Nowadays, rechargeable lithium batteries are the most mature energy storage systems. But high cost hinders its further application as large-scale energy storage device which is essential for the utilization of green and sustainable energy resources. Hence, various low cost battery systems have been proposed. Among them, rechargeable sodium batteries is a kind of promising candidate because sodium resources are unlimited, the second-lightest and smallest alkali metal next to lithium [1]. The working voltage of sodium batteries, however, is lower than it of lithium batteries because standard electrode potential of Na (-2.71 V vs. SHE) is about 0.33 V higher than Li (-3.04 V vs. SHE). Whereas, as another earth abundant alkali element, potassium (-2.93 V vs. SHE) has lower standard electrode potential than sodium. Consequently, research of rechargeable potassium batteries would be helpful for developing low cost large-scale energy storage system with high working voltage.

It cannot be denied that the radius of potassium is larger than sodium and lithium. Therefore, electrode material for potassium batteries should provide large migration tunnel for potassium ion. As a kind of good host for alkali ions, Prussian blue analogues provide open framework with large interstitial sites. Potassium insertion properties of Prussian blue thin film (40-300 nm) had been researched in aqueous solution by Murray and Neff for the first time [2,3]. Then, Eftekhari researched electrochemical performance of Prussian blue thin film (1 μm) positive electrode in non-aqueous Potassium secondary cell [4]. This thin film electrode exhibited about 80 mAh g^-1 reversible capacity and excellent cycle performance within 500 cycles. However, thin film electrode is far away from practical application. In this study, we synthesized K-rich Prussian blue powder with convenient precipitation method. Electrochemical properties of the product in non-aqueous potassium batteries will be shown in this presentation. For the practical application, potassium full cell with graphite negative electrode, which had been reported by us, will be also shown [5].

2. Experimental

2 mmol K₄Fe(CN)₆ and 4 mmol FeCl₂ were dissolved in to 40 and 80 ml of saturated KCl solutions. These two solutions were slowly dropped together with magnetic stirring under N₂ atmosphere at 60 ^oC. Then, the precipitate was centrifuged and washed thoroughly with 500 ml deionized water. The final product was obtained after drying in 80 ^oC oven for 12 h. Electrochemical measurements were carried out using 2032 coin cell with KFSI in EC:DEC (1:1) solution as electrolyte and potassium metal as negative electrode. Prussian blue electrode contains 70 % active material, 20 % Ketjen black carbon and 10 % PVdF binder.

3. Results and Discussion

Figure 1 shows the XRD pattern of K-rich Prussian blue. The sample is single phase product which belongs to monoclinic structure and space group of P2₁/n [6]. The lattice parameters are calculated as a=10.1303(1) Å, b=7.2574(7) Å, c=7.0208(1) Å and β=89.54(9)⁰. Figure 2 is the charge-discharge curves of sample in K half-cell with current density of 60 mA g^-1 in voltage range of 2.0-4.0 V. K-rich Prussian blue delivers 98 and 83 mAh g^-1 initial charge and discharge capacity, respectively. Meanwhile, the charge-discharge plateaus are located around 3.35~3.48 V, which relates with the redox reaction of Fe in nitrogen coordinate. After 100 cycles, the reversible capacity remains 75 mAh g^-1. Capacity retention is about 90 %. The as-prepared K-rich Prussian blue exhibits promising electrochemical performance in non-aqueous potassium battery. More details such like morphology, element composition and electrochemical mechanism of the sample will be given in the presentation.

References

1) K. Kubota and S. Komaba, Journal of The Electrochemical Society, 2015, 162, A2538-A2550

2) C. A. Lundgren and R. W. Murray, Inorg. Chem., 1998, 27, 933-939

3) V. Krishnan, A. L. Xidis and V. D. Neff, Analytica Chimica Acta, 1990, 239, 7-12

4) A. Eftekhari, Journal of Power Sources, 2004, 126, 221-228

5) S. Komaba and K. Kubota, Electrochemistry Communications, 2015, 60, 172-175

6) J. Song and J. B. Goodenough, Journal of the American Chemical Society, 2015, 137, 2658-2664