Research on optimizing the crystal structure and enhancing the performance of LLTO solid electrolyte through doping with high-electronegativity Fe3+

Abstract

The excellent bulk conductivity of the perovskite-type (Li_0.33La_0.56TiO₃, LLTO) solid electrolyte makes it one of the most valuable solid electrolyte materials. However, in the current research, there are few studies examining how electronegativity factors influence the crystal structure of LLTO. In this paper, the solid phase reaction method was adopted to adjust the lattice structure of the material by the chemical formula Li_0.33-xFe_xLa_0.56TiO₃(x = 0.01, 0.03,0.05, 0.07 mol) by substituting strong electronegativity Fe³⁺ ions for Li⁺ ions in LLTO. The effects of Fe³⁺ on the structure and properties of LLTO were studied. The results show that the addition of Fe³⁺ can successfully replace Li⁺ in the lattice, effectively improve the cubic phase content, crystal integrity, grain size uniformity and bulk density of LLTO, and further improve the grain conductance. However, increasing the doping amount of Fe³⁺ results in excessive unequal substitution, facilitating the formation of many cation vacancies in the lattice which is detrimental to the growth and perfection of cubic crystals. Also, excess Fe³⁺ causes exaggerated grain growth, reduction of grain size uniformity and the density of the material, thereby reducing the bulk conductance of the material. In this work, optimal comprehensive performance is achieved when the sample is doped with 0.01 mol of Fe³⁺ and sintered at 1220 °C for 6 h. The c-LLTO content reaches 93 %, bulk density of 4.75 g cm⁻³, and bulk conductivity of 1.42 mS cm⁻¹ which is 63.22 % higher than that of the undoped sample. This study demonstrates that the incorporation of Fe³⁺ significantly enhances the bulk conductivity of the material's grains.