Redox chemistry and the role role of trapped molecular O2 in Li-rich disordered rocksalt oxyfluoride cathodes

Research by the Peter Bruce Group and collaborators at Bath University, Diamond Light Source and STFC ISIS Facility, as reported in Journal of the American Chemical Society, explains that in the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyflurides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. 

However, a deep understanding of these phenomena and their redox chemistry remains incomplete.  Using the archetypal oxyfluoride, Li2MnO2F, they show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge.

The molecular O2 is trapped very rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment.  The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+.  

They furthermore derive the relationship between local co-ordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li-O-Li configurations.  

This study advances the fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.