Lowering the Activation Barriers for Lithium-Ion Conductivity through Orientational Disorder in the Cyanide Argyrodite Li₆PS₅CN

Abstract

Rapid advancements in safe and high-energy-density energy storage are predicated on identifying new solid-state ion conductors with low activation energies and high ionic conductivities for all-solid-state battery technologies. Halide argyrodites are among some of the top candidates for solid-state electrolytes, as they can achieve ionic conductivities that approach liquid electrolytes. Incorporating dynamic pseudohalide species in argyrodite solid electrolytes presents an exciting opportunity to exploit lattice dynamics as a design principle to modulate the ion conduction properties of solid-state ion conductors. In the present study, we have prepared the new argyrodite Li 6 PS 5 CN containing orientationally disordered cyanide ions. The new cyanide argyrodite Li 6 PS 5 CN exhibits an activation barrier to Li-ion transport of 471 ± 25 meV and a room-temperature ionic conductivity of 6(2) × 10 −5 S cm −1 in comparison to the activation barrier of 502 ± 16 meV and an ionic conductivity of 2.3(1) × 10 −4 S cm −1 measured for the bromide analogue Li 6 PS 5 Br. Structural studies of both compounds by high-resolution X-ray diffraction indicate that Li 6 PS 5 CN and Li 6 PS 5 Br adopt nearly identical crystal structures with similar lattice parameters, which indicates that lower activation barriers in Li 6 PS 5 CN arise due to the cyanide ion itself rather than due to changes in the geometry of conduction pathways in the local lithium environment. The orientational disorder of the quadrupolar cyanide ion in Li 6 PS 5 CN points to a complex interplay of lattice polarizability and molecular dynamics that lower the activation barrier for lithium-ion conductivity in the cyanide argyrodite.

Borohydride Substitution Effects of Li6PS5Cl Solid Electrolyte

“…A recent example is Li 6 PS 5 CN, where the CN − group takes the place of the original Cl sites for the "paddle-wheel" effect on Li conductivity. 11 Turning to the borohydride, Sakuda et al has reported the synthesis of argyrodite structure Li 6 PS 5 (BH 4 ) via a two-step ball-milling method. 9 Subsequently, Dao et al used a one-step ball-milling method to introduce a small amount of BH 4 − in the argyrodite Li 6 PS 5 Cl 1−x (BH 4 ) x (x = 0.17) and increased the ionic conductivity compared to Li 6 PS 5 Cl.…”

“…Therefore, it has been proposed to introduce the borohydride group into the halogen position of Li-argyrodite to improve the ionic conductivity. , This is because the rotational motion (the so-called “paddle-wheel” effect) of the polyanions could significantly accelerate the hopping of Li cations in the lattice. A recent example is Li 6 PS 5 CN, where the CN – group takes the place of the original Cl sites for the “paddle-wheel” effect on Li conductivity . Turning to the borohydride, Sakuda et al has reported the synthesis of argyrodite structure Li 6 PS 5 (BH 4 ) via a two-step ball-milling method .…”

Argyrodite-type advanced lithium conductors and transport mechanisms beyond paddle-wheel effect

“…Therefore, any ionic-conducting structures that can accommodate halogen site can be potential candidates for such a strategy. Indeed, we have seen this realized experimentally in Li/Na-rich antiperovskites (Li/Na) 3−x (O/S)X (X = halogen or cluster) 40 , 41 and lithium argyrodites 42 , 43 . More recently discovered solid electrolyte systems with good properties, such as Li x ScCl 3+x 37 , Na 3−x Y 1−x Zr x Cl 6 44 and Na 3−y PS 4−x Cl x 45 , can be also subject to such a strategy.…”

Ion Hopping: Design Principles for Strategies to Improve Ionic Conductivity for Inorganic Solid Electrolytes

“…The new argyrodite Li 6 PS 5 CN containing orientationally disordered cyanide ions exhibited a relative low lithium ion migration barrier, though it adopting the similar crystal structures with Li 6 PS 5 Br (Figure 8e). [106] This strategy could be innovatively applied in ionic conductors for multivalent metal ions. Yan et al predicted NH 3 in Mg(BH 4 ) 2 NH 3 exchanged and constructing a highly flexible structure, profiting from the di-hydrogen bonds and dramatically improving the Mg-ion conductivity ≈8 orders of magnitude higher than that of Mg(BH 4 ) 2 (Figure 8f).…”

“…e) Crystal structure of the cyanide argyrodite Li 6 PS 5 CN with orientationally disordered cyanide ions. Reproduced with permission [106]. Copyright 2021, American Chemical Society.…”