Metal-air batteries represent a promising class of energy storage technologies due to their high theoretical energy densities, which often exceed those of conventional lithium-ion batteries. However, their sustainability depends on multiple factors, including material availability, toxicity profiles, and recycling feasibility. This assessment focuses on these aspects to evaluate the environmental and resource implications of metal-air battery technologies.

Material availability is a critical factor in determining the scalability and long-term viability of metal-air batteries. The most common variants include zinc-air, aluminum-air, lithium-air, and iron-air batteries. Zinc is relatively abundant, with global reserves estimated at over 200 million metric tons, and is mined in over 50 countries. Aluminum is even more plentiful, being the third most abundant element in the Earth's crust. Lithium, while less abundant than zinc or aluminum, is increasingly extracted from brines and hard-rock deposits, though supply chain constraints may arise due to growing demand across battery technologies. Iron is the most abundant of these metals, ensuring minimal supply risks. The cathodes in metal-air batteries typically rely on atmospheric oxygen, eliminating the need for heavy or resource-intensive cathode materials. However, catalysts such as platinum or manganese oxide may be required to enhance oxygen reduction reactions, introducing additional material constraints.

Toxicity and environmental hazards associated with metal-air batteries vary by chemistry. Zinc-air batteries are generally considered low-toxicity, with zinc being an essential micronutrient and posing minimal environmental risks if managed properly. Aluminum is also non-toxic, though its production is energy-intensive and associated with significant greenhouse gas emissions. Lithium-air batteries raise concerns due to the reactivity of lithium metal and the potential formation of peroxides or superoxides, which can be hazardous if not contained. Iron-air systems are among the safest, with iron being non-toxic and environmentally benign. Electrolytes in metal-air batteries range from aqueous solutions, such as potassium hydroxide in zinc-air systems, to organic or ionic liquid electrolytes in lithium-air variants. Aqueous electrolytes are less hazardous but may require additives to prevent corrosion or evaporation. Non-aqueous electrolytes often involve flammable or reactive components, necessitating careful handling and disposal.

End-of-life recycling pathways for metal-air batteries are still under development, with some chemistries presenting clearer solutions than others. Zinc-air batteries benefit from established zinc recycling infrastructure, as zinc is widely reclaimed from industrial waste and consumer products. Recovery rates for zinc exceed 80% in regions with mature recycling systems. Aluminum is similarly recyclable, with nearly 75% of all aluminum ever produced still in use today due to efficient closed-loop recycling. Lithium-air batteries face greater challenges, as lithium metal recovery is less mature, and the reactive nature of the electrodes complicates disassembly. Iron-air systems are straightforward to recycle, with iron being one of the most recycled materials globally.

Electrolyte recycling remains a technical hurdle across most metal-air systems. Aqueous electrolytes can often be neutralized and treated as industrial wastewater, though recovery of alkaline components like potassium hydroxide is rarely practiced. Organic electrolytes pose greater difficulties due to their complex compositions and potential degradation products. Ionic liquids, while stable, require specialized processes for recovery and reuse. Catalyst recovery is another consideration, particularly for systems using precious metals like platinum, where recycling is economically incentivized but technologically demanding.

The sustainability of metal-air batteries also depends on energy inputs during production and recycling. Aluminum production is particularly energy-intensive, requiring approximately 15 kWh per kilogram of primary aluminum, compared to 4 kWh per kilogram for zinc. Recycling aluminum reduces this energy demand by up to 95%, underscoring the importance of closed-loop systems. Iron and zinc production are less energy-intensive, but recycling still offers significant energy savings. Lithium extraction and processing are energy- and water-intensive, particularly in brine operations, which may offset some of the environmental benefits of lithium-air batteries if not managed sustainably.

Emerging innovations aim to address these sustainability challenges. For zinc-air batteries, research focuses on improving electrode durability to extend lifespan and reduce material consumption. Aluminum-air systems are exploring alternative catalysts to replace precious metals, reducing costs and environmental impacts. Lithium-air technologies are investigating stable electrolyte formulations to mitigate safety risks and simplify recycling. Iron-air batteries, while less energy-dense, are gaining attention for their inherently sustainable material profile and compatibility with existing recycling infrastructure.

Regulatory frameworks will play a key role in ensuring the sustainable lifecycle management of metal-air batteries. Policies mandating recycling rates for critical metals, such as zinc and lithium, could drive investment in recovery technologies. Standardized labeling and collection systems would improve the efficiency of end-of-life processing, particularly for consumer applications. Hazardous material regulations must also evolve to address the unique risks posed by reactive electrolytes or metal electrodes.

In summary, the sustainability of metal-air batteries varies significantly by chemistry. Zinc-air and iron-air systems offer favorable profiles in terms of material availability, toxicity, and recyclability, while lithium-air and aluminum-air batteries present greater challenges due to material reactivity and energy-intensive production. Advances in recycling technologies and regulatory support will be essential to maximize the environmental benefits of these energy storage systems. Without concerted efforts to address end-of-life management and material sourcing, the potential of metal-air batteries may be limited by unsustainable practices.