Isotopic Labeling Reveals Active Reaction Interfaces for Electrochemical Oxidation of Lithium Peroxide

The unresolved debate on the active reaction interface of electrochemical oxidation of lithium peroxide (Li₂ O₂ ) prevents rational electrode and catalyst design for lithium-oxygen (Li-O₂ ) batteries. The reaction interface is studied by using isotope-labeling techniques combined with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and on-line electrochemical mass spectroscopy (OEMS) under practical cell operation conditions. Isotopically labelled microsized Li₂ O₂ particles with an Li₂ ¹⁶ O₂ /electrode interface and an Li₂ ¹⁸ O₂ /electrolyte interface were fabricated. Upon oxidation, ¹⁸ O₂ was evolved for the first quarter of the charge capacity followed by ¹⁶ O₂ . These observations unambiguously demonstrate that oxygen loss starts from the Li₂ O₂ /electrolyte interface instead of the Li₂ O₂ /electrode interface. The Li₂ O₂ particles are in continuous contact with the catalyst/electrode, explaining why the solid catalyst is effective in oxidizing solid Li₂ O₂ without losing contact.