Rethinking molecular O2

Abstract

Compared to conventional metal oxide cathodes found in Li-ion batteries, Li-rich layered oxides introduce an additional charge storage mechanism that involves not just transition metals, but also lattice oxygen, known as anionic redox. These dual (cationic and anionic) redox processes enhance Li-ion utilization and increase capacity, making these materials promising candidates for next-generation batteries. However, commercialization is limited by rapid performance degradation mainly related to complex oxygen evolution processes, affecting both capacity and discharge voltage. In particular, voltage decay, which is characterized by a gradual decline in discharge voltage, reduces the battery’s practical energy density and efficiency. Despite extensive debate, the role of anions in the redox processes is still uncertain. The most recent focus is on the involvement of trapped molecular O₂, based on high-resolution resonant X-ray inelastic scattering (RIXS) research. Now, Jean-Marie Tarascon and colleagues in France, China, and Russia challenge the prevailing view, arguing that molecular O₂ is not a direct product of the redox activity of the Li-stoichiometric or Li-rich layered oxides, raising questions about identifying speciation using RIXS.

The researchers survey RIXS data from various oxide-based cathodes under different conditions, suggesting that molecular O₂ is not formed through electrochemical redox reactions but rather is due to excitation by the RIXS measurement. The results reveal that molecular O₂ signals appear not only in Li-rich oxides capable of anionic oxygen redox, but also in O-redox-inactive materials like Li-stoichiometric layered and spinel oxides, indicating no correlation with voltage decay. By clarifying the origin of molecular O₂, this study provides valuable insights into oxygen redox reactions in Li-rich layered oxide cathodes and offers guidelines for developing more stable and efficient cathode materials for Li-ion batteries.