Research On V₂O₅Film Electrode With High Capacity Used For Lithium Ion Batteries

Vanadium pentoxide （V₂O₅） has become a very promising cathode material forlithium-ion batteries due to its high voltage, high lithium intercalation capacity,abundant resources and low price. However, the commercial application of V₂O₅waslimited in lithium-ion batteries because of poor stability of structure and lowconductivity of electron. In this paper, the V₂O₅thin films were prepared by sol-gelmethod, and electrochemical performances of V₂O₅thin films were researched.Firstly, the structure and morphology of V₂O₅thin films were characterized byThermogravimetric-Differential Thermal Analysis （TG-DTA）, X-ray diffraction （XRD）and Scanning Electron Microscopy （SEM）. The results showed that the structure ofV₂O₅was changed from amorphous into orthorhombic crystalline over350℃, thesynthesis temperature range of orthorhombic V₂O₅were350⁶50℃. The thin film ofV₂O₅was consisted of V₂O₅platelet particles with size of²00nm prepared by sol-gelmethod and annealed at500℃in the airfor2hours. However, its edge of platelet wasdissolved after used as cathode materials for lithium-ion batteries.Secondly, the electrochemical performances of V₂O₅thin films were studied bycyclic voltammetry （CV） and Chronopotentiometry （CP） Results showed that therewere two pairs of reduction/oxidation peaks on the CV curves of V₂O₅thin film,corresponding to two stable platform on its charge-discharge curves in the voltage rangeof-0.2V⁰.6V versus Ag/AgCl, indicating that the reversibility of electrode reactionwas better. It is suggested that there are two steps during the process of Li+intercalation/deintercalation in V₂O₅thin film. In discharge, the V⁵⁺is reduced to V⁴⁺at0.14V firstly, and then the V⁴⁺is reduced to V³⁺at0.34V. The initial discharge capacityof705mAh·g^-1was delivered with a current density of400mA·g^-1, whilecorresponding charge capacity was found to be664mAh·g^-1with an irreversiblecapacity of41mAh·g^-1. It demonstrated that the V₂O₅thin film has better reversibilityand stability.Thirdly, the effects of sol concentration and current density on the electrochemicalproperties of V₂O₅thin films were researched. Results showed that electrochemicalperformance with the sol concentration was improved observably with increases andthen deteriorated slowly. The best electrochemical performance was exhibited whensamples were prepared by concentration sol of0.008mol·L^-1. The V₂O₅thin film demonstrated a high initial discharge capacity of715mAh·g^-1with a current density of400mA·g^-1, and the discharge capacity of611mAh·g^-1after10cycles, with the capacityfading rate of1.5%per cycle. With the increase of current density, the initial dischargecapacity of V₂O₅thin film decreased, and the capacity fading rate increases. When thecurrent density up to1000mA·g^-1, the initial capacity is only339mAh·g^-1with thecapacity fading of2.5%per cycle.Finally, the long time charge/discharge of V₂O₅thin film was carried out. Theresult showed that the initial discharge capacity of731mAh·g^-1was delivered with acurrent density of400mA·g^-1, the discharge capacity of178mAh·g^-1after100cycles,with the capacity fading rate of75.6%. The high capacity may contribute to the largesurface area and the platelet-structure of V₂O₅. But it may also increase the dissolutionof V₂O₅and led to the decreases capacity.