In-depth Insight into the Effects of Oxygen Vacancies on the Excellent Li-storage Performances of CuNbO/N-doped Carbon Composite

Wadsley-Roth phase niobates demonstrate significant Li-storage advantages. Even though their Nb can fully transform into Nb during lithiation process, however, only partial Nb can further convert to Nb, leading to much lower practical capacities than the theoretical values according to the two-electron transfer per Nb. The specific mechanism to improve their conversion ratio of Nb to Nb during lithiation process has rarely been reported so far. Herein, the ultrafine oxygen-deficient CuNbO nanoparticles are closely connected by the N-doped carbon-based 3D conductive framework to form a cloud-like CuNbO/N-doped carbon composite (denoted as VU-CNO-NC) with nanoaggregate structure and porous structure. Based on density functional theory (DFT) calculations and ex situ X-ray photoelectron spectrometer (XPS), the oxygen vacancies in VU-CNO-NC can catalyze the conversion of Nb to Nb during lithiation process, which significantly enhance the conversion ratio of Nb to Nb to generate much higher capacity. This effect of oxygen vacancies has rarely been reported so far. Moreover, the oxygen vacancies, ultrafine primary nanoparticles, 3D conductive framework, porous structure, and nanoaggregate structure synergistically endow VU-CNO-NC with fast Li-storage kinetics and highly stable structure. Consequently, VU-CNO-NC not only shows high capacity (287 mAh g after 500 cycles at 1 C and 181 mAh g after 1000 cycles at 10 C) and excellent rate performance as anode material of lithium-ion batteries, but also endows hybrid lithium-ion capacitor with high energy density (126 Wh kg at 175 W kg) and remarkable capacity retention (87.3 % after 9000 cycles at 2 A g), demonstrating great application prospect.