Increasing the energy density of Li-ion batteries is a critical challenge if we are to improve the driving range of EVs. One of the greatest barriers to achieving this is the cathode material.
When conventional Li-ion cathodes are charged, Li+ ions are removed, and charge is compensated by oxidising the transition metal ions (i.e. in LiCoO2, Co3+ is oxidized to Co4+). In these materials, transition metal redox limits the capacity to store charge and hence the energy density. To increase the energy density of Li-ion batteries further, it is possible in certain Li-rich compositions (i.e. Li1.2Ni0.2Mn0.6O2) to store additional charge by oxidising the oxide ions. However, this oxygen redox reaction involves complex structural changes, and the mechanism has proved difficult to understand.
In this talk I will show, using high resolution RIXS, 17O NMR, neutron PDF, that O2- ions are oxidized to form molecular O2 trapped inside the structure. These O2 molecules are accommodated within voids formed in the bulk by reorganization of transition metal ions. During discharge, the trapped molecular O2 can be reduced back to O2-, but this process occurs at a lower voltage compared to the first charge, giving rise to voltage hysteresis. To overcome these challenges, it is necessary to suppress the formation of O2 and trap hole states on O2-. I shall show that this is possible with careful control of superstructure ordering leading to the formation of delocalised electron hole states and reversible, high voltage charge storage using oxygen redox.