Superionic Conduction in the Plastic Crystal Polymorph of Na4P2S6

Abstract

Sodium thiophosphates are promising materials for large-scale energy storage applications benefiting from high ionic conductivities and the geopolitical abundance of the elements. A representative of this class is Na₄P₂S₆, which currently shows two known polymorphs-α and β. This work describes a third polymorph of Na₄P₂S₆, γ, that forms above 580 °C, exhibits fast-ion conduction with low activation energy, and is mechanically soft. Based on high-temperature diffraction, pair distribution function analysis, thermal analysis, impedance spectroscopy, and ab initio molecular dynamics calculations, the γ-Na₄P₂S₆ phase is identified to be a plastic crystal characterized by dynamic orientational disorder of the P₂S₆ ^4- anions translationally fixed on a body-centered cubic lattice. The prospect of stabilizing plastic crystals at operating temperatures of solid-state batteries, with benefits from their high ionic conductivities and mechanical properties, could have a strong impact in the field of solid-state battery research.

Figure 1

Variable-temperature PXRD of Na₄P₂S₆ upon heating (bottom) and cooling (top) across the β–γ phase transition: contour plot of diffractograms (the color code, red to purple/blue, indicates the ranges from low to high diffraction intensities, respectively) and exemplary diffractograms (black) at 520 °C (β), 620 °C (γ), and 520 °C (β after heating above the phase transition). The selected diffractograms are additionally depicted in Supporting Information Figure S1.

Figure 2

(a) Temperature-dependent volume expansion and contraction across the β → γ phase transition relative to the room-temperature unit cell volume, V_rt, of Na₄P₂S₆. (b) Differential scanning calorimetry (DSC) curve.

Figure 3

(a) Experimental synchrotron PDF with PDFs simulated from the fitted structure models and P₂S₆^4– anion only and (b) Rietveld fit of the PXRD pattern for γ-Na₄P₂S₆ at 650 °C. The gray shaded region in (a) represents the unphysical interatomic distance range that is more highly affected by systematic errors from data processing. (c) Illustration of the PDF-refined γ-structure with a static approximation of the P₂S₆^4– anions in a 2 × 2 × 2 cell, and (d) in a single unit cell. (e) Overlay of bond valence energy landscape (red surface) and the crystal structure visualizing the three-dimensional Na⁺ conducting pathways.

Figure 4

(a) Arrhenius plot of ionic conductivity and activation energies derived from impedance spectroscopy. The heating and cooling segments are indicated by orange and blue circles. The activation energy was extracted from the orange (heating) data points, since the cooling data points possibly describe a mixed ionic–electronic conducting (MIEC) phase. The error bars arise from the standard deviation of ionic conductivity measured on three distinct pellets. (b) Mean-squared displacements (MSDs) of Na, P, and S at 1000 K from AIMD simulations. (c) Arrhenius plot of the ionic conductivity and activation energy from AIMD simulations.

Figure 5

(a) Scheme of the definition of the azimuthal angle (ϕ) and polar angle (θ) with respect to the P–P handle and scheme of the P–P orientation in Cartesian octants. (b) Orientation heat map of all P₂S₆^4– in a 2 × 2 × 2 cell at 1000 K in a 300 ps simulation. (c) ϕ and (d) θ as a function of time of an exemplary P₂S₆^4– at 1000 K.