(696d) Zinc Anode Discharge Study in Alkaline Electrolyte for Rechargeable Battery Applications

Metallic zinc (Zn) is a non-toxic, earth abundant metal with an equilibrium potential of -1.35 V vs. mercury-mercuric oxide (Hg|HgO) in concentrated alkaline electrolyte, a gravimetric discharge capacity of 820 mAh/g, and a volumetric discharge capacity of 5850 mAh/mL. During discharge, Zn oxidizes and forms a dissolved complex known as the zincate ion, Zn(OH)₄^2-, from which zinc oxide (ZnO) precipitates as a solid. On charge, Zn(OH)₄^2- is reduced to metallic Zn. Motivation to employ the Zn anode in rechargeable batteries is driven by a target overall battery energy density of 200 Wh/L that is sustainable for 500 or more charge-discharge cycles. In order to increase the percentage of discharge capacity of Zn accessed for an extended number of cycles, it is imperative to understand fundamentals of the discharge voltage profile.

After practical battery cycling of a porous particle Zn anode, the electrolyte within the pore space of the anode becomes saturated with zincate. In order to test hypotheses concerning failure mechanisms of porous particle Zn anodes, single-discharge tests with zincate saturated potassium hydroxide electrolyte were performed. These tests included a variety of anodes: porous particle Zn, metallic Zn mesh, metallic Zn mesh with a ZnO blocking layer, and Zn-plated Cu mesh. Furthermore, the anode’s access to electrolyte was varied. In some cases a reservoir of electrolyte was provided to the anode, while in others the anode was placed between two cathodes, thus limiting access of the anode to electrolyte.

The ZnO blocking layer was shown to inhibit discharge of the metallic Zn mesh. The voltage profile of a Zn mesh with a ZnO blocking layer was observed to be relatively flat up to a specific discharge capacity of 40 mAh/g, at which point the Zn overpotential increased by nearly 300 mV. A similar observation in the Zn overpotential for the plain Zn mesh was observed to occur at a specific discharge capacity of nearly 400 mAh/g. The ZnO blocking layer is believed to promote ZnO precipitation and thus cut off the remaining active Zn material from electrolyte, thereby halting the discharge reaction. Lack of access to electrolyte for a Zn-plated Cu mesh is observed to yield two potential plateaus during discharge. The second plateau occurs at a Zn overpotential of 100 mV. This second plateau may represent a separate electrochemical reaction from the active dissolution of zinc to the zincate ion which occurs at the initial discharge plateau (-1.35 V vs. Hg|HgO in concentrated alkaline electrolyte).