Nanoporous Nb₂O₅ with an Amorphous Structure for the Application as a Binder-Free Anode of a Hybrid Li-Ion Capacitor

The performance of hybrid Li-ion capacitors (LICs), which aim for high energy density while maintaining high power density, declines owing to the slow kinetics of battery-type anodes. In this study, the performance of LICs was improved by directly forming a nanoporous Nb 2 O 5 anode on a niobium (Nb) current collector through the etching of Nb via electrochemical anodization. This binder-free structure provides a large active surface and fast electron transport. In addition, the formed Nb 2 O 5 anode had an amorphous structure with many defects, which increased the number of active sites and the conductivities of Li ions and electrons. Consequently, the amorphous nanoporous Nb 2 O 5 (a-n-Nb 2 O 5 ) anode exhibited a high specific capacity of 233 mA h g –1 at 1 A g –1 and an excellent capacity retention rate of 92% after 1000 cycles. The charge storage of the a-n-Nb 2 O 5 anode exhibited pseudocapacitive intercalation behavior, demonstrating the rapid kinetics of the electrode. Moreover, the a-n-Nb 2 O 5 anode exhibited an excellent energy density, maintaining a high energy density of 123.8 W kg –1 at 831 W h kg –1 .