Rapid Redox Kinetics in Uniform Sandwich-Structured Mesoporous Nb 2 O 5 /Graphene/Mesoporous Nb 2 O 5 Nanosheets for High-Performance Sodium-Ion Supercapacitors

Abstract 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 Nb 2 O 5 /graphene/mesoporous Nb 2 O 5 (G@mNb 2 O 5 ) nanosheets as sodium storage materials. The mesoporous Nb 2 O 5 layers on graphene are constructed by several nanometer-sized Nb 2 O 5 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 ∼ 89% at 1 A g −1 . The availability of capacitive Na-ion storage system presented here is attractive for cost-effective energy storage applications.