Oxygen redox in transition metal oxide cathodes for lithium batteries
Peter G. Bruce, Departments of Materials and Chemistry, University of Oxford, UK
One of the grand challenges facing lithium-ion batteries is how to increase their energy density. In this regard the cathode, typically a layered lithium transition metal oxide represents a major
limitation. Current cathodes store charge on the transition metal ions, as lithium-ions are removed and reinserted on charge and discharge the transition metal ions are oxidised and reduced
correspondingly. Short-term efforts to increase energy storage are rightly focused on maximising the amount of transition metal capacity that is possible. However to go further we have to explore more radical solutions and these require a deeper understanding of the underpinning science. One such possibility is to invoke oxygen redox, where lithium-ion removal and reinsertion is charge balanced by the oxidation of O2- ions on charge and reduction back to O2- on discharge. Our exploration of what happens in oxides when O2- ions are oxidised has led to the discovery that O2 molecules are formed and trapped within voids inside the particles created by metal ion disordering within the transition metal layers.1,2 We have also shown that these trapped O2 molecules can be reduced back to O2- on discharge. The two processes follow different pathways, in particular the changing coordination of cations around the O2- ions explains the significant voltage hysteresis, typically 1eV, that is observed on the first cycle of a cell containing oxygen redox cathodes. The mechanism of oxygen redox involving O2-/O2, provides a unified understanding across a very wide range of 3, 4 and 5d transition metal oxides.3 It also explains why Li2MnO3 exhibits 100% O2 loss from the particles,4 whereas introducing relatively small amounts of Ni greatly suppresses O2 loss, converting this to trapped O2 which can undergo reversible O-redox.5 The mechanism of oxygen redox will be discussed and illustrated with several archetypal cathode materials such as Li[Li0.2Ni0.13Co0.13Mn0.54]O2 and Na0.6[Li0.2Mn0.8]O2.6 The implications of this mechanism for other features of oxygen redox materials will be discussed. References
1. House, R. A. et al. Nature Energy 8, 777–785 (2020).
2. House, R. A. et al. Nature Energy 6, 781-789 (2021).
3. House, R. A. et al. Nature Communications 12, 2975 (2021).
4. Guerrini N. et al. Chemistry of Materials, 32, 9, 3733-3740 (2020).
5. Boivin, E. et al. Advanced Functional Materials 2003660 (2020).
6. House, R. A. et al. Nature, 577, 502-508 (2020).