Effects of Discharge Products on Zinc-Air Flow Cells

Abstract

As one of the proposed post lithium-ion technologies, zinc-air batteries have received revived interest in the recent years, which is driven by the increased demand of high-performance, efficient, safe and environment-friendly energy storage solutions. These energy storage systems need to address variable power, capacity and profitability requests. Zinc-air flow batteries with higher specific energy density than lithium-ion and low-cost, highly available, eco-friendly active materials are suitable to fulfill these requirements. The energy density of most zinc-air flow batteries is limited by the low solubility of ZnO in the electrolyte. The use of a flow battery type with zinc-particles suspended in alkaline solution (zinc-slurry) in addition with high performance oxygen-reduction electrodes enables the development of high-power zinc-air batteries. This setup also enables the independent scaling of capacity and power.The cooperative project, ZnMobil, funded by the Federal Ministry of Economic Affairs and Energy in Germany, combines the experience of industrial and academic partners (Covestro Deutschland AG; Grillo-Werke AG; VARTA Microbattery GmbH; Accurec Recycling GmbH; Technical University Freiberg, Gottfried Wilhelm Leibniz University Hannover; University of Duisburg-Essen; the fuel cell research center ZBT GmbH) to develop a high-performance and low-cost zinc-air flow battery.The battery system consists of a 100 cm² copper plate as current collector for the zinc-suspension electrode and an oxygen reduction electrode with gas-diffusion layer (supplied by Covestro Deutschland AG). The zinc-slurry contains zinc particles (supplied by Grillo-Werke AG) suspended in alkaline solution (30 wt-% KOH) and stabilized with polyacrylic acid.Here we present a systematic study regarding the effects of the discharge products on the cell performance. Depending on the state of discharge and operation parameters the discharge product ZnO causes several important effects. The solid ZnO limits the discharge capacity in several ways. At high current densities, passivation is the main topic. If passivation is avoided, e.g. with the help of additives, the solid ZnO causes a viscosity increase until the cell is blocked. For this study we evaluated parameters like cell voltage, cell resistance, slurry viscosity, ZnO concentration in the electrolyte and other parameters in correspondence to the state of discharge. The understanding of these effects is essential to build a hydraulically rechargeable battery with high zinc usage and therefore high energy densities.Depending on parameters like current density and additive selection, it is possible to reach more than 70 % DOD, which corresponds to 287 mAh/gslurry or 574 mAh/gzinc. A demonstrator-cell was operated for more than 140 h. During this time it was successfully hydraulically recharged several times.