Synergistic Effects of Intrinsic Cation Disorder and Electron-Deficient Substitution on Ion and Electron Conductivity in La_1–<i>x</i>Sr_<i>x</i>Co_0.5Mn_0.5O_3−δ (<i>x</i> = 0, 0.5, and 0.75)

Abstract

The effects of intrinsic cation disorder and electron-deficient substitution for La_1–<i>x</i>Sr_<i>x</i>Co_0.5Mn_0.5O_3−δ (LSCM, <i>x</i> = 0, 0.5, and 0.75) on oxygen vacancy formation, and their influence on the electrochemical properties, were revealed through a combination of computer simulation and experimental study. First-principles calculations were first performed and found that the tendency of the oxygen vacancy formation energy was Mn³⁺-O*-Mn⁴⁺ < Co²⁺-O*-Co³⁺ < Co²⁺-O*-Mn⁴⁺, meaning that antisite defects not only facilitate the formation of oxygen vacancy but introduce the mixed-valent transition-metal pairs for high electrical conductivity. Detailed partial density of states (PDOS) analysis for Mn on Co sites (Mn_Co) and Co on Mn sites (Co_Mn) indicate that Co²⁺ is prone to being Co³⁺ while Mn⁴⁺ is prone to being Mn³⁺ when they are on antisites, respectively. Also it was found that the holes introduced by Sr tend to enter the Co sublattice for <i>x</i> = 0.5 and then the O sublattice when <i>x</i> = 0.75, which further promotes oxygen vacancy formation, and these results are confirmed by both the calculated PDOS results and charge-density difference. On the basis of microscopic predictions, we intentionally synthesized a series of pure LSCM compounds and carried out comprehensive characterization. The crystal structures and their stability were characterized via powder X-ray Rietveld refinements and in situ high-temperature X-ray diffraction. X-ray photoelectron spectroscopy testified to the mixed oxidation states of Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺. The thermal expansion coefficients were found to match the Ce_0.8Sm_0.2O_2−δ electrolyte well. The electrical conductivities were about 41.4, 140.5, and 204.2 S cm^–1 at doping levels of <i>x</i> = 0, 0.5, and 0.75, and the corresponding impedances were 0.041, 0.027, and 0.022 Ω cm² at 850 °C, respectively. All of the measured results testify that Sr-doped LaCo_0.5Mn_0.5O₃ compounds are promising cathode materials for intermediate-temperature solid oxide fuel cells