Pseudocapacitive Sodium Storage in Mesoporous Single-Crystal-like TiO₂–Graphene Nanocomposite Enables High-Performance Sodium-Ion Capacitors

Abstract

Sodium-ion capacitors can potentially combine the virtues of high power capability of conventional electrochemical capacitors and high energy density of batteries. However, the lack of high-performance electrode materials has been the major challenge of sodium-based energy storage devices. In this work, we report a microwave-assisted synthesis of single-crystal-like anatase TiO₂ mesocages anchored on graphene as a sodium storage material. The architecture of the nanocomposite results in pseudocapacitive charge storage behavior with fast kinetics, high reversibility, and negligible degradation to the micro/nanostructure. The nanocomposite delivers a high capacity of 268 mAh g^-1 at 0.2 C, which remains 126 mAh g^-1 at 10 C for over 18 000 cycles. Coupling with a carbon-based cathode, a full cell of sodium-ion capacitor successfully demonstrates a high energy density of 64.2 Wh kg^-1 at 56.3 W kg^-1 and 25.8 Wh kg^-1 at 1357 W kg^-1, as well as an ultralong lifespan of 10 000 cycles with over 90% of capacity retention.