Synergistic Effects of Intrinsic Cation Disorder and
Electron-Deficient Substitution on Ion and Electron Conductivity in
La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>Co<sub>0.5</sub>Mn<sub>0.5</sub>O<sub>3−δ</sub> (<i>x</i> =
0, 0.5, and 0.75)
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Abstract
The effects of intrinsic cation disorder
and electron-deficient substitution for La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>Co<sub>0.5</sub>Mn<sub>0.5</sub>O<sub>3−δ</sub> (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 Mn<sup>3+</sup>-O*-Mn<sup>4+</sup> < Co<sup>2+</sup>-O*-Co<sup>3+</sup> < Co<sup>2+</sup>-O*-Mn<sup>4+</sup>, 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<sub>Co</sub>) and Co
on Mn sites (Co<sub>Mn</sub>) indicate that Co<sup>2+</sup> is prone
to being Co<sup>3+</sup> while Mn<sup>4+</sup> is prone to being Mn<sup>3+</sup> 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 Co<sup>2+</sup>/Co<sup>3+</sup> and Mn<sup>3+</sup>/Mn<sup>4+</sup>. The thermal expansion coefficients were
found to match the Ce<sub>0.8</sub>Sm<sub>0.2</sub>O<sub>2−δ</sub> electrolyte well. The electrical conductivities were about 41.4,
140.5, and 204.2 S cm<sup>–1</sup> 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 cm<sup>2</sup> at 850 °C, respectively.
All of the measured results testify that Sr-doped LaCo<sub>0.5</sub>Mn<sub>0.5</sub>O<sub>3</sub> compounds are promising cathode materials
for intermediate-temperature solid oxide fuel cells