Development of Zinc Electrodes as Recyclable Fuel for Mechanically Rechargeable Zinc Air Batteries
Authors: Yahata Kosei , Ogawa Satoshi , Ajiki Yoshiharu 
Published: December 2024

Zinc air batteries are safe batteries because they are composed of an aqueous electrolyte. In addition, they also have an energy density several times higher than that of lithium-ion batteries. For this reason, the zinc-air battery is being considered for use as a rechargeable battery [1], but due to various issues, it has not been put to practical use. One of them is dendrite growth in the zinc anode during charging, which makes it difficult to create a rechargeable battery in a sealed space and requires a different methodology. In this study, instead of converting a zinc-air battery into a secondary battery, we focused on a mechanical-charge type, in which the active material is mechanically replaced. The fuel produced by this method is expected to be a medium that can efficiently store, and transport energy as shown in Fig.1. The key point of this system is that the zinc oxide is charged, and the zinc is regenerated. It has been reported that the regenerated zinc is not only prone to dendrites, but also takes on various shapes depending on the charging method [2].Therefore, we proposed the compression method as a method that minimizes changes in discharge characteristics due to shape changes, with the aim of establishing a resource recycling system that integrates current collectors and zinc.
The positive electrode (Ni plated SUS mesh) was placed along the inner wall of a PP cylindrical container (250 mL volume) as the charging device, and a brass mesh (#200) cut to 2 cm × 2 cm was used as the negative electrode collector. Electrolyte concentration and charging rate were varied as charging conditions for zinc. The electrolyte was 5wt%, 15wt%, 30wt%, and 48wt% KOH solution (250g), and the charging rates were 20 mA/cm2, 50 mA/cm2, 100 mA/cm2, and 200 mA/cm2. 12.5 g of zinc oxide powder was introduced into the electrolyte to generate zinc equivalent to 820 mAh. The generated zinc was taken out together with the current collector, rinsed twice with pure water, and then made into a zinc plate of about 0.5 mm using a flat plate press, and the moisture was dried in air, which was used as the zinc fuel electrode. As in the case of charging, a PP cylindrical container was used as the discharging device for this fuel, with a graphite rod as the reference electrode in the center, and the zinc fuel electrode as the working electrode and the zinc plate as the counter electrode on either side of the graphite rod. The discharge current was set at 100 mA and discharged until the potential of the zinc fuel electrode reached 0.5 V.
As is known from previous studies, various forms of zinc could be observed depending on the charging conditions, but by compressing the charged zinc, a discharge efficiency of about 80% of the theoretical capacity could be obtained under all conditions. The remaining 20% is considered to be capacity loss due to oxidation during the post-processes of rinsing, compressing, and drying after charging. Therefore, it is thought that the oxidized area on the zinc surface can be reduced by rinsing after compressing, resulting in a smaller capacity loss. Comparing the energy density calculated from the discharge capacity of the zinc anode fabricated without compression treatment and the zinc fuel electrode with compression treatment, it was confirmed that the zinc charged in the alkaline electrolyte is in a very sparse state and has a low volumetric energy density as it is, and that it can be made dense by compression to achieve a high energy density. The zinc fuel electrode was found to have a high energy density by compressing the zinc. In this zinc fuel electrode, the ratio of current collector to zinc is 25.3% and 74.7%, indicating that the current collector accounts for about a quarter of the total. Therefore, it is considered necessary to increase the charging capacity to obtain a higher energy density. However, it has been confirmed that zinc precipitation is biased toward the electrode edges when the charging capacity is increased, and the zinc spreads when the electrode is compressed, so the energy density cannot be increased even if the charging capacity is increased. Therefore, the lecture will also discuss methods to suppress the edge concentration of zinc that occurs when the charging capacity is increased, as well as a resource recycling system using zinc as fuel.
[1] Zequan Zhao, ACS Energy Lett., 4, (9), (2019)
[2] R. Y. Wang, Journal of The Electrochemical Society, 153, (5), (2006)

Figure 1