Synthesis And Electrochemical Properties Of LiV₃O₈ And V₂O₅ Cathode Materials For Lithium Ion Batteries

In the past few years, the severe weather changes, such as global warming and hazy, have made all the people realize the danger of the bad weather and the urgency of using clean energy. Lithium-ion battery is outstanding among the new energy resources (solar and wind power), owning to its excellent electrochemical performance, safety, light mass and high design flexibility. And it has been applied in portable electronic devices, electric vehicles, large power grid and other energy storage systems.Owning to the typical lamellar structure, Vanadium-based materials (Vanadate Vanadium, Ammonium Vanadate Oxidation, etc.) can be used in lithium-ion battery, supercapacitors and catalysis, presenting a wide application prospect. For the electrode materials, the structure, size and morphology have great influence on the electrochemical performance, and the electrode materials are strongly affected by the preparation technology. In our study, LiV3O8 and V2O5 with controlled size, structure and morphology were synthesized by controlling different preparation process. Meanwhile, the phase, size and morphology of the products were explored by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the electrochemical performance was tested by the constant current charge and discharge device test, cyclic voltammograms (CV) and impedance spectroscopy (EIS).(1) LiV3O8 was prepared by sol-gel method. The influence of thermal treatment temperature, thermal treatment time and the templates on the phase, size, morphology and performance were explored. With the increase of treatment temperature, the crystallinity and the size of the products were improved, and the morphology changed from particles to plates. The small particles were agglomerated gradually and the size of products was increased as the thermal treatment time prolonged. LiV3O8 was synthesized by adding oxalic acid and setting the molar radio of Li and V as 1.2:3.0 with following heating treatment at 400℃ for 2 h. The LiV3O8 has sheet structure and the thickness is around 100 nm. At 30 mA g-1, it shows a reversible capacity of 237.5 mAh g-1. After 50 cycles, it delivers 162.9 mAh g-1.(2) V2O5 was prepared by oil bath method by using deionized water as solvent and NH4VO3 as raw materials. V2O5 with different morphology, such as 3D flower-like microspheres, the mixture of flower-like microspheres and self-assembled sheets and microsheets, can be controlled by adjusting pH value. Among them, the microsheets were synthesized at 90℃for 2 h with thermal decomposition at 450℃ for 2 h, and show excellent cycling stability. At 1C, it shows a reversible capacity of 162.4 mAh g-1, and remains a capacity of 159.2 mAh g-1(98.0%).(3) In order to further improve the discharge capacity and rate performance of V2O5, the 3D urchin-like V2O5 microspheres assembled by nanorods, was prepared by oil bath method assisted by adding the same volume of organic alcohol. The formation mechanism of the 3D urchin-like V2O5 microspheres was also explored by studying the phase and morphology of the precursor. Experimental results show that the size of the urchin-like microsheres, the degree of density and the diameter of the self-assembled nanorods can be controlled by the concentration of the reactants, the kinds of organic alcohol and heating treatment process. Compared with the previous experiment, V2O5 with 3D structure was synthesized by adding ethylene glycol (EG), ethyl alcohol (EtOH) and polyvinyl alcohol (PVA) as templates in the solution. The deionized water was used as the solvent, EG with the same volume was added in the reaction solution and the pH was adjusted to 2.0. After thermal decomposition at 350℃ for 2 h, the V2O5 with self-assembled microspheres were prepared. At 1C, it has a capacity of 231.6 mAh g-1 and the capacity retention radio of 94.4%(218.6 mAh g-1). The V2O5 prepared by adding PVA shows better discharge capacity and cycling stability (0.1C:318.0 mAh g-1; 0.2C:256.9 mAh g-1; 0.5C:227.3 mAh g-1), while the V2O5 prepared by adding EtOH show better electrochemical performance at high rate (5C:81.2 mAh g-1).