Metal-air batteries represent a promising energy storage technology due to their high theoretical energy density and potential for cost-effective materials. However, their performance and longevity are significantly influenced by environmental humidity, which affects electrolyte stability, cathode reactions, and metal electrode corrosion. Understanding these effects and developing mitigation strategies are critical for advancing metal-air battery technology.

**Water Uptake in Electrolytes**
The electrolyte in metal-air batteries plays a central role in ion transport and electrochemical reactions. In aqueous electrolytes, humidity can lead to excessive water absorption, diluting the electrolyte and altering its ionic conductivity. Non-aqueous electrolytes, while less prone to dilution, may still experience water ingress, leading to hydroxide formation and parasitic reactions. For example, in lithium-air batteries, water contamination promotes the formation of lithium hydroxide instead of lithium peroxide, reducing battery efficiency.

In alkaline electrolytes, common in zinc-air batteries, water absorption can accelerate electrolyte carbonation when carbon dioxide from the air dissolves, forming carbonate precipitates that clog the air cathode. Studies have shown that even small increases in relative humidity (above 60%) can lead to measurable decreases in discharge capacity due to these side reactions.

**Cathode Flooding and Oxygen Transport**
The air cathode is particularly vulnerable to humidity. Excessive moisture causes electrode flooding, where water accumulates in the porous structure, blocking oxygen diffusion pathways. This flooding reduces the active sites available for oxygen reduction reactions, directly impairing battery performance. In extreme cases, complete pore blockage can occur, leading to sudden voltage drops during discharge.

The degree of flooding depends on the cathode’s hydrophobicity and pore structure. Electrodes with insufficient hydrophobic binders, such as polytetrafluoroethylene (PTFE), are more susceptible. Research indicates that optimizing the cathode’s microporous layer with graded hydrophobicity can mitigate flooding while maintaining oxygen permeability.

**Corrosion Acceleration in Metal Electrodes**
Humidity-induced corrosion is a major degradation mechanism in metal-air batteries. For zinc-air batteries, water exposure leads to zinc oxidation and hydrogen evolution, consuming active material and generating gas that may damage cell integrity. Aluminum and magnesium electrodes are even more reactive, forming passive oxide layers that increase polarization losses.

In non-aqueous systems, such as lithium-air batteries, trace water reacts with the lithium anode, forming lithium hydroxide and lithium oxide. This not only depletes the anode but also increases interfacial resistance. Corrosion rates are highly dependent on relative humidity, with studies demonstrating that lithium anodes exposed to 30% humidity exhibit significantly shorter cycle life compared to those in dry environments.

**Mitigation Strategies**
To address these challenges, several strategies have been developed to minimize humidity-related degradation while maintaining battery performance.

**Membrane Barriers**
Selective membranes can restrict water vapor transmission while allowing oxygen diffusion. Materials such as perfluorinated ionomers (e.g., Nafion) and hydrophobic microporous polymers have been tested for this purpose. These membranes act as humidity buffers, reducing water ingress without severely limiting oxygen supply. However, their effectiveness depends on thickness and permeability balance—thicker membranes improve water blocking but may increase oxygen diffusion resistance.

**Desiccant Materials**
Incorporating desiccants within the battery structure can locally reduce humidity levels. Molecular sieves and silica gel are commonly used due to their high water adsorption capacity. In zinc-air batteries, placing a desiccant layer near the air cathode has been shown to extend cycle life by maintaining lower internal humidity. The trade-off is the added weight and volume, which may reduce overall energy density.

**Hydrophobic Coatings**
Applying hydrophobic coatings to the metal electrode surface can slow corrosion. For example, zinc electrodes coated with conductive polymers or graphene oxide layers exhibit reduced water contact and suppressed hydrogen evolution. Similarly, lithium anodes protected by artificial solid-electrolyte interphases (SEI) with hydrophobic properties show improved stability in humid conditions.

**Electrolyte Additives**
Certain additives can stabilize electrolytes against humidity effects. For non-aqueous electrolytes, lithium salts with hydrophobic anions (e.g., lithium bis(trifluoromethanesulfonyl)imide) reduce water solubility. In alkaline electrolytes, corrosion inhibitors like inorganic phosphates or organic surfactants can suppress zinc dissolution.

**Conclusion**
Environmental humidity poses significant challenges for metal-air batteries, influencing electrolyte chemistry, cathode functionality, and metal electrode stability. Water management strategies, including selective membranes, desiccants, hydrophobic coatings, and electrolyte additives, are essential for improving reliability and cycle life. Future research should focus on optimizing these approaches to achieve a balance between humidity resistance and electrochemical performance, enabling broader adoption of metal-air battery systems in real-world applications.