3D ordered amorphous and porous TiO2 framework anode with low insertion barrier and fast kinetics for K-ion hybrid capacitors.

TiO2 is considered as a low cost, long-term stable, and safe anode for high power K-ion hybrid capacitors (KICs) due to its abundant reserve, small volume expansion rate, and sloping voltage plateau that avoids K-ion plating at high voltage polarization. However, the enhancement of its low capacity and sluggish kinetics caused by poor electroconductivity and high insertion barrier is still challenging to further develop high-performance KICs. Herein, the reduced graphene oxide (rGO) is embedded in the walls of 3D ordered macro-/mesoporous TiO2 (termed as TiO2@rGO framework) to create intimate TiO2/rGO interfaces, ensuring the effectively electron transportation during potassiation/depotassiation of TiO2 while maintaining rapid ions/electrolyte diffusion. Furthermore, the controlled amorphous TiO2 framework can further lower the lattice insertion energies, contributing to a fast accommodation of K-ion. As expected, the amorphous TiO2@rGO framework (TiO2@rGO-1) exhibits a superior rate capability (148.8 mAh g-1 at 5 A g-1) and cycling stability (171.2 mAh g-1 at 1 A g-1 after 800 cycles). The assembled KICs can reach a high energy/power density of 125.2 Wh kg-1/4267.4 W kg-1 as well as a long-term lifespan. This tactic provides a reliable and general way to design a TiO2-based anode with fast kinetics toward high-performance KICs.