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 Na19Si13P25, Na23Si19P33, Na23Si28P45, Na23Si37P57, LT-NaSi2P3 and HT-NaSi2P3, 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. Na-23 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 sigma(Na+) = 4 x 10(-4) Scm(-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.