Li₂₁Ge₈P₃S₃₄: New Lithium Superionic Conductor with Unprecedented Structural Type

Lithium superionic conductors are pivotal for enabling all-solid-state batteries, which aim to replace liquid electrolytes and enhance safety. Herein, we report the discovery of an unprecedented lithium superionic conductor, Li₂₁Ge₈P₃S₃₄, featuring a novel structural type and a new composition in the Li-Ge-P-S system. This material exhibits high lithium ionic conductivity of approximately 1.0 mS cm^-1 at 303 K with a low activation energy of 0.20(1) eV. It's unique crystal structure was elucidated using three-dimensional electron diffraction (3D ED) and further refined through combined powder X-ray and neutron diffraction analyses. The structure consists of alternating two-dimensional slabs: one of corner-sharing GeS₄ tetrahedra and the other of isolated PS₄ tetrahedra, enabling efficient lithium-ion transport through a tetrahedrally interconnected network of 1D, 2D, and 3D diffusion pathways. This distinctive structural motif provides a novel design strategy for next-generation solid electrolytes, broadening the structural landscape of lithium superionic conductors. With further advancements in compositional tuning and interfacial engineering, Li₂₁Ge₈P₃S₃₄ could contribute to the development of high-performance all-solid-state batteries.