Low-temperature paddlewheel effect in glassy solid electrolytes

Abstract

Glasses are promising electrolytes for use in solid-state batteries. Nevertheless, due to their amorphous structure, the mechanisms that underlie their ionic conductivity remain poorly understood. Here, ab initio molecular dynamics is used to characterize migration processes in the prototype glass, 75Li₂S-25P₂S₅. Lithium migration occurs via a mechanism that combines concerted motion of lithium ions with large, quasi-permanent reorientations of PS₄^3- anions. This latter effect, known as the 'paddlewheel' mechanism, is typically observed in high-temperature crystalline polymorphs. In contrast to the behavior of crystalline materials, in the glass paddlewheel dynamics contribute to Lithium-ion mobility at room temperature. Paddlewheel contributions are confirmed by characterizing spatial, temporal, vibrational, and energetic correlations with Lithium motion. Furthermore, the dynamics in the glass differ from those in the stable crystalline analogue, γ-Li₃PS₄, where anion reorientations are negligible and ion mobility is reduced. These data imply that glasses containing complex anions, and in which covalent network formation is minimized, may exhibit paddlewheel dynamics at low temperature. Consequently, these systems may be fertile ground in the search for new solid electrolytes.

Fig. 1. Computed structure of glassy Li₃PS₄ at 300 K and 1 bar generated from melt-and-quench ab initio MD. Green spheres represent lithium ions, phosphorus is magenta, and sulfur is yellow.

a Computational cell. A representative Li ion is identified with a black circle. The local solvation shell of this ion is magnified in (b), where numbers indicate the mean and standard deviation of nearest-neighbor sulfur distances in Å, averaged over 10 ps of MD.

Fig. 2. Characterization of the static structure of glassy Li₃PS₄ at ambient temperature.

a Partial pair distribution function, g(r). b Coordination number, n(r). c Total neutron-weighted pair distribution function, G’(r), (solid line) compared with experimental neutron diffraction data (diamonds) reported by Ohara et al.. Atom pair distances associated with the peaks in G’(r) are labeled.

Fig. 3. Detection of Li-ion migration events.

a Identification of specific Li ions that migrate at 300 K. Each rectangle individually plots h_i (Eq. 3) vs. simulation time for the 60 Li ions in the simulation cell. b Number of atoms participating in a cooperative migration event, and the time at which those events were observed. Blue and gray shading in panels (a), (b) identify, respectively, the Li ions that migrate during the first and second cooperative events shown in panel (b). c Displacements of cooperatively migrating Li ions participating in the first cooperative event (identified with blue shading in (a)). The inset in (b) shows the displacement of Li ions participating in the second cooperative event (gray shading in panels (a), (b)).

Fig. 4. Illustration of cation–anion cooperative motion at 300 K.

a Distinct colored spheres represent the positions of four different lithium ions superimposed at 40-fs intervals over a 10-ps trajectory; the initial and final positions of these ions are labeled “B” and “E”, respectively. Tetrahedral PS₄ anions are colored magenta (phosphorus) and yellow (sulfur). The initial positions of the anions at the start of the migration event are shown with partial transparency; opaque depictions indicate final positions. Numeric labels identify individual anions. For clarity, only a portion of the simulation cell is shown. b Angular (black) and linear (blue) displacements of the PS₄ anions as a function of simulation time. (Angular displacements are plotted for each of the four vectors parallel to a P–S bond in a PS₄ anion.) Yellow shading represents the time window over which a cooperative displacement occurs. c, d Diffusion mechanisms for the orange and blue Li ions from panel (a), illustrating the coupling of cation transport with the reorientation of anions. Black lines illustrate the evolution of the coordination environment of lithium as it moves from the beginning (B) and end points (E) of the migration displacement. Arrows identify the two anions (numbers 1 and 11) that exhibit the largest rotational displacements. The magnitudes of the largest anion reorientations are identified.

Fig. 5. Power spectra of the normalized velocity autocorrelation function for crystalline γ-Li₃PS₄ (top) and glassy (bottom) Li₃PS₄ at 300 K.

Lithium vibrations are shown in blue, and anion librations appear in red.

Fig. 6. Calculated Arrhenius plots and activation energies for anion rotational diffusion and Li translational diffusion in 75Li₂S–25P₂S₅ glass.

Data from the Green–Kubo formula (Eq. 5) are shown with diamonds; data from the Einstein formula (Eq. 6) appear as open blue circles. The dotted lines are a linear fit to the data at 1000, 850, and 700 K. Values at 300 K are extrapolated from the high-temperature data.