Mathematical modeling of the initial discharge of alkaline zinc-manganese dioxide batteries

Abstract

A simplified model is proposed to simulate discharge behavior of alkaline Zn-MnO2 batteries. The alkaline Zn-MnO2 battery system analyzed here consists of a zinc anode, an inert separator, and an MnO2 cathode. This simple model is based on macrohomogeneous porous electrode theory including the considerations of potential drop in the electrolyte, porosity change, composition change due to electrochemical or chemical reactions, charge-transfer effects, and particle-scale transport effects including a core-shell model of MnO2 discharge. The thesis focuses on the discharge behavior of MnO2 cathodes. Thus, the zinc anode is assumed as a reversible, nonpolarizable electrode with uniform current distribution and a mixed-reaction model is applied to describe the anode discharge. Newman's BAND method is used to solve the model numerically in Python. First, the model is developed in cylindrical coordinates to analyze the discharge behavior of primary AA battery cathodes. The effect of initial electrolyte concentration and discharge rate is investigated. Also, a secondary current distribution is evaluated in order to determine the reaction distribution of an annular porous electrode at the initial discharge stage. The results are considered as initial conditions for the numerical model. Conclusively, thinner electrode, high electrode conductivity, and slow kinetics lead to a more uniform current distribution at the initial discharge stage. Second, the model is developed in Cartesian coordinates to investigate the initial discharge behavior of prismatic, rechargeable Zn-MnO2 batteries. Prismatic Zn-MnO2 batteries are good candidates for grid-scale energy storage due to the low fabrication cost, high safety, high energy density, and environmental friendliness. Maintaining reversibility of cathode is an important task for rechargeable Zn-MnO2 batteries, as irreversible materials are prone to form in the MnO2 cathodes.. Local DOD (depth of discharge) y is an important parameter in this regard, as it has been found that when y is higher than 0.79 proton equivalents per Mn atom, Mn3O4 and ZnMn2O4 form, destroying MnO2 rechargeability. The local DOD (depth of discharge) y is evaluated at various discharge conditions (different cathode thicknesses & discharge rates) in order to map the design space where Zn-MnO2 batteries can maintain rechargeability during initial discharge.