High lithium ionic conductivity of γ-Li3PO4-type solid electrolytes in Li4GeO4-Li4SiO4-Li3VO4 quasi-ternary system

Lithium superionic conductors (LISICONs) are promising materials to realize high safety, high energy all-solid-state lithium batteries. LISICONs with a gamma-Li3PO4-type structure were developed for use in the Li4GeO4-Li4SiO4-Li3VO4 ternary system, and the relationship among their composition, phase formation, and ionic conductivity was analyzed. Ionic conductivity of 5.8 x 10 -5 S cm(-1) at 298 K, the highest value among the LISICONs reported to date, was obtained for Li-3.55(Ge0.45Si0.10V0.45)O-4 with a low activation energy of 0.37 eV. This was achieved by reducing the activation energy and increasing the pre-exponential factor through dual doping. The Ge4+, Si4+, and V5+ cations of Li-3.55(Ge0.45Si0.10V0.45)O-4 occupied the same site; thus, the solid-solution formation was confirmed. Li-3.55(Ge0.45Si0.10V0.45)O-4 functioned as a solid electrolyte in an all-solid-state cell, indicating the features of a pure lithium ionic conductor without electronic conduction. This work verified that the dual substitution strategy is beneficial for compositional and structural optimization, affording enhanced ion-conducting properties.