(84bb) Understanding the Role of Calcium Zincate (CaZn2(OH)6·2H2o) in Improving Cycle Life of Rechargeable Alkaline Zinc Batteries

There is a growing need for safe, affordable, low cost, and reliable grid scale energy storage as the world aims to cut down on fossil fuels and shift towards renewable sources of energy. Renewables are intermittent and need to be coupled with energy storage devices to allow energy generation to match hours of largest demand. Metallic zinc (Zn) is investigated and produced industrially for anodes in primary and rechargeable Zn batteries such as Zn/Ni, Zn/Air, Ag/Zn, and Zn/MnO₂. Zinc chemistries provide a high theoretical capacity, relative abundance, non-toxic, and non-flammable nature which make it safe for energy storage.

Metallic zinc itself has shown technological difficulties via poor reversibility when used alone as an anode in alkaline electrolytes. Failure mechanisms are passivation, shape change/redistribution, dendrite formation, hydrogen evolution, and the crossover of zincate (Zn(OH)₄) into the cathode. Zinc oxidizes that form during discharge include the zincate ion which is dissolved in the electrolyte and also solid zinc oxide (ZnO). Zincate ions that remain in solution, i.e., not lost to side products or irrecoverable ZnO precipitates or cross over to the cathode, are then able to participate in the cycling process back to metallic Zn on charge.

ZnO anodes with various additives have also been previously investigated to improve the cyclability of metallic zinc. The addition of calcium hydroxide (Ca(OH)₂) into ZnO anodes has shown great promise to improving the overall cyclability of the anode. It is widely understood that Ca(OH)₂ helps boost performance of ZnO anodes due to the in-situ formation of calcium zincate (CaZn₂(OH)₆·2H₂O) while cycling the cell. It has been shown that calcium zincate yields a lower solubility of zincate in alkaline KOH electrolytes compared to just ZnO. The lower solubility helps to reduce the dissolution of zincate ions which is susceptible to the losses mentioned above.

There has been some work done in the literature regarding anodes composed primarily of pure Calcium Zincate, with varying electrolyte amounts and other additives to produce varying performance. Making a direct comparison of these pure Calcium Zincate anodes with each other or with a commercial zinc anode is difficult to do because of these experimental differences that cause changes in performance. Ex-situ synthesized calcium zincate anodes show promise over metallic zinc and ZnO anodes at high zinc utilization, but the commercial production of Calcium Zincate powders is still relatively small. We would like to understand the optimal amount of Calcium Zincate powders to use as an additive to commercial metallic zinc anodes to minimize additives while improving the cycling performance.

Various anode formulations with different proportions of Zinc, Zinc Oxide, and Calcium Zincate were fabricated with standardized electrode size, theoretical capacity, separators, and electrolyte concentration at high 50% zinc utilization to understand the effects on battery cycling. These Zinc, Zinc Oxide, and Calcium Zincate anodes were cycled vs two sintered nickel cathodes galvanostatically between 1.2 to 1.98 volts and studied charged, discharged, and in between using cycle life, coulombic/energy efficiency, XRD, SEM, and EDAX mapping to understand morphology and chemical changes within the anodes. Deeper understanding of calcium zincate’s physical and chemical mechanism can help to explain the improved performance as an anode material and its addition can help to increase the cycle life and higher utilization of zinc in rechargeable secondary zinc batteries.

Acknowledgements:

This work was supported by the U.S. Department of Energy Office of Electricity. Dr. Imre Gyuk, Director of Energy Storage Research at the U.S. Department of Energy Office of Electricity, is thanked for his financial support of this project. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. This work was also supported by the United States Nuclear Regulatory Commission under grant number NRC- HQ-60-17-G-0030. The views expressed in this article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. This article has been co-authored by an employee of National Technology & Engineering Solutions of Sandia, LLC under Contract No. DE-NA0003525 with the U.S. Department of Energy (DOE). The employee owns all right, title and interest in their contribution to the article and is solely responsible for its contents. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this article or allow others to do so, for United States Government purposes. The DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan https://www.energy.gov/downloads/doe-public-access-plan.