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 (Pa3 ) and β-Li₈GeP₄ (P4 3n) 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.