Towards Solid State Zinc-Air Batteries—Enabling Bio-Based Polymer Gel Electrolytes

Electrochemical energy storage technologies can help mitigate the dependency from fossil sources and greenhouse gas emissions, enabling energy storage from renewable sources and their integration into the grid, thus driving energy transition worldwide. Presently, the most commercialized technology is lithium-ion, from small portable devices to electric vehicles, and up to stationary applications at grid level. However, its use in stationary energy storage systems involves a great cost, being necessary to seek new highly competitive technologies at lower cost. Therefore, it is strategically not convenient to rely only on this technology to implement electrochemical energy storage, but few valid alternative candidates can be foreseen considering the electrochemical window and energy density. Zn-air secondary battery is one of these; however, affordable development based on industrially and quantitatively relevant breakthroughs is required.<br/>Zn-air batteries based on liquid electrolyte have been widely investigated but due to the open nature of the technology (cell is open to air), electrolyte leaks and/or evaporates causing a decrease in the performance and subsequent cell failure. As a result, recent research has mainly focused on developing synthetic biodegradable polymer-based gel polymer electrolytes (GPEs) due to their low cost and affordability (e.g., alkaline GPEs such as polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyacrylamide (PAM), polyethylene oxide (PEO)). Much of the early, but also recent, work in metal air batteries (especially in Zn and Na air batteries) has been devoted to the study of biopolymer-based membranes, that act as separators and electrolytes to substitute liquid electrolytes.<br/>In this work, the development of an emerging solid electrolyte, based on a natural, linear, and biodegradable polymer will be presented. The biopolymer is directly jelled in the liquid electrolyte solution to for a stable gel, that not only presents outstanding ionic conductivity, (same magnitude order as 8M KOH solution liquid electrolyte) but also mechanical stability, even after being subjected to a constant discharge until battery failure. The gel does not liquefy, and behaves as an elastic material till up to a certain applied compression load. In this study, its manufacture, mechanical characterization and electrochemical performance will be discussed.