Batteries based on the shuttling
of multivalent (MV) ions are attractive
energy storage systems due to their potential to transfer multiple
electrons per working ion. Nevertheless, these batteries remain in
an early stage of development, and performance improvements are desired
for electrolytes that can transport MV ions efficiently and for cathode
materials that can store MV ions at high capacity. The present study
explores potential MV solid electrolytes (SEs) based on the anti-perovskite
(AP) structure. Ten SE compositions are considered: Mg3NX, Ca3NX (where X = P, As, Sb, or Bi), Ca3PSb, and Ca3AsSb. First-principles calculations are used
to predict several properties that are relevant for SE performance:
stability, band gaps, elastic moduli, ion migration barriers, and
defect formation energies. All compounds are predicted to be thermodynamically
stable at 0 K. Similar to the monovalent AP SEs, lattice distortions
in the MV systems decrease the energy barrier for percolating ion
migration. Large energies associated with the formation of vacancies
and interstitials imply that achieving high conductivities will require
defect concentrations that are controlled via doping or composition
variation. Of the compounds investigated, Mg3NAs, Ca3NAs, and Ca3PSb are the most promising. These SEs
are predicted to be stable against Mg or Ca anodes and have barriers
for vacancy migration that are less than ∼500 meV (less than
∼200 meV for interstitial migration). Stability against oxidation
is maintained up to 1.2–1.7 V, implying that interfacial coatings
may be needed to achieve compatibility with high-voltage cathodes.