Metal-air batteries represent a promising energy storage technology due to their high theoretical energy densities, particularly for applications requiring lightweight and compact systems. However, the metal anode interface presents significant challenges, including dendrite formation, parasitic reactions with atmospheric components, and poor cycling stability. Addressing these issues requires careful engineering of the anode-electrolyte interface through artificial solid-electrolyte interphase (SEI) layers, alloying strategies, and three-dimensional host structures. These approaches aim to stabilize the metal anode while preserving ionic conductivity and minimizing side reactions.

Artificial SEI layers are designed to mimic the protective films that form naturally on some metal electrodes but with enhanced properties. For zinc-air batteries, researchers have developed hybrid organic-inorganic SEI layers combining polymers with ceramic nanoparticles. These layers exhibit ionic conductivities ranging from 1 to 10 mS/cm while reducing water-induced corrosion by over 70%. The polymer matrix provides flexibility to accommodate volume changes during cycling, while the ceramic components block dendritic penetration. For lithium-air systems, thin lithium nitride or lithium fluoride layers created via plasma-enhanced deposition techniques demonstrate exceptional stability against oxygen and moisture, with interfacial resistances below 5 Ω·cm². These artificial SEI layers must balance mechanical robustness with ionic transport properties, as excessive thickness can impede battery performance.

Alloying strategies modify the bulk properties of the metal anode to improve interfacial stability. In magnesium-air batteries, the addition of 5-10% aluminum or zinc forms intermetallic phases that alter deposition behavior. The alloying elements increase the nucleation density during plating, leading to smoother surfaces and reducing dendrite initiation sites. Experimental results show magnesium-aluminum alloys can achieve over 200 cycles at 0.5 mA/cm² with 85% capacity retention, compared to 50 cycles for pure magnesium anodes. For aluminum-air systems, gallium and tin additions below 1% weight fraction lower the oxidation potential while maintaining high coulombic efficiency above 95%. The alloying approach must carefully control composition to avoid introducing new side reactions or significantly reducing energy density.

Three-dimensional host structures provide physical scaffolds to guide uniform metal deposition and prevent anode shape changes. Porous copper or nickel foams with pore sizes between 50-200 micrometers have demonstrated effectiveness for zinc anodes, enabling current densities up to 20 mA/cm² without dendrite formation. The host structures reduce local current density by increasing surface area while maintaining mechanical integrity during stripping and plating cycles. For lithium-air batteries, carbon-based hosts with lithiophilic coatings such as gold or silicon nanoparticles improve wetting behavior and lower nucleation overpotential. Testing shows these structures can maintain 80% capacity after 100 cycles at practical areal capacities above 3 mAh/cm². The host design must optimize pore connectivity to ensure electrolyte access while providing sufficient mechanical support.

Preventing parasitic reactions requires addressing both chemical and electrochemical degradation pathways. For sodium-air batteries, hydrophobic polymer coatings on the metal anode reduce water penetration by three orders of magnitude while allowing sodium ion transport. In aluminum-air systems, electrolyte additives like zinc oxide form protective layers that suppress hydrogen evolution, decreasing gas generation rates from 5 mL/cm²/day to below 0.5 mL/cm²/day. The challenge lies in developing interface modifications that are stable across the full operating voltage window of the battery while not introducing significant additional resistance.

Ionic conductivity maintenance remains critical for practical battery operation. Composite SEI layers containing lithium or sodium ion-conducting ceramics like LLZO or NASICON-type materials demonstrate bulk conductivities exceeding 10⁻⁴ S/cm at room temperature. Graded interface designs with gradually changing composition help minimize interfacial resistance spikes, with some systems achieving total interface resistances below 10 Ω·cm². For zinc-air batteries, pH-buffering additives in the electrolyte help maintain stable ionic transport by preventing passivation layer formation, enabling stable operation across 500 cycles at 60% depth of discharge.

The selection of interface modification techniques depends heavily on the specific metal-air chemistry and operating conditions. Zinc-air systems benefit most from three-dimensional hosts and corrosion inhibitors, while lithium-air batteries require ultra-stable SEI layers to prevent side reactions with oxygen species. Magnesium and aluminum systems show particular improvement from alloying approaches that address both deposition behavior and surface reactivity. Each approach presents trade-offs between protection level, ionic conductivity, and manufacturability that must be balanced for commercial viability.

Performance evaluation of modified interfaces requires specialized characterization techniques. Electrochemical impedance spectroscopy reveals interface resistance contributions across different frequency ranges, while in-situ microscopy tracks deposition morphology evolution. X-ray photoelectron spectroscopy depth profiling identifies chemical composition gradients in artificial SEI layers, and pressure monitoring quantifies gas generation rates from parasitic reactions. These measurements inform iterative improvements to interface designs.

Scaling these interface modifications for commercial production introduces additional considerations. Vapor deposition techniques for artificial SEI layers must achieve uniform coverage at high throughput, while alloying processes require precise composition control in bulk material production. Three-dimensional host structures need cost-effective manufacturing methods that maintain consistent pore architecture. The most promising solutions will combine multiple approaches—for example, alloyed metal anodes with nanostructured surfaces coated by thin protective layers—to address all aspects of interface instability simultaneously.

Continued progress in metal anode interface engineering will enable practical metal-air batteries with long cycle life and high efficiency. The optimal solutions will likely emerge from systematic studies comparing different modification strategies under identical testing conditions, allowing direct performance benchmarking. As understanding of interface phenomena deepens, targeted designs can be developed for specific application requirements ranging from electric vehicles to grid-scale storage. The field remains active with research efforts focused on overcoming the remaining challenges in metal-air battery commercialization.