Effect of chemical treatment and trivalent doping on the surface structure and surface chemistry of Li1-xNi0.5-yMn1.5+yO4 spinel
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The surface structure and surface chemistry of Li₁[subscript -x]Ni₀̣.₅[subscript -y]Mn₁.₅[subscript +y]O₄ was examined by first analyzing as-prepared Li[Mn₂]O₄, the basis cubic spinel structure without Ni or trivalent dopants. It was found that Li[Mn₂]O₄ undergoes a surface reconstruction, which results in the production of a thin, stable surface layer of Mn₃O₄, a subsurface region of Li₁[subscript -x][Mn₂]O₄ with retention of bulk Li[Mn₂]O₄. This observation is compatible with the surface disproportionation of Mn coupled with oxygen deficiency and a displacement of surface Li⁺ from the Mn₃O₄ surface phase to a subsurface layer. Li₁[subscript -x][Mn₂]O₄ was then subjected to chemical treatments to further understand and isolate the role of Li and oxygen in the surface reconstruction. An aqueous acid treatment, a non-aqueous chemical delithiation, and an oxygen plasma treatment were applied to Li[Mn₂]O₄. It was found that Mn₃O₄ is a robust surface phase in the Li₁[subscript -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. Analysis of as-prepared LiNi₀.₅Mn₁.₅O₄ revealed a surface composed of mixed Mn₃O₄ and a well-known Ni-rich rock-salt phase that occurs in the LiNi₀.₅Mn₁.₅O₄ system. Trivalent doping of LiNi₀.₅Mn₁.₅O₄ with Cr creates an ordered spinel/rock-salt phase that has not been previously observed in either the LiNi₀.₅Mn₁.₅O₄ or LiMn₂O₄ systems.
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