Correlated Migration Invokes Higher Na+-Ion Conductivity in NaSICON-Type Solid Electrolytes

Na super ion conductor (NaSICON), Na1+nZr2SinP3-nO12 is considered one of the most promising solid electrolytes; however, the underlying mechanism governing ion transport is still not fully understood. Here, the existence of a previously unreported Na5 site in monoclinic Na3Zr2Si2PO12 is unveiled. It is revealed that Na+-ions tend to migrate in a correlated mechanism, as suggested by a much lower energy barrier compared to the single-ion migration barrier. Furthermore, computational work uncovers the origin of the improved conductivity in the NaSICON structure, that is, the enhanced correlated migration induced by increasing the Na+-ion concentration. Systematic impedance studies on doped NaSICON materials bolster this finding. Significant improvements in both the bulk and total ion conductivity (e.g., sigma(bulk) = 4.0 mS cm(-1), sigma(total) = 2.4 mS cm(-1) at 25 degrees C) are achieved by increasing the Na content from 3.0 to 3.30-3.55 mol formula unit(-1). These improvements stem from the enhanced correlated migration invoked by the increased Coulombic repulsions when more Na+-ions populate the structure rather than solely from the increased mobile ion carrier concentration. The studies also verify a strategy to enhance ion conductivity, namely, pushing the cations into high energy sites to therefore lower the energy barrier for cation migration.