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 Nb2O5 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 Nb2O5 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 Nb2O5 (a-n-Nb2O5) 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-Nb2O5 anode exhibited pseudocapacitive intercalation behavior, demonstrating the rapid kinetics of the electrode. Moreover, the a-n-Nb2O5 anode exhibited an excellent energy density, maintaining a high energy density of 123.8 W kg–1 at 831 W h kg–1.