Development of solid electrolytes in Zn-air and Al-air batteries: From material selection to performance improvement strategies

Aqueous-based Zn-air and Al-air batteries are considered to be promising post-lithium energy storage technologies owing to their safety, environmental friendliness, affordability, and high energy density. Nevertheless, traditional liquid Zn-air and Al-air batteries have problems such as volatilization and leakage, as well as the realization of miniaturized, portable, and wearable electronic devices. The practice of optimizing the battery structure by replacing the flowing electrolyte with a solid type has emerged and made significant progress in the past ten years. Herein, this review provides a guiding and comprehensive summary of the basic understanding and manufacturing ideas of the solid electrolyte for Zn-air and Al-air batteries. First, two types of alkaline solid electrolytes are distinguished, including alkaline anion exchange membranes (AAEMs) and gel polymer electrolytes (GPEs). Then, three sorts of major framework materials (i.e., artificial organic polymer, biomass materials, and inorganic materials) are reviewed and discussed. Most importantly, the latest research progress and improvement strategies to enhance the electrolyte membrane performances involving conductivity, mechanical properties, and electrochemical stability are also highlighted. Finally, challenges and prospects for the future development of alkaline solid electrolytes are emphasized.