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 Na Si P , Na Si P , Na Si P , Na Si P , LT-NaSi P and HT-NaSi P , based entirely on earth-abundant elements. They have SiP tetrahedra assembled interpenetrating networks of T3 to T5 supertetrahedral clusters and can be hierarchically assigned to sphalerite- or diamond-type structures. 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 S cm . The conductivities increase with the size of the supertetrahedral clusters through dilution of Na -ions as the charge density of the anionic networks decreases.

Synthesis and Structure of the Sodium Phosphidosilicate Na2SiP2

“…Opposing the structure model, in the sodium spectrum only one signal is visible with a shoulder in the high field region at δ ( 23 Na) = 8.1 ppm instead of four distinct resonances. As it was shown for sodium based supertetrahedral ionic conductors, this observation can arise from similar chemical environment or from fast exchange processes. Since Na 2 SiP 2 comprises three different NaP x polyhedra, one single sodium resonance indicates fast dynamics.…”

“…Fast lithium and sodium ion conductors composed of earth abundant elements as potential components of all solid‐state batteries attract immense interest . Recently, several phosphidosilicates with isolated or condensed SiP 4 tetrahedra have demonstrated lithium or sodium ion conductivity, for example Li 8 SiP 4 , Li 2 SiP 2 ,, LiSi 2 P 3 , and the series Na 23 Si 9 n +19 P 12 n +33 ( n = 0–3) . These compounds have also been studied in the framework of the electrochemical mechanism of SiP 2 /Li anodes , .…”

“…These compounds have also been studied in the framework of the electrochemical mechanism of SiP 2 /Li anodes , . Except Li 8 SiP 4 with isolated [SiP 4 ] 8– tetrahedra, the crystal structures feature networks of interpenetrating T2–T5 supertetrahedra according to the Yaghi notation , , . It was shown that larger supertetrahedral entities favor the ion conductivity by diluting the alkali metal ions between the lower charged clusters .…”

“…Except Li 8 SiP 4 with isolated [SiP 4 ] 8– tetrahedra, the crystal structures feature networks of interpenetrating T2–T5 supertetrahedra according to the Yaghi notation , , . It was shown that larger supertetrahedral entities favor the ion conductivity by diluting the alkali metal ions between the lower charged clusters . Nevertheless there is currently no reason to believe that supertetrahedral structures are generally needed for facile ion movement in phosphidosilicates.…”

Polyanionic Frameworks in the Lithium Phosphidogermanates Li2GeP2 and LiGe3P3 – Synthesis, Structure, and Lithium Ion Mobility

“…The phosphide ion exhibits a higher charge compared to oxide and sulfide leading to an increased charge carrier concentration. These efforts resulted in the development of lithium phosphidotetrelates – named for their structural similarity to oxidosilicates – Li 8 SiP 4 , Li 2 SiP 2 , LiSi 2 P 3 , and α‐ and β‐Li 8 GeP 4 , which show conduction values of up to 0.1 mS · cm –1 …”