First-cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk

Robert A. House, Gregory J. Rees, Miguel A. Pérez-Osorio, John-Joseph Marie, Edouard Boivin, Alex W. Robertson, Abhishek Nag, Mirian Garcia-Fernandez, Ke-Jin Zhou and Peter G. Bruce ()
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Robert A. House: University of Oxford
Gregory J. Rees: University of Oxford
Miguel A. Pérez-Osorio: University of Oxford
John-Joseph Marie: University of Oxford
Edouard Boivin: University of Oxford
Alex W. Robertson: University of Oxford
Abhishek Nag: Diamond Light Source
Mirian Garcia-Fernandez: Diamond Light Source
Ke-Jin Zhou: Diamond Light Source
Peter G. Bruce: University of Oxford

Nature Energy, 2020, vol. 5, issue 10, 777-785

Abstract: Abstract 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.

Date: 2020
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DOI: 10.1038/s41560-020-00697-2

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