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 Na 19 Si 13 P 25 , Na 23 Si 19 P 33 , Na 23 Si 28 P 45 , Na 23 Si 37 P 57 , LT ‐NaSi 2 P 3 and HT ‐NaSi 2 P 3 , based entirely on earth‐abundant elements. They have SiP 4 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.