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 25oC 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 25oC 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