Origin of extra capacity in advanced Li-Rich cathode materials for rechargeable Li-Ion batteries

High-voltage Li-rich oxides are very attractive as cathode materials for Li-ion batteries due to their high energy density and promising specific parameters. In this paper detailed analysis of extra capacity based on the relationship between electrochemical performance and structure evolution of the new stoichiometric Li-Mn-Ni-O group has been discussed. Origin of the extraordinary reversible capacity (similar to 35 mAh.g(-1) at C/20) of Li [Li0.27Mn0.63Ni0.1]O-2 was examined with particular emphasis on changes in superstructure cation-ordering in transition metal (TM) layers, TM ions migration as well as oxygen participation during electrochemical processes. The composition, local geometry, chemical states variations of TMs and oxygen at different states of charge were investigated by means of synchrotron-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) techniques. Specific structural rearrangements during lithium (de)insertion were examined using operando-XRD, transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and selected area electron diffraction (SAED) investigations. Detailed analysis confirmed highly reversible C2/m -> C2/m + R-3 m <-> C2/m + Fd-3 m phase transitions during charge/discharge mechanisms as well as specific superstructure cation-ordering (Li1/3TM2/3) in TM layer, that could partially vanish reversibly upon delithiation process. In the pristine sample high density of planar defects between neighboring domains was observed towards the [100], [1 (1) over bar0] and [110] zone axis.