Novel states in transition metal oxides: a view based on high-efficiency RIXS of battery electrodes*

To meet today's energy storage requirements for sustainable energy applications, battery electrodes based on transition-metal oxides have been pushed towards very high voltages to achieve a high-energy and high-capacity battery. Such an approach drives the system into a highly oxidized states, which fundamentally leads to stability issues of the battery device. On the other hand, this unconventional state provides a unique playground for studying atypical states in transition metal oxides.
In this presentation, we will discuss spectroscopic studies of battery electrodes, with the focus on ultra-high efficiency mapping of resonant inelastic X-ray scattering (mRIXS). We show that the high voltage operation could trigger redox activities of oxygen (oxygen redox) that could be detected reliably by mRIXS. The spectroscopic results provide striking evidences that oxygen redox, if constrained in the lattice and under control, is not necessarily a detrimental activity as conventionally believed. Understanding and improving energy storage devices are therefore in keen need of collaborating efforts in both materials and fundamental physics.