Preparation of a new crystal form of manganese dioxide: λ-MnO2

Treatment of the spinel-type material LiMn2O4 with aqueous acid was found to result in conversion of the LiMn2O4 to nearly pure MnO2, while preserving the structural framework of the LiMn2O4. The resulting material, designated λ-MnO2, thus has a structure related to spinel, but with most of the lithium removed from the tetrahedral sites. The conversion of LiMn2O4 to λ-MnO2 results in some contraction of the lattice, as evidenced by a reduction in the lattice constant a0 from 8.24 to 8.03 Å. A mechanism for the conversion of LiMn2O4 to λ-MnO2 is proposed, which involves solid state diffusion of lithium in the structure, in a manner analogous to the proton diffusion which occurs during the cathodic reduction of γ-MnO2.