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. It is found that anion charge and lattice volume have great impacts on the activation energy barrier of lithium ion migration, which is validated by some reported sulfides. For the tetrahedrally occupied lithium, the less negative the anion charge is, the lower the migration energy barrier is likely to be. While for the octahedrally occupied lithium, the more negative anion charge is, the lower migration energy barrier is. Based on the full understandings of the anion sublattice model, general design strategies for developing lithium superionic conductors were proposed. Adjusting the electronegativity difference between the anion element and the nonmobile cation element by selecting the most suitable nonmobile cation element without changing the crystal structure framework can achieve low activation energy barriers for lithium ion migration. For the desired lithium superionic conductors with tetrahedrally occupied lithium, the fine nonmobile cation elements should give preference 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, the fine nonmobile cation elements should give preferences to the elements located at the left bottom of the periodic table with small electronegativities.