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, Na_0.75[Li_0.25Mn_0.75]O_2, 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 Li_1.2Ni_0.13Co0.13Mn_0.54O₂.

Resonant inelastic X-ray scattering and ¹⁷O magic angle spinning NMR spectroscopy show that molecular O₂, rather than 0₂^2-, forms within the particles on the oxidation of 0²- at 4.6V Li*/Li on charge.  These O₂ molecules are reduced back to O^2- 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.