Lithium superionic conductors (LISICONs) are promising materials to realize high safety, high energy all-solid-state lithium-ion batteries. Through a dual-cation-substitution strategy, we designed ternary solid solutions of Ge-Si-V derivatives with a γ-Li3PO4-type structure in the Li4GeO4-Li4SiO4-Li3VO4 quasi-ternary system, and the relationship among their compositions, phase formation, and ionic conductivity was analyzed. The samples were fabricated through a solid-state reaction at 1173 K. The crystalline phase of the samples was identified through X-ray diffraction (XRD). The crystal structure was elucidated with Rietveld refinement of Synchrotron X-ray diffraction data. The morphologies and elemental distributions of the synthesized samples were examined through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The ionic conductivities of the synthesized samples at various temperatures were determined by AC impedance spectroscopy. An all-solid-state battery using the discovered LISICON material was fabricated in an Ar-filled glovebox. The positive and negative electrodes were composed of a mixture of LiNbO3-coated LiCoO2 with Li10GeP2S12 and Li-In metal, respectively. A charge-discharge test was conducted between 1.9 and 3.6 V at an applied current of 0.03 mA cm−1 (0.05 C rate) at 373 K. This study determined the phase-formation regions in the Li4GeO4-Li4SiO4-Li3VO4 quasi-ternary system and demonstrated relatively high ionic conductivity of the γ-Li3PO4-type phase structures. A high conductivity with low activation energy was achieved in ternary compositions compared to that in any binary system. The bulk ionic conductivity of 5.8 × 10−5 S cm−1 at 298 K was obtained for Li3.55(Ge0.45Si0.10V0.45)O4 with a low activation energy of 0.37 eV. Li3.55(Ge0.45Si0.10V0.45)O4 maintained high ionic conduction property (~10−5 S cm−1) even for the total conductivity (sum of bulk and grain-boundary contributions). Ionic conductivity of the Li4GeO4-Li4SiO4-Li3VO4 system was visualized along with the phase-identification results, as shown in the provided figure. In this color contour map (Fig. 1), relatively high ionic conductivity is confirmed around the γ-Li3PO4-type phase area (the region around sample #9); Li3VO4, Li4SiO4, and Li4GeO4 solid solutions or their mixture exhibited low ionic conductivity. Compared to any binary composition system (i.e., Li-X-Y-O), the existing tie line between the corner of this triangular ternary composition system (i.e., Li-X-Y-Z-O) has a higher ionic conductivity. Particularly, the ionic conductivity of Li3.55(Ge0.45Si0.10V0.45)O4 at #9 is slightly higher than that of Li3.6(Ge0.6V0.4)O4, which is the composition showing the highest ionic conductivity reported to date of LISICONs in the binary systems. Besides, the activation energy of Li3.55(Ge0.45Si0.10V0.45)O4 was much lower than that of Li3.6(Ge0.6V0.4)O4. The activation energy decreased by ~17.8% compared with that of the Li3.6(Ge0.6V0.4)O4 (0.45 eV). Therefore, the novel material-search concept (more complex composition) is one of the positive directions for developing the LISICON-type ionic conductors. The absolute value of 5.8 × 10−5 S cm−1 exceeded that of the reported ternary material in the Li4GeO4-Li4PO4-Li3VO4 system (5.1 × 10−5 S cm−1) (ACS Appl. Energy Mater. 2 (2019) 6608–6615). In other words, the ionic conductivity of the discovered material is higher than that of all the practically existing LISICONs, including the ternary systems. Further optimization in the ternary system or a more complex composition system could be a promising way for the subsequent material search. The high ionic conductivity property (σ) was achieved by reducing the activation energy (E a) and increasing the pre-exponential factor (σ0) through dual doping, which determines the σ value as described in the Arrhenius equation: σT = σ0exp(−E a/kBT). Li3.55(Ge0.45Si0.10V0.45)O4 functioned as a solid electrolyte in an all-solid-state cell, indicating the features of a pure lithium ionic conductor without electronic conduction. The LISICON oxide prepared via a cold-press functioned as a solid-electrolyte separator similar to sulfide solid electrolytes in all-solid-state batteries without high-temperature sintering. This demonstration could be the first step to developing room-temperature-LISICONs-based all-solid-state lithium ionic batteries. This work verified that the dual-cation-substitution strategy is beneficial for compositional and structural optimization, affording enhanced ion-conducting properties. Additionally, the substitution of more kinds of cations with larger radii into LISICONs is proposed; further enhancement in ionic conductivity could be expected in the case of the LISICON with a larger lattice size. Ternary LISICON systems using cations with larger ionic radii (e.g., Al3+, Ga3+) are an attractive candidate for future research. Figure 1