Preparation of water–impermeable NASICON solid electrolyte separators for aqueous/nonaqueous hybrid sodium–ion cells using alumina as a sintering aid

The Na3Zr2(SiO4)2PO4 (NZSP)-based Na Super Ionic Conductor (NASICON) is widely known as a Na–ion conductive ceramic, and its stability in water makes it a promising solid electrolyte separator for aqueous and aqueous/nonaqueous hybrid Na–based cells. Some sintering additives have been used to enhance the densification of NASICON ceramics, while decreasing their stability against water because of their hygroscopicity. In this paper, we propose the use alumina (Al2O3) as a sintering aid for the preparation of dense NASICON ceramics. For this purpose, 5 mol.% of alumina powder was mixed with commercially available NZSP powder and the obtained composite green pellet was sintered at 1200 °C for 6 h in air. The crystal structure of the prepared NASICON ceramics, characterized via X–ray diffraction, significantly matched that of the pristine NZSP powder with a monoclinic (C2/c) structure. In the sintered sample, the boundaries of the NZSP particles were densified by a Na–Al–Si–P–based glass component. The sintered NASICON ceramic sample was used as a solid electrolyte separator in an aqueous/nonaqueous hybrid Na-ion cell. A hybrid cell constructed from a Na3LiTi5O12 negative electrode and 3 wt% H2O2-added 1.0 M Na2SO4 aqueous electrolyte/1.0 M sodium bis[(trifluoromethyl)sulfonyl]amide (NaTFSA)-based nonaqueous electrolyte exhibited a stable cell voltage of 2.4 V during several charge–discharge cycles. The Al2O3-added NZSP sintered ceramics can be used as solid electrolyte separators for aqueous/nonaqueous hybrid sodium-ion batteries.