Quantitative Measurement of Compositional Inhomogeneity in NMC Cathodes by X-ray Diffraction

A novel XRD analysis technique is described for quantitatively measuring compositional inhomogeneity in Li[Ni x Mn y Co z ]O 2 (NMC) cathode materials and NMC precursors. Single-phase rock salt precursors with varying degrees of compositional inhomogeneity were prepared by grinding mixtures of Ni, Mn and Co oxides for different times and then heating. These precursors were then heated with lithium to form cathode materials. A modified Williamson-Hall analysis was used to measure the degree of compositional inhomogeneity in the precursors and the final NMC materials. This analysis showed that precursors made with low grinding times had higher compositional inhomogeneity and that this compositional inhomogeneity was amplified in the final NMC, leading to interlayer mixing and poor electrochemical performance. Higher precursor grinding times lead to more compositionally homogeneous NMC, while even higher compositional homogeneity was achieved by NMC made from conventional hydroxide precursors, with correspondingly improved electrochemical performance. The ability described here to measure the degree of compositional homogeneity in NMC precursors and NMC cathode materials by simple XRD measurements presents a powerful tool for the research and development of NMC and other cathode materials.