(Battery Division Postdoctoral Associate Research Award Address Sponsored by MTI Corporation and the Jiang Family Foundation) Advances in Understanding the Rechargeable Zinc-air Batteries Chemistry

Abstract

As a possible post lithium-ion battery technology, metal-air batteries have revived interest recently. Among the different types of metal-air batteries, rechargeable zinc-air battery (ZAB) is a promising electrochemical energy storage device with the advantages of potential low cost, high safety, environmental friendliness and high energy density. However, current ZAB still suffer from the strongly alkaline electrolyte, which is chemically unstable towards the active cathode material (ambient air) and to a large extent causes electrochemical irreversibility at the Zn metal anode. Zn metal in alkaline environment suffers from formation of high surface area dendrites, well known from Li metal anodes, non-uniform electrodeposition/-dissolution, and persistent corrosion that consumes electrolyte. On the cathode side, the reaction between alkaline electrolytes and CO2 in air produces insoluble carbonate salts, which irreversibly consumes electrolyte, and also physically clogs and chemically deactivates the porous air cathode. Since the redox reaction of O2 occurs via a sluggish 4e-(electron) oxygen reduction reaction (ORR) in conventional alkaline electrolytes, bi-functional catalysts have to be used on the cathode.Herein, specific attention is given to the obstacles caused by the conventional alkaline electrolytes with the focus on the fundamental understanding in battery chemistry. Recently, we demonstrated a previous-unknown reversible zinc peroxide (ZnO2)/O2 chemistry for the rechargeable ZAB.[1] By comparing the hydrophobicity of anions of Zn salts, we selected the hydrophobic trifluoromethanesulfonate (OTf-) anion with a large size as a constituent of the electrolyte solute. Comprehensive characterization and simulations identified the critical role of hydrophobic OTf- anions in dictating the electrochemical double layer structure that favors the formation of ZnO2 and suppression of H2O-involved reactions. Leveraging the high reversibility of both air cathode and Zn metal anode in the Zn(OTf)2 electrolyte, the Zn-air full cell demonstrated excellent cycling performance in ambient air despite a simple cell structure. Such tailoring of interfacial structures via electrolyte properties provides a solution to the electrochemical irreversibility that has been plaguing not only alkaline ZABs, but essentially all metal-air batteries for centuries, especially those with promising high theoretical energy densities using materials with abundance, but being only feasible in alkaline electrolytes as either primary or mechanically rechargeable batteries.Keywords: Energy Storage, Zn-air batteries, Electrolytes, Battery ChemistryReferences [1] Wei Sun, Fei Wang, Bao Zhang, Mengyi Zhang, Verena Küpers, Xiao Ji, Claudia Theile, Peter Bieker, Kang Xu, Chunsheng Wang, Martin Winter, A rechargeable zinc-air battery based on zinc peroxide chemistry, Science 371, (2021) 46-51.