Modification Research On Anodic Active Materials With ZnO For Nikel-Zinc Secondary Cells

The Ni-Zn secondary battery has advantages of high specific energy, high specific power, low cost and low toxicity, which have urged researchers to renew the interest in such batteries. Nevertheless, the Ni-Zn secondary battery usually is hindered in widespread commercialization by their short cycle life and discharge capacity fading in comparison with other secondary batteries. These problems are primarily caused by the high solubility of zinc discharge products in the alkaline electrolyte and the formation of zinc dendrite of zinc electrode, especially after deep discharge. Increasing efforts have been devoted to overcome these difficulties, including:(1) additives to the anode and the electrolytes;(2) development and improvement of separators;(3) miscellaneous techniques such as pulse charging. Base on these points, we successfully synthesized ZnO/CeO2composite materials and ZnO coated with carbon materials, and systematicly investigated the effects of these materials on the electrochemical performances in Ni-ZN secondary batteries. We also investigated the differences in microscopic structure and electrochemical performance of ZnO coated with carbon samples by using different carbon sources.The samples of composite CeO2and ZnO material (CeO2/ZnO) were prepared by one step hydrothermal method. The X-ray diffraction (XRD) patterns reveal that CeO2does not enter the lattice of ZnO and no new phase is generated. Energy Dispersive Spectrometer (EDS) spectra of CeO2/ZnO demonstrates that the surface elemental composition of CeO2/ZnO contain Ce, Zn, O. The scanning electron microscopy (SEM) images of CeO2/ZnO and pure ZnO show that the grain sizes of CeO2/ZnO significantly increased after compositing and part of CeO2coating on the surface of ZnO. Tafel plot, cyclic voltammetry (CV) and galvanostatic charge-discharge measurement were utilized to examine the electrochemical performances of CeO2/ZnO. Compared with ZnO physical mixed with CeO2(ZMC), the CeO2/ZnO have a more positive corrosion potential in the zinc electrodes which indicates the CeO2/ZnO has a good anticorrosion ability. A lower charge platform and a flatter discharge platform of CeO2/ZnO indicate that the CeO2/ZnO have a better charge/discharge performance as anodic material for Ni/Zn cells. Most important of all, the CeO2/ZnO anodes show much more stable cycle stability. At the50th cycle, the discharge capacity of ZMC anode reduces to380.82mAh g-1with the capacity retention ratio of68%, while the capacity retention ratio of CeO2/ZnO anodes are96.7%,99.1%and95.2%, respectively.The ZnO samples coated with carbon are successfully synthesized by using a high energy ball milling method. The scanning electron microscopy (SEM) images and Energy Dispersive Spectrometer (EDS) spectra of the carbon-coated ZnO and pure ZnO show that the carbon-coated ZnO (carbon source:glucose, citric acid) samples and the untreated ZnO sample have similar particle size and crystal form. The particles have prismatic microstructure whose sizes are about100-200nm. However, the carbon-coated ZnO (carbon source:sucrose) sample has become agglomeration after calcination whose size has been increased to2-6um. The uncoated ZnO powders have more complete crystal shape and they are glazed quadrangular materials, while the carbon coated ZnO particles has a rough surface, which resulted from the growth of carbon coating on ZnO particles. X-ray diffraction (XRD) patterns of the carbon-coated ZnO and the pure ZnO show carbon formed on the surface of ZnO is amorphous. Tafel plot, cyclic voltammetry (CV), AC impedance spectroscopy and galvanostatic charge-discharge measurement are utilized to examine the electrochemical performances of the carbon-coated ZnO. The carbon-coated ZnO (carbon source:glucose) have the most positive steady-state potential and lowest corrosion current density in the zinc electrodes which indicates that it has a good anticorrosion ability. A lower charge platform and a higher discharge platform of carbon-coated ZnO indicate that it have a better charge/discharge performance as anodic material for Ni/Zn cells. A smaller ohmic resistance and charge-transfer resistance imply that the carbon film upon ZnO could greatly decrease the impedance of the reaction process. Meanwhile, the carbon-coated ZnO also showed more excellent cycling performance than pure ZnO. The reason of improvement about electrochemical performance can be ascribed as the unique structure of amorphous carbon layer.