Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li 6+ x P 1- x Si x O 5 Cl.

Argyrodite is a key structure type for ion-transporting materials. Oxide argyrodites are largely unexplored despite sulfide argyrodites being a leading family of solid-state lithium-ion conductors, in which the control of lithium distribution over a wide range of available sites strongly influences the conductivity. We present a new cubic Li-rich (>6 Li ⁺ per formula unit) oxide argyrodite Li ₇ SiO ₅ Cl that crystallizes with an ordered cubic ( P 2 ₁ 3) structure at room temperature, undergoing a transition at 473 K to a Li ⁺ site disordered F 4̅3 m structure, consistent with the symmetry adopted by superionic sulfide argyrodites. Four different Li ⁺ sites are occupied in Li ₇ SiO ₅ Cl (T5, T5a, T3, and T4), the combination of which is previously unreported for Li-containing argyrodites. The disordered F 4̅3 m structure is stabilized to room temperature via substitution of Si ⁴⁺ with P ⁵⁺ in Li _{6+ x} P _{1- x} Si _x O ₅ Cl (0.3 < x < 0.85) solid solution. The resulting delocalization of Li ⁺ sites leads to a maximum ionic conductivity of 1.82(1) × 10 ^-6 S cm ^-1 at x = 0.75, which is 3 orders of magnitude higher than the conductivities reported previously for oxide argyrodites. The variation of ionic conductivity with composition in Li _{6+ x} P _{1- x} Si _x O ₅ Cl is directly connected to structural changes occurring within the Li ⁺ sublattice. These materials present superior atmospheric stability over analogous sulfide argyrodites and are stable against Li metal. The ability to control the ionic conductivity through structure and composition emphasizes the advances that can be made with further research in the open field of oxide argyrodites.