Li-rich cathode materials are potential candidates for next-generation Li-ion batteries. However, they exhibit a large voltage hysteresis on the first charge/discharge cycle, which involves a substantial (up to 1 V) loss of voltage and therefore energy density. For Na cathodes, for example Na0.75[Li0.25Mn0.75]O2, voltage hysteresis can be explained by the formation of molecular O2 trapped in voids within the particles. Here we show that this is also the case for Li1.2Ni0.13Co0.13Mn0.54O2. Resonant inelastic X-ray scattering and 17O magic angle spinning NMR spectroscopy show that molecular O2, rather than O22−, forms within the particles on the oxidation of O2− at 4.6 V versus Li+/Li on charge. These O2 molecules are reduced back to O2− on discharge, but at the lower voltage of 3.75 V, which explains the voltage hysteresis in Li-rich cathodes. 17O magic angle spinning NMR spectroscopy indicates a quantity of bulk O2 consistent with the O-redox charge capacity minus the small quantity of O2 loss from the surface. The implication is that O2, trapped in the bulk and lost from the surface, can explain O-redox.
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Supporting research data has been deposited in the Oxford Research Archive and is available under the DOI https://doi.org/10.5287/bodleian:n6zY0x7nY.
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P.G.B. is indebted to the EPSRC, including the SUPERGEN programme (EP/L019469/1), the Henry Royce Institute for Advanced Materials (EP/R00661X/1, EP/S019367/1 and EP/R010145/1) and the Faraday Institution (FIRG007 and FIRG008) for financial support. We acknowledge Diamond Light Source for time on I21 under proposal MM23889-1. Support from the EPSRC (EP/K040375/1 ‘South of England Analytical Electron Microscope’) is also acknowledged. We acknowledge the resources provided by the Cambridge Tier-2 system operated by the University of Cambridge Research Computing Service (http://www.hpc.cam.ac.uk) funded by EPSRC Tier-2 capital grant EP/P020259/1, via the BATTDesign and AMAiB projects. The UK 850 MHz solid-state NMR Facility used in this research was funded by EPSRC and BBSRC (contract reference PR140003), as well as the University of Warwick, which included via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF).
The authors declare no competing interests.
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House, R.A., Rees, G.J., Pérez-Osorio, M.A. et al. First-cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk. Nat Energy 5, 777–785 (2020). https://doi.org/10.1038/s41560-020-00697-2