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

Abstract

Lithium argyrodite superionic conductors are currently being investigated as solid electrolytes for all-solid-state batteries. Recently, in the lithium argyrodite Li6PS5X (X = Cl, Br, and I), a site-disorder between the anions S2– and X– 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 a disorder in Li6PS5Br can be engineered via the synthesis method. By comparing fast cooling (i.e., quenching) to more slowly cooled samples, we find that the 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 ab initio molecular dynamics. Furthermore, we observe that after milling, a crystalline lithium argyrodite can be obtained within 1 min 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 via quenching is beneficial for ionic transport provides an additional approach for the optimization and design of lithium superionic conductors.

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, [40] providing further evidence that mechanochemical milling already acts as a primary synthesis step leading to low coherency products and possible prenucleation clusters.…”

Impact of Solvent Treatment of the Superionic Argyrodite Li6PS5Cl on Solid‐State Battery Performance

“…At this point, we feel a short discussion on the source of the argyrodite is needed. Li 6 PS 5 Cl can be synthesized by a high‐temperature reaction of mixtures of P 4 S 10 , Li 2 S, and LiCl as was done here, [ 61,64 ] by ball milling the same precursors (followed by short heat treatments), [ 77–79 ] or by solution chemistry from Li 2 S, Li X , and Li 3 PS 4 (which itself can be ball milled or made via solution) in a solvent such as ethanol. [ 51,80 ] It can also be bought commercially from a few suppliers, where the synthesis route is not always clear.…”

Sulfide and Oxide Inorganic Solid Electrolytes for All-Solid-State Li Batteries: A Review

“…They reported the presence of a three-dimensional (3D) pathway network for the long-range ion conduction of all Li 6 PS 5 X (X = Cl, Br, I) phases, which consisted of interconnected low-energy local pathway cages [180]. The experimentally measured ionic conductivity at 25 • C of Li 6 PS 5 Cl, Li 6 PS 5 Br, and Li 6 PS 5 I prepared by ball milling followed by heating at 550 • C in inert atmosphere are in the range 1.9 × 10 −4 -7.0 × 10 −3 S cm −1 and calculated activation energies in the range 0.26-0.41 eV (Table 1) [180][181][182][183][184][185][186]. Further, Boulineau et al [183] reported the effect of enhancement of the conductivity of Li 6 PS 5 Cl from 2× 10 −4 S cm −1 to 1.33 × 10 −3 S cm −1 when the ball milling time varies from 1 h to 10 h. Rao and Adams [181] compared the values of E a determined by both experimental and computational method for Li 6 PS 5 X with X= Cl, Br, I in the range 0.25-0.38 eV.…”

“…(iv) The conductivities of argyrodite electrolytes depend on the preparation method, grain boundary contributions, and conductivity measurement method and fabrication technique of pelletized samples, including sintering cold-pressed pellets that influences the density of the specimens [183]. Based on previous literature studies, conductivity values are also influenced by cooling rate [186], porosity, and pore distribution [190]. Lower Li + ion conductivities, in the range of 10 −5 -10 −4 mS cm −1 , were reported when the electrolytes were synthesized via the solution-based method, which were attributed to the presence of additional impurity phases in the compounds [189].…”