LiMO₂ (M=Mn, Fe, and Co): Energetics, polymorphism and phase transformation

LiMO₂ materials (M=Mn, Fe, and Co) with different structures were synthesized and their enthalpies of formation from oxides (Li₂O and M₂O₃, M=Mn and Fe), or from oxides (Li₂O and CoO) plus oxygen at 25 °C were determined by high-temperature oxide melt solution calorimetry. The relative stability of the polymorphs of the compound LiMO₂ was established based on their enthalpies of formation. Phase transformations in LiFeO₂ were investigated by differential scanning calorimetry and high-temperature oxide melt solution calorimetry. The phase transition enthalpies at 25 °C for β→α, γ→β, and γ→α are 4.9±0.7, 4.3±0.8 and 9.2±0.9kJ/mol, respectively. Thus the γ phase (ordered cations) is the stable form of LiFeO₂ at room temperature, the α phase (disordered cations) is stable at high temperature and the β phase may have a stability field at intermediate temperatures.