Solid electrolytes are fundamental to fuel cells, batteries, sensors and electrolysers. Among them, melilite oxides are promising oxide-ion solid electrolytes due to their unique layered tetrahedral networks. However, the defect chemistry in acceptor-doped melilites remains controversial and largely unchallenged for decades, particularly the long-standing assumption that oxygen vacancies can be created to prompt the oxide-ion transport. Herein, we provide robust experimental and theoretical evidence demonstrating that ionic transport in the acceptor-doped melilites is universally governed by interstitial cation migrations, rather than oxygen vacancies. In La1-xSr1+1.5xGa3O7, directional STEM-HAADF imaging, neutron and synchrotron X-ray powder diffraction, combined with pair distribution function analysis and reverse Monte Carlo modeling, demonstrate that the disordered interstitial Sr atoms in the average structure, together with correlated La/Sr disorder of local segregation in the local structure, enhance structural flexibility and create favorable cation migration pathways. High-fidelity machine-learning-potential molecular dynamics simulations further revealed that long-range Sr2+ migration is facilitated through a continuous “S-curve knock-on” mechanism between interstitial and lattice Sr sites. This study offers complementary insights into the defect chemistry and migration dynamics of interstitial cations in melilite solid electrolytes, laying a fundamentally important foundation of defect chemistry characterization for understanding ionic conduction and designing advanced solid electrolytes. Here authors demonstrate that ion transport in acceptor-doped melilite solid electrolytes is driven by interstitial cation migration rather than oxygen vacancies, revising the defect chemistry of this important family of energy materials.
Ma et al. (Sat,) studied this question.