(Digital Presentation) Electrochemistry and Structural Study of Manganese Hexacyanoferrate Cathode Material in Aqueous Zn-Ion Battery

Aqueous rechargeable Zinc-ion batteries (ARZIBs) have attracted extensive attention as one of the most promising post-lithium ion battery candidates for large-scale electrochemical applications, because of their low cost, intrinsic safety, and environmental friendliness. The metallic zinc, as an ideal anode material shows high theoretical gravimetric and volumetric capacity of 820 mAh g-1 and 5855 mAh cm-3, low electrochemical potential (-0.76 V vs. SHE) and high abundance.[1,2] Manganese hexacyanoferrate (MnHCF), one of the Prussian blue analogues (PBAs), has attracted widely attention as promising cathode material for Li-ion and post-Li ion batteries. MnHCF is composed of only highly abundant metals and displays large capacity and high discharge potential owning to the two-redox active sites [3-4]. Here, the electrochemical performance of MnHCF was studied in 3 M ZnSO4 aqueous electrolyte with Zn sheet as anode. The battery exhibits high specific capacity (176 mAhg-1) at C/20, and around 61% capacity retention after 50 cycles at C/5. In order to explain the capacity fading problems during cycling, the local geometric, electronic structures, as well as the framework structure change of MnHCF electrode were studied by means of ex-situ X-ray absorption spectroscopy (XAS) and synchrotron X-ray diffraction (XRD). Based on XAS data, no obvious change was observed at the Fe K-edge during cycling, and this indicates that there is no apparent change of the local Fe structural environment. However, the XAS spectra of Mn K-edge exhibit an apparent change after 10 cycles. The Zn K-edge shows a typical -Zn-NC-Fe- structural framework in the cycled samples that resembles the one of zinc hexacyanoferrate (ZnHCF), providing evidence for Zn-Mn partially replacement upon cycling, resulting in dissolution the Mn ion [5]. From the ex-situ XRD data, we found that this phase changes mainly concerns early cycles, because the XRD patterns of the 2nd and 10th cycle are almost identical. ZnHCF phase formed even during the first charge process. The more detailed phase transformation process is still under studying. By combing the ex-situ XAS and XRD data, the charge/discharge mechanism of MnHCF in aqueous Zn-ion battery can be more clearly illustrated. Trócoli, F. La Mantia, ChemSusChem. 8, 2015, 481–485. Tang, L. Shan, S. Liang, J. Zhou, Energy Environ. Sci. 12, 2019, 3288–3304. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, M. Giorgetti. Small Methods, 2019, 1900529. Mullaliu, M. Gaboardi, J. R. Plaisier, S. Passerini, and M. Giorgetti, ACS Applied Energy Materials 3, 2020, 5728. Li, R. Sciacca, M. Maisuradze, G. Aquilanti, J. Plaisier, M. Berrettoni, M. Giorgetti, Electrochim. Acta. 400 ,2021.