Lithium-rich manganese-based layered oxides Li[Li(x)Mn(y)TM(1-x-y)]O2 with TM standing for Ni, Co, or Fe are of great interest as cathode materials for lithium ion batteries. Indeed, among all of the materials, they offer the highest rechargeable capacity and energy density. However, when used, they suffer from complex evolutions that need to be understood before their practical use. Here we report on such evolutions studied using advanced transmission electron microscopy. Structural modifications are directly observed at the atomic scale using Cs corrected STEM HAADF imaging technique, and the chemical modifications are probed by the means of STEM EELS experiments. For the first time, segregation between nickel and manganese close the particle surface is pointed out. Finally, observed evolutions are correlated within a proposed mechanism that leads to the densification of the material. Our results allow understanding the link between the decrease of electrochemical performance and these evolutions occurring into the material upon cycling.

The evolutions of the structure occurring into the lithium rich cobalt free layered cathode material Li 1.2 Mn 0.61 Ni 0.18 Mg 0.01 O 2 upon the first electrochemical cycle were investigated by the means of high angle annular dark field (HAADF) imaging in a scanning transmission electron microscope and electron diffraction in a transmission electron microscope. They are coupled with electron energy loss spectroscopy (EELS) experiments in order to probe the chemical evolutions occurring during the first charge/discharge cycle. In the pristine material, the analysis of the HAADF images and electron diffraction patterns confirmed the ordering between the cations (Li or Ni with Mn) and the existence of disoriented domains stacked along the c axis. Moreover, the partial solid solution of Ni into Li 2 MnO 3 leading to a composite material is evidenced. Upon the first charge, a loss of material is shown to have occurred, and the presence of a defect spinel phase due to the transfer of transition metal cations to the interslab is clearly established. It is localized at the edge of the particles. This defect spinel phase apparition is confirmed by EELS experiments and identified as (Li)Mn (2−x) Ni x O 4 . After the first discharge, the spinel phase is still present, and structural discrepancies from one crystal to another are observed. Also, it seems that all the domains would not have the same behavior upon discharge.

The structure of Li 2 MnO 3 was investigated by the means of X-ray and electron diffraction as well as high resolution transmission electron microscopy experiments. Extra spots are present in the Li 2 MnO 3 electron diffraction patterns, and their origin is fully understood and explained here. They result from the existence of diffuse scattering lines observed along the c* monoclinic axis, intercepted by the Ewald's sphere, and not from double diffraction phenomenon nor from superstructure. Furthermore, the origin of these scattering lines is due to stacking faults of the ordered lithium/ manganese layers along the c-axis that were observed in images obtained using high resolution transmission electron microscopy.