Multivalent Ion Transport in Anti-Perovskite Solid Electrolytes

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.