Bulk Properties of Hong-Type NASICON as Solid Electrolyte: Vacancy Formation and Migration

Abstract

Hong-type sodium super ionic conductor (NASICON) compounds have been considered as strong candidates for solid electrolyte materials in sea water battery. However, there have been few theoretical studies on the bulk properties of Hong-type NASICON. Therefore, theoretical analysis on Hong-type NASICON electrolyte was performed to investigate the structural stability and migration path of Na ion by density functional theory (DFT) calculation. Four types of NASCION structure were constructed according to the ratio of Si and P (i.e., Si3, Si2P, SiP2, and P3) by following doping schema; Starting from Na4Zr2Si3O12, P was doped on tetrahedral Si site, and one Na was eliminated to maintain charge neutrality. Then, we calculated the vacancy formation energies for all elements of NASICON and migration barrier of Na ion. The result showed that the formation energy of neutral Si/P vacancy was calculated to be ~2 eV, while that of Zr and O was higher than 5 eV. In electronic structures, O and P vacancy induced vacancy state between valence band and conduction band, which could trap charge carriers. Especially, P vacancy induced the dumbbell shape of oxygen configuration, which could block the path of Na migration. Additionally, Na migration at the path 2->3 or 3->2 was calculated to be the dominant migration path with the lowest energy barrier in all types of NASICON.