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 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 R 3 ¯ 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.

中文翻译：

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Na3+Sc2Si P3-O12 (x=0.0, 0.2, 0.4, 0.8) NASICON 材料的结构和离子电导率：结合中子衍射、MAS 核磁共振和阻抗研究

摘要 了解影响离子传导的结构参数是设计改进 NASICON 材料的关键，该材料近年来作为钠离子电池的潜在固体电解质引起了极大的关注。在本文中，我们报告了对一系列基于钪的 NASICON 化合物的研究结果，即 Na3+xSc2SixP3-xO12 ( x = 0.0, 0.2, 0.4, 0.8) 已通过陶瓷途径制备。这些材料已通过粉末 X 射线和中子衍射以及室温 (RT) 下的固态 23Na、45Sc、31P 和 29Si 核磁共振 (NMR) 光谱进行表征。化合物在 RT 400 °C 温度范围内的电导率已通过阻抗谱测定。衍射图的 Rietveld 细化证实了 Na3Sc2P3O12 的单斜 C2/c 晶胞，但表明 Na3+xSc2SixP3-xO12 化合物的菱形 R 3¯c 晶胞。钠、钪、磷和硅原子周围的局部环境是从固态核磁共振推断出来的。23Na NMR 结果证明了室温下的钠离子迁移率。交流阻抗测量表明，样品的总电导率随着 P 对 Si 的替代增加而增加。但是，对于 x=0.8 的组合，它大大减少了。Na3.4Sc2Si0.4P2.6O12 化合物在 30 °C 下的钠离子电导率为 2.4 × 10 − 3 S cm-1，与性能最好的多晶 NASICON 相当。瓶颈区域的面积、Na-Na 跳跃距离等参数，钠离子传导的活化能由结构数据确定。已发现电导率与 Na 位点空位与 Na 位点占有率之比之间存在相关性。Na 离子电导率随 x 显着变化的可能原因也在结构和 Na 位点占有率的背景下进行了讨论。