ sheets containing CC dimers can be formed, the properties of which merit further investigation.