Current distribution in cylindrical and prismatic zinc anodes

Abstract

In this work experimental measurements of the Zn and ZnO volume fractions within discharging cylindrical Zn anodes are correlated to a mathematical model of the transport processes within the battery. The mathematical treatment is then extended to prismatic anodes for comparison. Zn is one of the most commonly used battery electrodes and has a low equilibrium potential, low equivalent weight, high specific energy and high volumetric energy density. The model presented here predicts the current distribution at different discharge rates in Zn anodes. Newman's BAND algorithm is used to solve the model equation numerically in Python. The model is first developed in cylindrical coordinates to analyse primary AA battery anodes. A macro homogenous porous electrode theory is adopted to describe the characteristics of the cell. The 〖Zn(OH)〗_4^(-2) ion (zincate) is assumed to be present only in the anode which gives rise to an additional boundary condition. All changes are assumed to occur in the radial direction. A dissolution-precipitation reaction mechanism explains dissolution of Zn and precipitation of ZnO in the anode. Precipitation of ZnO is a primary cause of cell failure in Zn anode batteries due to pore plugging. The model simulates data collected experimentally on commercial AA cells by energy dispersive x-ray diffraction, EDXRD, at discharge rates from 18 mA to 571 mA. EDXRD results show uniform consumption of Zn at low discharge rates. At higher discharge rates precipitation of ZnO is high at the anode-separator boundary due to a reaction profile concentrated at the front of the electrode. Formation of ZnO leads to reduced capacity and non-uniform current distribution inside the anode and complete utilization of Zn does occur, in turn reducing battery life. At lower discharge rates the dissolution of Zn is more uniform and the model simulates these results backing the EDXRD data. The model is then extended to Cartesian coordinates to analyse the current distribution in prismatic Zn anodes. Prismatic cells are compact and have a low production cost which makes them a good candidate for grid-scale energy storage. It is therefore important to understand the discharge kinetics in these cells. Prismatic cells show more uniform current distribution at low discharge rates compared to cylindrical cells.--Author's abstract