The Impact of La Doping on the Ionic Conductivity of Na3+xLaxZr2–xSi2PO12 Solid-State Electrolytes

Joint doping with trivalent and monovalent ions is a widely adopted strategy to enhance the ionic conductivities of NASICON-type solid-state electrolytes (SSEs) with the general formula Na1+xZr2SixP3–xO12, but the underlying microscopic mechanism remains unclear. In this study, we synthesized a series of Na3+xLaxZr2–xSi2PO12 (NLZSPx, 0 ≤ x ≤ 0.4) ceramics and characterized their electrical properties and structures by using electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and advanced solid-state nuclear magnetic resonance (SSNMR). Among all of the samples, the composition with x = 0.2 exhibited the highest ionic conductivity, achieving 1.05 × 10–3 S/cm at 25 °C, which is 3.65 times higher than that of the undoped sample. XRD and SSNMR results prove that La3+ ions did not integrate into the main phase lattice to replace Zr4+ ions. Instead, they formed a heterogeneous phase predominantly composed of Na3La(PO4)2 at the grain boundaries. The content of Na3La(PO4)2 and the composition of the NASICON main phase were accurately determined by SSNMR. This impurity resulted in changes in the Si/P and Na/Zr ratios within the main phase. Some comparison experiments are carried out and prove that these changes were not the principal drivers of the increased conductivity. SEM results further indicated that doping with an appropriate amount of La3+ ions significantly increased the sample densification, thus promoting Na+ ions transport between grain boundaries─this being the key factor driving the observed improvement in ionic conductivity.