Essential data for industrially relevant development of bifunctional cathodes and biopolymer electrolytes in solid-state zinc–air secondary batteries

Abstract

Presently, primary zinc–air batteries (ZABs), with a KOH-based liquid electrolyte, represent the first applied metal–air batteries and the most prevalent non-Li technology. The ZABs’ barrier to challenge Li–ion batteries is represented by rechargeability (needing a bifunctional cathode) and durability of the liquid electrolyte (due to leakage and/or evaporation). The liquid electrolyte should be replaced by a solid or gelled one but should not involve fossil-derived polymers or critical ceramic materials. Many naturally occurring biopolymers can be considered to prepare gelled electrolytes for ZABs, but focused literature about synthesis, properties, and applications in ZABs is still needed. Moreover, there is extensive literature about bifunctional cathodes for electrically rechargeable ZABs, but their assessment and performance for further industrialization are insufficient. The bottlenecks of sustainable gel electrolytes, extended cyclability, and relevant depth-of-discharge (DoD) per cycle should be met. In fact, industry seeks rechargeable materials, components, and assemblies capable of providing high current densities (e.g., >5–10 mA cm⁻²) in long cycles (e.g., >6–12 h) for as large as possible DoD (e.g., >5–10% per cycle, >100% total). The integration in the cells of gelled electrolytes and bifunctional cathode materials could overcome these problems if the correct calculations and testing are performed when carrying out experimental research. In this work, the actual state-of-the-art, key information, limitations, and calculations needed to assess a real promising cell integration between a biopolymer gel electrolyte and a cathode material in, at least, lab scale devices for rechargeability are reported. Finally, a wealth of experimental data spanning cyclability performance at low/medium drain rates (i.e., from 1–2 to 5–10 mA cm⁻²) at very short cycles (e.g., minutes) and long cycles (e.g., hours), enclosing a DoD analysis, are also shown, serving as a template for future studies.