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 O2- ions conduction in the Na0.96xCa0.04Nb0.96Zr0.04O3-δ solid-state ionic conductor is successfully achieved by the non-stoichiometric ratio strategy. Crystal and electrical property analyses reveal that both stoichiometric and non-stoichiometric 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 O2- ions as well as electrons can be effectively regulated. The O2- ions are the main charge carriers for Na-deficient samples, stoichiometric samples feature the mixed O2- ions and intrinsic electrons, while Na-excess samples have Na⁺ ions as charge carriers. This work highlights the important role of lattice defects and oxygen octahedral distortions/twisting on the conductivity, offering novel insights into the design of and Na+/O2- ions migration pathways.