**Introduction to Metal-Air Batteries**

Metal-air batteries have garnered significant attention in recent years due to their high energy density and potential to revolutionize energy storage. Among them, zinc-air and lithium-air batteries stand out as promising candidates, each with unique advantages and challenges. As the world seeks sustainable energy solutions, understanding the viability of these two types of metal-air batteries becomes crucial.

**Fundamentals of Zinc-Air and Lithium-Air Batteries**

Zinc-air batteries utilize zinc as the anode material and oxygen from the air as the cathode. The oxygen reduction reaction, which occurs at the cathode, is integral to the battery's operation. On the other hand, lithium-air batteries use lithium as the anode and also rely on oxygen from the air as the cathode. Both types aim to exploit the abundant and free nature of air to achieve an efficient and sustainable energy storage system.

**Energy Density Comparison**

One of the most critical aspects when comparing these two battery types is energy density. Lithium-air batteries theoretically offer a significantly higher energy density than zinc-air batteries. This high energy potential makes lithium-air batteries particularly appealing for applications like electric vehicles and portable electronics, where the weight and size of the battery are crucial considerations. However, achieving this theoretical energy density in practical applications remains a significant challenge due to issues like dendrite formation and electrolyte stability.

**Cost and Material Availability**

Zinc is more abundant and cheaper compared to lithium, making zinc-air batteries more cost-effective and sustainable in terms of raw material sourcing. The cost advantage of zinc-air batteries could make them a preferable option for large-scale applications such as grid energy storage systems. In contrast, the scarcity and high cost of lithium can be a limiting factor for the wide-scale adoption of lithium-air batteries, especially in developing regions.

**Technical Challenges and Innovations**

Both battery types face significant technical hurdles. Zinc-air batteries struggle with issues related to the stability of the zinc electrode and the efficiency of oxygen reduction catalysts. Efforts to improve the longevity and rechargeability of zinc-air systems are ongoing, with researchers exploring novel catalysts and electrode materials.

Lithium-air batteries, while promising with their high energy density, face even more daunting challenges. Problems such as the growth of lithium dendrites, electrolyte decomposition, and the clogging of air electrodes with reaction products need to be addressed. Innovations in solid-state electrolytes and the development of more robust cathode structures are crucial for the future of lithium-air batteries.

**Environmental Impact**

From an environmental standpoint, zinc-air batteries have an advantage due to the non-toxic and widely available nature of zinc. The environmental footprint of mining and processing zinc is relatively lower than that of lithium. In contrast, lithium mining and processing have raised environmental concerns, including water usage and habitat disruption.

**Applications and Market Prospects**

Zinc-air batteries have found use in hearing aids and other small-scale applications due to their low cost and reasonable energy density. The potential for scaling up to larger applications remains a point of interest for further research and development.

Lithium-air batteries, with their high theoretical energy density, are highly sought after for future electric vehicles. However, their practical application remains in the research and development phase, with commercial availability still some years away.

**Conclusion: Which is More Viable?**

Determining the more viable metal-air battery between zinc-air and lithium-air depends heavily on the application and specific requirements. Zinc-air batteries present a more practical solution currently due to their cost-effectiveness and environmental friendliness, especially for stationary energy storage. Conversely, lithium-air batteries hold promise for high-energy applications, provided that their significant technical challenges can be overcome.

Ultimately, both battery technologies are vital in the pursuit of renewable energy solutions, and ongoing research and innovation are key to unlocking their full potential in the energy landscape.