(549f) Investigation of Calcium Zincate (CaZn2(OH)6 ·2H2o) Anodes for Rechargeable Alkaline Zinc Nickel 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) has been 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)₄^2−) into the cathode. Zinc oxidizes during discharge to form the zincate ion which is dissolved in the electrolyte and zinc oxide (ZnO) can precipitate as a solid. 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.

Different additives to ZnO have been previously investigated, one of them being calcium hydroxide (Ca(OH)₂) which when added to ZnO has been shown to help improve the cyclability of the zinc. It is widely understood that Ca(OH)₂ helps boost performance due to the in-situ formation of calcium zincate (CaZn₂(OH)₆ ·2H₂O) while cycling the cell. It has been shown that calcium zincate has a lower solubility in alkaline KOH electrolytes compared to just ZnO. The lower solubility helps to reduce the dissolution of zinc ions and in turn reduces the formation of zincate ions which is susceptible to the losses mentioned above.

Calcium zincate has been synthesized ex-situ using ZnO and Ca(OH)₂ through various methods such as chemical precipitation, ball milling, hydrothermal, and alcohol thermal etc., which lead to different nano-morphologies of calcium zincate with varying particle sizes/distributions. These were used to make anodes with varying electrolyte amounts and other additives and have been reported with varying performance but without standardized experimental conditions to produce a direct controlled comparison.

We are interested in further understanding the physical and chemical mechanisms underlying the improved anode performance that calcium zincate has been shown to provide. We fabricated various calcium zincate anodes to investigate the electrochemical performance with standardized electrode size, separators, and electrolyte concentration with varying active mass loading, utilization of the theoretical capacity of the zinc, and charge/discharge rate to understand their effects. Calcium zincate anodes of various nano-morphologies were cycled vs two sintered nickel cathodes galvanostatically between 1.2 to 1.9 volts and the resulting material was analyzed with SEM and XRD measurements to understand the morphology and chemical changes during charge and discharge cycles. Deeper understanding of calcium zincate’s performance as an anode material will 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, Energy Storage Program through a contract from Sandia National Laboratories. The authors would like to thank Dr. Imre Gyuk, Manager of the DOE Energy Storage program, for funding this work. Sandia National Laboratories is a multi-program laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.