Microstructure And Electrochemical Performance Of Nanosized ZnO, Surface-modified ZnO As Anodic Materials For Zinc-nickel Batteries

Zinc-nickel battery has the advantages of high energy density, high power density, high working voltage, facile raw material, free of environment pollution in produce and application. However, cycle life of Zn-Ni battery remains relatively low due to the technical difficulties of the Zn electrode shape change, Zn dendrite formation, Zn densification and self-discharging that occur with increasing number of charge/discharge cycles. Thereby, the development of Zn-Ni battery is greatly limited and the massive production and utilization are currently impossible. Among the aforementioned problems, the most critical problems are shape change and dendrite formation. The mechanism of Zn-Ni battery shows that the shape change and dendrite formation are ascribed to the solution of ZnO in the alkaline electrolyte and Zn electrodeposition during the repeatedly charge/discharge. The dissertation focuses on two key factors influencing shape change and dendrite formation: the solution of ZnO and Zn electrodeposition, and put forward material design and surface modification of ZnO to improve cycle life of Zn-Ni battery. In the dissertation, the nanosized ZnO with different morphologies was prepared and their electrochemical performances and morphology evolution processes as the anode materials of Zn-Ni battery were systemically investigated. In addition, the influence of different surface modification on electrochemical performances of ZnO was also studied, as well as the working mechanism of tin ion as the electrolyte additive.ZnO nanoballs with 40-60 nm in average diameter were prepared using homogeneous precipitation process, ZnO nanorods with 20-30 nm in diameter and 30 average length-diameter ratio and ZnO nanopillar with 100 nm in average diameter were prepared using the plasma chemical vapour deposition process. The as-prepared nanosized ZnO showed better crystal structure and better particle morphology. The electrochemical performances of nanosized ZnO with different morphologies were investigated by the charge/discharge cycling test, slow rate cyclic voltammetery and electrochemical impedance spectroscopy, and so on. The ZnO nanoballs and nanorods exhibited higher utilization ratio of active materials and better cycle stability than ZnO nanopillar and conventional ZnO. The highest specific capacity of ZnO nanoballs was 630 mAh g^-1 and that of ZnO nanorods was 600 mAh g^-1. The discharge capacity delivered by ZnO nanoballs still achieved about 600 mAh g^-1 until the 250th cycle, and that delivered by ZnO nanorods exceeded 500 mAh g^-1 until the 175th cycle. The ZnO nanoballs and nanorods also showed higher midpointdischarge voltage than the conventional ZnO and ZnO nanopillar during the cycling tests. The reason is that the large specific surface of nanosized ZnO added the reaction area of active materials and decreased the resistance of the Zn electrode. The ZnO nanoballs and nanorods are the appropriate Zn electrode materials, and the performance of ZnO nanopillars is similar to that of conventional ZnO. The morphology evolution process of ZnO during the charge/discharge cycling test was investigated in detail by means of SEM, TEM and electron diffraction. ZnO with different morphologies showed similar electrodeposited growth habit, including the morphology retention on the initial stage and the formation of lamellar ZnO on the subsequent stage. Epitaxial growth and texture growth were propsed to illuminate the morphology evolution process of ZnO. Dendrite formation were the results of lamellar ZnO was parallel to the substrate, and the rapidest growth direction determined by the crystal growth habit was in accordance with that determined by concentration polarization induced by the liquid diffusionThe surface-modifed ZnO by Sn₆O₄(OH)₄ was prepared by a simple hydrolyzation of SnCl₂. Lots of nanosized Sn₆O₄(OH)₄ with 15 nm of average particle size were modified on the part surface of ZnO particlest. With increasing the Sn₆O₄(OH)₄ content, the electrochemical cycle stability of ZnO was enhanced, the average utilization ratio of ZnO increased, the weight change of the ZnO electrodes decreased. The improvement in the electrochemical properties resulted from the fact that Sn₆O₄(OH)₄ modified-ZnO decreased the direct contact of core ZnO with electrolyte, therefore, suppressed the dissolution of ZnO in the electrolyte. Nevertheless, surface modification decreased the discharge plateau voltage and increased the resistance of the Zn electrode, which resulted from surface modification decrease the electrochemical kinetics. XRD results before and after the charge/discharge test indicated that Sn₆O₄(OH)₄ was reduced into Sn and all the time existed as the form of metallic Sn. TEM after the cycling test showed that Sn still modified on the surface of ZnO. The ZnO modified with Sn₆O₄(OH)₄ was a new effective route for utilization of the electrode additives and the improvement of electrochemical performance of ZnO.Dendritic nanosized calcium zincate was modified on the surface of ZnO by the direct precipitation. Electrochemical measurements showed that the nanosized calcium zincate slightly improved the utilization ratio and cycling stability of ZnO. After 20 cycles, Ca(OH)₂·2Zn(OH)₂-modified ZnO showed higher specific capacity than the mechanical mixture ZnO and Ca(OH)₂ and ZnO without calcium additives. The Ca(OH)₂·2Zn(OH)₂ modified ZnO decreased the resistance of the Zn electrode and increased the dischargevoltage, as resulted from that calcium zincate could suppress the solution of ZnO into the electrolyte and maintain the electrochemical activity of ZnO. Surface modification with calcium zincate was an effective route to use calcium additive for ZnO.Amorphous carbon-coated ZnO with carbon thickness of 1-2 nm was prepared by a vapour deposition process. Electrochemical tests showed that although the carbon film could decrease charge the charge voltage and increase the discharge voltage, the utilization and cycling stability of ZnO were only slightly improved. It was the reason that the carbon film entirely covered ZnO particles, and then the contact between ZnO with the electrolyte was completely blocked. By the contrast of electrochemical performance and structure between Sn₆O₄(OH)₄-modified ZnO, Ca(OH)₂·2Zn(OH)₂-modified ZnO and carbon-coated ZnO, it was concluded that surface modification that partly covered ZnO particles was the most effective route to improve the cycling performance of Zn-Ni battery, and surface coating that entirely covered ZnO particles was not an effective approach.The effect of stannous ions as an electrolyte additive on the electrodeposition characteristics of Zn was investigated by chronoamperometry, the potential-step method and cyclic voltammetry. The chronoamperometry measurements showed that the addition of stannous ions inhibited the dendritic growth of Zn deposits. SEM observation also revealed that the Zn deposit was in the form of compact cylinders with rounded tops that consist of many small crystallites, rather than the classical dendrites with side branches. Furthermore, Sn²⁺ additive was found to suppress the corrosion reaction of Zn by 87% in the Sn²⁺-containing zincate electrolyte, comparing to that in the blank zincate electrolyte. The codepsotion effect was proposed to illuminate the inhibition effect of Sn²⁺ on the formation of Zn dendrites. The electrodeposited Sn was expected to deposit on the preferred growth sites. Because the nucleation overpotential of Zn on Sn was relatively high (～30 mV), the electrodeposited Sn interrupted further Zn deposition on the particular crystallite and led to nucleation of new Zn grains. Therefore, the electrodeposition process of Zn was changed, and dendritic growth was suppressed.