Interstitial cation defect chemistry and correlated disorder in melilite solid electrolytes

Area of Science:

• Materials Science
• Solid-State Chemistry
• Electrochemistry

Background:

• Melilite oxides are promising solid electrolytes for energy devices like fuel cells and batteries.
• The defect chemistry, specifically the role of oxygen vacancies in ionic transport, has been debated for decades.
• Previous assumptions favored oxygen vacancy migration in acceptor-doped melilites.

Purpose of the Study:

• To experimentally and theoretically investigate the defect chemistry and ionic transport mechanisms in acceptor-doped melilite oxides.
• To challenge the long-standing assumption of oxygen vacancy-mediated transport.
• To elucidate the role of interstitial cations in ionic conduction.

Main Methods:

• Directional scanning transmission electron microscopy with high-angle annular dark-field (STEM-HAADF) imaging.
• Neutron and synchrotron X-ray powder diffraction.
• Pair distribution function analysis and reverse Monte Carlo modeling.
• Machine-learning-potential molecular dynamics simulations.

Main Results:

• Ionic transport in acceptor-doped melilites is universally governed by interstitial cation migrations, not oxygen vacancies.
• Disordered interstitial Sr atoms and La/Sr disorder create favorable cation migration pathways.
• A "S-curve knock-on" mechanism facilitates long-range Sr2+ migration.

Conclusions:

• The study provides robust evidence for interstitial cation migration as the dominant ionic transport mechanism in melilite solid electrolytes.
• This finding necessitates a re-evaluation of defect chemistry models for these materials.
• Understanding these cation migration dynamics is crucial for designing next-generation solid electrolytes for energy applications.