Fast Sodium-Ion Conductivity in Supertetrahedral Phosphidosilicates.

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.