Changes in the Crystal Structure and Electrochemical Properties of LixNi0.5Mn0.5O2 during Electrochemical Cycling to High Voltages

Abstract

cells were cycled galvanostatically to 4.5 and at different current densities. Cycling to was shown to not only improve the reversible capacities and rate capability upon subsequent cycling in the voltage range of 4.5 and , but also to lead to the appearance of a reversible process at . electrodes exposed initially to were found to exhibit stable, reversible capacities of upon subsequent cycling to , which were considerably higher than those of electrodes cycled only to for the identical number of cycles. The changes in the crystal structure of layered were investigated by synchrotron X-ray powder diffraction and electron diffraction studies. Rietveld refinement analyses of X-ray diffraction data showed that octahedral nickel migrated from the lithium layer to transition metal layer, and to tetrahedral sites occurred upon cycling to . Increasing the charging voltage to appeared to remove all of the tetrahedral nickel and further increased the occupancy of octahedral nickel in the transition metal layer. The formation of having hexagonal close-packed oxygen array was revealed in the electrode charged to by X-ray and electron diffraction. The changes in the cation arrangements of the structure are discussed with respect to lithium reactivity, reversible capacities, and lithium mobility.