High-power lithium-ion capacitor using orthorhombic Nb₂O₅ nanotubes enabled by cellulose-based electrospun scaffolds

Orthorhombic Nb₂O₅-based nanoarchitectures have shown promise as electrode materials for Li-ion capacitors because they improve lithium ion transport and conductivity of Nb₂O₅ with high theoretical capacity. However, despite the several advantages of nanotubes, the facile synthesis of Nb₂O₅ nanotubes remains challenging. Herein, we present Nb₂O₅ nanotubes as an efficient ion- and electron-conducting electrode using environmentally friendly cellulose as sacrificial templates. The rational synthetic design based on sol–gel interactions with hydroxyl groups on the cellulosic surface enables uniformly covering the electrospun-cellulose one-dimensional templates with Nb₂O₅, resulting in unusual nanotubular Nb₂O₅ crystals featuring crystallographic iso-orientation owing to a controlled two-step calcination. Such crystallographically oriented nanotubes are favorable to fast Li⁺ intercalation kinetics for pseudocapacitive behavior with efficient lithium ion channels. The orthorhombic Nb₂O₅ nanotubes provide electrodes with a high initial capacity (163 mAh g⁻¹), excellent rate capability (95 mAh g⁻¹ at 5 A g⁻¹), and stable cycle performance (81.8% retention after 1000 cycles at 5 A g⁻¹) without any carbon composite and are thus superior to previously reported Nb₂O₅ electrodes. This simple strategy can open a new avenue for fabricating nanotubes of various transition metal oxides.