Research by the Peter Group Group and collaborators at the Diamond Light Source, as reported in Nature Energy, explains that 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 02 trapped in voids within the particles. They 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 022-, forms within the particles on the oxidation of 02- at 4.6V 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 02, trapped in the bulk and lost from the surface, can explain O-redox.