A new class of lithium ion conductors with tunable structures and compositions: Quaternary diamond-like thiogermanates

The new Li2CoGeS4 compound crystallizes in the Pn space group with the wurtz-kesterite structure, according to single crystal X-ray diffraction. The structure of Li2CoGeS4 and the high degree of phase-purity in which it is prepared are supported by high-resolution synchrotron X-ray powder diffraction. Varying the divalent ion in Li2-II-GeS4 materials yields three different structure types, all of which are derived from hexagonal diamond. These structural variations give rise to Li+-encompassing [II-GeS4]2- nets with different topologies that offer diversity in lithium ion diffusion pathways. In the first systematic study of the lithium ion conductivity in quaternary diamond-like materials, wurtz-kesterite-type Li2CoGeS4 and Li2FeGeS4 (Pn), lithium cobalt(II) silicate-type Li2MnGeS4 (Pna21), and wurtz-stannite-type Li2CdGeS4 (Pmn21) are presented as environmentally stable lithium ion conductors. These materials are comprised of cubic diamond-like [CoGeS4]2- and [FeGeS4]2- anionic frameworks, ABW-like [MnGeS4]2-, and square lattice-like [CdGeS4]2-. As assessed using impedance spectroscopy, Li2FeGeS4 exhibits the most promising Li+ ion conductivity of 1.8(3) × 10-4 S/cm at 100°C, while Li2CdGeS4 shows the lowest activation energy for lithium ion conduction, EA = 0.74(2) eV