Research Of Cathode Materials Na₃V₂（PO₄）₃

With the rapid development of portable electronic devices and hybrid electric vehicles, rechargeable batteries with high storage capacity and cycling stability have become the versatile,clean and promising energy storage systems.Among the rechargeable batteries, the lithium ion battery with the highest energy density is a prime candidate.However, the disadvantages such as high cost and scarce resource of lithium limit its application.Therefore, the employment of other guest ions as alternatives to lithium ions is favorable to further development of low-cost power sources.The abundant resource and low cost of sodium make sodium-ion batteries a promising alternative to lithium-ion batteries.However, the larger ionic radius of Na+ than that of Li+ leads to its poor kinetics and inferior rate capability.Therefore, it is still a big challenge to discover new sodium intercalation hosts with fast ion intercalation capability.Na₃V₂（PO₄）₃ is gradually obtaining the attention by people with the NASCION structure.It is becoming more and more wide for the application of Na₃V₂（PO₄）₃ electrode materials.We prepared new Na₃V₂（PO₄）₃ meterials by hydrothermal sol-gel method and the crystal structure of Na₃V₂（PO₄）₃ is identified by X-ray diffraction（XRD）. Then,we prepared reference sample to measure their electrochemical characteristics.Compared with various electrochemical properties, the results show that the improved method has a greater influence on the improvement of material properties. Therefore,it is feasible to the improved method which we reported and the Na₃V₂（PO₄）₃ is promising as the cathode materials for hybrid ion batteries.Based on the research of the three-dimensional hierarchical porous structure,the construction of a 3DHP structure for more complex materials, such as polyanion-based materials,is rarely reported.But this structure can effectively improve the electrochemical performance of positive electrode materials and cycling stability.Therefore, for the first time, we report the design of a new structure, named “honeycomb-type hierarchical porous microball,”for the polyanion-based electrode material in a sodium ion battery. We choose Na₃V₂（PO₄）₃ as a model material, and thisstrategy can also be applied to other polyanions.With the NASCION structure, Na₃V₂（PO₄）₃ has properties of high capacity, good thermal stability and large interstitial channels, which make it a good candidate for sodium de/intercalation. In this study,the honeycomb-structured Na₃V₂（PO₄）₃/C is fabricated by a simple one-pot synthesis with cetyltrimethylammonium bromide（CTAB）.The CTAB is simultaneously employed as carbon source and soft template to construct the hierarchical porous architecture.The honeycomb-structured sample at the 5 C and 20 C rates, the capacities of 97.2 and 80.2 m Ah/g, which are much higher than those of the reference samples.After long-term cycles,the capacity retentions of the honeycomb-structured composite are respectively 93.6% and 81.9% at 1 C and 20 C.