Effect of Additives Reducing Solubility of ZnO on the Decomposition Kinetics of a Supersaturated Zincate Solution

Anodic polarization of zinc electrode during discharging of alkaline Ni-Zn accumulators is accompanied by dissolution of the oxidation product into the alkaline electrolyte. The maximum amount of dissolved zinc species could be much higher than the equilibrium state, which is known as the state of saturation. Supersaturated electrolyte (SZS – supersaturated zincate solution) could subsequently decompose to a solid ZnO precipitate. The decomposition process consists of initial formation of ZnO nucleation centers followed by increased precipitation. It is clear that if the rate of decomposition is high, the ZnO will precipitate not only on the surface of the negative electrode, but in the whole volume of the cell. Rate of the decomposition process is influenced by temperature, initial supersaturation concentration of zincate, concentration of hydroxide (pH) and also by presence of specific additives. In relation to the last mentioned, for example the study of SiO₃^2- additive effect on decomposition kinetics shows that SiO₃^2- causes complete inhibition of decomposition by its adsorption on the surface of ZnO.

Actual research in the field of Ni-Zn batteries deals with electrolytes with reduced ZnO solubility, which limit the amount of zinc transferred from electrode to electrolyte and back during cycling and thus reduce the electrode shape changes. These electrolytes are based on the alternative anion groups such as F^-, CO₃^2-, or PO₄^3-. In many cases the battery development also considers the organic additives into the base electrolyte, which serve as Zn dendrite inhibitors. These additives are mostly surfactants, which means that they have also the ability to adsorb on the surface of any particles.

With respect to above-mentioned fact we focused our attention on the measurement of decomposition kinetic rate of SZS with regard to different additives, which are mostly used in the Ni-Zn batteries. Precipitation of ZnO is accompanied by the release of hydroxyl ions that linearly increase electrolyte conductivity as the decomposition process is ongoing. For this reason we determine the decomposition rate by conductometric measurements using a four electrode platinum-glass conductivity cell Knick Messcell 227. Conversion characteristic between the conductivity and concentration of dissolved zinc over the saturation state were determined by complexometric titration for the each type of measured electrolyte.

We trust, that our result, which describes the decomposition kinetic in low solubility electrolyte, can contribute to the better understanding of the complex phenomena of Ni-Zn battery.