Due to the extensive application of green energy technologies, the demand of large-scale energy storage systems is increasing. As far as now, the most successful and widely used battery system is Li-ion battery (LIB). When the energy storage system is enlarged, however, the cost performance becomes a more dominant factor, which is not apparently the advantage of LIB. Therefore, many novel battery systems with low cost materials have been developed recently, such as Na-ion batteries and aqueous batteries. However, the working voltage of these batteries is limited because of the 0.3-V higher potential of Na⁺/Na than that of Li⁺/Li or limited electrochemical window of H₂O. Consequently, energy density of these battery systems is not satisfied. On the other hand, potassium is an earth abundant element with a lower standard electrode potential than that of lithium in PC solution [1]. Hence, the cost benefit and high working voltage are expected at the same time in K-ion batteries. We have studied the KIB materials since 2013 and recently reported graphite negative electrode delivering 250 mAh/g with excellent reversibility and rate-capability based upon stage-1 KC₈ formation by electrochemical potassium intercalation [1]. Obviously, another key to obtain the successful K-ion batteries is development of the promising positive electrode materials. Considering the larger ionic size of K⁺_­, the insertion materials should provide the large diffusion tunnels. As a suitable positive electrode, Prussian blue analogues with open framework structure exhibit excellent electrochemical performance in Na-, K-ion aqueous and non-aqueous batteries in previous reports [2,3]. Herein, we propose non-aqueous K-ion batteries by developing hexacyanoferrate(II) compounds, K_1.75Mn[Fe^II(CN)₆]_0.93·0.16H₂O and K_1.64Fe[Fe^II(CN)₆]_0.89·0.15H₂O (denoted as K-Mn[Fe(CN)₆] and K-Fe[Fe(CN)₆], respectively) as affordable positive electrode materials. Furthermore, we demonstrate 4-volt class K-ion full cells with K_1.75Mn[Fe^II(CN)₆]_0.93·0.16H₂O and graphite electrodes for the first time to prove their feasibility as a high energy density battery system.

K-rich Prussian blues studied here were synthesized by precipitation method. Electrochemical measurements were carried out using 2032 coin cell with 0.7 M KPF₆ in EC: DEC (1:1) solution as electrolyte and potassium metal as negative electrode. Prussian blue electrode consisted of 70 % active material, 20 % Ketjen black and 10 % PTFE binder.

As shown in Figure 1a, K-Fe[Fe(CN)₆] exhibits two voltage plateaux during charge at 3.5 and 4.1 V and corresponding discharge plateaux at 3.4 and 3.9 V, while K-Mn[Fe(CN)₆] exhibits higher discharge plateaux at 4.0 and 3.9 V. Thanks to the higher working voltage and reversible capacity, K-Mn[Fe(CN)₆] electrode shows higher energy density (vs. Na as negative electrode) than K-Fe[Fe(CN)₆], which is 520 Wh kg^-1. Such high energy density is comparable to that of commercial LiCoO₂ in Li cell. As potassium metal is violently reactive with water than lithium and sodium metals, a metallic K battery is highly dangerous and unrealistic for practical use. We demonstrated K-ion full cells performance, i.e. K⁺-shuttlecock cell consisting of graphite/K-Mn[Fe(CN)₆] electrodes on Al current collectors, as shown in Figure 1b. The full cell delivers a reversible capacity of 110 mAh (g of K-Mn[Fe(CN)₆])^-1 with a mean operating voltage of 3.5 V. The K-ion battery also demonstrates reversible charge-discharge profiles and acceptable capacity retention/efficiency over 60 cycles. In this presentation, we will present new potassium insertion materials, binders, and electrolytes for advanced battery demonstration, of which the charge/discharge voltage is compatible with the voltage of conventional Li-ion battery.

References

[1] S. Komaba, T. Hasegawa, M. Dahbi, and K. Kubota, Electrochem. Commun., 60, 172 (2015).

[2] A. Eftekhari, J. of Power Sources, 126, 221 (2004).

[3] C. Wessells, S. Peddada, R. Huggins, Y. Cui, Nano Lett., 11, 5421 (2011).