Modeling Secondary Zinc-Air Batteries with Advanced Aqueous Electrolytes

Abstract

Advances in 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. Modelling and simulation of zinc air batteries with 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. 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 irreversibly forms carbonate species and degrades cell performance by consuming hydroxide. Our simulations and experimental results show that as carbonate species form, the hydroxide concentration decreases linearly with time. The depletion of hydroxide decreases the ionic conductivity and slows down zinc dissolution leading 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 utilize an ammonium chloride buffer solution to stabilize the pH during discharge. However, even small changes in pH may significantly alter the dominant aqueous zinc species. Due to this effect, the final discharge product may shift from ZnO to Zn(NH3)2Cl2 or Zn(OH)1.6Cl0.4∙(H2O)0.2, causing serious losses in the conductivity of the electrolyte and theoretical specific energy of the cell. We present the first model-based analysis of zinc-air batteries with near-neutral electrolytes. This work was supported by the EU Horizon 2020 project Zinc Air Secondary (ZAS!)