Metal-air batteries are a promising energy storage technology due to their high theoretical energy density, particularly for applications requiring lightweight and long-lasting power sources. A critical component influencing their performance is the electrolyte, which facilitates ion transport between the anode and cathode while ensuring stability and compatibility. Three primary electrolyte formulations—aqueous, organic, and hybrid—are employed in metal-air batteries, each with distinct advantages and limitations in terms of ionic conductivity, electrochemical stability, and interaction with metal anodes.

Aqueous electrolytes are widely used in metal-air batteries due to their high ionic conductivity and cost-effectiveness. Typically composed of alkaline solutions such as potassium hydroxide (KOH) or neutral saline solutions, aqueous electrolytes offer ionic conductivities in the range of 0.1 to 1 S/cm, which is significantly higher than non-aqueous alternatives. This high conductivity enables efficient charge transfer, reducing internal resistance and improving rate capability. However, aqueous electrolytes face challenges related to electrochemical stability. The narrow voltage window of water, approximately 1.23 V, limits the operational voltage of the battery and can lead to parasitic reactions such as hydrogen evolution at the anode and oxygen evolution at the cathode. Additionally, metal anodes like zinc or aluminum may corrode in aqueous environments, reducing Coulombic efficiency and cycle life. Despite these drawbacks, aqueous electrolytes remain attractive for primary metal-air batteries and certain rechargeable systems where cost and safety are prioritized.

Organic electrolytes, often based on aprotic solvents like dimethyl sulfoxide (DMSO) or acetonitrile, address some limitations of aqueous systems by offering a wider electrochemical stability window, typically between 2.5 and 4 V. This broader window allows for higher operating voltages, which can enhance energy density. Organic electrolytes also mitigate corrosion of metal anodes, as they are less reactive than aqueous solutions. However, organic electrolytes exhibit lower ionic conductivity, usually in the range of 0.01 to 0.1 S/cm, due to the lower dielectric constant and higher viscosity of organic solvents. This reduced conductivity can lead to higher polarization and lower power density. Another challenge is the solubility of discharge products, such as lithium peroxide in lithium-air batteries, which can clog the cathode and limit rechargeability. Furthermore, organic solvents are often volatile and flammable, raising safety concerns for large-scale applications. Research efforts focus on optimizing solvent-salt combinations to improve ionic transport while maintaining stability.

Hybrid electrolytes aim to combine the benefits of aqueous and organic systems while minimizing their respective drawbacks. These electrolytes typically consist of a dual-phase system, such as an aqueous anode compartment separated from an organic cathode compartment by a selective membrane. The hybrid approach can leverage the high ionic conductivity of aqueous electrolytes near the anode while utilizing the broad voltage window of organic electrolytes near the cathode. For example, in lithium-air batteries, a hybrid electrolyte might pair a lithium-conducting aqueous solution with an organic catholyte to prevent anode corrosion and enhance oxygen reduction kinetics. Ionic conductivities in hybrid systems vary depending on the composition but often fall between those of purely aqueous and purely organic electrolytes. Stability is improved compared to fully aqueous systems, though challenges remain in managing the interface between the two phases and preventing crossover of reactive species. Hybrid electrolytes also introduce additional complexity in cell design, which can increase manufacturing costs.

Compatibility with metal anodes is a critical consideration for all three electrolyte types. Aqueous electrolytes often require additives or protective coatings to suppress corrosion and dendrite formation on metal anodes like zinc or lithium. Organic electrolytes, while less corrosive, may still necessitate the use of stabilizing salts or artificial interphases to enhance anode compatibility. Hybrid systems must carefully balance the requirements of both anode and cathode environments to ensure uniform performance. For instance, in zinc-air batteries, a hybrid electrolyte might employ a neutral pH aqueous solution near the zinc anode to prevent passivation while using an alkaline catholyte to optimize oxygen reduction.

In summary, aqueous electrolytes excel in ionic conductivity and cost but suffer from limited stability and anode compatibility issues. Organic electrolytes offer wider voltage windows and better anode stability but at the expense of lower conductivity and safety concerns. Hybrid electrolytes represent a middle ground, though their complexity and cost may hinder widespread adoption. The choice of electrolyte depends on the specific requirements of the metal-air battery system, including energy density, cycle life, and safety considerations. Ongoing research aims to refine these formulations to unlock the full potential of metal-air batteries for diverse applications.