As one of the proposed post lithium-ion battery technologies, metal-air batteries have received revived interest recently. Among the different types of metal-air batteries, rechargeable zinc-air battery (RZAB) is a promising electrochemical energy storage device with the advantages of high theoretical energy density of 1086 Wh kg^-1, high abundance, low toxicity and intrinsic safety.¹ However, current RZAB still suffer from poor energy efficiency and cycle life, owing mainly to the zinc-anodes, electrolytes, and air-cathodes. To improve the low energy efficiency, many efforts have been made to promote both of the sluggish oxygen reduction/evolution reactions in the air-cathodes by engineering bifunctional catalyst materials with high reactivity. Although significant progress has been made in developing suitable catalyst materials for the air-cathode, many technical challenges still remain in electrolyte to achieve the long cycle life while maintaining high performance.

Aqueous electrolytes, predominantly alkaline electrolytes, have been adopted by the RZAB system since its birth. Potassium hydrogen (KOH) is the most commonly used alkaline electrolyte because of its high ionic conductivity, high activity for both the zinc and air electrodes as well as good low temperature performance.² Despite its desirable properties, the usage of KOH in RZAB raises several technical problems: i) zinc dendrite formation, shape change, surface passivation from complicated reaction between zinc and OH^- as well as the dissolution and migration of Zn(OH)₄^2-; ii) corrosion of the carbon-based air cathode in concentrated alkaline electrolyte; iii) formation of insoluble K₂CO₃ arising from the reaction of aerial CO₂ with KOH; iv) reduced cell shelf due to the high corrosive ability of concentrated alkaline electrolyte to the stainless steel used in the experimental cell component.

In this work, specific attention is given to the obstacles caused by the alkaline electrolytes with the focus on the fundamental understanding of zinc-anode reaction mechanisms, carbonization of electrolyte as well as the precipitation of carbonates on the air cathode. Furthermore, the effects of the practical operating conditions on battery performance and durability are discussed, including the experimental cell configuration for RZAB as well as the contaminants commonly encountered in ambient air (e.g. carbon dioxide). The approaches to overcome the challenges are also presented and analyzed for facilitating further research and development of the electrolyte for RZAB.

Reference

1. Li, Y.; Dai, H., Recent advances in zinc-air batteries. Chemical Society reviews 2014, 43 (15), 5257-75.
2. R. Mainar, A.; Leonet, O.; Bengoechea, M.; Boyano, I.; de Meatza, I.; Kvasha, A.; Guerfi, A.; Alberto Blázquez, J., Alkaline aqueous electrolytes for secondary zinc-air batteries: an overview. International Journal of Energy Research 2016, 40 (8), 1032-1049.