First-cycle voltage hysteresis in Li-rich 3dcathodes associated with molecular O2trapped in the bulk

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[Li0.25Mn0.75]O-2, voltage hysteresis can be explained by the formation of molecular O(2)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(17)O magic angle spinning NMR spectroscopy show that molecular O-2, rather than O-2(2-), forms within the particles on the oxidation of O(2-)at 4.6 V versus Li+/Li on charge. These O(2)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.O-17 magic angle spinning NMR spectroscopy indicates a quantity of bulk O(2)consistent with the O-redox charge capacity minus the small quantity of O(2)loss from the surface. The implication is that O-2, trapped in the bulk and lost from the surface, can explain O-redox. Understanding the severe voltage hysteresis in the first cycle of Li-rich cathodes is essential to realize their full potential in batteries. P. G. Bruce and colleagues report the formation of molecular O(2)on charging rather than other oxidized O species is the cause for the voltage hysteresis.