Synergetic interface engineering and space-confined effect in CoSe2@Ti3C2Tx heterostructure for high power and long life sodium ion capacitors

The intriguing characteristics of two-dimensional (2D) heterostructures stem from their unique interfaces, which can improve ion storage capability and rate performance. However, there are still challenges in increasing the proportion of heterogeneous interfaces in materials and understanding the complex interaction mechanisms at these interfaces. Here, we have successfully synthesized confined CoSe2 within the interlayer space of Ti3C2Tx through a simple solvothermal method, resulting in the formation of a superlattice-like heterostructures of CoSe2@Ti3C2Tx. Both density functional theory (DFT) calculations and experimental results show that compared with CoSe2 and Ti3C2Tx, CoSe2@Ti3C2Tx can significantly improve adsorption of Na+ ions, while maintaining low volume expansion and high Na+ ions migration rate. The heterostructure formed by MXene and CoSe2 is a Schottky heterostructure, and its interfacial charge transfer induces a built-in electric field that promotes rapid ion transport. When CoSe2@Ti3C2Tx was used as an anode material, it exhibits a high specific capacity of up to 600.1 mAh/g and an excellent rate performance of 206.3 mAh/g at 20 A/g. By utilizing CoSe2@Ti3C2Tx as the anode and activated carbon (AC) as the cathode, the sodium-ion capacitor of CoSe2@Ti3C2Tx//AC exhibits excellent energy and power density (125.0 Wh kg−1 and 22.5 kW kg−1 at 300.0 W kg−1 and 37.5 Wh kg−1, respectively), as well as a long service life (86.3 % capacity retention over 15,300 cycles at 5 A/g), demonstrating its potential for practical applications.