Rapid redox kinetics in uniform sandwich-structured mesoporous Nb2O5/graphene/mesoporous Nb2O5 nanosheets for high-performance sodium-ion supercapacitors

Sodium-ion hybrid supercapacitors have received great attracts for next-generation energy storage applications due to their high energy and power densities, as well as the abundant sodium resource. However, electrodes based on transition-metal oxides often suffer from low reversible capacity and sluggish redox kinetics, which seriously constrains the rate and cycling performance of the devices. Herein, a facile two-step hydrolysis synthesis is used to prepare uniform sandwich-like mesoporous Nb2O5/graphene/mesoporous Nb2O5 (G@mNb(2)O(5)) nanosheets as sodium storage materials. The mesoporous Nb2O5 layers on graphene are constructed by several nanometer-sized Nb2O5 particles. In virtue of the structural features, the G@mNb(2)O(5) nanosheets electrode demonstrates high-rate capacity (293 and 125 mA h g(-1) at 50 and 2000 mA g(-1), respectively) and stable cycling performance due to the rapid redox kinetics, including significantly increased surface pseudocapacitive contribution, improved sodium-ion diffusion coefficient, and short characteristic relaxation process. By employing activated carbon as cathode, a full sodium-ion hybrid device successfully demonstrates a high energy density of 56.1 Wh kg(-1) at 120 W kg(-1), and 9.7 Wh kg(-1) at 7200 W kg(-1), as well as a stable capacitance retention of similar to 89% at 1 A g(-1). The availability of capacitive Na-ion storage system presented here is attractive for cost-effective energy storage applications.