Non-aqueous metal-air batteries represent a promising energy storage technology due to their high theoretical energy densities, particularly lithium-air and zinc-air systems. A critical component influencing their performance is the electrolyte, which must address multiple challenges, including oxygen solubility, stability against reactive intermediates, and suppression of parasitic reactions. Advanced electrolyte formulations based on organic solvents, ionic liquids, and functional additives have been explored to enhance efficiency and cycle life.

Organic solvents have been widely investigated for non-aqueous metal-air electrolytes due to their ability to dissolve lithium or zinc salts while facilitating oxygen transport. Common aprotic solvents include carbonates (ethylene carbonate, dimethyl carbonate), ethers (tetraethylene glycol dimethyl ether), and sulfones (dimethyl sulfoxide). Ether-based solvents exhibit higher oxygen solubility (~10 mM) compared to carbonates (~5 mM), improving discharge capacity. However, ethers are susceptible to nucleophilic attack by superoxide radicals (O2•−), leading to decomposition. Sulfones demonstrate superior stability against O2•− but suffer from high viscosity, limiting oxygen diffusion rates. Recent studies have shown that fluorinated solvents, such as bis(2,2,2-trifluoroethyl) ether, enhance oxidative stability while maintaining reasonable oxygen solubility (~8 mM).

Ionic liquids (ILs) offer advantages over conventional organic solvents, including negligible volatility, non-flammability, and wide electrochemical windows. Imidazolium, pyrrolidinium, and phosphonium-based ILs have been tested in metal-air systems. For example, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) exhibits high oxygen solubility (~20 mM) and moderate viscosity. However, imidazolium cations are prone to reduction at the metal anode, forming resistive layers. Pyrrolidinium ILs, such as N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ([Pyr13][FSI]), show improved stability but require additives to mitigate side reactions. Phosphonium ILs demonstrate exceptional stability against O2•− but face challenges in achieving high ionic conductivity.

Additives play a crucial role in stabilizing electrolytes by scavenging reactive species or forming protective interphases. Lithium nitrate (LiNO3) is commonly added to suppress anode passivation by promoting the formation of a stable solid electrolyte interphase (SEI). Redox mediators, such as 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ), facilitate oxygen reduction and evolution reactions (ORR/OER) by shuttling electrons between electrodes and oxygen species. Water scavengers like molecular sieves or aluminum oxide minimize proton-induced side reactions that lead to carbonate precipitation in lithium-air systems. Recent work has explored polymeric additives, such as poly(ethylene oxide), to improve electrolyte viscosity and electrode wetting without sacrificing oxygen permeability.

Enhancing oxygen solubility is critical for maximizing discharge capacity. Strategies include the use of perfluorinated compounds, which exhibit high oxygen affinity due to their low polarizability. For instance, perfluorohexane can increase oxygen concentration by up to 30% when used as a co-solvent. Another approach involves tuning solvent mixtures to optimize oxygen diffusivity and solubility. Binary or ternary blends of ethers, sulfones, and fluorinated solvents have demonstrated synergistic effects, achieving oxygen solubilities exceeding 15 mM while maintaining electrochemical stability.

Superoxide radical stability is a persistent challenge in non-aqueous metal-air electrolytes. O2•−, generated during ORR, initiates solvent decomposition pathways that degrade battery performance. Strategies to mitigate this include the use of stable anion receptors, such as tris(pentafluorophenyl)borane, which coordinates O2•− and reduces its reactivity. Another approach involves incorporating stable radical scavengers like hindered phenols or nitroxides, which quench O2•− before it attacks the solvent. Recent studies highlight the role of high donor number solvents (e.g., dimethylacetamide) in stabilizing O2•− through solvation effects, though these solvents often exhibit poor anodic stability.

Electrolyte formulation must also address metal anode compatibility. In lithium-air systems, uncontrolled lithium dendrite growth and interfacial reactions lead to capacity fade. Hybrid electrolytes combining ILs with organic solvents have shown promise in forming stable SEI layers while maintaining oxygen transport. For zinc-air batteries, alkaline-resistant additives, such as potassium hydroxide with zincate inhibitors, prevent dendritic growth and hydrogen evolution. The use of hydrophobic ionic liquids has also been explored to minimize water-induced corrosion at the zinc anode.

Long-term stability remains a key hurdle for non-aqueous metal-air electrolytes. Decomposition products, such as lithium carbonate or zinc hydroxide, accumulate over cycles, increasing impedance and reducing efficiency. In-situ characterization techniques, including Fourier-transform infrared spectroscopy and X-ray diffraction, have identified key degradation pathways. For example, nucleophilic attack by O2•− on carbonate solvents generates lithium alkyl carbonates, which precipitate on cathode surfaces. Advanced formulations incorporating sacrificial additives, such as lithium iodide, decompose preferentially to form protective layers that limit further electrolyte breakdown.

Future directions in electrolyte development focus on multi-functional designs that simultaneously address oxygen transport, radical stability, and interfacial compatibility. Computational screening of solvent and additive combinations has accelerated the identification of promising candidates. Machine learning models trained on experimental datasets predict properties such as oxidative stability and oxygen solubility with high accuracy, guiding experimental validation. Another emerging area is the use of localized high-concentration electrolytes, where dilute solvation structures improve ion transport while maintaining stability.

Performance metrics for advanced electrolytes include cycle life, round-trip efficiency, and rate capability. State-of-the-art non-aqueous metal-air systems achieve up to 200 cycles with 80% capacity retention when paired with optimized electrolytes. Round-trip efficiencies exceeding 75% have been reported using redox mediators and stable solvent blends. Rate capability remains limited by oxygen diffusion, with current densities typically below 0.5 mA/cm² for sustained operation.

In summary, non-aqueous metal-air battery electrolytes require careful balancing of multiple properties to achieve practical performance. Organic solvents, ionic liquids, and additives each contribute to addressing specific challenges, but no single formulation yet meets all requirements for commercial viability. Ongoing research focuses on hybrid systems, advanced additives, and computational optimization to unlock the full potential of this technology.