Origin of improved Na+ ionic conductivity in the NASICON-type solid state electrolyte with Sm modification

The NASICON-type (Na 1+x Zr 2 Si x P 3-x O 12 , 0≤x ≤ 3) sodium-ion solid state electrolyte (SSE) has emerged as an alternative for liquid electrolyte due to its good chemical/electrochemical stability, high thermal stability and high safety. However, the insufficient ionic conductivity and poor understanding of Na + transport mechanism hinder the further development of sodium-ion SSE. Herein, we report Sm modified Na 3+x Zr 2-x Sm x Si 2 PO 12 (0≤x ≤ 0.4) electrolytes with a highest ionic conductivity of 2.25 × 10 −3  S cm −1 at 25 °C when x = 0.2. The introduction of Sm element adjusts the Si/P ratio in NASICON phase and densifies the electrolyte pellets by formation of Na 3 Sm(PO 4 ) 2 . To get a deep insight into the enhanced mechanism of Na  +  transport in grains, low-temperature electrochemical impedance spectra (LT-EIS) and Jonscher's power law relationship are employed to analyze the electrical behavior of grains. The results demonstrate that the regulation of Si/P ratio can evoke lower migration energy, more charge carrier concentration and higher hopping frequency , which synergistically improve the ionic conductivity in grains.