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, Li x NiO 2, Li x CoO 2, and Li x Mn 2 O 4 . The decomposition of Li x NiO 2 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 Li x CoO 2, but the high migration barrier for Co may inhibit the layered-to-spinel transformation and lead to decomposition into LiCoO 2 and Co 3 O 4 . For the stable spinel LiMn 2 O 4, high temperature is needed to provide enough thermodynamic driving force for its endothermic decomposition reaction. The fully charged λ-Mn 2 O 4 transforms kinetically into the stable phase β-MnO 2 first and then decomposes at elevated temperature into the lower-valence oxides, α-Mn 2 O 3 and Mn 3 O 4 . 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.