Recent progress of electrolytes and electrocatalysts in neutral aqueous zinc-air batteries

Rechargeable Zn-air batteries (R-ZABs) are promising electrochemical energy storage devices with the advantages of high theoretical energy density, low cost of Zn metal and environment-friendliness. ZABs with neutral electrolytes can avoid or mitigate many side reactions harassing alkaline ZABs because neutral electrolyte is mild and green, which is conducive to battery life. However, the development of neutral ZABs is still in an initial stage. In this review, we would like to summarize the recent progress of neutral electrolytes and electrocatalysts, which are applied in or beyond ZABs. The working mechanism of ZABs is first introduced, especially oxygen electrochemistry on cathode. After elucidating the disadvantages of alkaline electrolytes, several neutral and near-neutral electrolytes are then summarized on developments as well as challenges, such as inorganic/organic salt solutions, high concentration electrolytes and quasi-solid electrolytes. Next, different cathode structures and their impacts on catalysts are presented. The latest progress of different catalysts in neutral electrolytes are classified and summarized, including metal oxides, metal phosphides/sulfides, metal phosphates, single-atom catalysts, etc. Finally, we conclude with a summary and propose a brief outlook for the future developments of electrolytes and oxygen catalysts applied in neutral ZABs.