Investigation of the Na Intercalation Mechanism into Nanosized V₂O₅/C Composite Cathode Material for Na-Ion Batteries

There is a significant interest to develop high-performance and cost-effective electrode materials for next-generation sodium ion batteries. Herein, we report a facile synthesis method for nanosized V₂O₅/C composite cathodes and their electrochemical performance as well as energy storage mechanism. The composite exhibits a discharge capacity of 255 mAh g^-1 at a current density of 0.05 C, which surpasses that of previously reported layered oxide materials. Furthermore, the electrode shows good rate capability; discharge capacity of 160 mAh g^-1 at a current density of 1 C. The reaction mechanism of V₂O₅ upon sodium insertion/extraction is investigated using ex situ X-ray diffraction (XRD) and synchrotron based near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Ex situ XRD result of the fully discharged state reveals the appearance of NaV₂O₅ as a major phase with minor Na₂V₂O₅ phase. Upon insertion of sodium into the array of parallel ladders of V₂O₅, it was confirmed that lattice parameter of c is increased by 9.09%, corresponding to the increase in the unit-cell volume of 9.2%. NEXAFS results suggest that the charge compensation during de/sodiation process accompanied by the reversible changes in the oxidation state of vanadium (V⁴⁺ 虠 V⁵⁺).