Influence of Lattice Polarizability on the Ionic Conductivity in the Lithium Superionic Argyrodites Li₆PS₅X (X = Cl, Br, I)

Abstract

In the search for novel solid electrolytes for solid-state batteries, thiophosphate ionic conductors have been in recent focus owing to their high ionic conductivities, which are believed to stem from a softer, more polarizable anion framework. Inspired by the oft-cited connection between a soft anion lattice and ionic transport, this work aims to provide evidence on how changing the polarizability of the anion sublattice in one structure affects ionic transport. Here, we systematically alter the anion framework polarizability of the superionic argyrodites LiPSX by controlling the fractional occupancy of the halide anions (X = Cl, Br, I). Ultrasonic speed of sound measurements are used to quantify the variation in the lattice stiffness and Debye frequencies. In combination with electrochemical impedance spectroscopy and neutron diffraction, these results show that the lattice softness has a striking influence on the ionic transport: the softer bonds lower the activation barrier and simultaneously decrease the prefactor of the moving ion. Due to the contradicting influence of these parameters on ionic conductivity, we find that it is necessary to tailor the lattice stiffness of materials in order to obtain an optimum ionic conductivity.

Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor

“…As has been shown recently, the softness of a lattice may influence v 0 . 43 In the present case, besides such an influence, effects of migration entropy, affected by both vibrational or configurational contributions, is expected to control the prefactor mostly. Frenkel defects show a considerable high migration entropy that may especially affect the samples with x = 0.5 and x = 0.8.…”

Na3SbSe4−xS x as Sodium Superionic Conductors

“…This conductivity is also comparable to those of recently reported sulfides such as Na 3 SbS 4 23 and Na 3 P 0.62 As 0.38 S 4 25 . This high ionic conductivity can be explained by the large unit cell of Na 3 SbSe 4 and the high polarizability of Se 2− , which lowers the electrostatic binding energy and weakens the attraction between Na + and the tetrahedron of [SbSe 4 3− ] 29,30 .…”

Li+ Ion Conductors with Adamantane‐Type Nitridophosphate Anions β‐Li10P4N10 and Li13P4N10X3 with X=Cl, Br

“…Solid-state MAS (magic-angle spinning) NMR methods: 6 Li, 7 Li, and 31 Ps olid-state MAS NMR spectra of both polymorphs were recorded on aD SX Advance spectrometer (Bruker) with am agnetic field of 11.7 T. The sample was placed in aZ rO 2 rotor with an outer diameter of 4, 2.5, or 1.3 mm, which was mounted in ac ommercial MAS probe (Bruker) under inert conditions. The rotor was spun at ar otation frequency of 10, 20, or 50 kHz.…”

“…[30] However,i th as recently been shown in the case of the lithium argyrodites Li 6 PS 5 X (X = Cl, Br,I )t hat the prefactor and the activation energy of ionic conduction are closely linked, and that al ow activation energy alone may not be sufficient to achieve high ion conductivity. [31] In this contribution,w er eport ar e-investigation of cubic aLi 10 P 4 N 10 .B yl owering the synthesis temperature,w eh aves ucceeded in synthesizingt he new polymorph b-Li 10 P 4 N 10 ,a nd have elucidated the phase transition between the a-a nd bpolymorphs. Furthermore,w eh ave synthesized the first quaternary lithium nitridophosphate halides Li 13 P 4 N 10 X 3 with X = Cl, Br.P hase-pure products of all of the title compounds have enabled ion conductivity measurements and investigations concerning their stability with respecttoL imetal.…”