Lithium Phosphidogermanates α- and β-Li₈GeP₄—A Novel Compound Class with Mixed Li⁺ Ionic and Electronic Conductivity

To utilize all-solid-state batteries as high power and energy density energy storage devices it is necessary to improve the current electrolyte materials and the cell architecture. In this work, we present α- and β-Li8GeP4 as potential compounds for the use in cathode composites of solid-state batteries because of their ability to conduct both Li+ ion and electrons. Each polymorph was synthesized via mechanical alloying of the elements and subsequent annealing. Structural analysis of α- and β-Li8GeP4 via X-ray diffraction reveals isolated [GeP4]8– tetrahedra. α-Li8GeP4 (Pa3̅) and β-Li8GeP4 (P4̅3n) are isotypic with Li8SiP4 and Li8SnP4, respectively. The lithium ion mobility indicated by partially filled octahedral voids was investigated by temperature-dependent nuclear magnetic resonance and electrochemical impedance spectroscopy and reveals low activation energies for lithium hopping in the range from 34 and 42 kJ mol–1 as well as high ionic conductivities of up to 8 × 10–5 S cm–1 and electronic conductivities of up to 4 × 10–7 S cm–1 at 293 K. These results combine a new substance class with a systematic synthesis approach for materials with high ionic carrier densities.