Anion Charge-Lattice Volume Maps for Searching Lithium Superionic Conductors

The effects of anion charge and lattice volume (lithium-anion bond length) on lithium ion migration have been investigated by utilizing the density functional theory calculations combined with the anion framework models, e.g. fcc, hcp and bcc. It is found that the anion charge and lattice volume have great impacts on the activation energy barrier (E_a) of lithium ion migration, which is validated by some reported sulfides. For the tetrahedrally occupied lithium, the less negative anion charge is, the lower the lithium ion migration barrier is likely to be. While for the octahedrally occupied lithium, the more negative anion charge is, the lower the lithium ion migration barrier is. The large lattice volume (lithium-anion bond length) can lower E_a to a certain extent. Lithium ion direct migrations along the direct Tet-Tet pathway in the bcc- or hcp-type anion framework are less sensitive to anion charge and lattice volume than other pathways. Most importantly, based on the full understandings of anion framework model, new design rules for developing alkali-metal superionic conductors were proposed. Getting the desired electronegativity difference between the anion element and non-mobile cation element by selecting the most suitable non-mobile cation element without changing the crystal structure framework can eventually achieve low E_a for alkali-metal ion migration. For the desired alkali-metal superionic conductors with tetrahedrally occupied alkali-metal ions, the fine non-mobile cation element should give preferences to those elements located at the right top of the periodic table of elements with large electronegativities. For the alkali-metal superionic conductors with octahedrally occupied alkali-metal ions, the fine non-mobile cation element should give preferences to the elements located at the left bottom of the periodic table with small electronegativities.