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 AxCoO2 and AxTiS2 (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.