Trigonal polymorph of Li2MnO3

We report the discovery of a trigonal polymorph of the prospective Li ion battery material Li2MnO3 and its synthesis in bulk, single crystal form. Crystal growth of trigonal Li2MnO3 is strongly dependent upon the quality of a polycrystalline LiMnO2 precursor consumed in the synthesis process. The crystal structure of the new trigonal phase is composed of ordered honeycomb layers of LiO6 and MnO6 octahedra segregated by layers of LiO6 octahedra and represents an ordered stacking variant of the known monoclinic polymorph. Diffuse reflectance spectroscopy reveals a direct optical gap of 2.47 ± 0.11 eV and a series of charge excitations that are well explained by the expected Mn4+ 3d3 valence. Density functional theory calculations are in excellent agreement with the spectroscopic measurements and find a near degeneracy in the formation energies of the two polymorphs. Furthermore, our results suggest that the trigonal structure resolves the compositional and structural disorder often manifested in the monoclinic phase.