Enhanced ionic conductivity in Na₃Zr₂Si₂PO₁₂ NASICON-type solid electrolytes by adding Mg²⁺-ions

The unique NASICON-type 3D structure of Na₃Zr₂Si₂PO₁₂ (NZSP) material offers ideal Na⁺ conductivity at high temperatures, making it a subject of widespread interest among researchers in fields such as solid state batteries and solid state capacitors. However, the low-symmetry monoclinic phase structure and high grain boundary resistance of NZSP limit its ionic conductivity and application at room temperature. In this work, we optimize Na⁺ conductivity of NZSP by doping bivalent Mg²⁺ (∼ 2.4 × 10⁻³ S/cm, when x = 0.25). The analysis reveals that this way increases the content of the R3c phase which has a higher ionic conductivity and reduces grain boundary resistance. Simultaneously, the internal voids of NZSP decrease along with the original square-like grain size. and the more spaces for Na⁺ transport are provided because the cell volume of NZSP can be effectively enhanced by doping Mg²⁺. In addition, the doped samples exhibit electronic conductivity 4–5 orders of magnitude lower than their ionic conductivity. This investigation provides some references for the mechanism of ionic conductivity regulation and the technical production of NASICON-type ceramic electrolyte.