New Chemical Route for the Synthesis of β‑Na_0.33V₂O₅ and Its Fully Reversible Li Intercalation

To obtain good electrochemical performance and thermal stability of rechargeable batteries, various cathode materials have been explored including NaVS₂, β-Na_0.33V₂O₅, and Li_<i>x</i>V₂O₅. In particular, Li_<i>x</i>V₂O₅ has attracted attention as a cathode material in Li-ion batteries owing to its large theoretical capacity, but its stable electrochemical cycling (i.e., reversibility) still remains as a challenge and strongly depends on its synthesis methods. In this study, we prepared the Li_<i>x</i>V₂O₅ from electrochemical ion exchange of β-Na_0.33V₂O₅, which is obtained by chemical conversion of NaVS₂ in air at high temperatures. Crystal structure and particle morphology of β-Na_0.33V₂O₅ are characterized by using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy techniques. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, in combination with electrochemical data, suggest that Na ions are extracted from β-Na_0.33V₂O₅ without irreversible structural collapse and replaced with Li ions during the following intercalation (i.e., charging) process. The thus obtained Li_<i>x</i>V₂O₅ delivers a high discharge capacity of 295 mAh g^–1, which corresponds to <i>x</i> = 2, with crystal structural stability in the voltage range of 1.5–4.0 V versus_. Li, as evidenced by its good cycling performance and high Coulombic efficiency (under 0.1 mA cm^–2) at room temperature. Furthermore, the ion-exchanged Li_<i>x</i>V₂O₅ from β-Na_0.33V₂O₅ shows stable electrochemical behavior without structural collapse, even at a case of deep discharge to 1.5 V versus Li