High Lithium Content and Site Disorder in the Transition Metal Oxide Argyrodites Li₇TiO₅X (X = Cl^-, Br^-)

Sulfide lithium argyrodites are a key materials family that are studied as solid electrolytes in commercial all-solid-state batteries (ASSBs), while their oxide analogues remain relatively unexplored. This study presents the discovery of Li₇TiO₅X (X = Cl^-, Br^-), the first lithium argyrodite materials in which a transition metal is used as the framework-forming cation, expanding the chemical space that is accessible for oxide argyrodites. Incorporation of Ti⁴⁺ enables the lithium content to be maximized to 7 Li⁺ per formula unit. Interestingly, even with the high lithium content, Li₇TiO₅Cl retains a Li⁺ site disordered cubic F4̅3m structure at room temperature with Li⁺ occupancy of the T5, T5a, and T3 positions, and exhibits an ionic conductivity of 2.2(2) × 10^-6 S cm^-1 with the lowest reported activation energy (0.36(2) eV) for bulk Li⁺ ion transport in an oxide argyrodite. Conversely, Li₇TiO₅Br adopts the same F4̅3m symmetry at room temperature but with an ordered arrangement of Li⁺ positions via full occupancy of the T5a and T3 positions, and thus has an ionic conductivity that is 3 orders of magnitude lower (∼10^-9 S cm^-1) and a much higher activation energy (0.58(2) eV) than Li₇TiO₅Cl. Order-disorder behavior is observed below 250 K in Li₇TiO₅Cl, where a Li⁺ site ordering pattern is observed that is distinct from Li₇TiO₅Br and all sulfide argyrodites, yielding a tetragonal symmetry (I4̅) for only the second time to date in the argyrodite structure type. This unique order-disorder behavior, alongside the ability to incorporate transition metal cations within this material family emphasizes the potential to access much greater structural diversity via the expansive chemical space that is available for exploration in oxide argyrodites.