Rapid microwave synthesis of carbon-bridged Nb2O5 mesocrystals for high-energy and high-power sodium-ion capacitors

Sodium-ion capacitors (SICs) have been considered as low-cost candidates for hybrid energy-storage devices with high power/energy densities. However, kinetics imbalance at high rates between battery-type anodes and capacitor-type cathodes is a significant challenge. Here, we report the synthesis of carbon-bridged Nb2O5 mesocrystals (meso-Nb2O5@C) for sodium storage with ultrafast kinetics through a fast microwave-assisted method and subsequent heat treatment. Such a unique meso-Nb2O5@C electrode exhibits ultrafast sodium storage capability, benefiting from superior electronic conductivity by carbon coating/bridging and fast redox kinetics by mesocrystalline design with abundant boundaries and a uniform nanocrystalline orientation. Furthermore, the long-term cyclability of meso-Nb2O5@C (80.5% capacity retention upon 10 000 cycles at 20C) verifies high structural stability of meso-Nb2O5@C. The as-assembled SIC with a meso-Nb2O5@C anode and an activated carbon cathode displays outstanding electrochemical performance with a high energy density of 28.4 W h kg(-1) at 15 600 W kg(-1). This work demonstrates that engineering carbon-bridged mesocrystals offers an unexpected route towards low-cost, high-energy, and high-power energy storage devices.