Studies On NaVPO₄F And Its Doped Compounds For The Cathode Materials Of Sodium-ion Battery

Lithium-ion rechargeable batteries are now well established as power sources for portable electronic equipments such as cellular phone, camcorder and laptop computer. If sodium-ion rechargeable batteries with good performance characteristics could be developed, it would have some significant advantages over lithium-ion batteries, notably a reduction in raw materials cost and the ability to utilize electrolyte systems of lower decomposition potential (due to the higher half-reaction potential for sodium relative to lithium). So, sodium-ion batteries will be a kind of promising novel batteries.In this thesis, the cathode materials of sodium-ion battery, NaVPO₄F, were prepared under the protection of argon atmosphere by two step high temperature solid-state methods. The structure and performance of the prepared cathode material was characterized by Flourier-Infrared Spectra (FT-IR), Atomic Absorption Spectra (AAS), Thermogravimetric Analysis(TG/DTG), X-ray Diffractometer (XRD) , Scanning Electron Microscope (SEM), the galvanostatic charge/discharge, Cyclic voltammograms (CVs) and Electrochemical Impedance Spectroscopy(EIS). The results showed that NaVPO₄F with a good crystal stability can be obtained about a temperature of 600℃. Its crystal system was monoclinic, and it was in accord with VPO4 precursor. The SEM micrograph showed that the size of NaVPO₄F is micron-class, and the distribution of particle was uniform, The first charge and discharge capacity of material was 87mAh/g。The cyclic voltammogram showed two couples of peaks in cathodic sweep and anodic sweep. It agreeed well with the two voltage plateaus of the curve of the charge/discharge curves for NaVPO₄F.However, the discharge capacity of NaVPO₄F was declined to less than by 50% of its initial discharge capacity after 30 charge/discharge cycles. In this thesis, NaVPO₄F were synthesized with a high temperature solid state reaction, and Cr was doped into this material to prepare NaV_1-xCr_xPO₄F(x=0-0.1) powders. In the FT-IR spectrum of Cr doped materials, it was observed that the absorbance of peak increased comparing to the un-doped materials, and the band peak was moving to higher wave numbers as the doped amount of Cr increase. So, it explained that the strength of V-O band increased with the doped Cr, and the crystal cell shrunk. So that, it could be expected that the stability of materials would be enhanced and the cycle performance would be better with the introduction of Cr. The XRD results clearly confirmed that Cr substitution for V sites was successful and the single-phase solid solution was formed...