Layered Li-transition metal (TM) oxides are very promising materials for new Li ion battery cathodes. Compounds with an increased content of Li and Mn are particularly interesting because they exhibit a high capacity of 200-300 mAhg-1 even after an initial drop in capacity during the first charging cycle. The structure of such compounds and their capacity degradation over multiple cycles is not fully understood due to the complexity of the crystallographic phases, the ambiguities in the diffraction data and the presence of additional phases.
The investigated compound Li1.2Ni0.13Mn0.56Co0.13O2 is derived from LiNi0.33Mn0.33Co0.33O2, (NMC). Due to the higher capacity of the Li-rich compound, it will be referred to as high energy NMC (HENMC). The chemical formula of HENMC can be rewritten as Li2MnO3 · LiNi0.33Mn0.33Co0.33O2 which implies that this compound is a mixture of trigonal NMC and monoclinic Li2MnO3. In NMC, there are two different cation layers which are exclusively occupied by Li and TM respectively. In Li2MnO3 one third of the TM sites are occupied by Li. HENMC must be either a phase mixture with distinct domains of the two oxides or a solid solution of the two, with “Li rich” positions in the TM layers and an overall reduced, monoclinic symmetry (Figure 1). The exact nature of HENMC and similar materials is crucial to understand the de-lithiation processes during charging cycles.
The layered structure and its relation to other phases was investigated with a combination of electron diffraction in bright-field and scanning modes with high resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) imaging. Due to the high structural similarity of NMC and Li2MnO3, all lattice distances which can be found for NMC can also be found for Li2MnO3; as seen in the respective electron diffraction patterns (Figure 2). HAADF-STEM imaging reveals the presence of Li-rich positions in the TM layers as indicated by a lower intensity of the related atom columns (Figure 3). Due to the stacking faults of the TM layers, the interpretation of contrast in any other orientation becomes by far less intuitive as we will demonstrate. A LiTM2O4 spinel phase is found on the surface quite frequently. Its relation to the bulk phase was derived from high resolution images. STEM-EELS revealed the presence of surface reduction due to oxygen depletion in HENMC.
This work thus provides structural information about the synthesized material in its pristine state to facilitate understanding the changes the material undergoes during the (de-)lithiation segments of electrochemical cycling. Further investigations on such effects are currently being carried out and will be discussed.
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