Computationally Guided Discovery of the Sulfide Li₃AlS₃ in the Li–Al–S Phase Field: Structure and Lithium Conductivity

Abstract

With the goal of finding new lithium solid electrolytes by a combined computational-experimental method, the exploration of the Li-Al-O-S phase field resulted in the discovery of a new sulfide Li₃AlS₃. The structure of the new phase was determined through an approach combining synchrotron X-ray and neutron diffraction with ⁶Li and ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy and revealed to be a highly ordered cationic polyhedral network within a sulfide anion <i>hcp</i>-type sublattice. The originality of the structure relies on the presence of Al₂S₆ repeating dimer units consisting of two edge-shared Al tetrahedra. We find that, in this structure type consisting of alternating tetrahedral layers with Li-only polyhedra layers, the formation of these dimers is constrained by the Al/S ratio of 1/3. Moreover, by comparing this structure to similar phases such as Li₅AlS₄ and Li_4.4Al_0.2Ge_0.3S₄ ((Al + Ge)/S = 1/4), we discovered that the AlS₄ dimers not only influence atomic displacements and Li polyhedral distortions but also determine the overall Li polyhedral arrangement within the <i>hcp</i> lattice, leading to the presence of highly ordered vacancies in both the tetrahedral and Li-only layer. AC impedance measurements revealed a low lithium mobility, which is strongly impacted by the presence of ordered vacancies. Finally, a composition-structure-property relationship understanding was developed to explain the extent of lithium mobility in this structure type.