Fast Sodium-Ion Conductivity in Supertetrahedral Phosphidosilicates

Abstract

Fast sodium-ion conductors are key components of Na-based all-solid-state batteries which hold promise for large-scale storage of electrical power. We report the synthesis, crystal-structure determination, and Na⁺ -ion conductivities of six new Na-ion conductors, the phosphidosilicates Na₁₉ Si₁₃ P₂₅ , Na₂₃ Si₁₉ P₃₃ , Na₂₃ Si₂₈ P₄₅ , Na₂₃ Si₃₇ P₅₇ , LT-NaSi₂ P₃ and HT-NaSi₂ P₃ , based entirely on earth-abundant elements. They have SiP₄ tetrahedra assembled interpenetrating networks of T3 to T5 supertetrahedral clusters and can be hierarchically assigned to sphalerite- or diamond-type structures. ²³ Na solid-state NMR spectra and geometrical pathway analysis show Na⁺ -ion mobility between the supertetrahedral cluster networks. Electrochemical impedance spectroscopy shows Na⁺ -ion conductivities up to σ (Na⁺ )=4×10^-4 S cm^-1 . The conductivities increase with the size of the supertetrahedral clusters through dilution of Na⁺ -ions as the charge density of the anionic networks decreases.

Keywords: crystal structures; ion conductivity; phosphidosilicates; sodium; supertetrahedra.