Two-Dimensional Nb-Based M₄C₃ Solid Solutions (MXenes)

Herein, two new two-dimensional Nb₄C₃-based solid solutions (MXenes), (Nb_0.8,Ti_0.2)₄C₃T_x and (Nb_0.8,Zr_0.2)₄C₃T_x (where T is a surface termination) were synthesized - as confirmed by X-ray diffraction - from their corresponding MAX phase precursors (Nb_0.8,Ti_0.2)₄AlC₃ and (Nb_0.8,Zr_0.2)₄AlC₃. This is the first report on a Zr-containing MXene. Intercalation of Li ions into these two compositions, and Nb₄C₃T_x was studied to determine the potential of those materials for energy storage applications. Lithiation and delithiation peaks at 2.26 and 2.35 V, respectively, appeared in the case of Nb₄C₃T_x, but were not present in Nb₂CT_x. After 20 cycles at a rate of C/4, the specific capacities of (Nb_0.8,Ti_0.2)₄C₃T_x and (Nb_0.8,Zr_0.2)₄C₃T_x were 158 and 132 mAh/g, respectively, both slightly lower than the capacity of Nb₄C₃T_x.