Electrochemically Produced Zinc Oxide Electrode in Rechargeable Alkaline Batteries
Authors: Kolhekar Snehal , Turney Damon , Yadav Gautam Ganapati , Nyce Michael , Banerjee Sanjoy 
Published: February 2020

The zinc electrode is a commonly used negative electrode in a host of commercially important battery systems such as Zinc- Manganese dioxide (Zn-MnO2) and Silver – Zinc (Ag2O-Zn) batteries. Out of these the Zn-MnO2 battery is a commercially successful non-rechargeable battery because of its inherent safety, low cost and relatively high energy density. Recent experimental work has shown that a rechargeable Zn-MnO2 battery is capable of giving 3000 cycles when cycled at 10% of the MnO2 theoretical 1 e- capacity and 1.5% Zn 2e- capacity. However, at higher zinc utilization the battery is short-lived. Dendrite shorting, shape change and MnO2 poisoning are some of the main mechanisms of zinc electrode failure. These failure mechanisms are closely related to the high electrochemical solubility and the unrestrained movement of zincate ions, formed at the anode on discharge. Therefore, it is imperative to immobilize zincate ion either via complexation or intercalation.

Hydrotalcite-like layered materials are recently reported as anode active materials for Nickel -Zinc batteries and have been suggested to capture zinc ions into their structure. We report the cycling performance of Zinc Aluminum Bismuth hydrotalcite for 900 cycles at 10% zinc utilization in 6 M potassium hydroxide solution saturated with zinc oxide against Nickel oxyhydroxide cathode. X-ray diffraction results show the presence of zinc oxide on discharge and do not support the presence of a layered hydrotalcite after cycling, as has been reported in literature. This is attributed to the instability of the hydrotalcite crystal structure in concentrated alkaline electrolytes. The results of this study show improved performance because of zinc oxide saturated electrolyte.

To inherently reduce the tendency of the zinc electrode for dendrite shorting and shape change we investigate a fundamental question of the difference between the cycling of a zinc electrode versus the cycling of an electrochemically formed zinc oxide electrode. A novel, first of its kind, battery cycling protocol is reported where manganese dioxide cathode is cycled against a zinc oxide anode formed in-situ. This is made possible by first deeply discharging the manganese dioxide against a metallic zinc electrode resulting in complete conversion of metallic zinc to zinc oxide. Subsequent cycling of the electrochemically formed zinc oxide anode and cathode is carried out at a shallow depth of discharge. Adopting this strategy has resulted in improved cycle life at 12% utilization of the zinc electrode. We dissect the cells at different cycles to see an evolution of the active material in the cell with metallic zinc and the cell with electrochemically formed zinc oxide. Bulk electrolyte analysis for zincate ion shows that the zinc oxide cell is under-saturated while the metallic zinc cell is saturated with zincate ions. Optical Microscopy images of the zinc oxide anode show the presence of finer zinc particles embedded inside a zinc oxide matrix. Preliminary analysis results show that cycling of electrochemically produced zinc oxide rather than metallic zinc anode reduces shape change and dendrites because of reduction of zincate ions in the bulk electrolyte.