Effect of Particle Size and Anion Vacancies on Electrochemical Performances of Potassium Manganese Hexacyanoferrate for Potassium-Ion Batteries

K-ion battery (KIB) has recently attracted much attention as a potential high-voltage and high-power battery owing to low standard electrode potential of K/K + and fast ionic diffusion of K + ion in electrolyte solutions, respectively. 1 Among positive electrode materials reported so far, K x Mn[Fe(CN) 6 ] y (KMnHCF) is a promising material because of a high working potential (ca. 3.8 V vs. K/K + ). 2 Although the theoretical capacity of stoichiometric K 2 Mn[Fe(CN) 6 ] is as high as 155 mAh g –1 , the reported reversible capacities range from 40 to 140 mAh g –1 . 1 Moreover, the reason for the different capacities is still unclear. Previous studies on K x Fe[Fe(CN) 6 ] y proposed the potential two factors which may affect the electrochemical performance. These are particle size 3 and number of [Fe(CN) 6 ] n - anion vacancies. 4 However, the difficulty of independent controlling the particle size and number of anion vacancies has hindered understanding the impact of each factor. In this study, we investigated the effect of the factors on electrochemical performance by controlling the particle size and number of the anion vacancies via chelate assisted and ionic exchange synthesis, respectively. KMnHCFs were synthesized by direct precipitation with chelating agents or by ionic exchange from the NaMnHCF. Samples with small (S-KMnHCF) and large particle size (L-KMnHCF) precipitated in 0.2 M and 1 M potassium citrate solutions, respectively. Samples with a large number of vacancies (IE-KMnHCF) were obtained by the ionic exchange method. All the samples were carefully dehydrated at 200 °C under vacuum to ignore the effect of interstitial water. The samples were characterized by powder X-ray diffraction (XRD), inductively coupled plasma emission spectroscopy (ICP-AES), scanning electron microscopy (SEM), and galvanostatic charge/discharge measurements. Figure 1a shows the XRD patterns and their fitted pattern by Rietveld refinement of S-, L-, and IE-KMnHCFs. The diffraction patterns of all the samples were successfully fitted with the monoclinic structures ( P 2 1 / n ), and no crystalline impurities were confirmed. Rietveld refinement and ICP-AES revealed that the composition of S-, L-, and IE-KMnHCF were K 1.9 Mn[Fe(CN) 6 ] 1.0 , K 1.8 Mn[Fe(CN) 6 ] 0.99 □ 0.01 (□ = anion vacancy), and Na 0.10 K 1.6 Mn[Fe(CN) 6 ] 0.85 □ 0.15 , respectively. Thus, S- and L-KMnHCF have negligible numbers of [Fe(CN) 6 ] 4- vacancies, while IE-KMnHCF has many anion vacancies. SEM image of Figs. 1b–1d display that the particle size of S-KMnHCF was 100–200 nm, whereas the particle size of L- and IE-KMnHCF was approximately 1–2 μm. Figure 2 shows the initial charge/discharge curves for S-, L-, and IE-KMnHCF in K cells. S-KMnHCF delivered a large reversible capacity of 137 mAh g -1 , whereas L-KMnHCF showed a small reversible capacity of 49 mAh g -1 . In contrast, IE-KMnHCF exhibited a reversible capacity of 117 mAh g -1 despite the large particle size. The reason behind the relatively large capacity of IE-KMnHCF would be enhanced potassium ion diffusion caused by the [Fe(CN) 6 ] 4- vacancies. 4 In the presentation, we will also discuss the impact of the anion vacancies on K + ion diffusion barrier and structural changes during charge and discharge process. References T. Hosaka, K. Kubota, A. S. Hameed and S. Komaba, Chem. Rev., DOI: 10.1021/acs.chemrev.9b00463, in press . X. Bie, K. Kubota, T. Hosaka, K. Chihara and S. Komaba, J. Mater. Chem. A , 5 , 4325 (2017). G. He and L. F. Nazar, ACS Energy Lett. , 2 , 1122 (2017). M. Ishizaki, H. Ando, N. Yamada, K. Tsumoto, K. Ono, H. Sutoh, T. Nakamura, Y. Nakao and M. Kurihara, J. Mater. Chem. A , 7 , 4777 (2019). Figure 1