Synergetic interface engineering and space-confined effect in CoSe@TiCT 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 CoSe within the interlayer space of TiCT through a simple solvothermal method, resulting in the formation of a superlattice-like heterostructures of CoSe@TiCT. Both density functional theory (DFT) calculations and experimental results show that compared with CoSe and TiCT, CoSe@TiCT can significantly improve adsorption of Na ions, while maintaining low volume expansion and high Na ions migration rate. The heterostructure formed by MXene and CoSe is a Schottky heterostructure, and its interfacial charge transfer induces a built-in electric field that promotes rapid ion transport. When CoSe@TiCT 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 CoSe@TiCT as the anode and activated carbon (AC) as the cathode, the sodium-ion capacitor of CoSe@TiCT//AC exhibits excellent energy and power density (125.0 Wh kg and 22.5 kW kg at 300.0 W kg and 37.5 Wh kg, 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.