ϒ-Li3PO4-Type Solid Electrolytes in Li4GeO4-Li4SiO4-Li3VO4 Quasi-Ternary System with High Lithium Ionic Conductivity

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-Li 3 PO 4 -type structure in the Li 4 GeO 4 -Li 4 SiO 4 -Li 3 VO 4 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 LiNbO 3 -coated LiCoO 2 with Li 10 GeP 2 S 12 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 Li 4 GeO 4 -Li 4 SiO 4 -Li 3 VO 4 quasi-ternary system and demonstrated relatively high ionic conductivity of the γ-Li 3 PO 4 -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 Li 3.55 (Ge 0.45 Si 0.10 V 0.45 )O 4 with a low activation energy of 0.37 eV. Li 3.55 (Ge 0.45 Si 0.10 V 0.45 )O 4 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 Li 4 GeO 4 -Li 4 SiO 4 -Li 3 VO 4 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 -Li 3 PO 4 -type phase area (the region around sample #9); Li 3 VO 4 , Li 4 SiO 4 , and Li 4 GeO 4 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 Li 3.55 (Ge 0.45 Si 0.10 V 0.45 )O 4 at #9 is slightly higher than that of Li 3.6 (Ge 0.6 V 0.4 )O 4 , which is the composition showing the highest ionic conductivity reported to date of LISICONs in the binary systems. Besides, the activation energy of Li 3.55 (Ge 0.45 Si 0.10 V 0.45 )O 4 was much lower than that of Li 3.6 (Ge 0.6 V 0.4 )O 4 . The activation energy decreased by ~17.8% compared with that of the Li 3.6 (Ge 0.6 V 0.4 )O 4 (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 Li 4 GeO 4 -Li 4 PO 4 -Li 3 VO 4 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 (σ 0 ) through dual doping, which determines the σ value as described in the Arrhenius equation: σ T = σ 0 exp(− E a / k B T ). Li 3.55 (Ge 0.45 Si 0.10 V 0.45 )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. 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., Al 3+ , Ga 3+ ) are an attractive candidate for future research. Figure 1