Improvement of lithium ion conductivity for A-site disordered lithium lanthanum titanate perovskite oxides by fluoride ion substitution

Abstract

Lithium ion conducting perovskite oxides, namely La_0.56−yLi_0.33+3yTiO₃ (y = 0, 0.01, 0.02, 0.04) with various La³⁺ ion concentrations and La_0.56−yLi_0.33TiO_3−3yF_3y (y = 0.01, 0.017, 0.033, 0.05) with various F⁻ ion concentrations, were prepared. When y = 0.017, the Li-ion conductivity of La_0.56−yLi_0.33TiO_3−3yF_3y was 2.30 × 10⁻³ S cm⁻¹ at 30 °C, which is one of the highest Li-ion conductivities in these systems. The F⁻ substitution was effective for decreasing the activation energy for lithium ion migration. To understand the effect of F⁻ ion substitution on lithium ion conduction, synchrotron X-ray absorption fine structure (XAFS) techniques were carried out. It was found with the XAFS analysis that the local distances around Ti⁴⁺ ions increased and the Debye–Waller factors decreased with F⁻ substitution. These local structural changes were expected to expand the bottleneck for lithium ion hopping and to decrease the activation energy.