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 (Nb⁵⁺/Nb⁴⁺ and Nb⁴⁺/Nb³⁺ 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 Nb₁₂O₂₉ materials with rich oxygen vacancies (Nb₁₂O_29-x@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 Nb₁₂O₂₉ material. As a result, the as-prepared Nb₁₂O_29-x@C exhibits high reversible capacity (226 mAh g^-1 at 0.1 A g^-1), excellent high-rate performance (83 mAh g^-1 at 5.0 A g^-1), and durable cycling life (98.1% capacity retention at 1.0 A g^-1 after 3000 cycles). The lithium storage mechanism and structural stability of Nb₁₂O_29-x@C were also revealed by in situ X-ray diffraction (XRD), ex situ X-ray photoelectron spectroscopy (XPS), and ex situ Raman spectroscopy. When applied as the anode of lithium-ion capacitors (LICs), the as-built LIC achieves high energy density (72.4 Wh kg^-1) within the voltage window of 0.01-3.5 V, demonstrating the practical application potential of the Nb₁₂O_29-x@C materials.