Modeling Zinc-Air Batteries with Aqueous Electrolytes

Abstract

Emerging markets such as electric mobility and renewable power generation are driving a demand for high-performance electrochemical energy storage. Zinc-air batteries are a promising technology due to their high theoretical specific energy, use of cheap materials, and superior operational safety. But they suffer from effects such as poor cycling stability and self-discharge due to carbonate formation in the alkaline electrolyte. The EU Horizon 2020 project Zinc Air Secondary (ZAS!) aims to overcome these limitations and develop a high-performance rechargeable zinc-air battery. Modelling and simulation of novel electrolytes provide crucial support towards achieving this goal. We have developed a 1D finite volume continuum model implemented in MATLAB. Our model includes a thermodynamically consistent description of mass transport in concentrated electrolytes, multi-phase coexistence in porous media, and reaction kinetics with considerations for anode passivation due to types I and II ZnO, among other effects. Within this framework, we simulate performance on mesoscopic and macroscopic scales. Zinc-air batteries have long been commercialized as primary batteries for hearing-aids. The contamination of potassium hydroxide electrolyte due to carbon dioxide from ambient air is known to limit the lifetime of alkaline zinc-air batteries to just a few months. This reaction is irreversibly forms carbonate species and degrades cell performance by consuming hydroxide. Our simulations show that as carbonate species form, the hydroxide concentration decreases linearly with time. The depletion of hydroxide decreases the ionic conductivity and in combination with the logarithmic dependence of anode overpotential on hydroxide concentration, this effect leads to a marked decrease in cell potential over time. Carbon dioxide reactions do not occur in non-alkaline electrolytes. Near-neutral chloride aqueous electrolytes have been proposed to improve zinc-air battery lifetime. These electrolytes do exhibit superior lifetime performance, but also have lower nominal conductivity values. This inhibits ionic transport in the electrolyte, and can be rate-limiting for large geometries. We present the first model-based analysis of zinc-air batteries with near-neutral electrolytes proposed to solve carbon dioxide poisoning.