The Origin of Anion Redox in High Energy Density Lithium-Ion Battery Cathodes
ORAL · Invited
Abstract
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. High resolution resonant inelastic X-ray scattering (RIXS) at the O K-edge has been critical to determining the nature of oxidised oxygen. Characteristic peaks arising from vibrational excitations of molecular O2 trapped in the bulk of the cathode are detected in the emission spectra. The mechanism of oxygen redox will be discussed and illustrated with several archetypal cathode materials such as Li[Li0.2Ni0.13Co0.13Mn0.54]O2, Na0.6[Li0.2Mn0.8]O2, and 4d and 5d transition metal oxides.2-5 The implications of this mechanism for the design of better oxygen redox materials will be discussed.
The importance of adopting the correct measurement protocols such that RIXS operates as an analytical tool and is non-destructive will be considered.
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, 577, 502-508 (2020).
4. House, R. A. et al. Nature Energy, 8, 351–360 (2023).
5. House, R. A. et al. Nature Communications 12, 2975 (2021).
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. High resolution resonant inelastic X-ray scattering (RIXS) at the O K-edge has been critical to determining the nature of oxidised oxygen. Characteristic peaks arising from vibrational excitations of molecular O2 trapped in the bulk of the cathode are detected in the emission spectra. The mechanism of oxygen redox will be discussed and illustrated with several archetypal cathode materials such as Li[Li0.2Ni0.13Co0.13Mn0.54]O2, Na0.6[Li0.2Mn0.8]O2, and 4d and 5d transition metal oxides.2-5 The implications of this mechanism for the design of better oxygen redox materials will be discussed.
The importance of adopting the correct measurement protocols such that RIXS operates as an analytical tool and is non-destructive will be considered.
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, 577, 502-508 (2020).
4. House, R. A. et al. Nature Energy, 8, 351–360 (2023).
5. House, R. A. et al. Nature Communications 12, 2975 (2021).
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Presenters
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Peter Bruce
University of Oxford
Authors
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Peter Bruce
University of Oxford
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Robert A House
University of Oxford
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Mirian Garcia-Fernandez
Diamond Light Source
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Ke-Jin Zhou
Diamond Light Source