Sodium-air batteries represent a promising alternative to lithium-air systems due to sodium's natural abundance and lower cost. The chemistry primarily involves the formation of sodium superoxide (NaO2) or sodium peroxide (Na2O2) during discharge, depending on reaction conditions. These discharge products dictate the battery's theoretical energy density, reversibility, and cycle life. The choice between NaO2 and Na2O2 formation is influenced by electrolyte composition, oxygen partial pressure, and catalytic effects at the electrode surface.

The electrochemical reactions in non-aqueous Na-air batteries proceed through a two-electron transfer process. The primary discharge product, NaO2, forms via a one-electron reduction of oxygen, while Na2O2 results from a two-electron reduction. NaO2 offers advantages in terms of lower overpotential and better reversibility compared to Na2O2. The theoretical energy density for NaO2-based systems reaches approximately 1100 Wh/kg, while Na2O2-based systems achieve around 1600 Wh/kg. However, the practical energy density is significantly lower due to parasitic reactions and incomplete rechargeability.

Electrolyte selection critically impacts the discharge mechanism and cycle stability. Non-aqueous electrolytes, typically composed of sodium salts such as NaPF6 or NaClO4 dissolved in organic solvents like diglyme or tetraglyme, facilitate NaO2 formation. These solvents must exhibit high oxygen solubility, chemical stability against superoxide species, and low volatility. Aqueous electrolytes, though less common due to sodium's high reactivity with water, can form NaOH as a discharge product, but this leads to irreversible side reactions and poor cyclability.

A persistent challenge in Na-air batteries is the formation of sodium carbonate (Na2CO3) from CO2 contamination. Even trace amounts of CO2 in the oxygen feed or electrolyte decomposition lead to irreversible Na2CO3 deposition on the cathode, reducing active sites and increasing polarization. Unlike Li-air systems, where Li2CO3 formation is also problematic, sodium carbonate is more soluble in certain organic electrolytes, but its accumulation still degrades performance over time. Strategies to mitigate this include the use of CO2 scrubbers in the oxygen supply and the development of stable electrolyte formulations resistant to nucleophilic attack by superoxide.

Comparing Na-air with Li-air systems reveals key differences in reaction pathways and product stability. Lithium-air batteries typically form Li2O2 as the primary discharge product, which is more stable than NaO2 but suffers from higher overpotentials during recharge. The lower electronegativity of sodium compared to lithium results in weaker metal-oxygen bonds, making NaO2 easier to decompose upon charging. However, sodium's higher solubility in organic electrolytes can lead to increased anode dendrite formation and electrolyte depletion, further complicating long-term cycling.

Cycle life remains a major limitation for Na-air batteries, with most laboratory prototypes achieving fewer than 100 cycles before significant capacity fade. The causes include cathode passivation by discharge products, electrolyte decomposition, and sodium anode degradation. Unlike lithium, sodium does not form a stable solid-electrolyte interphase (SEI) in many organic electrolytes, leading to continuous side reactions and poor Coulombic efficiency. Research efforts focus on optimizing electrolyte additives to stabilize the anode interface and enhance oxygen reduction kinetics at the cathode.

The role of catalysts in Na-air systems is another area of investigation. While platinum and gold nanoparticles improve oxygen reduction and evolution kinetics in Li-air batteries, their effectiveness in Na-air systems is less pronounced due to differences in intermediate adsorption energies. Transition metal oxides and perovskites show promise in reducing overpotentials, but their long-term stability under cycling conditions requires further validation.

In summary, Na-air batteries present a compelling opportunity for large-scale energy storage, but their practical implementation hinges on overcoming fundamental challenges. The balance between NaO2 and Na2O2 formation must be carefully controlled through electrolyte engineering and oxygen partial pressure management. CO2 contamination remains a critical issue, necessitating innovative solutions to prevent carbonate accumulation. While Na-air systems exhibit certain advantages over Li-air chemistries, such as lower charge overpotentials for NaO2 decomposition, their cycle life and energy efficiency still lag behind commercial requirements. Future advancements in cathode design, electrolyte formulation, and anode protection will determine whether Na-air batteries can transition from laboratory curiosities to viable energy storage solutions.

The development of Na-air batteries is still in its early stages compared to more mature lithium-based technologies. However, the potential for lower costs and sustainable materials continues to drive research interest. By addressing the chemical and electrochemical hurdles outlined here, Na-air systems may eventually find applications in grid storage or specialized electronic devices where weight and cost considerations outweigh the need for ultra-high energy density.