Anion Charge and 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 sublattice 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 (Ea) 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. There are opposite effects of anion charge on Ea and optimum lattice volumes for minimum Ea of lithium ion migration along the Tet-Oct-Tet and Oct-Tet-Oct pathways in the hcp-type sublattices. Based on the full understandings of anion sublattice model, general design strategies for developing lithium superionic conductors were proposed. Adjusting the 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 sublattice can achieve low Ea for lithium ion migration. For the desired lithium superionic conductors with tetrahedrally occupied lithium 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 lithium superionic conductors with octahedrally occupied lithium 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.