The Re-Discovery of Scandium-Substituted Na3Zr2(SiO4)2(PO4)

Abstract

In recent years, the requirement of energy storage for renewable clean energy, like solar and wind energy, has attracted more and more interest of the relevant research globally. As one of the choices, room-temperature Na-ion batteries (NIBs) show advantages of unlimited sources, low price of starting materials and easy recycling,[1] which promise them high potential for energy storage applications. NIBs in all-solid-state design are regarded as the batteries of the next generation because of their non-leakage, non-volatilization, non-flamma­bility, separator-free design and adaptability to temperature changes.[2] Trivalent-cation-substituted Na3Zr2(SiO4)2(PO4)3 in NASICON structure are regarded as promising electrolyte materials for all-solid-state NIBs because of their high Na-ion conductivity and reliable chemical and physical stability. In this study, scandium is chosen as the substitution element for Na3Zr2(SiO4)2(PO4) (Sc substituted Na3Zr2(SiO4)2(PO4)3, NSZSP), because among all the trivalent cations, the ionic radius of Sc3+ (74.5 pm) is the closest to that of Zr4+ (72.0 pm), indicating that the substitution only generates a deficiency in positive charge, but does not cause distortions in the crystal structure. NSZSP has received hardly any attention so far. Only one reference reported this kind of materials.[3] However, the reference mainly targeted the performance of NSZSP at high temperatures (~300°C). Only a few conductivities at room temperatures were mentioned, and they are rather moderate (maximum 5.0 × 10-4 S cm-1). In this study, a solution-assisted solid-state reaction method was invented by which a series of NSZSP nano-powders with favorable microstructures were prepared. With increasing of scandium substitution, the conductivity of sintered NSZSP samples also increases. Na3.4Sc0.4Zr1.6(SiO4)2(PO4) shows the highest conductivity of the system so far, more scandium substitution decreases the conductivity because there exists a most suitable ratio between occupied and vacant Na sites. The total-ionic conductivity of Na3.4Sc0.4Zr1.6(SiO4)2(PO4) reaches 4.0 × 10-3 S cm-1 at 25 °C. To our knowledge, it is not only the best value for all materials with NASICON type structure, but also the best reported value for all polycrystalline oxidic Na-ion conductors. It is even comparable with common liquid electrolytes of NIBs. The possible reason for such high conductivity is discussed in specific. [1] D. Larcher, J. M. Tarascon, “Towards greener and more sustainable batteries for electrical energy storage”, Nat Chem 2015, 7, 19-29. [2] A. Hayashi, K. Noi, A. Sakuda and M. Tatsumisago, “Superionic glass-ceramic electrolytes for room-temperature rechargeable sodium batteries”, Nat. Commun. 2012, 3, 856-861. [3] M. A. Subramanian, P. R. Rudolf and A. Clearfieled, “The Preparation, Structure, and Conductivity of Scandium-Substituted NASlCONs”, J. Solid State Chem. 1985, 60, 172-181.