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

Lithium-ion batteries are currently one of the most popular types of rechargeable battery commonly used in consumer electronics. 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 (due to the higher half-reaction potential for sodium relative to lithium). If 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 reactions. The structure and performance of as-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(CV) 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. The SEM micrograph showed that the size of NaVPO₄F is micron-class, and the distribution of particle was uniform, the first discharge capacity of material was 86.3 mAh/g and the discharge capacity of NaVPO₄F was declined to 58.4%of its initial discharge capacity after 30 charge/discharge cycles.. The cyclic voltammogram showed two couples of peaks in cathodic sweep and anodic sweep. It agreed well with the two voltage plateaus of the curve of the charge/discharge curves for NaVPO₄F.In this thesis, NaV_1-x Fe xPO₄F (x=0-0.1) powders were synthesized with a high temperature solid state reaction. In the FT-IR spectrum of Fe 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 Fe increase. So, it explained that the strength of V-O band increased with the doped Fe, and the crystal cell shrunk. Moreover, it could be expected that the stability of materials would be enhanced and the cycle performance would be better with the introduction of Fe. The XRD results clearly confirmed that Fe substitution for V sites was successful and the single-phase solid solution was formed when the doped amount of Fe was increase to x=0.06. A small amount of Fe doping gave rise to the electrochemical cycling properties of NaV_1-x Fe xPO₄F. The as-prepared Fe-doped materials had a better cycle stability than the un-doped one, an initial reversible discharge capacity of NaV_0.96Fe_0.04PO₄Fand NaV_0.94Fe_0.06PO₄F are 81.6 mAh/g and 74.5mAh/g , respectively; the discharge capacity of NaV_0.96Fe_0.04PO₄F and NaV_0.94Fe_0.06PO₄F are 66.7 mAh/g and 68.4 mAh/g after 20 charge/discharge cycles. However, the first discharge capacity of NaVPO₄F is 86.3 mAh/g, the discharge capacity of NaVPO₄F is 62.9 mAh/g after 20 charge/discharge cycles.In addition, NaV_1-xAl_xPO₄F (x=0,0.02)powders were also synthesized with a high temperature solid state reaction. In the FT-IR spectrum of Al doped materials, it was observed that the absorbance of peak increased comparing to the un-doped materials, the crystal cell shrunk. So, it could be expected that the stability of materials would be enhanced and the cycle performance would be better with the introduction of Al. The XRD results clearly confirmed that the crystal phases of both samples are identified to be a monoclinic structure. The SEM results indicated that the Al doping sample had a smaller and uniform particle size than the un-doped sample; the electrochemical performance of the NaV_0.98Al_0.02PO₄F is known to be good, because the insertion and de-insertion of sodium ions during the charge and discharge processes can be facilitated when electrode materials are smaller in size. A small amount of Al doping gave rise to the electrochemical cycling properties of NaV_1-xAl_xPO₄F. The as-prepared Al-doped materials indicate a better cycle stability than the un-doped one. The initial reversible capacity of NaV_0.98Al_0.02PO₄F is 80.4 mAh/g,the discharge capacity of NaV_0.98Al_0.02PO₄F is 68.3mAh/g after 30 charge/discharge cycles, and the capacity retention at the 30th cycle is 85%.