Enhancing Fast Lithium Ion Conduction in Li₄GeO₄–Li₃PO₄ Solid Electrolytes

Codoping the lithium superionic conductor (LISICON) Li3.75Ge0.75P0.25O4 system with various aliovalent cations M (M = Mg2+, B3+, Al3+, Ga3+, and V5+) was conducted via a solid-state reaction method following the chemical formula Li3.75±y(Ge0.75P0.25)1–xMxO4 to obtain high-conductivity lithium ionic conductors. The highest ionic conductivity (5.1 × 10–5 S cm–1) was obtained at 25 °C for Li3.53(Ge0.75P0.25)0.7V0.3O4, which also exhibited low activation energy of 0.43(2) eV. Rietveld refinement using neutron diffraction data revealed that the Ge4+, P5+, and V5+ cations occupied the same crystallographic site, and their ratios were consistent with the nominal ratios of Li3.53(6)Ge0.5264(17)P0.1743(12)V0.2993(7)O4. Compared with the framework of γ-Li3PO4, which has only two fully occupied crystallographically different tetrahedral site lithium ions (Li: 8d, Li2: 4c), two additional partially occupied crystallographic octahedral lithium ion sites that dominantly contribute to lithium conduction were detected. The codoping strategy contributed to the compositional and structural optimization of the LISICON system, affording enhanced ion-conducting properties.