Double Transition Metal Ti2NbC2Tx MXene with Modulated Electronic Structure for Advanced Lithium-ion Capacitors

Double transition metal (DTM) MXenes are garnering increasing attention owe to their wide diversity, controllable properties, tunable electronic structure and surface chemistry. Nevertheless, research on the exciting DTM MXenes is still in its infancy. Herein, we present a novel out-of-plan ordered DTM MXene, Ti₂NbC₂T_x, achieved by introducing Nb species into the M (Ti) site of Ti₃C₂T_x MXene. In which, Ti and Nb atoms occupy the outer and middle transition metal layers, respectively. This structure endows the as-synthesized Ti₂NbC₂T_x MXene with significantly higher chemical affinity and absorbability for lithium ions than Ti₃C₂T_x, showing a high reversible capacity of up to 272 mAh g^-1 at 0.1 A g^-1 and exceptional long-term stability (no capacity loss after 1000 cycles). Moreover, the lithium-ion capacitors (LICs) assembled with Ti₂NbC₂T_x MXene anode and activated carbon (AC) cathode exhibit high energy and power densities of 39 Wh Kg^-1 and 4600 W kg^-1, respectively, surpassing the most state-of-the-art MXene-based LICs. This work demonstrates the significant potential of DTM MXenes in advancing energy storage applications.