Thermally Induced Structural Reordering in Li- and Mn-Rich Layered Oxide Li Ion Cathode Materials

In recent years, a considerable amount of effort has been put into a better understanding of the correlation of structure and electrochemical properties in Li- and Mn-rich NCM layered oxide Li ion battery cathode materials, such as the intensely investigated Li1.2Ni0.15Co0.1Mn0.55O2 composition. A gradual transformation from a trigonal R3̅m layered structure toward a cubic Fd3̅m spinel structure during electrochemical cycling results in an unwanted decay of the mean charge and discharge voltages, called “voltage fade”. This transformation proceeds via an interim phase, which is characterized by the local deordering of cations and the introduction of various lattice defects after the formation of the initially well-ordered material. In this study, these structural changes are studied in detail on a long-range atomic scale by synchrotron radiation powder diffraction as well as on a very local atomic scale by 7Li nuclear magnetic resonance and X-ray absorption spectroscopy. A structural reordering was induced by a mild thermal treatment (150–300 °C) in lithiated (discharged to 2.0 V) as well as in delithiated (charged to 4.7 V) electrodes, which results either in a partial recovery of the initial well-ordered state or in an intensification of the structural degradation toward a spinel-type cation ordering, respectively. The structural reordering thus obtained was again studied on a long-range and local atomic scale and correlated with the electrochemical properties. To complement the experiment, an electrochemically highly fatigued electrode (300 cycles, discharged to 2.0 V) showing a pronounced voltage fade was thermally treated, which resulted again in a partial recovery of the initial well-ordered structure and its accompanying electrochemical properties. Finally, the results are summarized in a model explaining the influence of the local cation ordering, lattice defects, and the oxygen sublattice on the electrochemical properties, such as the oxygen redox activity and the voltage fade.