Exploring Oxygen Electrocatalysis in Phosphonium Ionic Liquids Towards Sodium–Oxygen Batteries

Sodium–oxygen (Na–O 2 ) batteries are attractive candidates for high‐energy storage systems; however, their long‐term operation is hindered by the instability of highly reactive intermediates such as the superoxide anion (O 2 •− ). Ionic liquids (ILs) have emerged as promising electrolytes not only because of their intrinsic safety but also due to their ability to stabilise electrogenerated species. Building on our previous studies of the oxygen reduction reaction (ORR) in neat triisobutyl(methyl)phosphonium ([P 1i4i4i4 ] + )‐based ILs and inspired by encouraging reports on sodium electrochemistry, we here investigate the electrochemical behaviour of [P 1i4i4i4 ] + ‐based electrolytes combined with NaFSI and NaFTFSI (FSI = bis(fluorosulfonyl)imide anions) and FTFSI = fluorosulfonyl)(trifluoromethanesulfonyl)imide) salts. A comprehensive cyclic voltammetry study, complemented by impedance spectroscopy, Fourier transformed infrared, and 23 Na nuclear magnetic resonance, identified the optimal electrolyte composition in terms of salt concentration and anion. Notably, the use of [P 1i4i4i4 ][FSI] with 30 mol% NaFSI enabled a Na–O 2 cell to sustain more than 100 cycles, representing the highest cycling stability reported to date for an IL‐based electrolyte. These results underscore the critical role of electrolyte formulation on ORR/OER pathways and provide valuable electrochemical insights for the rational design of IL‐based electrolytes for Na–O 2 batteries.