Effect of proton-exchange reactions on the ionic conductivity of Li29-3xAlxZr9Nb3O40, an ordered rock-salt oxide

Abstract

Li_29-3xAl_xZr₉Nb₃O₄₀, an ordered rock-salt oxide, was synthesized by a two-step solid-state method, and sintered bodies with relative densities of approximately 90 % were obtained. The sample with x = 0 exhibited a conductivity of 1.5 × 10⁻⁴ Scm⁻¹ at 500 °C, with an activation energy of 0.99 eV, and lithium conductivity showed no improvement by Al³⁺ substitution. After exposing Li₂₉Zr₉Nb₃O₄₀ to a humidified atmosphere, its surface became strongly alkaline and the lattice constant changed, indicating the formation of lithium hydroxide on the surface by the exchange reaction between Li⁺ and H⁺ in water vapor. The sample exposed to a humidified atmosphere for 1 week exhibited a high conductivity of 4.6 × 10⁻³ Scm⁻¹ at 25 °C, which decreased to 1.75 × 10⁻⁶ Scm⁻¹ at 150 °C. Our results indicate the occurrence of fast ion diffusion at the surface of the ordered rock-salt phase.