Enthalpies of formation of layered LiNi_xMn_xCo_1-2xO₂ (0 ≤ x ≤ 0.5) compounds as lithium ion battery cathode materials

Layer-structured mixed transition metal oxides with the formula LiNi_xMn_xCo_1-2xO₂ (0 ≤ x ≤ 0.5) are considered as important cathode materials for lithium-ion batteries. In an effort to evaluate the relative thermodynamic stabilities of individual compositions in this series, the enthalpies of formation of selected stoichiometries are determined by high temperature oxide melt drop solution calorimetry and verified by ab-initio calculations. The measured and calculated data are in good agreement with each other, and the results show that LiCoO₂-LiNi_0.5Mn_0.5O₂ solid solution approaches ideal behavior. By increasing x, i.e. by equimolar substitution of Mn⁴⁺ and Ni²⁺ for Co³⁺, the enthalpy of formation of LiNi_xMn_xCo_1-2xO₂ from the elements becomes more exothermic, implying increased energetic stability. This conclusion is in agreement with the literature results showing improved structural stability and cycling performance of Ni/Mn-rich LiNi_xMn_xCo_1-2xO₂ compounds cycled to higher cut-off voltages.