Rechargeable zinc-air batteries with neutral electrolytes: Recent advances, challenges, and prospects

Rechargeable zinc-air batteries (R-ZABs) are attractive for many essential energy storage applications - from portable electronics, electric vehicles to incorporation of renewable energy due to their high energy storage density, abundant raw materials, and inherent safety. However, alkaline electrolytes cause critical obstacles in realizing a long battery life. Thus, neutral electrolytes are attracting growing interest. However, the current understandings of R-ZABs in neutral/near-neutral electrolytes are far behind those in alkaline electrolytes. This review summarizes the latest research progress of neutral electrolytes used in R-ZABs, including aqueous inorganic and organic salt solutions, water-in-salt electrolytes, and quasi-solid electrolytes based on polymer hydrogels. Research efforts in improving the stability of Zn anodes in neutral electrolytes are also reviewed. Reaction mechanisms of oxygen reduction and evolution reactions in alkaline and neutral electrolytes are compared in the context of R-ZABs, together with a summary of potential oxygen electrocatalysts applicable in neutral conditions. Different device configurations are introduced. We further provide our perspectives on future research directions of R-ZABs with neutral electrolytes.