Rapid Crystallization and Kinetic Freezing of Site-Disorder in the Lithium Superionic Argyrodite Li6PS5Br

Abstract

<p>Lithium argyrodite superionic conductors are currently being investigated as solid electrolytes for all-solid-state batteries. Recently, in the lithium argyrodite Li<sub>6</sub>PS<sub>5</sub>X (X = Cl, Br, I), a site-disorder between the anionsS<sup>2–</sup>and X<sup>–</sup>has been observed, which strongly affects the ionic transport and appears to be a function of the halide present. In this work, we show how such disorder in Li<sub>6</sub>PS<sub>5</sub>Br can be engineered <i>via</i>the synthesis method. By comparing fast cooling (<i>i.e. </i>quenching) to more slowly cooled samples, we find that anion site-disorder is higher at elevated temperatures, and that fast cooling can be used to kinetically trap the desired disorder, leading to higher ionic conductivities as shown by impedance spectroscopy in combination with <i>ab-initio</i>molecular dynamics. Furthermore, we observe that after milling, a crystalline lithium argyrodite can be obtained within one minute of heat treatment. This rapid crystallization highlights the reactive nature of mechanical milling and shows that long reaction times with high energy consumption are not needed in this class of materials. The fact that site-disorder induced <i>via</i>quenching is beneficial for ionic transport provides an additional approach for the optimization and design of lithium superionic conductors.</p>

On the Local Charge Inhomogeneity and Lithium Distribution in the Superionic Argyrodites Li6PS5X (X = Cl, Br, I)

“…9,[24][25][26][27][28]32,41,43 The hereobtained X − /S 2− site-disorder is quantified in Figure 3b. In agreement with the literature, a higher degree of disorder can be found for the chloride compared to the bromide counterpart, which can be attributed to the similar ionic radii of S 2− and Cl − , 9,25,29 compared to the more dissimilar radii of S 2and Br − , 9,24,27,36 while no site-disorder is observed for Li6PS5I. 9,17,34 The extent of the site-disorder does not change with temperature, indicating that within this modifications to the degree of disorder due to changing the temperature treatment are not occurring within our investigated temperature-range.…”

“…Li6PS5Br and Li6PS5Cl, however, typically adopt anion-disordered structures, in which the X − and S 2− anions are significantly disordered across the Wyckoff 4a and 4d sites, 9,17 with the precise degree of anion-disorder tunable by adjusting synthesis protocols. 36 The origin of this anion-site-disorder has not been unequivocally resolved, but theoretical and experimental analyses suggest that anion mixing is facilitated by the similar ionic radii for S 2− versus Br − or Cl − . 9,37 The absence or presence of site-disorder in Li6PS5X argyrodites strongly affects their lithium transport properties.…”

“…(anion-ordered) is a poor Li-ion conductor compared to Li6PS5Cl and Li6PS5Br (both aniondisordered), which exhibit ×10 3 higher room-temperature ionic conductivities. 9,13,[38][39][40] While the correlation between anion disorder and fast lithium transport is well documented, 9,[24][25][26][27][28][29][30][31][32][33]36,41 the effect of anion site-disorder on the microscopic dynamics and local structure of the mobile lithium ions is less well understood. In Li6PS5X argyrodites, the anionic framework contains 132 tetrahedral voids per unit cell (4 formula units) that may accommodate the corresponding 24 lithium ions.…”

Mechanochemical Synthesis: A Tool to Tune Cation Site Disorder and Ionic Transport Properties of Li3MCl6 (M = Y, Er) Superionic Conductors

“…Exceeding the range of 8 Å, the remaining, more crystalline precursor ErCl 3 dominates the G ( r ) due to the low coherence length of the locally formed Li 3 ErCl 6 . This rapid crystallization within 1 min after mechanochemical milling, while surprising, was recently found to also occur in Li 6 PS 5 Br, providing further evidence that mechanochemical milling already acts as a primary synthesis step leading to low coherency products and possible prenucleation clusters.…”

Insights into the Lithium Substructure of the Superionic Conductors Li3YCl6 and Li3YBr6

“…With the already discussed increasing electrostatic repulsion resulting from the large yttrium occupancy, the anisotropic thermal displacement ellipsoids are being rotated away from the yttrium octahedra (Figure 10a,c) and thereby being pushed towards into the tetrahedral position. 28,29 Contrary to this, the lower occupancy in the mechanochemically synthesized compound allows for lithium to displace to the energetically more stable octahedral sites (Figure 10b,d). Regarding all structural observations, it is still not clear why the transport properties of Li3YBr6 deviate so significantly with different synthesis procedures.…”