Development of a Lab-Scale, Rechargeable, Aqueous Coin Cell and Methods for Measuring the Self-Discharge Rate of Zinc Electrodes
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Zinc-air cells are promising as a cheap, safe and sustainable energy storage technology, but few publicly funded institutions currently research them. One reason for this is a lack of a simple, standardized cell design to facilitate the comparison of data sets between labs. In the Li-ion battery research community, a coin cell format is one of the accepted standard cell designs and any researcher attempting to shift from Li-ion to Zn-air research will want to use their existing coin cell infrastructure. Coin cells require small amounts of material, can have good reproducibility, are easily fabricated in large quantities and have small space requirements that allow many cells to be tested simultaneously under controlled conditions. If thin electrodes are used, concerns over bulk electrode issues can be alleviated, making coin cells a good research tool for testing new active materials, electrode material recipes, electrolytes and separators. Due to identical reactions at the zinc electrode, nickel-zinc (Ni-Zn) cells could potentially be used to study zinc electrodes without the complications of an air electrode. It was shown that to adapt coin cells for use with Ni-Zn, nickel should be used for all positive-side components, including the current collector, while tin should be used for all negative-side components. Additionally, a pressure-release valve, non-woven separator and microporous separator are required for long cycle life. Ni-Zn coin cells created in this work achieved Ni active material utilizations over 100% and cycle lives of over 300 cycles with un-optimized electrode materials. The procedures and equipment developed for Ni-Zn coin cells were also used to create a 3-electrode Ni-Zn coin cell, which demonstrated that future cycle life experiments on rechargeable Zn electrodes should not use Ni electrodes as a counter electrode as was done in this work. A Zn-air coin cell was created to demonstrate that Zn-air coin cells can be made when durable bi-functional air electrodes are acquired in the future. A promising alternative technology, aqueous LiMn2O4-Zn, was also tested in coin cells and showed to require overcharging every cycle to achieve a long cycle life. The self-discharge rate of rechargeable Zn electrodes is an issue that is poorly measured in the literature. A new experimental method for directly measuring the self-discharge rate of rechargeable Zn electrodes in any cell format is presented here. The rate determining step for (galvanic) Zn corrosion was determined to be H2O adsorption onto the current collector during H2 evolution.