Al Doped into Si/P Sites of Na3Zr2Si2PO12 with Conducted Na3PO4 Impurities for Enhanced Ionic Conductivity

Natrium superionic conductor (NASICON) is a promising solidstate electrolyte because of its high stability under air as well as its safety. Doping is an effective way to improve its ionic conductivity, but there is limited information about the explanation of the doping sites. In this work, Al-doped NASICONs are designed. When Al doping is 0.3 (NAl(0.3)ZSP), the ionic conductivity is the highest and is 5.08 x 10(-5) S cm(-1) at 30 degrees C, which is 3.3 times that of undoped NASICON. NAl(0.3)ZSP consists of a NASICON structure (monoclinic and rhombohedral phases), an amorphous glassy phase, and Na3PO4 impurities. After Al doping, more Si/P sites are occupied by Al; thus, the ratio of Na3PO4 impurities increases. Na3PO4 at the grain boundary is beneficial for grain boundary resistance decrease, contributing to the decrease of the total resistance. Our work first provides a detailed explanation of doped-Al sites and interprets their effects on ionic conductivity.