Synthesis And Electrochemical Lithium Storage Performances Of Rod-like Sodium Vanadium Oxides

As cathode materials for Lithium-ion batteries(LIBs), sodium vanadium oxides have the advantages of low-cost raw materials, high specific capacity, excellent cyclic stability, good rate capability and so on. Sodium vanadium oxides are regarded as one of the promising cathode materials for high power LIBs. However, their commercial applications are still hampered duo to the complex preparation of the electrode materials. In this paper, the preparation, microstructure and electrochemical performance of sodium vanadium oxides were systematically investigated. The relationship between the composition, microstructure and electrochemical performances of sodium vanadium oxides was also discussed to reveal their electrochemical lithium storage mechanism and capacity deterioration reason. The results are very important for us to further study and develop high-performance cathode materials of sodium vanadium oxide. The main results are as follows:1) Sodium fluoride and ammonium metavanadate(the molar ratio is 1:3) were used as raw materials to synthesize the rod-like Na V3O8 by a low-temperature solid-state calcination route. The measurements of X-ray diffraction(XRD) and scanning electron microscopy(SEM) showed that the purity, the crystallinity and the rod size of Na V3O8 increased with the calcination temperatures. The relationship between the morphologies and electrochemical lithium storage performances of Na V3O8 was studied. It was found that the lithium storage capacity decreased with the increasing rod sizes, but the cycling performances and rate capability enhanced withthe rod sizes. Among them, the rod-like Na V3O8 synthesized at 450 °C showed the best comprehensive performances for lithium storage. This sample delivered a specific discharge capacity of 226 m A h g-1 at the current density of 30 m A g-1 and exhibited good cycle stability over 100 cycles at 100 and 300 m A g-1. The study of lithium storage mechanism demonstrated that the lithiation of Na V3O8 was a single-phase reaction. During the lithium storage, Na V3O8 was converted to Li x Na V3O8, in which the maximum value of x was 2.5 in this work. The observation of in-situ transmission electron microscope(in-situ TEM) revealed that the lattice expansion of Na V3O8 during the Li+ ions intercalation resulted in cracks appeared at the rod edge of rod-like Na V3O8. As a result, during the repeated Li+ ions intercalation/deintercalation, the fracture of rod-like Na V3O8 was the main reason for the cycling capacity deterioration.2) The rod-like Na V6O15 materials can be synthesized by change the molar ratio of sodium fluoride and ammonium metavanadate to 1:6. The measurements of XRD and SEM showed that the purity, the crystallinity and the rod size of Na V6O15 increased with the calcination temperatures. In comparison with the electrochemical lithium storage performances of the synthesized Na V6O15 products at various calcination temperatures. It was found that the rod-like Na V6O15 synthesized at 450 °C showed the best comprehensive performances for lithium storage. This sample delivered a specific capacity of 325 m Ah g-1 at the current density of 30 m A g-1. However, the cycling stability is poor, after 100 cycles at the current densities of 100 m A g-1, this sample delivered a specific capacity of 114 m Ah g-1, and the capacity retention rate is 66%. The ex-situ scanning electron microscope(ex-situ SEM) results revealed that the destruction of rod-like Na V6O15 during the repeated Li+ ions intercalation/ deintercalation was the main reason for the cycling capacity deterioration.3) The rod-like Na1.25V3O8 materials can be synthesized by change the molar ratio of sodium fluoride and ammonium metavanadate to 1.25:3. The measurements of XRD and SEM demonstrated that the purity, the crystallinity and the rod size of Na1.25V3O8 increased with the calcination temperatures. In comparison with the electrochemical lithium storage performances of the synthesized Na1.25V3O8 products at various calcination temperatures. It was found that the rod-like Na1.25V3O8 synthesized at 450 °C showed the best comprehensive performance of lithium storage. After 100 cycles at the current densities of 30, 100 and 300 m A g-1, this sample delivered the discharge specific capacity of155, 135 and 132 m Ah g-1, and the capacity retention rate is 78%, 91% and 98%, respectively. The cycling stability of charge/discharge enhanced with the current density increasing. The ex-situ SEM results indicated that the excellent cycle stability can be attributed to the superior structural stability of Na1.25V3O8 nanorods.