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, Li1.2Ni0.13Co0.13Mn0.54O2. 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 O-2 loss, when the delithiation depth corresponds to x = 0.4 in LixNi0.13Co0.13Mn0.54O2 (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: P3m1). 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 LixNi0.13Co0.13Mn0.54O2. These results provide fundamental insights and guidance for mitigating the energy efficiency of Li-rich layered cathodes.