Evolutions of Li1.2Mn0.61Ni0.18Mg0.01O2 during the Initial Charge/Discharge Cycle Studied by Advanced Electron Microscopy

The evolutions of the structure occurring into the lithium rich cobalt free layered cathode material Li1.2Mn0.61Ni0.18Mg0.01O2 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 Li2MnO3 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)NixO4. 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.