Thermodynamic Stability of Transition-Metal-Substituted LiMn_2−xM_xO₄ (M=Cr, Fe, Co, and Ni) Spinels

The formation enthalpies from binary oxides of LiMn₂O₄, LiMn_2−xCr_xO₄ (x=0.25, 0.5, 0.75 and 1), LiMn_2−xFe_xO₄ (x=0.25 and 0.5), LiMn_2−xCo_xO₄ (x=0.25, 0.5, and 0.75) and LiMn_1.75Ni_0.25O₄ at 25 °C were measured by high temperature oxide melt solution calorimetry and were found to be strongly exothermic. Increasing the Cr, Co, and Ni content leads to more thermodynamically stable spinels, but increasing the Fe content does not significantly affect the stability. The formation enthalpies from oxides of the fully substituted spinels, LiMnMO₄ (M=Cr, Fe and Co), become more exothermic (implying increasing stability) with decreasing ionic radius of the metal and lattice parameters of the spinel. The trend in enthalpy versus metal content is roughly linear, suggesting a close-to-zero heat of mixing in LiMn₂O₄—LiMnMO₄ solid solutions. These data confirm that transition-metal doping is beneficial for stabilizing these potential cathode materials for lithium-ion batteries.