About the Thermodynamics of LixCoO2 in the Metal-Insulator Two-Phase Region Derived from DFT Calculations

Although Li_xCoO₂ has been in use as cathode material in Lithium-ion batteries for quite some time the mechanisms responsible for its cycling behavior are not completely understood. In practice only about 50 % of its theoretical capacity is accessible without rapid deterioration of cathode performance due to changes in the crystal structure. Moreover, its performance as a cathode material is known to be significantly influenced by the metal-insulator transition within a compositional range of x in [0.75; 0.94]. This concentration range marks the 2-phase region within which the cathode potential exhibits a plateau in agreement with the Gibbs’ phase rule.

Up to now this transition has never been modeled by ab initio calculations due to the lack of feasible straight-forward methods for strongly correlated materials to account for the interactions of the open-shell d-electrons as they exist in Li_xCoO₂. In this work the ambition is to capture the 2-phase region by running a number of calculation series on various compositions within the 2-phase-region by using the GGA+U approach. The space of magnetic configurations is sampled and results are analyzed with respect to metallic and semiconducting properties. Convex hulls are drawn through the acquired data points in a total energy versus Lithium-concentration plot for the metallic and semiconducting states. The common tangent of both convex hulls will pinpoint the borders of the two-phase region.

The successful DFT-modeling of the metal-insulator transition is of particular interest since effects of dopants on this region could be studied. Especially the effect on the potential and the compositional range of the region would be leading aspects to look at since a higher potential and a broader two-phase region are beneficial for the cathode’s overall performance.