Thermodynamic Stability of Transition-Metal-Substituted LiMn2-x Mx O4 (M=Cr, Fe, Co, and Ni) Spinels

The formation enthalpies from binary oxides of LiMn2 O4 , LiMn2-x Crx O4 (x=0.25, 0.5, 0.75 and 1), LiMn2-x Fex O4 (x=0.25 and 0.5), LiMn2-x Cox O4 (x=0.25, 0.5, and 0.75) and LiMn1.75 Ni0.25 O4 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 LiMn2 O4 -LiMnMO4 solid solutions. These data confirm that transition-metal doping is beneficial for stabilizing these potential cathode materials for lithium-ion batteries.