Structure and Sodium Ion Transport in Na_11+xSn_2+x(Sb_1–yP_y)_1–xS₁₂

Abstract

Sulfidic sodium ion conductors are currently investigated for the possible use in all-solid-state sodium ion batteries. The design of high-performing electrolytes in terms of temperature-dependent ionic transport is based upon the fundamental understanding of structure–transport relationships within the given structural phase boundaries inherent to the investigated materials class. In this work, the Na+ superionic structural family of Na11Sn2PS12 is explored by using the systematic antimony substitution with phosphorus in Na11+x Sn2+x (Sb1–y P y )1–x S12. A combination of Rietveld refinements against X-ray synchrotron diffraction data with electrochemical impedance spectroscopy is used to monitor the changes in the anionic framework, the Na+ substructure, and the ionic transport. A new simplified descriptor for the average Na+ diffusion pathways, the average Na+ polyhedral volume, is introduced, which is used to correlate the contraction of the overall lattice and the found activation barriers in the system. This study exemplifies how substitution affects diffusion pathways in ionic conductors and widens the knowledge about the related structural motifs and their influence on the ionic transport in this novel class of ionic conductors.

Influence of Reduced Na Vacancy Concentrations in the Sodium Superionic Conductors Na11+xSn2P1−xMxS12 (M = Sn, Ge)

“…As previously observed in the Na11Sn2PS12 structural family, the reduced activation energy leads to an overall reduced room temperature in-grain sodium ionic conductivity. 25,28,35 This work further highlights how aliovalent substitution and changing Na + carrier densities influence the ionic transport properties in this class of materials.…”

“…Expressed as the occupancies on the six distinct Na sites and the coordination polyhedra around them, the Na + substructure is extracted from the structural models refined against low-temperature synchrotron X-ray diffraction. Whereas previous studies investigated the complex Na(X)S n polyhedral connectivity, it has shown useful to separate the six Na-S polyhedra present into two locally connected clusters as highlighted by bond valance maps depicted in Figure 4b; 25,28,31 (I) In the Nyquist representation of the exemplarily shown impedance spectra, (Figure 5a) two processes can be distinguished. At lower temperatures (≤ −40 °C) the two processes are well resolved and can be fit (as shown in Figure 5a) with equivalent circuits of two series elements consisting of a resistance in parallel with a constant phase element (CPE).…”

“…From linear regression the in-grain activation energies EA,in-grain and respective Arrhenius pre-factors σ0 are obtained and used to extrapolate σin-grain to room temperature, as previously shown to be a reliable approach. 25,27,28 Errors related to σin-grain(298 K) are obtained from error propagation of the linear regression to the Arrhenius behavior. 39 All resistances with respective Arrhenius plots and capacitances of the investigated samples in-grain and total conductivities are presented in the Supporting Information (Figure S6 and Table S14 & S15).…”

“…An expanding unit cell within Na11Sn2PS12-type materials has previously been associated with a decreasing activation energy in the anionic substitution Na11.1Sn2.1P0.9Se12 and cationic substitution Na11+xSn2+x(Sb1−yPy)xS12. 25,28 Previously, it was attempted to capture this size effect by; (I) the "Na + channel"-volume, 32 defined as difference of unit cell volume and sum of MS4 tetrahedral volumes following the reduction of predicted activation energies along Na11Tt2PS12 Tt = Si, Ge, Sn. 32 And (II) the "average Na-S polyhedral volume", 25 defined as the average of the Na(X)Sn polyhedral volumes around the Na(X) sites X = 1 to 6, weighted by their multiplicities (see Equation S1).…”

“…Especially on the Na(4) and Na(5) sites, large thermal displacement ellipsoids were reported and a splitting of the Na(4) site has been used to describe the Na + distribution in that octahedron. 25,27 Furthermore, the quasi-cubic 8b site hosts the Na (6), which bridges the Na (1) and Na(2) positions and was shown to take part in the diffusion mechanism by molecular dynamics simulations. 24,[30][31][32][33] Lastly, another unoccupied 32g site in an octahedral void has been discussed as the intermediate site in alternative diffusion pathways on the basis of bond valence sum analyses.…”