Oxygen defected T-Nb2O5-x confined in necklace-like N-doped carbon fibers for Li+/Na+ capacitor

Optimizing the transmission dynamics and stability of orthorhombic (T-) Nb 2 O 5 is the development direction of hybrid supercapacitors. In this work, Necklace-like T-Nb 2 O 5-x @NC assembled by Yolk-Shell nanosphere is applied for self-supporting anode of the Li + /Na + capacitor. Each Yolk-Shell T-Nb 2 O 5-x @NC unit effectively alleviates the volume effect of Nb 2 O 5 during Li + /Na + embedding repeatedly, thus achieving good structural stability. In addition, oxygen vacancy optimizes the transmission barrier of Li + /Na + and its conductivity. T-Nb 2 O 5-x @NC units are stringed along the carbon fibers, which are further interconnected to form integrated 3D conductive network for excellent electron conductivity. As a result, T-Nb 2 O 5-x @NC-7, as the optimal sample, has high specific capacity (up to 208.3 mAh g −1 at 0.1 A g −1 in Li half-cell) and long cycle life (92.3% after 2000 cycles in Li capacitor). Also, it exhibits superior electrochemical performance in Na energy storage (84.2% after 2000 cycles). • Yolk-shell structure greatly relieved the volume expansion effect. • Oxygen vacancy reduces the ion transport barrier. • The independent unmatched electrons generated optimize its conductivity. • Carbon interconnected network improves electron conductivity.