Copper hexacyanoferrate (CuHCF) is a prototypical example of Prussian Blue Analogue (PBA) compound that has attracted significant attention for electrochemical applications and specifically for reversible ion insertion due to its open framework and characteristic wide cavities that can conveniently host a number of different cations, e.g. alkali metal ions and divalent cations.

Among possible uses of CuHCF, rechargeable cells - particularly aqueous batteries - constitute a category of upcoming technologies that have created remarkable interest as possible advanced charge storage devices with the prospect of large-scale stationary storage of electricity for grids and renewables¹. In this scenario, CuHCF represents an intriguing compound not only from an applied perspective of electrochemical charge storage, but also from a fundamental point of view due to its complex structure and properties that intrinsically influence corresponding cell performances.

The use of CuHCF as environmentally friendly positive electrode in aqueous Zn-ion batteries (ZIBs) with a weakly acidic pH is considered an attractive route to combine the high specific capacity of metallic zinc (820 mAhg^-1) with a smooth ion insertion into the spacious channels of the CuHCF framework to reach enhanced power capabilities and sustain high charge/discharge rates². Such an approach has not only the advantage of employing abundant, cost-effective and non-toxic materials, but also contributes to improved safety, as the electrolyte is non-flammable and strongly acidic/alkaline species are ruled out. Besides, CuHCF exhibits one of the highest working potentials (≈1.7 V vs. Zn²⁺/Zn) in these rechargeable aqueous cells². CuHCF possesses a moderate capacity of ≈60 mAhg^-1, however, its negligible structural variations during ion insertion/de-insertion and minor volume changes make it suitable for efficient use in these batteries³ for stationary storage or UPS systems, avoiding detrimental strain effects and hence possible mechanical and electrical connection failures in resulting electrodes.

Nevertheless, aqueous Zn/CuHCF cells are currently hampered by a series of challenges that need to be addressed to provide reliable operation. Uneven Zn deposition/stripping is a well-known source of capacity loss and detrimental effects on the cell behavior, typically leading to internal short-circuits due to uncontrolled growth of Zn dendrites that can intrude the separator. Despite these limitations, formation and growth of Zn dendrites are rather understood processes that can be handled and mitigated by proper strategies^4,5. Conversely, possible limitations and degradation mechanisms for CuHCF upon cycling are intrinsically more subtle and less understood, due to its complex nature and concurrent interplay of several species in the electrochemical reactions. In fact, various pathways for capacity loss have been suggested for this electrode material^6-10, whose aspects are crucial for a correct functioning of the cell and may limit its ageing, especially in view of heavy-duty usage.

Here, the challenges and opportunities for efficient utilization of CuHCF in aqueous ZIBs with a mildly acidic ZnSO₄ electrolyte will be presented with a focus on the materials ageing process and limiting factors that could undermine its suitability for upcoming rechargeable water-based ZIB technologies.

Figure 1. (a) Schematic drawing of a Zinc/CuHCF battery. (b) Charge-discharge voltage profiles of a Zn/CuHCF cell cycled between 1.00 and 2.15 V at 0.6 mAcm^-2 showing the evolution of the cumulative cell capacity. The inset in (b) highlights the shapes of the individual charge-discharge curves. Note a slight polarization of 0.15 V.

Acknowledgements

M.V. acknowledges the ÅForsk Foundation for funding, the support by the Swedish Energy Agency, the Swedish Electromobility Centre and StandUp for Energy.

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