We reported the high working voltage of K-ion battery (KIB) comparable to Li-ion and higher than Na-ion thanks to the standard potential of potassium is lower than those of lithium and sodium in non-aqueous electrolyte [1,2]. In 2015, our group reported graphite negative electrode delivering 250 mAh g^-1 or higher with excellent reversibility [1]. Furthermore, a Prussian blue analogue (PBA) of K₂Mn[Fe(CN)₆] demonstrated highly reversible potassium insertion/extraction and 4 V operation, realizing a 4 V-class K₂Mn[Fe(CN)₆]/graphite full cell [3]. The previous studies revealed that a three-dimensional open framework structure of PBAs is suitable for insertion/extraction of large K⁺ ion. Addition to the PBAs, polyanionic compounds are potential positive electrode materials for KIBs due to their open framework structure and inductive effect. In this talk, our recent studies on potassium insertion into various polyanionic compounds, such as heterosite FePO₄, KFePO₄, KMnPO₄, K₂FeP₂O₇, K₂MnP₂O₇, KVPO₄F, KVOPO₄, monoclinic KFeSO₄F, and orthorhombic KFeSO₄F (o-KFeSO₄F) [4,5], will be presented to give a perspective of potassium insertion material design.

Because K⁺ ion is too large to occupy the octahedral site in heterosite FePO₄ prepared from olivine-type LiFePO₄, the crystallinity of heterosite FePO₄ significantly decreases after potassium insertion. On the other hand, KVPO₄F, KVOPO₄, and o-KFeSO₄F possess the KTiOPO₄-type structure providing large channels to diffuse K⁺ ion and show the reversible potassium insertion/extraction properties and excellent rate performance. These results indicate that the crystal structure plays a key factor for reversible potassium insertion and rate performance. Moreover, the average discharge voltage of KVPO₄F and KVOPO₄ is as high as 4 V in a K half-cell. That of o-KFeSO₄F is 3.6 V, which is notably high redox potential of Fe²⁺/Fe³⁺ due to the strong inductive effect of SO₄^2- and F^- on the Fe^2+/3+ couple. The cycling stability of these high-voltage electrode materials is highly improved by using highly concentrated potassium salt solutions [6]. From these results, we will present our future insight into electrochemical potassium insertion chemistry for high voltage battery applications.

References

[1] S. Komaba et al., Electrochem. Commun., 60, 172 (2015).

[2] K. Kubota, T. Hosaka, S. Komaba et al., Chem. Rec., 18, 459 (2018).

[3] X. Bie, K. Kubota, S. Komaba et al., J. Mater. Chem. A, 5, 4325 (2017).

[4] K. Chihara, S. Komaba et al., Chem. Commun., 53, 5208–5211 (2017).

[5] T. Hosaka, T. Shimamura, S. Komaba et al., Chem. Rec., in-press

[6] T. Hosaka, Kei Kubota, S. Komaba et al., Chem. Commun., 54, 8387 (2018).