Abstract:

Sodium batteries have drawn increasing attention from multiple researchers owing to the abundant reserves and low cost of sodium resources. However, traditional sodium batteries based on organic solvent electrolyte systems have safety risks. Thus, the utilization of solid electrolyte materials instead of organic electrolytes could effectively resolve safety issues and ensure the safe performance of the battery. Solid sodium-ion battery is a promising energy storage device. The sodium ion solid-state electrolytes mainly includes Na-β-Al₂O₃, Na super ionic conductor (NASICON), sulfide, polymer, and borohydride. Inorganic solid electrolytes have the advantage of ionic conductivity compared with polymer solid electrolyte. This paper summarizes the research progress on three common inorganic sodium ion solid electrolytes: Na-β-Al₂O₃, NASICON, and sulfide. Research efforts have mainly focused on increasing ionic conductivity and interface stability. Na-β-Al₂O₃ has been successfully commercialized in high-temperature Na-S and ZEBRA batteries with molten electrodes. Pure β″-Al₂O₃ is difficult to prepare owing to its low thermodynamic stability. The synthesized β″-Al₂O₃ based on traditional solid-state reaction generally contains impurities such as β-Al₂O₃ and NaAlO₂ (around the boundaries). Further improvements are required to develop favorable methods for fabricating pure β″-Al₂O₃ with high production yield, low cost, and well-controlled microstructure. NASICON, one of the most promising ionic conductors for solid sodium-ion batteries, has attracted considerable attention for its high ionic conductivity at room temperature. The general method to enhance ionic conductivity is to increase the bottleneck size by introducing proper substituents. However, the substitution of synthetic elements could result in different optimal calcination temperatures, which would lead to a change in the density of ceramic sintering. β″-Al₂O₃ and NASICON have higher ionic conductivity at room temperature but cannot achieve good performance in the field of high power densities and long-term cycling owing to the poor interface contact with electrode materials. Because the high polarizability and large ionic radius of sulfur atoms weaken the interaction between skeleton and sodium ions, sulfide solid electrolytes often provide higher ionic conductivity at room temperature than analogous oxides. At the same time, sulfide solid electrolytes can be easily pressed into a mold at room temperature. However, sulfide electrolytes have low chemical stability in air because of hydrolysis by water molecules with the generation of H₂S gas, which should be handled in inert gas atmosphere. In conclusion, this review discusses the recent progress in different aspects of ionic conductivity and interface stability.

Key words: Solid sodium-ion battery, Safety, Inorganic sodium ion solid electrolyte, Ionic conductivity, Interface