Orthorhombic LiMnO2 cathodes suffer severe capacity fading due to accelerated manganese dissolution at elevated temperatures. Fluorine-modified LiMnO2 cathodes prepared by a solid-state reaction show less cation mixing and a well-defined crystallinity with a larger grain/powder morphology. The cathodes exhibit improved capacity retention and rate capability at the elevated temperature (55°C) compared with undoped counterparts.