Stacking-Sequence Changes and Na Ordering in Layered Intercalation Materials

The performance of Na-ion batteries is sensitive to the nature of cation ordering and phase transformations that occur within the intercalation compounds used as electrodes. In order to elucidate these effects in layered Na intercalation compounds, we have carried out a first-principles statistical mechanics study of Na ordering and stacking-sequence preferences in the model compound Na_xTiS₂. Our calculations predict a series of structural phase transitions at room temperature between O₃, P₃, O₁, O₁–O₃ staged hybrid, and O₁–P₃ staged hybrid. We further explore the ordering of Na ions in P₃ and O₃ and find that these host structures favor very distinct Na-vacancy patterns. Low energy orderings on the honeycomb lattice in P₃ consist of triangular island domains with vacancies coalescing at antiphase boundaries. This results in a devil’s staircase of ground-state Na orderings within P₃ that are unlike the orderings possible in the triangular lattice of Na sites in O₃. Here, we explore the role that antiphase boundaries play in mediating Na diffusion in the P₃ host.