Aqueous Sodium-Ion Battery As Post Lithium-Ion Battery
Recently, aqueous electrolyte has been recognized as an attractive alternative and much attention has been paid to its development, because it has 3 big advantages about the conductivity, nonflammability and cost. In addition, water is an ideal solvent that dissolves a lot of various salts. Here, the most important thing is the selection of electrode active materials to avoid decomposition of the aqueous electrolyte. An aqueous Li-ion battery with LiMn2O4 cathode and VO2 anode was first reported by J. R. Dahn's group . Since them, several combinations of cathodes and anodes have been reported as aqueous Li-ion battery. Theoretically, higher cell voltage more than 1.2 V cannot be obtained in the aqueous battery. So, it is unnecessary to dare to use Li in the aqueous batteries. Nevertheless, there is few report about aqueous Na-ion battery, in comparison with aqueous Li-ion battery. As one of the few exceptions, aqueous sodium-ion hybrid device with Na0.44MnO2cathode and active carbon anode was first proposed by Carnegie Mellon University, 4 years ago .
In this presentation, we introduce the aqueous Na-ion battery with pyrophosphate Na2FeP2O7 cathode and NASICON-type NaTi2(PO4)3 anodes. Because these polyanionic electrodes have flat voltage plateaus at 3.0 V and 2.1 V vs. Na/Na+, respectively. These redox voltages are almost located within the potential window of the aqueous electrolyte. In addition, Fe and Ti are the first and the second cheapest redox couple in transition metals, respectively.
 W. Li, J. R. Dahn and D. S. Wainwright, Science, 264 (1994) 1115.
 A. D. Tevar and J. F. Whitacre, J. Electrochem. Soc., 157 (2010) A870.