ϒ-Li 3 PO 4 -Type Solid Electrolytes in Li 4 GeO 4 -Li 4 SiO 4 -Li 3 VO 4 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