Finding the Right Blend: Interplay Between Structure and Sodium Ion Conductivity in the System Na5AlS4–Na4SiS4

Abstract

The rational design of high performance sodium solid electrolytes is one of the key challenges in modern battery research. In this work, we identify new sodium ion conductors in the substitution series Na 5-x Al 1-x Si x S 4 (0 ≤ x ≤ 1), which are entirely based on earth-abundant elements. These compounds exhibit conductivities ranging from 1.64 · 10 −7 for Na 4 SiS 4 to 2.04 · 10 −5 for Na 8.5 (AlS 4 ) 0.5 (SiS 4 ) 1.5 (x = 0.75). We determined the crystal structures of the Na + -ion conductors Na 4 SiS 4 as well as hitherto unknown Na 5 AlS 4 and Na 9 (AlS 4 )(SiS 4 ). Na + -ion conduction pathways were calculated by bond valence energy landscape (BVEL) calculations for all new structures highlighting the influence of the local coordination symmetry of sodium ions on the energy landscape within this family. Our findings show that the interplay of charge carrier concentration and low site symmetry of sodium ions can enhance the conductivity by several orders of magnitude.

Influence of Structural Distortion and Lattice Dynamics on Li-Ion Diffusion in Li3OCl1–xBrx Superionic Conductors

“…They claimed the distortions make the ground state unstable and lead to a smooth energy landscape. A similar phenomenon can be found in Na 9 (AlS 4 )­(SiS 4 ) . However, the condition in which distortions have a negative influence on ionic conductivity is also reported.…”

The dielectric relaxation behavior induced by sodium migration in the Na2CoSiO4 structure within a three-dimensional Co–O–Si framework

“…First, NCS-SS (with the Pna2 1 space group) had lower local Na coordination symmetry than NCS-CP (with the Pbca space group) which was caused by the different silicon and cobalt sources. 29,30 Second, the NCS-SS material presented two different charge carriers Na(1) and Na(2) in two equivalent cavities. However, NCS-CP presented four charge carriers Na(1), Na(2), Na(3) and Na(4) in two non-equivalent cavities.…”

Na7TaP4: A Ternary Sodium Phosphidotantalate Containing [TaP4]7– Tetrahedra

“…13,14 Good ionic conductivities of up to 0.02 mS cm −1 were recently reported for Na 5−x Al 1−x Si x S 4 , which contain AlS 4 and SiS 4 tetrahedra. 15 The sodium ionic conductivity of phosphide-based materials is almost unexplored, but recently a sodium phosphidosilicate with a sodium ion conductivity of 0.4 mS cm −1 was reported. 16,17 After the discovery of the exceptional lithium conductivity in Li 9 AlP 4 , we tried to expand the class of phosphidoaluminates to sodium ion conductors, and from the reaction of the elements Na, Al, and P in tantalum ampules, we obtained the first sodium phosphidotantalate.…”

“…Up to now, the highest sodium ionic conductivity (3.7 mS·cm –1 ) combined with a low activation energy (0.19 eV) was obtained for Na 3 SbSe 4 , an Sb- and Se-substituted version of Na 3 PS 4 . Recently, with Na 11 Sn 2 PS 12 , the first sodium-based thiophosphate with an ionic conductivity higher than 1 mS·cm –1 was found. , Good ionic conductivities of up to 0.02 mS cm –1 were recently reported for Na 5– x Al 1– x Si x S 4 , which contain AlS 4 and SiS 4 tetrahedra …”