Synthesis of High Na sup(+) Ion-Conducting NASICON-based electrolytes and insights from diffraction and positron annihilation lifetime spectroscopy studies

Abstract

Na superionic conductor (NASICON) Na sub(3)Zr sub(2)Si sub(2)PO sub(12) is a promising but not applicable electrolyte for solid-state batteries due to its relatively low conductivity. Here, we have pursued two distinct synthesis routes, the conventional solid-state reaction method (SSR) and the lesser-utilized hydrothermal combined solid-state reaction method (HT-SSR), for the preparation of NASICON materials of Na sub(3)Zr sub(2)Si sub(2)PO sub(12), Na sub(3.4)Zr sub(2)Si sub(2.4)P sub(0.6)O sub(12), and Na sub(3.4)Zr sub(1.8)Sc sub(0.2)Si sub(2.2)P sub(0.8)O sub(12). We show a significant enhancement of the ionic conductivity by simultaneously adopting the HT-SSR method, substituting Zr sup(4+) with Sc sup(3+), and tuning the Si/P ratio. The highest value of 3.2 mS/cm at 25 degrees C is achieved for HT-SSR-prepared Na sub(3.4)Zr sub(1.8)Sc sub(0.2)Si sub(2.2)P sub(0.8)O sub(12). It also displays good electrochemical stability with Na metal and consistent cycling performance in a solid Na|Na sub(3.4)Zr sub(1.8)Sc sub(0.2)Si sub(2.2)P sub(0.8)O sub(12)|Na symmetrical cell. Besides, we have investigated the effects of the synthesis method, Si/P ratio, and Sc sup(3+) substitution in Na sub(3)Zr sub(2)Si sub(2)PO sub(12) on the ionic conduction behavior by X-ray and neutron diffraction, impedance spectroscopy, and scanning electron microscopy. To obtain in situ information about the imperfections of the lattice in the materials, we utilized the positron annihilation lifetime spectroscopy (PALS) technique, which has not been previously applied to NASICON materials.