Comprehensive review of Li-O₂ batteries: Electrolytes, additives, and electrodes

Lithium-oxygen batteries (LOBs) have gained significant interest due to their ultra-high theoretical energy density (3458 Wh kg⁻¹), abundant oxygen supply, and low environmental footprint. Despite this potential, practical application and commercialization remain limited by rapid capacity fading, electrolyte degradation, lithium anode instability, and cathode corrosion. This review provides a critical assessment of these challenges and the strategies developed to address them. We examine the four principal electrolyte systems used in LOBs, specifically aqueous, aprotic (non-aqueous), hybrid, and solid-state electrolytes. Fundamental reaction mechanisms, as well as the structure and properties of discharge products, such as lithium superoxide (LiO₂), lithium peroxide (Li₂O₂), and lithium hydroxide (LiOH) in LOBs, are discussed. The roles of lithium salts, electrode materials, and functional additives, ranging from immobile heterogeneous catalysts to mobile redox mediators, are further analyzed. The review also expands on the contamination effects of H₂O, CO₂, and N₂ in aprotic systems. Particular emphasis is placed on emerging semiconductor photocathodes and electrolyte systems, including room-temperature ionic liquids (RTILs), RTIL-based mixed solvents, solvated ionic liquids, and inorganic molten salts, which offer unique advantages in terms of safety, conductivity, and electrochemical stability. Finally, we highlight the use of multinuclear magic-angle spinning (MAS) nuclear magnetic resonance (NMR) approaches (^6,7Li, ¹H, ¹³C, and ¹⁷O) and advanced 2D homonuclear and heteronuclear correlation NMR techniques to investigate the evolution of electrochemical and decomposition products during galvanostatic cycling.