(Invited) Research Development on K-Ion Batteries
Authors: Komaba Shinichi , Hosaka Tomooki , Kubota Kei , Abdulrahman Shahul Hameed 
Published: May 2020

In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to Li-ion batteries (LIBs). Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Besides being free from any scarce/toxic elements, the low standard electrode potentials of K/K+ electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K+ ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than with Li+ ions; this is essential to realize high-power KIBs. Over the past few years, many potassium insertion materials have been evaluated in K cells. Figure 1 summarizes the yearly number of scientific papers on K batteries. The number of papers on K systems drastically increased in recent years. Thus, we have recently published a comprehensive review to provide a guideline for research on KIBs (1). This talk presents an overview of our recent development on electrode and electrolyte materials.
Previous studies on positive electrode materials for KIBs have shed light on the importance of the crystal structures, redox center, and anions in the framework. Although transition metal layered oxides are the most promising positive electrode materials for LIBs and NIBs, we found that typical transition metal layered oxide materials like P2-K
x
CoO2 exhibit small capacities, low operating potentials, and multiple phase transitions due to K+/vacancy ordering (2). PBAs are one of a promising group of positive electrode materials in terms of energy density, cycle performance, and rate performance (3, 4). Their 3D open framework structure provides suitable channels and interstitial sites for large K+ ion diffusion and insertion. Interestingly, unlike layered transition metal oxides, PBAs tend to exhibit higher K+ ion insertion potentials than those of Li+ and Na+ ions (5). Similar to PBAs, polyanionic compounds with 3D open framework structures are other possible positive electrode material candidates, owing to their high ionic conductivities and high operation potentials. Previous studies have revealed suitable crystal structures, including the KTiOPO4-type structure for K+ ion diffusion (6). Moreover, an inorganic-organic hybrid material called metal organic phosphates open framework (MOPOF) has been demonstrated as a 4 V-class positive electrode for KIBs with high rate capability (7).
For negative electrodes, graphite is a promising candidate for KIBs (8). Our current studies are confirming that the physical properties of graphite, such as the d002 and Lc values, affect the electrochemical performance and that the optimum properties should be different from those of the Li system. Therefore, the development of graphite material suitable for K+ insertion is also an interesting and practically important topic. Furthermore, many studies of negative electrode materials have highlighted the properties of SEI, which depend on the electrolyte composition; this is important to demonstrate satisfactory cycles and rate capabilities (1). Therefore, recent studies have been focusing on the development of suitable electrolytes. The use of proper electrolytes, such as superconcentrated KFSA electrolytes (9), dramatically improved the electrochemical performance of the positive and negative electrodes. Thus, we believe that the development of a suitable electrolyte for KIBs will achieve a breakthrough in the research field of KIBs. Based on these results, we will provide insights into electrode reactions and solid ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs, compared to those of LIBs and NIBs.
References:
1. T. Hosaka, K. Kubota, A. S. Hameed and S. Komaba, Chem. Rev., in press (2019).
2. Y. Hironaka, K. Kubota and S. Komaba, Chem. Commun., 53, 3693 (2017).
3. L. Xue, Y. Li, H. Gao, W. Zhou, X. Lü, W. Kaveevivitchai, A. Manthiram and J. B. Goodenough, J. Am. Chem. Soc., 139, 2164 (2017).
4. X. Bie, K. Kubota, T. Hosaka, K. Chihara and S. Komaba, J. Mater. Chem. A, 5, 4325 (2017).
5. K. Kubota, M. Dahbi, T. Hosaka, S. Kumakura and S. Komaba, Chem. Rec., 18, 459 (2018).
6. T. Hosaka, T. Shimamura, K. Kubota and S. Komaba, Chem. Rec., 19, 735 (2019).
7. A. S. Hameed, A. Katogi, K. Kubota and S. Komaba, Adv. Energy Mater., in press, 1902528 (2019).
8. S. Komaba, T. Hasegawa, M. Dahbi and K. Kubota, Electrochem. Commun., 60, 172 (2015).
9. T. Hosaka, K. Kubota, H. Kojima and S. Komaba, Chem. Commun., 54, 8387 (2018).