Theoretical Study on Hong-Type NASICON Structure for the Application on Sodium-ion Battery

Abstract

Sodium-ion batteries are promising alternatives to lithium-ion batteries for large-scale applications due to low price and high abundance of Na compared to the Li. NASICON (Na Super Ionic CONductor) solid electrolytes are commonly used for sodium-ion batteries, because of its good performance in ionic conductivity and stability. However, only few researches have been done on the fundamental study of ion migration and stability of the NASICON structure. In this study, we investigated the basic properties of NASICON structure to investigate vacancy formation energy and migration barrier for Na+ ion using density functional theory (DFT) calculation. NASICON refers to ceramic solid with the chemical formula Na1+xZr2¬SixP3-xO12 (0 < x < 3) and is known to exhibit a maximum ionic conductivity around x = 2, which is called Hong-type NASICON. The crystal structure of NASICON consists of covalent networks between ZrO6 octahedra and PO4/SiO4 tetrahedra that share common corners. From a starting material Na4Zr2Si3O12, P was doped on tetrahedral Si site and one Na was eliminated to maintain the charge neutrality. Note that doping was performed in primitive unit cell to reduce the number of doping cases effectively. On those systems, Na+ ion migration in the path of [100] direction was expected to be dominant because of its low energy barrier and Hong-type showed the best ionic conductivity. Also, vacancy formation energies for the atom elements in NASICON were estimated. The results showed that the formation of tetrahedral Si/P vacancy was relatively easy in all doped structures. Especially, the P vacancy induced the dumbbell shape configuration of oxygens, which could block the path of Na+ ion migration. Consequently, we successfully identified optimal ion migration path and structural vulnerability of NASICON.