Sodium-ion batteries are receiving intense attention as an inexpensive yet high-performance rechargeable battery to possibly replace current lithium-ion chemistries in future. A myriad of positive and negative electrode materials have so far been reported for sodium-ion batteries and, interestingly, some of them exhibit superior performance (e.g., voltage, capacity, kinetics, or cycleability) than state-of-the-art electrodes for lithium-ion batteries. On the other hand, there are only a few studies on liquid electrolyte design for sodium-ion batteries. In particular, high reductive/oxidative stability is strongly required for electrolyte materials to achieve prolonged charge-discharge cycling of hard carbon anodes and high-voltage cathodes. Recently, superconcentrated electrolytes are being recognized as a new class of highly stable electrolytes for lithium-based batteries.^1-3 In the present work, we report basic physicochemical and electrochemical properties of sodium salt-based superconcentrated electrolytes. First, we study the effects of cations, anions, and solvents on the ionic conductivity and viscosity of electrolytes at various concentrations. Based on the results, important factors to dominate the physicochemical properties are discussed. Next, we study charge-discharge behavior of hard carbon and sodium metal electrodes in sodium salt-based superconcentrated electrolytes. We have found much better cycleability of the electrodes in superconcentrated electrolytes than in conventional dilute electrolytes, suggesting that the sodium salt-based superconcentrated electrolytes have enhanced reductive stability.

1. K. Yoshida et al., J. Am. Chem. Soc., 133, 13121 (2011).

2. Y. Yamada et al., J. Am. Chem. Soc., 136, 5039 (2014).

3. Y. Yamada et al., J. Electrochem. Soc., 162, A2406 (2015).