Metal-air batteries represent a promising energy storage technology due to their high theoretical energy density, particularly for applications such as electric vehicles and grid storage. However, their operational reliability and safety depend on addressing several critical failure modes and implementing robust safety protocols. Unlike lithium-ion batteries, where thermal runaway dominates safety concerns, metal-air systems face distinct challenges that require specialized mitigation strategies.

One of the most prevalent failure modes in metal-air batteries is electrolyte leakage. The aqueous or non-aqueous electrolytes used in these systems can evaporate or leak, leading to rapid performance degradation. In zinc-air batteries, for example, water loss from the electrolyte increases internal resistance and reduces capacity. Similarly, in lithium-air batteries, organic electrolytes may decompose upon exposure to atmospheric moisture or oxygen. Preventing leakage demands hermetic sealing and advanced membrane technologies that permit oxygen diffusion while blocking electrolyte escape.

Another critical issue is anode passivation, particularly in zinc-air and aluminum-air batteries. During discharge, the metal anode can form oxide or hydroxide layers that impede ion transport and reduce efficiency. In aluminum-air batteries, the accumulation of aluminum hydroxide on the anode surface increases polarization losses, while zinc-air systems suffer from zinc oxide buildup. Strategies to mitigate passivation include alloying the anode with additives like indium or gallium, or employing pulsed discharge techniques to disrupt oxide layer formation.

Cathode flooding is a unique challenge in metal-air systems. The air cathode, which facilitates oxygen reduction, can become flooded with electrolyte, blocking oxygen diffusion pathways. This reduces the battery's power output and accelerates degradation. Hydrophobic coatings on cathode materials, such as polytetrafluoroethylene (PTFE), help maintain a balance between electrolyte retention and oxygen permeability. Additionally, optimizing pore structure in the cathode substrate can mitigate flooding while ensuring sufficient ionic conductivity.

Dendrite formation is a shared concern with lithium-ion batteries but manifests differently in metal-air systems. In rechargeable zinc-air batteries, zinc dendrites can grow during cycling, piercing the separator and causing internal short circuits. Unlike lithium-ion dendrites, which are primarily driven by uneven lithium plating, zinc dendrites form due to localized pH changes and uneven zinc deposition. Incorporating additives like polyethylene glycol or using three-dimensional anode structures can homogenize zinc deposition and suppress dendrite growth.

Corrosion of the metal anode in standby or idle states is another failure mode. Aluminum and zinc anodes corrode spontaneously in the presence of electrolyte, generating hydrogen gas and reducing coulombic efficiency. This self-discharge is exacerbated at elevated temperatures. Inhibitors such as sodium stannate or organic corrosion inhibitors are added to the electrolyte to slow down parasitic reactions. In some designs, electrolyte circulation systems remove dissolved metal ions to prevent precipitation and further corrosion.

Oxygen crossover presents a challenge in rechargeable metal-air batteries. During charging, oxygen evolved at the cathode can migrate to the anode, oxidizing the metal and reducing efficiency. This is particularly problematic in lithium-air batteries, where oxygen reacts with the lithium anode to form lithium peroxide. Protective coatings on the anode, such as ceramic or polymer layers, can block oxygen diffusion while allowing lithium ion transport.

Safety protocols for metal-air batteries differ significantly from those for lithium-ion systems. While lithium-ion batteries require rigorous thermal management to prevent runaway reactions, metal-air batteries prioritize preventing environmental exposure and managing gas evolution. Enclosures must be airtight to prevent electrolyte drying and oxygen intrusion, yet capable of venting excess pressure from hydrogen or oxygen accumulation. Pressure relief valves and gas recombination systems are often integrated to manage internal gases safely.

Unlike lithium-ion batteries, where thermal runaway is driven by exothermic decomposition of the electrolyte and cathode, metal-air systems are less prone to catastrophic thermal events. However, they can still overheat due to high current densities or internal shorts. Temperature sensors and current interrupt devices are employed to disconnect the battery if abnormal conditions are detected. For large-scale systems, passive cooling methods like heat sinks or phase-change materials are sufficient, as active cooling is rarely required.

Handling and storage protocols for metal-air batteries emphasize preventing accidental activation. Since these batteries rely on atmospheric oxygen, they are often stored with air cathodes isolated or in a dry state. For example, aluminum-air batteries are activated by adding electrolyte immediately before use. This reduces corrosion during storage but requires careful handling to avoid electrolyte spills or premature activation.

Recycling and disposal of metal-air batteries present unique considerations. Spent aluminum or zinc anodes can be recycled metallurgically, but electrolytes and air cathodes may contain hazardous materials. Neutralization of alkaline electrolytes is necessary before disposal, while carbon-based cathodes may require high-temperature treatment to remove organic residues. Regulatory frameworks for metal-air battery disposal are still evolving, with current guidelines often adapted from other battery chemistries.

In summary, metal-air batteries face distinct failure modes such as electrolyte leakage, anode passivation, cathode flooding, and corrosion, which demand tailored solutions. Safety protocols focus on environmental sealing, gas management, and preventing unintended activation rather than mitigating thermal runaway. As research progresses, advancements in materials and system design will further enhance the reliability and safety of this high-energy-density technology. The development of standardized testing and certification methods specific to metal-air systems will be crucial for their commercialization and widespread adoption.