We show that the monoclinic and orthorhombic ordered-rocksalt polymorphs of LiMnO2 can be synthesized by high-temperature firing in air atmosphere, without requiring reducing ambients as previously used. For undoped LiMnO2, the orthorhombic phase is stable up to 1000 degreesC in air, while the monoclinic (layered) phase is obtained at higher temperature. Aluminum doping further stabilizes the monoclinic phase over the orthorhombic phase. Control of cooling conditions is crucial to avoiding oxidative decomposition into LiMn2O4 and Li2MnO3. The stability of LiMnO2 in the alpha-NaFeO2 structure over the corrugated layered structure at higher temperatures can be explained based on the temperature dependence of cooperative Jahn-Teller distortion.