Electrochemical Behavior of Calcium Ions on Prussian Blue Analogue Electrodes
Authors: Tojo Tomohiro , Sugiura Yosuke , Oshida Noriyuki , Tawa Hayato , Inada Ryoji , Sakurai Yoji 
Published: February 2020

Beyond lithium ion batteries have attracted considerable attention due to a rapidly increasing demand for high energy storage systems installed in electric vehicles and natural energy power plants. Especially, multivalent ion batteries (MIBs) have the potential of leading to a higher specific capacity and energy density because of one or more electron reaction. The multivalent ions such as magnesium ions, calcium ions, and aluminum ions are adapted to the working ions for MIBs in which their ions can provide redox reaction of two or three electron in principle. However, the development of MIBs has been stunted by a lack of an optimal electrode material for reversible insertion and extraction of the working ion, in addition to the larger ionic radius compared to that of lithium ion. Thus, MIBs would necessitate the electrode material with an open framework or a large d-spacing to allow the movement of multivalent ions.
Prussian blue analogues (PBAs) have recently demonstrated the reversible insertion and extraction of monovalent and multivalent ions in both aqueous and non-aqueous electrolytes because of their large interstices.[1,2] The large sites (A sites) have been reported to accommodate large-sized ions and water molecules. In addition, PBA electrodes were observed to show excellent rate performance and cycle performance due to the reversible conversion reaction and the high structural stability.[3] Therefore, PBAs possess an attractive potential for consideration as compatible electrodes for MIBs, particularly calcium ion batteries (CIBs) which have some advantages in cell voltage, thermal stability and costs. In addition, Ca2+ ions are thought to be faster diffusion derived from the lower charge density on its larger radius than that of Mg2+ ions,[4]resulting in a higher reversible capacity.
In this study, prussian blue analogues (MFe-PBAs) were prepared by a wet chemical precipitation method, changing the M species for Ni, Mn, and Co. The electrochemical behaviors of Ca2+ ions into MFe-PBA electrodes were investigated by galvanostatic charge/discharge curves and cyclic voltammograms (CVGs). All electrochemical measurements were performed in three-electrode cells with MFe-PBA electrodes as working electrodes, silver/silver-ion electrodes as references, and porous carbon electrodes as counter electrodes. The working electrodes were prepared by compressing a mixture of MFe-PBA powder, carbon additives, and polytetrafluoroethylene (PTFE) at 70:25:5 wt.%. The cells were cycled with a potential scan rate of 0.05 mV/s in a potential limit between -1.0 and 1.0 V vs. Ag/Ag+, and were discharged and charged at 25 μA/cm2 in the same potential range. The electrolyte employed was 0.5 mol/L calcium bis-trifluoromethylsulfonylimide (Ca(TFSI)2) dissolved in acetonitrile (AN).
The XRD patterns of the as-synthesized powder all exhibited main diffraction peaks corresponding to the peaks of the similar open framework structure without other crystal structures. CVGs on PBA electrodes showed pair peaks attributed to Ca2+ insertion (reductive reaction) and Ca2+ extraction (oxidative reaction) around -0.15 and 0.6 V vs Ag/Ag+, respectively. Potential plateaus upon a Ca2+ insertion process were observed in discharge-charge curves between -0.2 and 0.2 V vs. Ag/Ag+ which corresponds to the reductive peak position in CVGs. However, the plateaus during Ca2+ extraction were not clearly observed in discharge-charge curves. This suggests a higher excess voltage during Ca2+ extraction. On the other hand, PBA electrodes for CIBs showed reversible capacities of 40 mAh/g without a large structural distortion in PBAs. From the viewpoint of Ca2+behaviors on the PBA, such open framework structures possess excellent potential for beyond lithium ion batteries.
In this presentation, I will discuss the electrochemical characteristics and the structural analysis mentioned above.
References
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[4] R. D. Shannon, Acta Cryst. A32 (1976) 751-767.