Aqueous alkali metal-ion batteries have attracted much attention, because they have three major advantages, namely non-flammability, high ionic conductivity and low cost. In addition, water can dissolve various salts in large amounts to provide a variety of electrolytic solutions. On the other hand, the 1.2 V theoretical electrochemical window of water restricts the selection of the cathode and anode active materials. However, three years ago, this issue was first addressed by U.S. Army Research Laboratory [1]. They successfully expanded the cell voltage of the aqueous Li-ion battery more than 2 V by introducing highly concentrated electrolytes, which effectively suppressed undesired electrolysis of water. To confirm the electrochemical window expansion of the concentrated electrolyte even in Na system, we also tried to realize a high voltage aqueous Na-ion battery by concentrated aqueous electrolyte. In preliminary measurement, 1.4 V average discharge voltage was obtained in the combination of Prussian blue-type Na₂Mn[Fe(CN)₆] (NMHCF) cathode and NASICON-type NaTi₂(PO₄)₃ (NTP) anode with concentrated (17 mol/kg) NaClO₄ aqueous electrolyte [2]. However, according to a CV measurement, the electrochemical window was almost 2.8 V [2]. Therefore, in order to further raise the cell voltage, instead of NTP anode based on Ti⁴⁺/Ti³⁺ redox, Prussian blue-type KMn[Cr(CN)₆] (potassium manganese hexacyanochromate; KMHCC) anode based on Cr³⁺/Cr²⁺ redox was adopted. Then, 2 V class aqueous Na-ion battery was successfully obtained [3].

As this analogy, we have newly found the rechargeable operation of the aqueous K-ion battery in the combination of Prussian blue-type K₂Mn[Fe(CN)₆] (potassium manganese hexacyanoferrate; KMHCF) cathode and KMHCC anode with highly concentrated potassium triflate (19 mol/kg KSO₃CF₃; KOTf) aqueous electrolyte. The average cell voltage was approximately 1.5 V and the specific capacity was 118 mAh/g-KMHCF and 60 mAh/g-KMHCC, respectively. However, the two similar full cells suffered from capacity fading caused by degradation of Prussian blue-type structure induced by side reaction such as electrolysis of water solvent or electrolyte salt decomposition. The mechanism of degradation and our trial to improve the cyclability by changing the condition such as charging rate, cell configuration, electrolyte or its additives will be discussed in presentation.

References

[1] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, and K. Xu, Science, 350 (2015) 938-943.

[2] K. Nakamoto, R. Sakamoto, M. Ito, A. Kitajou, and S. Okada, Electrochemistry, 85(4) (2017) 179-185.

[3] K. Nakamoto, R. Sakamoto, Y. Sawada, M. Ito, and S. Okada, Small Methods, (2018) 1800220.