Effect of cooling rates on phase separation in 0.5Li₂MnO ₃·0.5LiCoO₂ electrode materials for Li-ion batteries

The results of a detailed structural investigation on the influence of cooling rates in the synthesis of lithium- and manganese-rich 0.5Li ₂MnO₃·0.5LiCoO₂ composite electrode materials, which are of interest for Li-ion battery applications, are presented. It is shown that a low-temperature, intermediate firing step, often employed in cathode synthesis, yields a minor secondary component representing a polydisperse distribution of lattice parameters, not found in the absence of low-temperature treatments. However, regardless of the heating and cooling conditions employed, all samples present two distinctly different local environments as evidenced by X-ray absorption fine structure spectroscopy (XAFS) and nuclear magnetic resonance (NMR) analysis. Transmission electron microscopy (TEM) data show discrete domain structures that are consistent with the XAFS and NMR findings. Furthermore, high resolution synchrotron X-ray diffraction (HR-XRD), as well as the XAFS and NMR data show no discernible differences between sample sets heated in similar fashion and subsequently cooled at different rates. The results contradict recent reports, using X-ray diffraction, that rapidly quenched samples of the same composition are true solid solutions. This apparent discrepancy is assigned, in part, to the inherent nature of conventional diffraction, which firmly elucidates the average long-range structure but does not capture the local domain microstructure of these nanocomposite materials. The combined use of HR-XRD, XAFS, NMR, and TEM data indicate that charge ordering, which is initiated at relatively low temperatures, is the dominant force that produces a nanoscale, inhomogeneous composite structure, irrespective of the cooling rate.