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(0.33)V2O5, and Li(x)V2O5. In particular, Li(x)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(x)V2O5 from electrochemical ion exchange of β-Na(0.33)V2O5, which is obtained by chemical conversion of NaVS2 in air at high temperatures. Crystal structure and particle morphology of β-Na(0.33)V2O5 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(0.33)V2O5 without irreversible structural collapse and replaced with Li ions during the following intercalation (i.e., charging) process. The thus obtained Li(x)V2O5 delivers a high discharge capacity of 295 mAh g(-1), which corresponds to x = 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(x)V2O5 from β-Na(0.33)V2O5 shows stable electrochemical behavior without structural collapse, even at a case of deep discharge to 1.5 V versus Li.