Zinc-nickel Battery Electrode Materials Of Zinc Oxide And Surface Coating And Its Electrochemical Performance

Zinc/Nickel (Zn/Ni) secondary battery has attractive advantages of high energy and power densities, low cost and no toxicity, is a new promising power source and has wide application for electric tools and new portable devices. However, widespread commercialization of the Zn/Ni battery has been prevented due to the serious problems such as Zn dendritic growth and shape change. By far, most researches on this field have focused on additives to the electrolytes or the zinc electrode, selection of stable separators and other techniques. Nevertheless, problems such as Zn dendritic growth and shape change have close relations with physical and chemical properties of ZnO, which results from dissolution of ZnO in alkalinity electrolyte and deposition of zincate during the charge/discharge cycles. Therefore, studying the structure and properties of ZnO is an effective method to enhance the electrochemistry performance of Zn/Ni secondary battery. This dissertation puts forward material design and surface modification of ZnO to improve cycle life of Zn/Ni battery. ZnO nanowires and surface-modified ZnO with nanosized Bi compounds are prepared, electrochemistry performance of them used as zinc electrode active material are investigated and the shape evolution of ZnO nanowires is discussed.ZnO nanowires were fabricated by a low temperature hydrothermal approach of ZnCl₂ and Na2CO3, and have diameters of 50-80 nm, lengths of 5-8 um and the length-diameter ratios of about 100. The electrochemical performance of ZnO nanowires were studied by the constant current charge-discharge test and the cyclic voltammetry test. The results of constant current charge-discharge test revealed that ZnO nanowires had better cycle stability, higher discharge midpoint voltage and lower charge midpoint voltage than the conventional ZnO. The discharge capacity of ZnO nanowires still remained 605 mAh g^-1 until the 75th cycle, while that of conventional ZnO was only 172 mAh g^-1. The fading rate of ZnO nanowires was only 8.2 % and displayed the ameliorative electrochemistry performance. The improved electrochemistry performance had relations with nanometer size effect and shape effect of zinc oxide. The cyclic voltammetry test also showed the ZnO nanowires had the higher electrochemistry activities compared to the conventional ZnO. By analysing the successional variation process of ZnO nanowires'microscopic shape, we found that the morphology of ZnO nanowires did not essentially change, only shortened and thickened, and turned into nanorods. This was the reason that part of long ZnO nanowires dissolved and ruptured to broke nanorods in the charging /discharging process, and high surface activeness of nanorods caused the epitaxial growth.What's more, ZnO nanowires was parallel to Zinc electrode interior and surface, so the rapidest growth direction determined by the crystal growth habit was vertical or inclined to the accelerated growth direction induced by concentration polarization which was aroused by liquid-side mass transfer. The two growth modes competed and inhibited mutually, at last the zinc dendrite was impeded effectively. ZnO nanowires were nanomaterials after the certain charging and discharging cycles, so they still had higher electrochemistry activeness.Surface-modified ZnO with nanosized Bi2O3/BiO compounds was prepared by the hydrolyzation reaction of Bi(NO3)3. The characteristics of surface-coated ZnO were analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD), and the results revealed that some nanoparticles with about 50 nm in diameter were modified on ZnO. The constant current charge-discharge test and cyclic voltammetry of the ZnO electrode showed that bismuth modifying on the surface of ZnO enhanced the cycle stability of the electrode, maintained the electrochemical activity of ZnO, and increased the discharge capacity and average utilization respectively. In comparison with the untreated ZnO, discharge capacity and average utilization of ZnO coated by 9.3 wt.% Bi increased 132 mAh g^-1 and 27 %. The enhancement in the electrochemical performance was owing to the fact that Bi modified on ZnO decreased the contact area of active materials with the electrolyte, therefore, slowed the dissolution of ZnO in the electrolyte and maintained the electrochemical activity of ZnO. Nanosized bismith compounds were deoxidized to the metal bismith after the initial charge/discharge cycles, then bismith compounds kept the form of metallic Bi to modify on the surface of ZnO, which could raise the zinc electrode conductivity, promote zincate to deposit evenly and suppresse the zinc electrode corrosion. Compared with conventional mechanical mixture between ZnO and Bi2O3, surface modification with nanosized Bi compounds is one efficiency method to improve electrochemical performance of ZnO .