1, University of Texas at Austin, Austin, Texas, United States
Li[Mn2]O4 (LMO) is a well-known cathode material for Li-ion batteries, but it is plagued with cyclability problems associated with the surface disproportionation of Mn (2Mn3+ → Mn2+ + Mn4+) and consequent loss of Mn2+ to the organic liquid electrolyte during electrochemical cycling.
In this paper, we use a combination of high-angle annular dark-field (HAADF) aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to identify the atomic surface structure and composition of LMO. We confirm the underlying spinel structure and for the first time we find, in as-processed LMO, a surface structure composed of Mn3O4 and a lithium-rich Li1+xMn2O4 subsurface layer which occurs as a result of the surface reconstruction.
In addition, we have applied an aqueous acid treatment, a non-aqueous chemical delithiation, and an oxygen plasma treatment to LMO in order to understand how this surface reconstruction is affected by chemical treatments. We find that Mn3O4 is a robust surface phase in the Li1-x[Mn2]O4 system regardless of the chemical treatment and level of lithiation. The surface Mn3O4 phase is cubic whereas bulk Mn3O4 undergoes a cooperative Jahn-Teller distortion to tetragonal symmetry. Thicker Mn3O4 surface layers are tetragonal.