Correlation between oxygen vacancy, microstrain, and cation distribution in lithium-excess layered oxides during the first electrochemical cycle

Dynamic structural changes during the first electrochemical charge and discharge cycle in the Li-excess layered oxide compound, Li[Li _1/5Ni_1/5Mn_3/5]O₂, are studied with synchrotron X-ray diffraction (SXRD), aberration corrected scanning transmission electron microscopy (a-S/TEM), and electron energy loss spectroscopy (EELS). At different states of charge, we carefully examined the crystal structures and electronic structures within the bulk and have found that increased microstrain is accompanied with the cation migration and a second phase formation which occurs during the first cycle voltage plateau as well as into the beginning of the discharge cycle. The evidence indicates that the oxygen vacancy formation and activation may facilitate cation migration and results in the formation of a second phase. The EELS results reveal a Mn valence change from 4+ to 3+ upon oxygen vacancy formation and recovers back to 4+ at the discharge. The oxygen vacancy formation and activation at the partially delithiated state leads to the generation of several crystal defects which are observed in TEM. Identification of the correlation between microstrain and oxygen vacancy formation during the first electrochemical cycle clarifies the complex intercalation mechanisms that accounts for the anomalous capacities exceeding 200 mAh/g in the Li-excess layered oxide compounds.