Non-monotonic first-cycle irreversible capacity governed by delithiation depth in Li-rich layered cathodes

Abstract

Li-rich layered oxides are promising high-energy-density cathodes for lithium-ion batteries. However, their ultimate energy density remains obscure due to an incomprehensive understanding of the first-cycle irreversible capacity. Here, we report an intriguing non-monotonic irreversible capacity behavior governed by the first-cycle delithiation depth (i.e., the extent of anionic redox reaction) in the archetypical Li-rich chemistry, Li_1.2Ni_0.13Co_0.13Mn_0.54O₂. In contrast to the previous belief that the irreversible capacity increases with the depth of charging in Li-rich cathode materials, the irreversible capacity reaches a maximum, excluding the unrecoverable capacity via O₂ loss, when the delithiation depth corresponds to x = 0.4 in Li_xNi_0.13Co_0.13Mn_0.54O₂ (i.e., half of the lithium extraction in the Li₂MnO₃ phase). We also demonstrate that such non-monotonic irreversible capacity is fully recoverable and strongly correlates with the discharge capacity that is kinetically limited during deep discharge down to extremely low voltages. Operando synchrotron X-ray diffraction reveals that such kinetic-related irreversible capacity during deep discharge is associated with a metastable phase transition to an overlithiated Li₂MO₂ 1T structure (space group: Pm1). Scanning transmission X-ray microscopy (STXM) combined with O K-edge X-ray absorption spectroscopy (XAS) confirms that the unrecoverable oxygen release from the particle surface is triggered when x < 0.2 in Li_xNi_0.13Co_0.13Mn_0.54O₂. These results provide fundamental insights and guidance for mitigating the energy efficiency of Li-rich layered cathodes.