Highly oxidized states of oxide-based battery cathodes

The improvement of energy density of electrochemical energy storage devices, i.e., batteries, is bottlenecked by transition-metal (TM) oxide based cathodes. At this time, the very first generation of oxide cathode, LiCoO₂, remains the choice for the record-high energy density accessible in practical batteries after more than three decades of intensive efforts. It is clear that conventional concepts have to be innovated, and the selection and optimization of oxide-based components in energy applications need to be guided by fundamental understandings. In this talk, we will first explain that pushing a rechargeable battery system into the high-energy operation always spontaneously triggers a highly oxidized oxide state in the battery cathode during high voltage charging. The highly oxidized TM oxides is a complex system, in which conventional ionic crystal models fail and a significant amount of oxygen activities are involved in the electrochemical operations, the so-called oxygen redox reaction. We show that soft X-ray resonant inelastic X-ray scattering (RIXS) is a powerful tool for detecting and distinguishing different types of oxygen activities; however, reliable experimental data indicate a very complex nature of these highly oxidized states of oxide cathodes, which calls for collaborations in Physics, Chemistry and Material Science scientists to tackle the fundamental understanding and practical optimization of highly oxidized oxide systems towards high energy-density batteries.