A Raman-Based Investigation of the Fate of Li₂MnO₃in Lithium- and Manganese-Rich Cathode Materials for Lithium Ion Batteries

The relationship between structure and electrochemical performance of lithium- and manganese-rich cathode materials with the general formula xLi 2 MnO 3 •(1-x)LiMO 2 is under intensive study world-wide in the context of its importance to the development of high energy/high capacity lithium batteries. One of the issues raised in these studies is the fate of the Li 2 MnO 3 component as a function of voltage and repeated cycling. We have performed Raman spectroscopy based measurements that shed light on the transformations of the Li 2 MnO 3 phase as a function of state-of-charge. We find that on charging the Li 2 MnO 3 phase appears to de-lithiate at a rate at least equal to that of the LiMO 2 phase, whereas, on discharge a Li 2 MnO 3 -like component reforms later in the discharge than does the LiMO 2 phase. The absence of X-ray diffraction evidence for the presence of Li 2 MnO 3 after the first charge/discharge cycle can be reconciled by the possibility that the C2/m structured Li 2 MnO 3 exists in domains thick enough to diffract; but during subsequent discharging, a perturbed C2/m phase forms in conjunction with the re-lithiated LiMO 2 phase in thin sheets (electrochemically induced topotaxy) that are not thick enough to diffract but form in sufficient volume to give Raman scattering similar to that of Li 2 MnO 3 .