Revealing the Reconstructed Surface of Li[Mn2]O4

The spinel Li[Mn-2]O-4 is a candidate cathode for a Li-ion battery, but its capacity fades over a charge/discharge cycle of Li1-x[Mn-2]O-4 (0 < x < 1) that is associated with a loss of Mn to the organic-liquid electrolyte. It is known that the disproportionation reaction 2Mn(3+) = Mn2+ = Mn4+ occurs at the surface of a Mn spinel, and it is important to understand the atomic structure and composition of the surface of Li[Mn-2]O-4 in order to understand how Mn loss occurs. We report a study of the surface reconstruction of Li[Mn-2]O-4 by aberration-corrected scanning transmission electron microscopy. The atomic structure coupled with Mn-valence and the distribution of the atomic ratio of oxygen obtained by electron energy loss spectroscopy reveals a thin, stable surface layer of Mn3O4, a subsurface region of Li1+x[Mn-2]O-4 with retention of bulk Li[Mn-2]O-4. This observation is compatible with the disproportionation reaction coupled with oxygen deficiency and a displacement of surface Li+ from the Mn3O4 surface phase. These results provide a critical step toward understanding how Mn is lost from Li[Mn-2]O-4, once inside a battery.