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 (Li2O2) prevents rational electrode and catalyst design for lithium-oxygen (Li-O-2) 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 Li2O2 particles with an (Li2O2)-O-16/electrode interface and an (Li2O2)-O-18/electrolyte interface were fabricated. Upon oxidation, O-18(2) was evolved for the first quarter of the charge capacity followed by O-16(2). These observations unambiguously demonstrate that oxygen loss starts from the Li2O2/electrolyte interface instead of the Li2O2/electrode interface. The Li2O2 particles are in continuous contact with the catalyst/electrode, explaining why the solid catalyst is effective in oxidizing solid Li2O2 without losing contact.