A medium-entropy garnet-type oxide as a solid electrolyte with enhanced air stability for Li-ion batteries

Abstract

Garnet-type oxides are commonly used as the solid electrolytes for all-solid-state Li-ion batteries. However, the widely utilized Ta-doped Li₇La₃Zr₂O₁₂ (LLZO) readily reacts with CO₂ and H₂O in air, leading to a decrease in ionic conductivity. In this study, a novel medium-entropy garnet-type oxide, Li_6.5La₃Zr_0.5Ta_0.5Nb_0.5Y_0.5O₁₂ (LLZTNYO), was successfully synthesized using a conventional solid-phase synthetic method. Ta, Nb, and Y were strategically substituted with Zr to significantly enhance conductivity, improve stability in air, and lower the sintering temperature. Neutron powder diffraction was used to resolve the unusual local structural properties of LLZTNYO. LLZTNYO achieved a high Li-ion conductivity of 1.87 × 10⁻⁴ S cm⁻¹ and maintained a constant Li-ion conductivity for 30 days in an air atmosphere without decay, demonstrating excellent air stability. The density functional theory calculations suggest that the multi-doping strategy can effectively suppress hydration reactions and thus enhance the stability of the solid electrolyte against water. Furthermore, the Li//LLZTNYO//LiFePO₄ solid state battery exhibited high capacity up to 167 mA h g⁻¹ with excellent cycling retention of 95% after 200 cycles at 0.1C, positioning LLZTNYO as a practicable material for use as a solid electrolyte for Li-ion batteries.