A Study of Zn Electrode Operation in Alkaline Electrolytes; First Principles to Battery Electrode

Recently there has been growing interest in alternatives to Li-ion battery systems. Metal-air batteries are a considered a promising alternative. Zn-air systems possess a number of advantages over Li-based systems however face a number of challenges particularly at the negative Zn electrode. Dendrite growth, passivation and electrode shape change all lead to declining performance and ultimately failure of the cell. This study of Zn electrochemistry alkaline electrolytes was done using in situ electrochemical Scanning Transmission Electron Microscopy (ec-STEM) and Electrochemical Quartz Crystal Microbalance (EQCM) to study fundamental processes during Zn deposition, dissolution and electrode passivation. Using ecS/TEM experiments to study Zn deposition a mechanism for Zn dendrite propagation during cycling via particle detachment during discharge steps was developed and proposed. This was also used to explain capacity fade during cells over long term cycling as detached particles are not able to contribute to the capacity of the cell. EQCM was used to study dissolution and passivation of Zn in 1, 4 and 8 M Potassium Hydroxide solutions. Native oxide layers were found to present in the solutions saturated with Zn(OH)₄²⁻ [Zinc Tetrahydroxide] ions while the 4 and 8M solutions half saturated with Zn(OH)₄²⁻ were found not to form an oxide layer and only corrosion of the Zn electrode occurred. Mechanisms of passivation was determined to shift with pH of the solution. At low concentrations of OH⁻ [Hydroxide] in the solution an adsorption mechanism where passivating ZnO is formed directly on the species occurs. At high OH⁻ concentrations the dissolution and precipitation mechanism of passivation occurs. At intermediate concentrations both participate in the passivation of the electrode. Indicating that researchers need to build their passivation management strategies based on the solution they are using. Finally carbon felts were studied for their potential as negative electrodes in a Zn-air flow battery. It was found that they exhibit high current densities at low overpotential, good cycling stability and lifetime. It was observed that IR drop in the felt determines where deposition occurs and flow conditions need to be optimized.