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

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 Li6PS5CN containing orientationally disordered cyanide ions. The new cyanide argyrodite Li6PS5CN 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 Li6PS5Br. Structural studies of both compounds by high-resolution X-ray diffraction indicate that Li6PS5CN and Li6PS5Br adopt nearly identical crystal structures with similar lattice parameters, which indicates that lower activation barriers in Li6PS5CN 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 Li6PS5CN points to a complex interplay of lattice polarizability and molecular dynamics that lower the activation barrier for lithium-ion conductivity in the cyanide argyrodite.