Structure and ionic conductivity of Na3+xSc2SixP3-xO12 (x=0.0, 0.2, 0.4, 0.8) NASICON materials: A combined neutron diffraction, MAS NMR and impedance study

• Published in: Solid state sciences Vol. 111
• Main Authors: Santhoshkumar, B., Rao, P. Lokeswara, Ramanathan, K.V., Bera, A.K., Yusuf, S.M., Hathwar, Venkatesha R., Pahari, Bholanath
• Format: Journal Article
• Language: English
• Published: Elsevier Masson SAS 01-01-2021
• Subjects: Ionic conductivity; MAS NMR; NASICON; Neutron diffraction; Sodium batteries; Neutron diffraction; NASICON; MAS NMR; Ionic conductivity; Sodium batteries
• ISSN: 1293-2558; 1873-3085
• DOI: 10.1016/j.solidstatesciences.2020.106470

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 Na3+xSc2SixP3-xO12 (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 23Na, 45Sc, 31P, and 29Si nuclear magnetic resonance (NMR) spectroscopy at room temperature (RT). The electrical conductivity of the compounds in the temperature range of RT ̶400 °C has been determined by impedance spectroscopy. Rietveld refinement of the diffraction patterns confirms monoclinic C2/c unit cell for the Na3Sc2P3O12 but indicates a rhombohedral R3‾c unit cell for Na3+xSc2SixP3-xO12 compounds. The local environments surrounding sodium, scandium, phosphorus and silicon atoms have been deduced from solid-state NMR. 23Na 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 2.4×10-3 S cm−1 at 30 °C has been measured in Na3.4Sc2Si0·4P2·6O12 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.
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•Impedance study of Na3+xSc2SixP3-xO12 NASICON-type solid materials revealed very high Na-ion conductivity values.•Highest Na-ion conductivity of 2.4×10-3 S cm−1 at 30 °C has been measured in Na3.4Sc2Si0·4P2·6O12 compound.•The role of structure in determining the ion conduction has been examined by NPD, MAS NMR, and SEM measurements.•A correlation has been found between the conductivity and the ratio of Na site vacancy to Na site occupancy.•The Na3+xSc2SixP3-xO12 materials offers a greater prospect in Na-ion battery applications.