Towards High‐Voltage Aqueous Metal‐Ion Batteries Beyond 1.5 V: The Zinc/Zinc Hexacyanoferrate System
By Leyuan Zhang, Liang Chen, Xufeng Zhou, Zhaoping Liu. 2015
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Many studies focus on cubic MeHCFs as hosts for alkaline cations such as Li + , Na + and K + . Recently, Cui's group found that nickel hexacyanoferrates can also act as hosts for divalent alkaline earth cations: Mg 2+ , Ca 2+ , Sr 2+ and Ba 2+ . [ 26 ] Zinc hexacyanoferrates (ZnHCFs) exhibit different open-framework structure as compared to cubic MeHCFs. For rhombohedral ZnHCFs as shown in Figure 1 a, the FeC 6 octahedra are linked to ZnN 4 tetrahedra via CN ligands to form a porous 3D framework with large open sites in which alkaline cations A (A = Na + , K + and Cs + ) and water molecules are located. Such a framework endows ZnHCFs with the possibility of behaving as intercalation hosts for various cations. The feasibility of ZnHCFs as Na + -intercalation compounds has been demonstrated by Chio and co-workers. [ 27 ] However to the best of our knowledge, no attempts to utilize ZnHCFs as intercalation hosts for divalent cations such as Zn 2+ have been reported. Here we demonstrate the fi rst use of ZnHCFs as cathode materials for aqueous zincion batteries. When combined with a zinc anode, zinc hexacyanoferrate yields an average operation voltage of ca. 1.7 V, which is the highest recorded for aqueous zinc-ion batteries and it also delivers a specifi c energy density of 100 Wh Kg −1 based on the total mass of the active electrode materials. Two zinc hexacyanoferrates (K 2 Zn 3 [Fe(CN) 6 ] 2 , KZnHCF and Zn 3 [Fe(CN) 6 ] 2 , ZnHCF) with the rhombohedral structure were synthesized using high temperature co-precipitation method developed in our group (see Experimental Section). To confi rm their crystal structures, X-ray diffraction (XRD) analyses are conducted (Figure 1 b). The XRD peaks of KZnHCF and ZnHCF are well-indexed to K 2 Zn 3 [Fe(CN) 6 ] 2 (H 2 O) 9 (JCPDS # 33–1061) and Zn 3 [Fe(CN) 6 ] 2 (JCPDS # 38–0688), respectively, which indicates that two compounds crystalize in the same group of R3 C without any impurities. From their scanning electron microscopy (SEM) images, it can be seen that KZnHCF particles have a narrow size distribution with an average size of 200 nm, while ZnHCF particles exhibit a wider size distribution ranging from 200 nm to several μm. The molar K:Zn:Fe ratios of two samples determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) are found to be 0.94:1.44:1 for KZnHCF and 0.08:1.44:1 for ZnHCF, respectively. These ratios are also very close to the nominal ones (1:1.5:1 and 0:1.5:1), and agree well with the data in the literature. [ 28 ]
Subfields: Electrical and Electronic Engineering
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