Synthesis and characterization of solid sodium ion conductors based on phosphido-metallates of Groups 3 to 5 as well as 13 and 14

• Applicants: Professor Dr. Thomas F. Fässler; Professor Dr. Leo van Wüllen
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 565473508

The storage of electrical energy plays a central role in the transformation towards more sustainable energy sources, and rechargeable solid-state batteries represent an important progress in battery technology. The ever-increasing demand for lithium-ion batteries is restricted by the limited availability of lithium, so alternative materials are being sought. Sodium-ion batteries have similar electrochemical properties to lithium-ion batteries, which makes them an attractive option for scalable energy storage systems such as grid storage and other stationary applications due to the abundant sodium resources available worldwide. In addition to suitable anode and cathode materials, fast sodium ion conductors are required for realization. Based on extensive work by the two applicant groups on the development of the new substance class of lithium ion-conducting phosphide tetrelates (Tt) and trielates (Tr) with Tt = Si, Ge, Sn and Tr = Al, Ga, In, this research project aims to find and characterize new compounds for new, good to very good sodium ion conductors and to elucidate the mechanisms of sodium ion conduction. While oxidic and sulfidic sodium ion conductors are intensively investigated, only a few compounds have been described in the Na-Tt-P and Na-Tr-P system so far, in particular compounds of the simple type Na8TtP4 or Na9TrP4, which have very simple building units (tetrahedral units). They allow rapid derivatization thanks to a wide range of substitution options. Our preliminary studies show, a first representative, Na8SnP4, has very good Na ion conductivity. In this project, the substance class in the Na-M-P M = Tt, Tr, and early transition metals is systematically developed synthetically, using the experience and structural analogies of the Li derivatives, but also analogies to sulfidic representatives. Ball milling methods and high-temperature reactions are used for synthesis. Powder and single-crystal X-ray diffraction methods as well as temperature-dependent synchrotron and neutron diffraction methods are used to determine the atomic structure including possible disorder of the cations, the minimum entropy method and bond valence sum (BVS) calculations for the experimental and theoretical determination of ion migration paths and temperature-dependent impedance measurements and static 6Li-NMR spectroscopy to determine conductivity and activation energies. Advanced methods of solid-state NMR spectroscopy (including investigation of frequency-dependent static 23Na NMR relaxation times, MAS NMR line shape analysis and 2D exchange NMR spectroscopy as well as spin-alignment echo NMR spectroscopy) to determine different correlation time ranges are used to characterize the dynamic processes (cation movement, anion reorientation). In addition to a deeper understanding of sodium ion mobility, we expect new sodium compounds with very good ionic conductivity properties.

• DFG Programme: Research Grants