Electrochemical Activity of Oxygen in Li-ion Battery Cathodes from X-ray Spectroscopy and Modeling

As demand for better performance in energy storage increases, a clear understanding of the charge compensating mechanism in Li-ion battery cathodes is essential for developing next generation battery chemistries and materials. X-ray core level spectroscopies, e.g. x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS), allow for experimental measurement of the electronic structure of battery materials before and after charge, providing the clearest physical picture of electrochemical device operation.

Here, we present numerical modeling of the XAS/RIXS of various battery chemistries compared to experimental measurements in situ. Modeling spectroscopic changes before and after discharge using exact diagonalization demonstrates that multiplet and correlation effects prove crucial in these materials, where reversible changes in the transition metal L-edge spectroscopy erroneously point to cationic redox. We address the common concept of transition metal redox, highlight the essential role of oxygen in both reversible and irreversible processes, and the break down in the standard paradigm of cationic/anionic redox.