Crystal structure changes of LiMn0.5Ni0.5O2 cathode materials during charge and discharge studied by synchrotron based in situ XRD

The structural changes of LiMn0.5M0.5O2, used as a cathode material in a lithium half-cell, have been studied during charge-discharge using synchrotron based in situ X-ray diffraction spectroscopy. A phase transition from a hexagonal structure (H1) to another hexagonal structure (H2) was observed during a C/17 charge to 4.7 V. The new H2 unit cell has a shorter a axis and a longer c axis, as was observed in the LiNiO2 cathode material during charge. However, the second phase transition that occurs in LixNiO2 at voltages higher than 4.3 V, to a third hexagonal structure, H3, with much shorter c axis, was not observed. During discharge, the LixMn0.5M0.5O2 structure reversibly changed back from H2 to H1. After that, another hexagonal structure H0 with a slightly larger a parameter was formed following further discharging. Extended discharge at a voltage near 1 V resulted in an amorphous structure. (C) 2002 Elsevier Science B.V. All rights reserved.