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).
Conventional positive (cathodes) electrode materials for lithium batteries, use mixed-conducting lithium containing transition metal oxides, metal phosphates etc. which are able to store both lithium and electrons by changing their oxidation state. In this presentation, we discuss the facile synthesis of materials suitable for alternative electrode concepts including Transition metal fluorides MF2 (M=Fe, Mn, Zn) were obtained by reacting Fe, Mn, Zn-Metal salts with fumaric acid as a one-dimensional metal organic framework, and a polymer (PVDF) as Fluorine source, final products are obtained by heating at 600°C, 6h in Ar gas. The MOF were initially prepared by a simple chimie douce method. Incorporation of the MOF with a fluorinated polymer and the eventual decomposition lead to carbon coated metal fluoride nanoparticles of high surface area of >200 m2/g for FeF2. The obtained materials will be characterized in detail by X-ray diffraction, Scanning and Transmission electron microscope (SEM/TEM) are used to evaluate the structure and morphology, X-ray photoelectron spectroscopy are used understand structure, vibrational bands and oxidation state of the materials and BET surface area method. Electrochemical studies were carried out in the voltage, range 4 to 1.0 vs. Li, at current rate of 50 mA/g (0.1 C) using 1MLiPF6 (EC;DEC) as liquid electrolyte and tested with Li-metal as a counter and reference electrodes. The cyclic voltammetry at scan rate of 0.075 mV/sec at room temperature (24°C). Galvanostatic cycling of FeF2 demonstrates that the material exhibit stable and good reversible capacity of 580 mAh/g during the first cycle and slight capacity fading has been observed after 20 cycles. Further studies on rate performance is being carried up to 2.5 C rate. Whereas MnF2 and ZnF2 showed reversible capacity of 220 and 200 mAh/g and retained a capacity around 100 mAh/g after 20 cycles and showed lower capacity than FeF2. Discuss the structural, reaction mechanism of FeF2 during charge-discharge cycling by in situ/operando X-ray diffraction and electrochemical impedance spectroscopy studies discuss in detail. Keywords: Metal fluorides (MF2 M=Fe, Zn, Mn); Electrochemical properties; Insitu studies; Electrochemical impedance spectroscopy; energy storage
K-ion batteries (KIBs) potentially operate at high-voltage and high-current thanks to the lower standard electrode potential of K/K+ and the lower solvation energy than those of Li/Li+ in organic electrolytes and fast K-ion diffusion.[1] To realize practical applications, high energy density positive electrode materials are, however, indispensable. In this study, two vanadium based positive electrode materials with layered structure, namely, K2[(VO)2(HPO4)2(C2O4)] and and K x VOPO4 are synthesized and their electrode performances are examined in potassium cells. Highly reversible Li insertion/extraction oxalatophosphate material was reported with reversible capacities of >100 mAh g-1 and with a high voltage operation of ~3.8 V vs. Li/Li+. [2, 3] Hence, reversible K extraction/insertion and high voltage operation are expected in K cells. K2[(VO)2(HPO4)2(C2O4)] was prepared by a simple precipitation method at room temperature. As shown in Fig. 1, K2[(VO)2(HPO4)2(C2O4)] delivers a reversible capacity of ~90 mAh g-1 in the K-half cell at 0.1 C in the voltage range of 2.5 - 4.5 V vs. K/K+ and excellent cycle stability is demonstrated. Furthermore, high rate capability with 76 mAh g-1 at 10 C is achieved by optimization of the electrolyte composition. Another vanadium-based layered positive electrode material, K x VOPO4 was also prepared by a precipitation method at room temperature. By optimization with carbon-coating and ball-milling, the material delivers a reversible capacity of 80 mAh g-1 at 3.7 V for over 200 cycles. Fig.1: Voltage profiles and capacity retention plots of K2[(VO)2(HPO4)2(C2O4)] in a K-half cell. References [1] K. Kubota, S. Komaba, et al., Chem. Rec., 2018, 18, 459-479. [2] A.S. Hameed et al., Sci. Rep., 2015, 5, 16270. [3] A.S. Hameed et al., J. Mater. Chem. A, 2013, 1, 5721-5726. Figure 1
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