Profiling the nanoscale gradient in stoichiometric layered cathode particles for lithium-ion batteries

Abstract

Chemical and structural evolution in battery materials influences properties relevant to ionic and electronic transport and ultimately impacts the battery performance. Although chemical and structural gradients have been observed in several cathode materials, the origin(s) of these phenomena are poorly understood. Via high-throughput core-level spectroscopies {i.e., X-ray absorption spectroscopy (XAS), depth-profiled X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS)}, as well as scanning transmission electron microscopy (STEM), the present study seeks to achieve mechanistic understanding for these phenomena in a stoichiometric Rm layered cathode material (e.g., LiNi_xMn_xCo_1−2xO₂, NMC). We observed that the surfaces of particles in the composite electrode are complicated by the presence of a surface reaction layer resulting from electrolyte decomposition. In large particle ensembles, the global nickel oxidation state switches between Ni²⁺ and Ni^2+x (x = 1–2) during charging/discharging processes, and hole states are also created at the O2p level due to the TM3d–O2p hybridization states. In primary particles, the surface is less oxidized than the bulk counterpart of the same particle whenever the particle has been cycled. This is partially attributed to the reconstruction from an Rm structure to an Fmm structure at the surfaces of NMC particles. This work provides a unique insight into correlating crystal structures with charge compensation mechanisms and performance fading in stoichiometric layered cathode materials.