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 NaxTiS2. 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. We explore the role that antiphase boundaries play in mediating Na diffusion in the P3 host.