STRUCTURE AND ELECTROCHEMISTRY OF LIXMNYNI1-YO2

We show that the solid-solution series LixMnyNi1-yO2 can be prepared as a single phase for x near 1 and 0 less-than-or-equal-to y less-than-or-equal-to 0.5. Using powder X-ray diffraction and Rietveld profile refinement, we show that these solid solutions are isostructural with LiNiO2 but with Mn and Ni sharing the Ni sites. However, subtle changes in the diffraction profiles show that as y increases some mixing of Li atoms into Ni, Mn sites and Ni, Mn atoms into Li sites occurs. In addition, the ratio of Li atoms to Ni, Mn atoms in the material increases with y at fixed x, suggesting the formation of Mn4+ as y increases. The cation mixing and the presence of Mn4+ profoundly affects the electrochemical deintercalation of Li from LixMnyNi1-yO2. Electrochemical measurements using Li/LixMnyNi1-yO2 cells show that as y increases the amount of Li which can be reversibly cycled in these materials decreases, thus limiting the practical usefulness of these materials.