Tailoring sodium and oxygen mixed-ion conduction in the A-site non-stoichiometric NaNbO3-based ceramics

Tailoring the electronic or ionic conduction properties of solid-state electrolytes with precision is essential to fulfilling the functional demands of electrochemical energy storage and conversion technologies. Here, the synergistic regulation of Na⁺ and O^2- ions conduction in the Na_0.96xCa_0.04Nb_0.96Zr_0.04O_3-δ conductor is achieved by the non-stoichiometric ratio strategy. Crystal structure and electrical property analyses reveal that all samples feature a Pbma orthorhombic structure. With rising Na content, the defect characteristics shift from vacancies to interstitials, the NbO₆ octahedra experience a change from being compressed to normal and then to being obliquely flattened, which leads to an expansion of the corresponding interstitials in the Na-O-Na and Na-O-Nb networks. By changing the structures of these three types of polyhedron and networks, the conduction channels of Na⁺ and O^2- ions as well as electrons can be effectively regulated. The O^2- ions are the main charge carriers for Na-deficient samples, stoichiometric samples feature the mixed O^2- ions and intrinsic electrons, while Na⁺ ions become the dominant carriers for Na-excess samples. This work highlights the important role of lattice defects and oxygen octahedral distortions/twisting on the conductivity, offering insights into the design of and Na⁺/O^2- ions migration pathways in solid-state ion conductors.