Oxide single crystals with high lithium-ion conductivity as solid electrolytes for all-solid-state lithium secondary battery applications

Li6.5La3Zr1.5Nb0.5O12 and Li6.5La3Zr1.5Ta0.5O12 single-crystal rods were grown by floating zone melting. A typical single-crystal rod is 8 mm in diameter and 70 mm in length. Crystallized Li6.5La3Zr1.5Nb0.5O12 and Li6.5La3Zr1.5Ta0.5O12 have cubic structures in the I21d space group. The crystal structures of Li6.5La3Zr1.5Nb0.5O12 and Li6.5La3Zr1.5Ta0.5O12 belong to the cubic I213 space group, while the final structure refinements were performed based on an approximate space group Ia3d. The lattice parameter (a) of Li6.5La3Zr1.5Nb0.5O12 was 12.9130(8) Å (1 Å = 0.1 nm), while that of Li6.5La3Zr1.5Ta0.5O12 was 12.9455(6) Å. The crystallographic reliability factors (R-values) of the Li6.5La3Zr1.5Nb0.5O12 and Li6.5La3Zr1.5Ta0.5O12 crystal structures were 7.09% (wR = 7.94%) and 8.82% (wR = 7.45%), respectively, based on the single-crystal neutron diffraction data. Li ions in the crystal structures occupied both distorted tetrahedral 96h sites and distorted octahedral 96h sites. From the results of alternating current impedance measurements, we estimated the total Li-ion conductivity of Li6.5La3Zr1.5Nb0.5O12 and Li6.5La3Zr1.5Ta0.5O12 at 298 K to be 1.39 × 10−3 and 1.27 × 10−3 S cm−1, respectively. These are the highest conductivities reported to date for Li6.5La3Zr1.5Ta0.5O12 members. The bulk properties of garnet-type electrolyte single crystals were advantageous for bulk conductivity and lacked the grain boundaries characteristic of sintered polycrystalline bodies. These solid electrolytes represent a game-changing technology for realizing advanced battery systems with strong safety features.