Manganese migration in Li1-xMn2O4 cathode materials

Abstract

LiMn2O4 has been considered one of the most promising cathode materials for Li-ion batteries due to its thermal stability, abundance, environmental affinity and the possibility to exchange Li-ions in a three-dimensional fashion. However, it still suffers from major problems such as capacity fading and voltage decay, which has been associated to phase transformations and dissolution of transition metals. In this report, we use scanning transmission electron microscopy and differential phase contrast (DPC) to obtain new insights in the structural characteristics of Li1-xMn2O4 to better understand the mechanisms behind the structural transformation in LiMn2O4. We exploit the fact that DPC has the ability to observe simultaneously light and heavy elements, measure projected electric fields and charge distribution at the atomic level. This approach allow us to monitor the migration of very low amounts of Mn to the Li atomic columns at the surface and subsurface regions, which otherwise will be very challenging to observe using other techniques such as HAADF and ABF. These observations not only provide fundamental understanding of the structure of LiMn2O4, but also reveal DPC as a novel technique to determine local structural changes in materials consisting of heavy and light elements, localise light elements, monitor low concentrations of substitutional species and identify phase transformations.