Operando RIXS investigation of O-redox in Li-ion cathodes

Oxygen redox in oxide cathodes for Li-ion batteries allows for higher energy densities, but its mechanisms are still debated due to challenges in identifying the oxidized oxygen species. Recent studies claim that oxygen redox involves "trapped" molecular O2 in nanovoids (M—O-O species) based on high-resolution RIXS investigations. However, our latest RIXS experiments at ESRF demonstrate that molecular O2 signals indeed appear in both O-redox active and inactive cathodes, thereby challenging this view. Further theoretical investigations suggest that the RIXS signatures do not indicate M—O-O species, i.e., the predicted electrochemical products, but rather resemble free O2 molecules likely dissociated from M—O-O species due to the core excitation process in RIXS. To prove this striking finding, we propose performing operando RIXS to observe the evolution of molecular O2 during electrochemical cycling, which is believed to be the key to unveiling the real origin of molecular O2.