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.