A First-Principles Approach to Studying the Thermal Stability of Oxide Cathode Materials

We present a new method for predicting the thermodynamics of thermal degradation of charged cathode materials for rechargeable Li batteries and demonstrate it on three cathode materials, LixNiO2, LixCoO2, and LixMn2O4. The decomposition of LixNiO2 is a two-step process: the first step is a kinetically controlled exothermic conversion in which the layered structure transforms to the stable spinel structure. The second step is an endothermic decomposition of the spinel into a rocksalt phase, accompanied by the loss of oxygen. The heat generation for the overall reaction from the layered to the rocksalt structure is exothermic when x < 0.5 and endothermic when x > 0.5. From the calculated phase diagram, a similar mechanism is expected for LixCoO2, but the high migration barrier for Co may inhibit the layered-to-spinel transformation and lead to decomposition into LiCoO2 and Co3O4. For the stable spinel LiMn2O4, high temperature is needed to provide enough thermodynamic driving force for its endothermic decomposition reaction. The fully charged λ-Mn2O4 transforms kinetically into the stable phase β-MnO2 first and then decomposes at elevated temperature into the lower-valence oxides, α-Mn2O3 and Mn3O4. The calculated decomposition heat for the three systems is in good agreement with experiments. When present, the electrolyte can act as a sink for the oxygen released from the cathode. Although oxygen release from the cathode is generally endothermic, its combustion with the electrolyte leads to a highly exothermic reaction.