Analytical electron microscopy of Li1.2Co0.4Mn0.4O2 for lithium-ion batteries

The structure of as-prepared Li1.2Co0.4Mn0.4O2 is studied using a combination of techniques that includes high-resolution electron microscopy (HREM), high-angle annular dark-field imaging in scanning transmission electron microscopy (HAADF-STEM), selected-area electron diffraction (SAED) and electron energy loss spectroscopy (EELS). Electron diffraction and imaging revealed Li ordering in the transition metal planes consistent with the presence of locally monoclinic (Li2MnO3-like) regions with three different topotaxial orientations with respect to a parent rhombohedral (LiCoO2-like) structure. EELS analyses revealed compositional variations too small to be compatible with perfect, or close to perfect, segregation into pure LiCoO2 and Li2MnO3 over length scales longer than the sample thickness. We propose a structural model in which Mn and Co segregate within TM planes to form LiMn6 and Co clusters, which overlap within EELS sampling volumes causing the observed Co/Mn homogeneity.

Research Highlights
► The overall layered-NaCl structure of LiCoO2 is preserved in Li1.2Co0.4Mn0.4O2.
► Excess Li atoms in Li1.2Co0.4Mn0.4O2 form locally monoclinic Li2MnO3-like areas.
► The coherence length of monoclinic areas is longer within than across TM planes.
► The Co/Mn ratio is fairly homogeneous across Li2MnO3- and LiCoO2-like areas.
► HAADF-STEM contrast is determined by the overlap of misaligned Li2MnO3-like areas.