Abstract:

Zinc-air batteries have gained popularity due to their high capacity, low cost, and low self-discharge performance. Zinc-air button batteries are the preferred power source for hearing aids, particularly in the medical field. German company Varta, American company Rayovac, and Chinese company ZeniPower are the main manufacturers of zinc-air hearing-aid button cells. The commercial air electrodes of zinc-air batteries are frequently manufactured using a wet process that involves preparing catalyst powders, which is time-consuming and energy-intensive. This study introduced a novel method using a 3000 rpm high-speed cutting machine without adding any solvent for the granulating process. Then after being treated by 12 mesh sieve, highly free-flowing dry catalyst powders were obtained for further treatment. Scanning electron microscope shows that the polytetrafluoroethylene (PTFE) binders in the catalyst powders are widely and completely fibrillated, which is beneficial to binding the catalyst powders together tightly even in the long term of storage. The above catalyst powders were then pressed into a roll-to-roll catalyst layer by self-made rollers and relevant machines. The current collector, waterproof and air permeable films, and the catalyst layer were pressed together to obtain the solvent-free fabrication of commercial zinc-air electrodes. A traditional commercial wet process was utilized to prepare the catalyst powders and air electrodes to make a comparison. It demonstrates that the discharge capacity of zinc-air batteries using solvent-free electrodes is 5% higher than the wet method, which could be attributed to the solvent-free fabrication process's widespread and complete formation of strong fibrillated PTFE fibers. Different electrode densities and thicknesses, as well as different waterproof and air permeable films, were also investigated. It shows that electrodes with a low density of 1.15 g·cm^-3 have better capacity performance after storage, possibly because there is higher bonding strength between the catalyst layer and PTFE film in the air electrode. Electrodes with larger thicknesses as 0.52 mm contribute to higher power density resulting from more three-phase active sites in the catalyst layer. Because of the high strength of carbon-fluorine bonds, pure PTFE film functioned as waterproof and air permeable films with better anti-leakage performance. To summarize, this solvent-free process is simple, energy-efficient, and appropriate for commercializing metal-air batteries. More applications in various types of metal-air batteries, such as large-size, solid-state, flexible, and secondary batteries, could benefit from this commercialized technology in the future.

Key words: zinc-air batteries, air electrodes, solvent-free fabrication, roll to roll, commercial application