An oxygen-deficient vanadium oxide@N-doped carbon heterostructure for sodium-ion batteries: insights into the charge storage mechanism and enhanced reaction kinetics

Vanadium oxides are a class of promising anode candidates for sodium-ion batteries (SIBs) with high theoretical capacity and low cost. However, their practical application has been impeded by the low electronic conductivity, sluggish ionic transport and large volume change upon sodiation. Herein, a novel, high-performance anode for SIBs based on an oxygen-deficient vanadium oxide (VO/V2O3) heterostructure embedded in porous nitrogen-doped carbon (denoted as VNC) derived by a very simple and localized phase transition from a vanadium glycolate precursor has been demonstrated. The strong synergy of the hierarchically porous framework, VO/V2O3 phase heterojunctions with oxygen vacancy defects, and interfacial coupling with N-doped carbon (NC) efficiently promotes the electron/ion transport and alleviates the volume change during the sodiation/de-sodiation process. Electrochemical evaluation reveals that this novel VNC anode material manifests high sodium-ion storage capability and stability. Ex situ X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses show that (a portion of) the vanadium oxides can be converted into vanadium metal and sodium oxides. First-principles density functional theory (DFT) calculations reveal that coupling with NC effectively enhances the interfacial interactions and charge transfer between vanadium oxides and carbon, as well as the conversion reaction kinetics during sodiation/desodiation. The present work offers a viable strategy for the rational design and exploration of novel heterostructure composite electrodes for a wide array of beyond-lithium-ion batteries.