Stability of Prismatic and Octahedral Coordination in Layered Oxides and Sulfides Intercalated with Alkali and Alkaline-Earth Metals

Many layered oxide and sulfide intercalation compounds used in secondary batteries undergo stacking-sequence-change phase transformations during (de)intercalation. However, the underlying reasons why different intercalants result in different stacking-sequence changes are not well understood. This work reports on high-throughput density functional theory calculations on the prototype systems A x CoO 2 and A x TiS 2 (where A = [Li, Na, K, Mg, and Ca]), which show that a few simple rules explain the relative stability among the O1, O3, and P3 stacking sequences. First, for large intercalants (Na, K, and Ca), P3 stacking is favorable at intermediate concentrations ( x ∼ 0.5) as its intercalant site topology minimizes in-plane electrostatic repulsion. At the extreme compositions ( x ∼ 0 and x ∼ 1), O1 or O3 are stable, with more ionic compounds preferring O3 and covalent ones O1. These rules explain why stacking-sequence changes are much more common in Na materials than Li ones.