In situ and ex situ XRD investigation of Li[CrxLi1/3-x/3Mn2/3-2x/3]O2 (x=1/3) cathode material

The structural evolution of Li[CrxLi1/3-x/3Mn2/3-2x/3]O-2 ( x = 1/3) during charge-discharge cycling is investigated by in situ and ex situ X-ray diffraction (XRD) methods. Rietveld refinements of ex situ XRD patterns for Li-y[CrxLi1/3-x/3Mn2/3-2x/3]O-2 (x = 1/3) taken during the first extraction of lithium show that the transition metal atoms move first to tetrahedral sites in the lithium layers and then to the octahedral sites in the lithium layers. Although previous X-ray absorption near edge and extended X-ray absorption fine structure spectroscopy results showed the migration of Cr, they did not distinguish whether the tetrahedral sites occupied by Cr atoms were located in the transition metal layers or in the lithium layers. The in situ and ex situ XRD measurements reported here show that the transition metal migration process is not fully reversible and that a significant amount of transition metal remains within the lithium layers even after lithium is reinserted during discharge. This is the reason why Li/Li[CrxLi1/3-x/3Mn2/3-2x/3]O-2 (x = 1/3) cells have a very large irreversible capacity. The experiments also demonstrate that the first charge of Li/Li[CrxLi1/3-x/3Mn2/3-2x/3]O-2 (x = 1/3) cells is different from the following cycles, due to the irreversibility of the transition metal migration. (C) 2003 The Electrochemical Society.