Micro/Nanoscale Mg And Al: Preparation And Application In Primary Batteries

Recently, electrochemical storage and generation of energy in primary batteries have attracted considerable research attention due to their high energy density, superior discharge performances, and low cost. Active metals (e.g., Li, Mg, Al and Zn) can be used in various primary batteries. Among the metallic materials, Mg and Al are especially suitable for being anodes in power devices because of the advantages of high theoretical energy density, high theoretical voltage, non-toxicity for the environment, and safety. However, some problems still remain for the two materials such as low anode utilization of Mg or Al sheet material, and low actual energy density, which hinder their widespread applications in the consumer market. The scientific issues associated with the practical problems can be grouped into three categories:low kinetics of active materials, passivation of the anode surfaces, and unsuitable electrolyte leading to a high corrosion rate of the anodes. In addition, micro/nanoscale materials own a series of specific physical and chemical properties that are different from their bulk counterparts because of unique crystallographic structure, which are paving a good way for the influence of the electrochemical capacity. Therefore, enhancing the electrochemical properties of the micro/nanoscale electrode materials and using an appropriate electrolyte can greatly improve the performances of primary batteries.In this thesis, the developments of typical primary batteries were reviewed, especially for magnesium based and aluminum based batteries. The aims of the present study were to focus on the preparation processes, the structural characterization and the electrochemical properties of microscale and nanoscale Mg and Al as anode materials. The main content follows:(1) The morphology-controlled synthesis of magnesium micro/nanomaterials and their electrochemical performance as the anode of primary Mg/MnO2 batteries have been reported. Mg micro/nanoscale materials with controllable shapes have been prepared via a conventional vapor-transport method under an inert atmosphere by adjusting the heating temperatures, heating time, flow-rates of argon gas, and the deposition temperatures. Extensive analyzing techniques including SEM, XRD, TEM/HRTEM, and BET were carried out to characterize the as-obtained samples. The results show that the as-prepared Mg samples are microspheres or micro/nanospheres with specific surface areas of 0.61-1.92 m2 g-1. The electrochemical properties of the as-prepared Mg and commercial Mg powders were further studied in terms of linear sweep voltammograms, impedance spectra, and discharge capabilities. By comparing the performances of different inhibitors in electrolytes, it was found that NaNO2 (2.6 mol L-1) as an inhibitor in the Mg(NO3)2 (2.6 mol L-1) electrolyte affords an Mg electrode with high current density and low corrosion rate. In particular, the Mg sample consisting of microspheres with a diameter of 1.5-3.0μm and nanospheres with a diameter of 50-150 nm exhibited superior electrode properties including negative initial potential (—1.08 V), high current density (163 mA cm-2), low apparent activation energy (5.1 kJ mol-1), and high discharge specific capacity (784 mAh g-1). The mixture of Mg microspheres and nanospheres is promising for the application in Mg/MnO2 primary batteries because of the sufficient contact with the electrolyte and greatly reduced charge transfer impedance and polarization.(2) Various Al micro/nanoscale materials were successfully prepared via a conventional physical vapor deposition through the evaporation of commercial Al powders at certain experimental conditions. The mixture of microspheres and nanoparticles were fabricated via using anodic aluminum oxide (AAO) template as substrate even at a wide range of heating temperatures of 750-950℃, because the AAO substrate can play a crucial role in affecting the amount of nucleation in the initial depositing stage. After studying the effect of experimental parameters on the morphologies of micro/nanoscale materials, the optimal condition for synthesizing Al nanorods with uniform diameters of 30-90 nm were selected at a heating temperature of 1000℃for 10 h, a depositing temperature of 300℃with a constant argon gas of 1000 cm3·min-1 on the stainless steel mesh substrate. The HRTEM image of part of an Al nanorod shows that clear fringes with an interplanar spacing is 0.23 nm, which is in accordance with the d-spacings between the (111) crystal planes, indicating that the Al nanorod grows along the [111] direction. The electrode made from the as-deposited Al nanorods with the composition of 65 wt% Al,25 wt% carbon, and 10 wt% PTFE exhibits superior electrochemical properties to that made from commercial Al powders. Furthermore, the laboratorymade Al/air battery with the as-prepared Al nanorods displays high operating voltage and discharge specific capacity, which is important for developing long-life battery systems.