Nanoporous Nb<sub>2</sub>O<sub>5</sub> with an Amorphous\nStructure for the Application as a Binder-Free Anode of a Hybrid Li-Ion\nCapacitor

The performance of hybrid Li-ion capacitors (LICs), which\naim for\nhigh energy density while maintaining high power density, declines\nowing to the slow kinetics of battery-type anodes. In this study,\nthe performance of LICs was improved by directly forming a nanoporous\nNb<sub>2</sub>O<sub>5</sub> anode on a niobium (Nb) current collector\nthrough the etching of Nb via electrochemical anodization. This binder-free\nstructure provides a large active surface and fast electron transport.\nIn addition, the formed Nb<sub>2</sub>O<sub>5</sub> anode had an amorphous\nstructure with many defects, which increased the number of active\nsites and the conductivities of Li ions and electrons. Consequently,\nthe amorphous nanoporous Nb<sub>2</sub>O<sub>5</sub> (a-n-Nb<sub>2</sub>O<sub>5</sub>) anode exhibited a high specific capacity of 233 mA\nh g<sup>–1</sup> at 1 A g<sup>–1</sup> and an excellent\ncapacity retention rate of 92% after 1000 cycles. The charge storage\nof the a-n-Nb<sub>2</sub>O<sub>5</sub> anode exhibited pseudocapacitive\nintercalation behavior, demonstrating the rapid kinetics of the electrode.\nMoreover, the a-n-Nb<sub>2</sub>O<sub>5</sub> anode exhibited an excellent\nenergy density, maintaining a high energy density of 123.8 W kg<sup>–1</sup> at 831 W h kg<sup>–1</sup>.