Effect of Li2O additions upon the crystal structure, sinterability and electrical properties of yttria stabilized zirconia electrolyte

Abstract

The crystal structure, lattice constant, sinterability, microstructure, conductivity and cell performance of 8 mol% yttria stabilized zirconia (YSZ) with varying ratios of Li₂O (n mol%, n = 0, 0.25, 0.5, 1, 1.5, 1.7, 2, 2.5, 3), i.e., n mol% Li₂OYSZ, were investigated. The results show that the Li⁺ can dissolve into the ZrO₂ cubic lattice and cause lattice contraction due to the small radius of Li⁺. The YSZ retained its cubic lattice until Li₂O addition of 1.7 mol%, and the cubic-to-monoclinic transition was detected in cases of Li₂O content more than 1.7 mol%, leading to the increase of the lattice parameter. Li₂O addition of 0.25 and 0.5 mol% decreased the sintering temperature from the typical 1400 °C to 1265 °C and 1250 °C, and promoted both the densification and electrical conductivity of the YSZ samples. The conductivities of 0.25 mol% Li₂OYSZ and 0.5 mol% Li₂OYSZ samples at 800 °C were as high as 0.0275 S cm⁻¹ and 0.0313 S cm⁻¹, which are 1.26 and 1.43 times greater than that of pure YSZ, respectively. Then the densification of the YSZ samples reduced gradually at Li₂O content greater than 1 mol%. For n mol% Li₂OYSZ (n ≥ 1.7) samples, a dense and pure cubic phase ceramic could be achieved at sintering temperatures above 1400 °C due to the vaporization of Li₂O at such high temperatures resulting from its high vapor pressure. Solid oxide fuel cells (SOFCs) with 0.25 mol% Li₂OYSZ and 0.5 mol% Li₂OYSZ electrolyte displayed open circuit voltages higher than 1.0 V at 800 °C, indicating the absence of electronic conductivity, and meanwhile exhibited much increased maximum power densities than that of the cell with pure YSZ electrolyte, implying that inclusion of a small amount of Li₂O into YSZ electrolyte could be a desirable improvement approach for zirconia-based SOFC.