The increase of energy consumption and the demand for environmental energy resources have generated numerous investigations concerning several types of rechargeable batteries. In particular, research on rechargeable sodium-ion batteries (SIBs) has been promoted to utilize large energy storage systems (ESSs) as an alternative to lithium-ion batteries (LIBs) due to considerations of price and abundance in the world. While interest in SIBs has grown, concerns over safety using both Na metal as an anode and ammable organic solvents as electrolytes similar to the Li case have also arisen. Safety and a low price are the most important aspects of large-scale battery systems; hence, aqueous SIBs are attractive candidates to satisfy both these safety and cost requirements. In addition, aqueous solutions have further advantages of easy preparation, non-toxicity, and higher ionic conductivity than the currently employed organic solvents. Despite these advantages of aqueous electrolytes, aqueous batteries have been troubled with the following drawbacks – a narrow potential window limited by the decomposition of water, the dissolution of electrode materials in aqueous solutions, and a short lifetime due to side reactions. Since Li et al. rst introduced the feasibility of aqueous LIBs based on intercalation compounds, various types of electrode materials have been scrutinized for aqueous rechargeable batteries. Whereas most of the studies in aqueous systems have been focused on Li compounds, only a few sodium ion insertion materials in