Two-Dimensional Substitution Series Na₃P_1–xSb_xS_4–ySe_y: Beyond Static Description of Structural Bottlenecks for Na⁺ Transport

Abstract

Highly conductive solid electrolytes are fundamental for all solid-state batteries with low inner cell resistance. Such fast solid electrolytes are often found by systematic substitution experiments in which one atom is exchanged for another, and corresponding changes in ionic transport are monitored. With this strategy, compositions with the most promising transport properties can be identified fast and reliably. However, the substitution of one element does not only influence the crystal structure and diffusion channel size (static) but also the underlying bonding interactions and with it the vibrational properties of the lattice (dynamic). Since both static and dynamic properties influence the diffusion process, simple one-dimensional substitution series only provide limited insights to the importance of changes in the structure and lattice dynamics for the transport properties. To overcome these limitations, we make use of a two-dimensional substitution approach, investigating and comparing the four single-substitution series Na3P1–x Sb x S4, Na3P1–x Sb x Se4, Na3PS4–y Se y , and Na3SbS4–y Se y . Specifically, we find that the diffusion channel size represented by the distance between S/Se ions cannot explain the observed changes of activation barriers throughout the whole substitution system. Melting temperatures and the herein newly defined anharmonic bulk modulusas descriptors for bonding interactions and corresponding lattice dynamicscorrelate well with the activation barriers, highlighting the relevance of lattice softness for the ion transport in this class of fast ion conductors.

On the Discrepancy between Local and Average Structure in the Fast Na+ Ionic Conductor Na2.9Sb0.9W0.1S4

“…Till et al investigated the isoelectronic substitution series of Na 3 P x Sb 1−x S 4 and obtained tetragonal structures for all compositions. 25 Thus, mere polyanion disorder is unlikely to lead to a cubic average structure. In contrast to isoelectronic substitution, the aliovalent substitution of SbS 4 3− by WS 4 2− leads to an increase in the Na + vacancy density (2), and with it much faster ionic transport (see below).…”

Investigating the Li+substructure and ionic transport in Li10GeP2−xSbxS12(0 ≤x≤ 0.25)

“…The (PS 4 ) 3− tetrahedron shows no volume changes, while the ( M /P)S 4 n − tetrahedron increases in volume since the smaller phosphorus ( r (P 5+ ) = 0.17 Å) 30 is replaced by the bigger antimony ( r (Sb 5+ ) = 0.50 Å). 31 However, as indicated the overall structural changes are small in the order of maximum 2%.…”

“…The tendency of symmetry reduction when substituting with antimony could be caused by the need of the structure to adjust the unit cell to the radii difference of 0.33 Å. 30,31 Herein, primarily the lattice parameter c is changed. The lattice parameters of the nominal composition Li 10 GeP 1.85 Sb 0.15 S 12 agree well with the reported values by Liang et al 29 Already published literature reports that the P(2) site (Wyckoff 2b) is only occupied by phosphorus, while the M(1)/P(1) site (Wyckoff 4d) shows cation disorder of P 5+ ,Ge 4+ and M n+ in LGPS.…”

“…The introduction of Sb(+V) into the LGPS structure is expected to introduce two changes intrinsic to P(+V) and Sb(+V): (1) the smaller P(+V) (r = 0.17 Å) 30 is replaced by the bigger Sb(+V) (r = 0.50 Å). 31 The widening of the unit cell seems to have a positive effect on the conductivity as shown by Liang et al, 29 however it remains elusive how exactly the framework and the Li + sublattice is affected. (2) The magnitude of the electronegativity on the phosphorus sites may be weakened due to the introduction of the more electropositive antimony (EN(P) = 12.8 eV e À1 vs. EN(Sb) = 11.2 eV e À1 ) (see Fig.…”

Relating critical phonon occupation to activation barrier in fast lithium-ion conductors

“…bonding) are important in some fast ionic conductors. [15][16][17][18] In this case, 𝐸 A is correlated to frequencies of atomic vibrations (i.e. phonon modes), particularly those of the mobile ion.…”