(Invited) Ordering and Structural Transformations upon Li, Na, K and Mg Intercalation in Transition Metal Oxides and Sulfides

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Variations in size and electropositivity among Li, Na, K and Mg can lead to very different electrochemical behavior when these species intercalate into transition metal oxides and sulfides. First-principles statistical mechanics approaches are capable of predicting a wide variety of thermodynamic and kinetic properties associated with intercalation processes. We have performed a systematic first-principles study of the electrochemical properties of AxMO2 and AxMS2 where A = Li, Na, K, Mg and where M are different transition metals such as Co, Ti, Ni and Mn. Many differences in the predicted electrochemical properties of these compounds can be attributed to variations in the size of the A cation. Larger cations tend to induce a multitude of stacking sequence change phase transformations. They also lead to stronger ordering tendencies in the layered compounds as the larger separation between MO2 and MS2 slabs results in a reduction of screening between positive cations. We find a rich hierarchy of ordered phases in Na and K containing intercalation compounds that are made up of well-ordered domains separated by anti-phase boundaries. These ordered phases have important consequences for diffusion mechanisms in layered intercalation compounds. The multitude of stacking sequence change transformations also have consequences for cycle life performance.