Structure and electrochemistry of Li xMn yNi 1− yO 2

Abstract

We show that the solid-solution series Li x Mn y Ni 1− y O 2 can be prepared as a single phase for x near 1 and 0≤ y ≤0.5. Using powder X-ray diffraction and Rietveld profile refinement, we show that these solid solutions are isostructural with LiNiO 2 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 Mn 4+ as y increases. The cation mixing and the presence of Mn 4+ profoundly affects the electrochemical deintercalation of Li from Li x Mn y Ni 1− y O 2. Electrochemical measurements using Li/ Li x Mn y Ni 1− y O 2 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.