Structure and ionic conductivity of Na sub(3+x) Sc sub(2) Si sub(x) P sub(3-x) O sub(12) (x=0.0, 0.2, 0.4, 0.8) NASICON materials: A combined neutron diffraction, MAS NMR and impedance study

Abstract

A knowledge of structural parameters that play a role in influencing ionic conduction is the key to the design of improved NASICON materials which have attracted significant attention in recent years as potential solid electrolytes for sodium ion batteries. In this article, we report the results of our study on a series of scandium-based NASICON compounds, namely Na sub(3+x) Sc sub(2) Si sub(x) P sub(3-x) O sub(12) (x=0.0, 0.2, 0.4, 0.8) that have been prepared by the ceramic route. These materials have been characterized by powder X-ray and neutron diffraction, as well as solid-state sup(23) Na, sup(45) Sc, sup(31) P, and sup(29) Si nuclear magnetic resonance (NMR) spectroscopy at room temperature (RT). The electrical conductivity of the compounds in the temperature range of RT -400 degrees C has been determined by impedance spectroscopy. Rietveld refinement of the diffraction patterns confirms monoclinic C2/c unit cell for the Na sub(3+x) Sc sub(2) Si sub(x) P sub(3-x) O sub(12) but indicates a rhombohedral R3c unit cell for Na sub(3+x) Sc sub(2) Si sub(x) P sub(3-x) O sub(12) compounds. The local environments surrounding sodium, scandium, phosphorus and silicon atoms have been deduced from solid-state NMR. sup(23) Na NMR results gave evidence of Na-ion mobility at RT. AC impedance measurements show that the total conductivity of the sample increases with increasing substitution of P with Si. However, it is much reduced for x = 0.8 composition. Na-ion conductivity of 2.4 x 10 sup(-3) S cm sup(-1) at 30 degrees C has been measured in Na sub(3.4) Sc sub(2) Si sub(0.4) P sub(2.6) O sub(12) compound, which is comparable to the best-performing polycrystalline NASICONs. Parameters such as the area of the bottleneck region, Na-Na hopping distances, and the activation energy for Na-ion conduction have been determined from the structural data. A correlation has been found between the conductivity and the ratio of Na site vacancy to Na site occupancy. Possible reasons for the significant variation of the Na-ion conductivity with x are also discussed in the context of the structure and Na site occupancy.