Novel Nonstoichiometric Niobium Oxide Anode Material with Rich Oxygen Vacancies for Advanced Lithium-Ion Capacitors.

Given the inherent features of open tunnel-like structures, moderate lithiation potential (1.0-3.0 V vs Li/Li), and reversible redox couples (Nb/Nb and Nb/Nb redox couples), niobium-based oxides with Wadsley-Roth crystallographic shear structure are promising anode materials. However, their practical rate capability and cycling stability are still hindered by low intrinsic electronic conductivity and structural stability. Herein, ultrathin carbon-confined NbO materials with rich oxygen vacancies (NbO@C) were designed and synthesized to address above-mentioned challenges. Computational simulations combined with experiments reveal that the oxygen vacancies can regulate the electronic structure to increase intrinsic electronic conductivity and reduce the Li diffusion barrier. Meanwhile, the carbon coating can enhance structural stability and further improve the electronic conductivity of the NbO material. As a result, the as-prepared NbO@C exhibits high reversible capacity (226 mAh g at 0.1 A g), excellent high-rate performance (83 mAh g at 5.0 A g), and durable cycling life (98.1% capacity retention at 1.0 A g after 3000 cycles). The lithium storage mechanism and structural stability of NbO@C were also revealed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. When applied as the anode of lithium-ion capacitors (LICs), the as-built LIC achieves high energy density (72.4 Wh kg) within the voltage window of 0.01-3.5 V, demonstrating the practical application potential of the NbO@C materials.