Insight into the capacity degradation and structural evolution of single-crystal Ni-rich cathodes

Single-crystal Ni-rich cathodes are a promising candidate for high-energy lithium-ion batteries due to their higher structural and cycling stability than polycrystalline materials. However, the phase evolution and capacity degradation of these single-crystal cathodes during continuous lithation/delithation cycling remains unclear. Understanding the mapping relationship between the macroscopic electrochemical properties and the material physicochemical properties is crucial. Here, we investigate the correlation between the physical-chemical characteristics, phase transition, and capacity decay using capacity differential curve feature identification and in-situ X-ray spectroscopic imaging. We systematically clarify the dominant mechanism of phase evolution in aging cycling. Appropriately high cut-off voltages can mitigate the slow kinetic and electrochemical properties of single-crystal cathodes. We also find that second-order differential capacity discharge characteristic curves can be used to identify the crystal structure disorder of Ni-rich cathodes. These findings constitute a step forward in elucidating the correlation between the electrochemical extrinsic properties and the physicochemical intrinsic properties and provide new perspectives for failure analysis of layered electrode materials.