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