Structural Ambiguity in Li- and Mn-Rich Transition Metal Oxides: Trigonal, Monoclinic, or Both?

Li- and Mn-rich transition metal oxides have been extensively studied recently for their potential application in Li-ion batteries. These materials have attracted a lot of interest due to the high capacity offered by them. However, the structure of these materials in their pristine state is not clearly understood. Several reports have assigned their structure to be trigonal (R-3m), monoclinic (c2/m), or a combination of both (composite). The present study discusses the structure of Li_1.2(Ni_0.13Mn_0.54Co_0.13) O₂ prepared with two different morphologies: plates and needles, using the results obtained from aberration corrected scanning transmission electron microscopy (STEM) imaging, electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (XEDS). 3-dimensional information was obtained by imaging the material at different zone axes and it was found that the primary particles are made up of a single phase, consisting of domains that correspond to variants of monoclinic structure, save for rare localized defects and a thin surface layer on certain crystallographic facets. These findings were further confirmed on two commercially available Li-and Mn-rich transition metal oxides, namely TODA HE5050 and Envia HCMR XLE2 . It will be shown how diffraction-based techniques can lead to ambiguous interpretation of structure in this class of materials. The study not only solves the ambiguity in the structure determination of this class of complex oxides but also highlights the importance of atomic resolution imaging performed on multiple zone axes when it is used for similar investigations.