Fast Lithium-Ion Conduction in Phosphide Li₉GaP₄

Lithium-ion conductors are currently tested for their possible usage in all-solid-state lithium-ion batteries. In order to design high-performance solid electrolytes, the fundamental understanding of the relationships of the atomic structure and the transport properties such as temperature-dependent ionic conductivity is a basic prerequisite. Therefore, systematic investigations of closely related structures are essential. Phosphide-based materials are promising candidates for solid electrolytes, and recently, we have shown that the superionic conductor Li₉AlP₄ with an ionic conductivity of 3 mS cm^–1 at room temperature can be obtained by the substitution of Si by Al in Li₈SiP₄. Now, we present the heavier gallium homologue Li₉GaP₄, which reveals a similarly high superionic conductivity of 1.6 mS cm^–1 and a low activation energy. Li₉GaP₄ is easily accessible via ball milling of the elements and subsequent annealing at quite moderate temperatures. The single-crystal X-ray structure determination reveals that Li₉GaP₄ is isotypic to Li₉AlP₄ and crystallizes in the cubic space group P4̅3n (no. 218) with a lattice parameter of a = 11.868(1) Å. Temperature-dependent single-crystal X-ray diffraction reveals that lithium is not located at the center of the octahedral voids of the slightly distorted cubic close packing of P atoms but occurs with split positions. Impedance spectroscopy and temperature-dependent static ⁷Li NMR experiments reveal activation energies of 36 and 25 kJ mol^–1, respectively.