Elettra | XRF, XAFS, MCX Structural evolution of manganese prussian blue analogue in aqueous ZnSO4 electrolyte Aqueous Rechargeable Zinc-ion Batteries (ARZIBs) have attracted extensive attention as one of the most promising post-lithium ion battery candidates for the large-scale electrochemical energy storage, because of their low cost, intrinsic safety, and environmental friendliness. Manganese Hexacyanoferrate (MnHCF), as one of the Prussian Blue Analogues (PBAs), exhibits high potential for the grid-scale ARZIBs application. Until now, the intercalation mechanism of Zn2+ into MnHCF has not been clearly illustrated. In this work, combining different synchrotron x-ray techniques: X-ray Fluorescence (XRF), X-ray Absorption Spectroscopy (XAS), X-ray Diffraction (XRD), the structural and microscopic evolution of MnHCF in 3M ZnSO4 electrolyte is comprehensively studied, and a thorough understanding of the intercalation/release dynamic, in terms of local and long-range domain is provided. The elemental distribution and structural information of Fe, Mn, Zn inside MnHCF electrodes is obtained from the XRF elemental maps and XAS. The XRF maps in Figs. 1 (a-c) showing the Mn distribution in 1st charged (C1), 1st discharged (D1) and 10th charged (C10) electrodes. For the C1 electrode, the Mn is still quite homogeneously distributed, compared to D1 electrode, where the severe dissolution and replacement occurred on Mn site. An evenly distributed Mn was observed in C10 electrode. Thanks to the relatively stable content of Fe, which provides a reliable term of comparison to observe elemental trends among the samples. Our data shown that the Mn/Fe ratio increased from 0.088 in D1 to 0.56 in C10, which can be explained by the redeposition of Mn. Compared to Mn3+HCF (Fig. 1 d), the energy shift and spectral shape of C1, C2 and C10 electrodes look totally different, which indicates that the Mn inside these electrodes differ from the characteristic of Mn3+-NC-Fe framework. To further confirm the existing state of Mn inside the cycled electrode, an in-depth analysis of Extended X-ray Absorption Fine Structure (EXAFS) signals was conducted. The EXAFS fitting results display that the MnHCF model fitting applies only on pristine electrode, fails at Mn K-edge EXAFS for C1, D1 and C10 electrodes. Based on the Linear Combination Fitting (LCF) results of Mn K-edge XANES data, the contribution proportion of the MnO2 phase increased from 0.075 in C1 to 0.57 in C10 state. Based on this information and the strategy available in the literature, we have conducted a 2-models fitting to the EXAFS signal for the C10 sample, as it is displayed in Fig. 1 (e). The two chosen models were MnHCF and MnO2. The agreement of the theoretical signal to the experimental one is striking, and far better than the one model fitting for sample C10. The EXAFS fitting results confirm the rearrangement of Mn site, evidencing the cleavage of the Mn-N bond with the formation of a Mn-O bond, in an octahedral environment. To identify the phase and crystal structure evolution of MnHCF during the insertion of Zn2+, ex-situ and operando XRD data were collected. To better understand the transformation process of MnHCF during the initial cycle, an operando XRD experiments was performed, and the peak evolutions are obviously visible from the contour map (Fig. 1 f). During the 1st cycle, firstly the electrode was charged to 1.9 V, which corresponds to the extraction of Na+ from the structure, the oxidation of Fe and Mn will happen at around 1.57 V and 1.80 V, respectively. Some new XRD peaks at around 11.2 °, 15.9 ° and 22.6 °, appeared at the final stage of the charging process, which is related to the Mn oxidation process (>1.8 V). These experimental findings suggest that the structural transformation started mainly during the oxidation process of the Mn. At the end of the 1st charge, the newly formed peaks are coexisting with the previous ones, but with less intensity at 11.2 °and 15.9 °. During the discharge process, those new peaks became dominant. The XRD pattern became quite stable by 9 cycles, no apparent changes were observed in the 10th cycle operando XRD data. After Pawley refinement, the phase transformation of MnHCF during cycling can be illustrated as this: during the 1st cycling process, the monoclinic MnHCF transformed to rhombohedral and cubic Zinc Hexacyanoferrate (ZnHCF) phase, while during the following cycles, the cubic ZnHCF phase became the only stable phase. In conclusion, combining multiple x-ray techniques, the local and long-range structural changes of MnHCF electrode in aqueous ZnSO4 electrolytes were comprehensively studied. The EXAFS fitting results confirm the cleavage of the Mn-N bond, with the formation of a Mn-O bond, in an octahedral environment, and MnO2 as a main manganese- “survival” component in the cathode was proposed. Although no MnO2 reflection peaks were observed from the XRD data, the typical redox peaks of MnO2 were observed from the electrochemical tests. The XRD results confirmed that the phase transformation of MnHCF took place exclusively during the 1st cycle, and a mixture of rhombohedral and cubic ZnHCF phases were formed during the first charge process. Later, the new cubic ZnHCF phase became the only stable phase participating in the subsequent cycling and exhibiting high cycling stability. The signature and evolution of the electrochemical data is fully consistent with the interpretation of the proposed structural and electronic modification upon the Zn ion insertion, providing a full picture of the structureproperty-performance relationship in this class of materials. This research not only provides a deeper understanding of the working principle of MnHCF in ARZIB, but also offers strategies and methodologies for the development and interpretation of other battery systems in the future research. Elettra Highlights 2024–2025 98
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