Flexible CNT@Porous carbon sponge cathode with large mesopores for high-rate zinc-ion hybrid capacitors

Zinc ion hybrid capacitor (ZIHC) is promising in achieving both high energy density and power density at a large scale. However, the poor rate capability of carbon-based cathodes limits the fast-charging performance of ZIHCs due to mismatch between micropores in the carbon matrix and large hydrated Zn ions. Herein, we prepare a flexible carbon nanotube@porous carbon (CNT@PC) sponge with large mesopores (average mesopore size of 14.3 nm), which can achieve a high specific capacity of 175.3 mAh/g (0.1 A/g) and a high rate capability of 93.1 mAh/g (50 A/g). The effects of pore size of PC cathodes on rate capability are investigated by various electrochemical methods. Large mesopores facilitate rapid ion transport with low IR drop and ion diffusion resistance, and high ion diffusion coefficient and surface capacitive contributions. CNT@PC sponge-based ZIHC delivers high energy densities of 150.8 Wh/kg (80 W/kg) and 74.4 Wh/kg (40 kW/kg). The flexible pouch cell can maintain high performance in various bending states. Multilayer stacked sponges show increased mass loading and high areal capacities. This work provides insight into the pore structure design for high-rate ZIHC PC cathodes.