Vacuum-free spectroscopy reveals metastable oxygen redox intermediates in oxygen-redox cathodes

Capturing metastable reaction intermediates through spectroscopy is a fundamental challenge across many fields, as their intrinsic metastability often leads to transformation while being measured. This challenge is acute in oxygen redox in battery intercalation cathodes, where metastable intermediates, despite their long lifetime, remain difficult to capture reliably. Here, we demonstrate that vacuum conditions destabilize oxidized oxygen intermediates and drive their transformation to thermodynamically stable O ₂ in Li _1.13 Ni _0.13 Mn _0.57 O ₂ . To overcome these issues, we employ vacuum-free and cryogenic hard X-ray Raman spectroscopy, circumventing both vacuum and radiation-induced effects. This approach enables direct and reliable detection of metastable oxygen-redox intermediates in this system, while also capturing diverse oxygen-redox features across different cathodes and stoichiometries. Our findings demonstrate a robust method to identify redox intermediates, reconciling experimental observations and theoretical predictions and sharpening our understanding of oxygen-redox mechanisms.