Zinc-air batteries have gained attention as a promising energy storage technology due to their high theoretical energy density, low cost, and environmental friendliness. A critical component influencing their performance is the electrolyte, which facilitates ion transport while maintaining stability during operation. Aqueous and quasi-solid electrolytes are widely studied for zinc-air batteries, each presenting unique advantages and challenges related to ionic conductivity, zinc solubility, carbonate formation, and water evaporation.

Aqueous electrolytes in zinc-air batteries are typically classified into alkaline and neutral types. Alkaline electrolytes, such as potassium hydroxide (KOH), offer high ionic conductivity, often exceeding 500 mS/cm at concentrations of 6–8 M. This facilitates efficient oxygen reduction and evolution reactions at the air electrode. However, alkaline electrolytes introduce challenges, including zinc anode corrosion and dendrite formation due to high zinc solubility. Zincate ions (Zn(OH)₄²⁻) form in alkaline media, leading to shape change and passivation, which reduce cycle life. Neutral saline electrolytes, such as zinc sulfate or zinc chloride solutions, mitigate dendrite growth and corrosion but suffer from lower ionic conductivity, typically below 100 mS/cm. The oxygen reaction kinetics are slower in neutral conditions, limiting power density.

Carbonate formation is a major issue in aqueous electrolytes, particularly in alkaline systems. Atmospheric CO₂ reacts with hydroxide ions to form carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻), which precipitate as insoluble salts, clogging the air electrode pores and reducing performance. Strategies to minimize carbonate formation include using CO₂ scrubbers in the air intake or employing electrolyte circulation systems to remove carbonates. Neutral electrolytes are less prone to carbonate formation but still require careful management of pH to prevent zinc hydroxide precipitation.

Water evaporation is another challenge in aqueous zinc-air batteries, especially in open-system designs where the electrolyte is exposed to air. Evaporation leads to concentration changes, increased internal resistance, and eventual drying of the cell. Hydrophobic membranes or electrolyte additives can reduce water loss, but these solutions often trade off against oxygen diffusion efficiency.

Quasi-solid electrolytes, including hydrogel polymers and hybrid systems, have emerged as alternatives to liquid electrolytes. Hydrogels, such as polyacrylamide or polyvinyl alcohol networks saturated with alkaline or neutral solutions, combine the ionic conductivity of liquids with the dimensional stability of solids. These materials typically exhibit conductivities in the range of 10–200 mS/cm, depending on the polymer matrix and liquid electrolyte content. Hydrogels also suppress zinc dendrite growth by providing a mechanically resistant barrier while maintaining interfacial contact with the electrodes.

Hybrid electrolytes incorporate inorganic fillers or ion-conductive polymers to enhance performance. For example, adding silica nanoparticles or cellulose nanofibers improves mechanical strength and reduces water evaporation. Hybrid systems may also include pH buffers, such as borate or phosphate salts, to stabilize the electrolyte against acid-base fluctuations during cycling. These modifications help extend cycle life by preventing drastic pH changes that accelerate electrode degradation.

pH stabilization is critical in both aqueous and quasi-solid electrolytes. In alkaline systems, pH drift occurs due to zincate formation and oxygen reactions, leading to localized high or low pH zones that degrade electrodes. Buffering agents, such as potassium borate or potassium phosphate, help maintain pH within a stable range. Neutral electrolytes require similar stabilization to prevent zinc hydroxide precipitation at high pH or hydrogen evolution at low pH.

The table below summarizes key properties of different electrolyte types:

Electrolyte Type Ionic Conductivity (mS/cm) Zinc Solubility Carbonate Formation Water Evaporation
Alkaline (KOH) >500 High Severe Moderate
Neutral Saline <100 Low Mild Moderate
Hydrogel Polymer 10–200 Medium Mild Low
Hybrid Electrolyte 50–300 Medium-Low Mild Low

Despite progress, challenges remain in optimizing these electrolytes for long-term operation. Future research may focus on advanced hydrogel formulations with self-healing properties or hybrid electrolytes incorporating ionic liquids for wider electrochemical stability. The balance between ionic conductivity, stability, and manufacturability will determine the viability of aqueous and quasi-solid electrolytes in commercial zinc-air batteries.

In summary, aqueous and quasi-solid electrolytes each present trade-offs in zinc-air battery performance. Alkaline electrolytes offer high conductivity but suffer from carbonate formation and zinc corrosion, while neutral electrolytes reduce side reactions at the cost of slower kinetics. Quasi-solid systems improve stability and water retention but require further development to match the conductivity of liquid electrolytes. Advances in material design and pH management will be essential to overcoming these limitations and enabling practical zinc-air battery systems.