Fast Lithium Ionic Conductors Li14SiP6, Li14GeP6, and Li14SnP6 – Structure-Property-Relationships in the Newly Discovered Family of Lithium Phosphidotetrelates
Authors: Strangmueller Stefan , Eickhoff Henrik , Mueller David , Klein Wilhelm , Raudaschl-Sieber Gabriele , Kirchhain Holger , Sedlmeier Christian , Baran Volodymyr , Senyshyn Anatolyi , Deringer Volker L. , van Wüllen Leo , Gasteiger Hubert A. , Fässler Thomas Friedrich 
Published: December 2020

A broad repertoire of potential solid-state electrolytes (SSEs) is a prerequisite for the development and optimization of high-energy-density all-solid-state batteries (ASSBs). In this context, the key challenge in solid state chemistry and material science is to get access to new materials with improved properties which allow for detailed investigation of structure-property relationships.[1]

Recently, a new class of promising lithium ion conductors has been studied intensively. The so-called lithium phosphidotetrelates consisting of lithium phosphidosilicates, phosphidogermanates, and phosphidostannates as well as the closely related lithium phosphidoaluminates are suitable candidates for the systematic investigation of structure-property-relationships of next generation lithium ion conductors.[1-5] As the name implies, the materials are structurally related to (oxo-)silicates, thiosilicates and thiophosphates. In contrast to the latter, which are well-known for some of the most prominent sulfide-based lithium super ionic conductors, lithium phosphidotetrelates are basically built up by tetrahedrally coordinated [SiP4]8−, [GeP4]8−, and [SnP4]8− units. The substitution of sulfur by phosphorus allows for much higher charge carrier concentration within the structure and thus higher ionic conductivities (Figure 1: a) Structure of Li14SiP6. b) Arrhenius and Nyquist plot (inset) of Li14SiP6.).[4] Hence, lithium phosphidotetrelates and phosphidoaluminates offer a large structural variety combined with low activation energies and fast lithium ion conductivity up to 3 mS cm−1 at room temperature.[5]

Here, the preparation and characterization of new lithium-rich phosphidotetrelates is reported. Characterization of the materials applying X-ray diffraction (powder and single crystal) and solid-state MAS NMR measurements as well as elastic coherent neutron scattering experiments enabled the thorough investigation of the structural and thermal behavior of the compounds. Activation energies, ionic and electronic conductivities have been determined using solid-state 7Li NMR measurements and electrochemical impedance spectroscopy. Furthermore, diffusion pathways were analyzed by temperature-dependent powder neutron diffraction measurements in combination with MEM and DFT calculations to extend the knowledge about the material properties. Finally, the structural differences and the consequential material properties are analyzed with respect to the resulting structure-property-relationship which is crucial for further development of high-performance lithium ion conductors.

References:

[1] Toffoletti, L.; Kirchhain, H.; Landesfeind, J.; Klein, W.; van Wüllen, L.; Gasteiger, H. A.; Fässler, T. F. Lithium Ion Mobility in Lithium Phosphidosilicates: Crystal Structure, 7Li, 29Si, and 31P MAS NMR Spectroscopy, and Impedance Spectroscopy of Li8SiP4 and Li2SiP2. Chem. Eur. J.
2016, 22 (49), 17635-17645.

[2] Eickhoff, H.; Toffoletti, L.; Klein, W.; Raudaschl-Sieber, G.; Fässler, T. F., Synthesis and Characterization of the Lithium-Rich Phosphidosilicates Li10Si2P6 and Li3Si3P7. Inorg. Chem.
2017, 56 (11), 6688-6694.

[3] Eickhoff, H.; Strangmüller, S.; Klein, W.; Kirchhain, H.; Dietrich, C.; Zeier, W. G.; van Wüllen, L.; Fässler, T. F., Lithium Phosphidogermanates α- and β-Li8GeP4—A Novel Compound Class with Mixed Li+ Ionic and Electronic Conductivity. Chem. Mater.
2018, 30 (18), 6440-6448.

[4] Strangmüller, S.; Eickhoff, H.; Müller, D.; Klein, W.; Raudaschl-Sieber, G.; Kirchhain, H.; Sedlmeier, C.; Baran, V.; Senyshyn, A.; Deringer, V. L.; van Wüllen, L.; Gasteiger, H. A.; Fässler, T. F., Fast Ionic Conductivity in the Most Lithium-Rich Phosphidosilicate Li14SiP6. J. Am. Chem. Soc.
2019, 141 (36), 14200-14209.

[5] Restle, T. M. F.; Sedlmeier, C.; Kirchhain, H.; Klein, W.; Raudaschl-Sieber, G.; Deringer, V. L.; van Wüllen, L.; Gasteiger, H. A.; Fässler, T. F. Fast Lithium Ion Conduction in Lithium Phosphidoaluminates. Angew. Chem., Int. Ed.
2020, 59 (14), 5665-5674.

Figure 1