Abstract
The zinc/copper hexacyanoferrate (Zn/CuHCF) cell has gained attention as an aqueous rechargeable zinc-ion battery (ZIB) owing to its open framework, excellent rate capability, and high safety. However, both the Zn anode and the CuHCF cathode show unavoidable signs of aging during cycling, though the underlying mechanisms have remained somewhat ambiguous. Here, we present an in-depth study of the CuHCF cathode by employing various X-ray spectroscopic techniques. This allows us to distinguish between structure-related aging effects and charge compensation processes associated with electroactive metal centers upon Zn2+ ion insertion/deinsertion. By combining high-angle annular dark-field-scanning electron transmission microscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy, and elemental analysis, we reconstruct the picture of both the bulk and the surface. First, we identify a set of previously debated X-ray diffraction peaks appearing at early stages of cycling (below 200 cycles) in CuHCF. Our data suggest that these peaks are unrelated to hypothetical ZnxCu1–xHCF phases or to oxidic phases, but are caused by partial intercalation of ZnSO4 into graphitic carbon. We further conclude that Cu is the unstable species during aging, whose dissolution is significant at the surface of the CuHCF particles. This triggers Zn2+ ions to enter newly formed Cu vacancies, in addition to native Fe vacancies already present in the bulk, which causes a reduction of nearby metal sites. This is distinct from the charge compensation process where both the Cu2+/Cu+ and Fe3+/Fe2+ redox couples participate throughout the bulk. By tracking the K-edge fluorescence using operando XAS coupled with cyclic voltammetry, we successfully link the aging effect to the activation of the Fe3+/Fe2+ redox couple as a consequence of Cu dissolution. This explains the progressive increase in the voltage of the charge/discharge plateaus upon repeated cycling. We also find that SO42– anions reversibly insert into CuHCF during charge. Our work clarifies several intriguing structural and redox-mediated aging mechanisms in the CuHCF cathode and pinpoints parameters that correlate with the performance, which will hold importance for the development of future Prussian blue analogue-type cathodes for aqueous rechargeable ZIBs.
Highlights
The use of renewable energies is imperative to lower the greenhouse gas emissions and progressively develop a society free of fossil fuel dependence.[1]
We provide a highly detailed electrochemical and structural characterization study of the CuHCF cathode employed in aqueous zinc-ion batteries (ZIBs)
We reveal that a set of previously unidentified X-ray diffraction (XRD) peaks that appear at early stages of cycling originate from the intercalation of ZnSO4 (Zn2+/SO42−) and/ or crystallization of these ionic species in the surface regions of graphitic carbon and are unrelated to CuHCF
Summary
The use of renewable energies is imperative to lower the greenhouse gas emissions and progressively develop a society free of fossil fuel dependence.[1]. We investigate the zinc/copper hexacyanoferrate (Zn/ CuHCF) system, a PBA suggested as a possible ZIB cathode material in 2015 by Troć oli and La Mantia[18] and Jia et al.,[17] being known at the time to host other cations.[19,23−25] CuHCF has a typical gravimetric capacity of around 60 mAh g−1, a high operating potential of ∼1.7 V, and negligible structural changes and volume expansion during Zn2+ ion insertion (i.e., near-zero strain) This allows for fast ion insertion and makes this type of ZIBs more suitable for high power applications compared to, for example, manganese dioxide batteries (Zn/MnO2), which exhibit generally higher gravimetric capacities instead. The detailed analyses, combined with our tailored X-ray approach, pave the way toward the development of aimed methodologies for accurate insights into the key components of these ZIBs, enabling future improvements of this challenging rechargeable Zn-ion technology
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