Modifying the properties of fast lithium-ion conductors - The lithium phosphidotetrelates Li₁₄SiP₆, Li₁₄GeP₆, and Li₁₄SnP₆

A broad repertoire of potential solid-state electrolytes is a prerequisite for the development and optimization of high-energy-density all-solid-state batteries. An isovalent substitution of suitable elements is a very successful tool to get access to new materials with improved properties, which allow for a detailed investigation of structure-property relationships. Here, we present the two new lithium phosphidotetrelates Li14GeP6 and Li14SnP6 with ionic conductivities of σ ∼1 mS cm-1 at room temperature. To evaluate the rules for the structure-property relationships, all experimental data of lithium phosphidogermanate Li14GeP6 and lithium phosphidostannate Li14SnP6 are compared to the recently reported lithium phosphidosilicate Li14SiP6. The isotypic compounds Li14TtP6 (Tt = Si, Ge, Sn) are accessible via a straightforward and simple synthesis, starting from ball milling of the elements, followed by annealing of the obtained mixtures. Because of the high Li and low Tt content, all of these compounds are considered as lightweight materials with a density of 1.644-2.025 g cm-3. The materials were analyzed applying powder X-ray diffraction, differential scanning calorimetry, 6Li, 31P, and 119Sn solid-state magic angle spinning NMR as well as temperature-dependent 7Li NMR experiments, and electrochemical impedance spectroscopy.