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

抄録

Li_6.5La₃Zr_1.5Nb_0.5O₁₂ and Li_6.5La₃Zr_1.5Ta_0.5O₁₂ 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 Li_6.5La₃Zr_1.5Nb_0.5O₁₂ and Li_6.5La₃Zr_1.5Ta_0.5O₁₂ have cubic structures in the I2₁d space group. The crystal structures of Li_6.5La₃Zr_1.5Nb_0.5O₁₂ and Li_6.5La₃Zr_1.5Ta_0.5O₁₂ 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 Li_6.5La₃Zr_1.5Nb_0.5O₁₂ was 12.9130(8) Å (1 Å = 0.1 nm), while that of Li_6.5La₃Zr_1.5Ta_0.5O₁₂ was 12.9455(6) Å. The crystallographic reliability factors (R-values) of the Li_6.5La₃Zr_1.5Nb_0.5O₁₂ and Li_6.5La₃Zr_1.5Ta_0.5O₁₂ 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 Li_6.5La₃Zr_1.5Nb_0.5O₁₂ and Li_6.5La₃Zr_1.5Ta_0.5O₁₂ 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 Li_6.5La₃Zr_1.5Ta_0.5O₁₂ 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.