Clarifying the Limitations of Copper Hexacyanoferrate in Rechargeable Aqueous Zinc-Ion Batteries Via in Situ X-Ray Techniques
Authors: Gorlin Mikaela , Ojwang Dickson O. , Lee Ming-Tao , Renman Viktor , Tai Cheuk-Wai , Valvo Mario 
Published: February 2024

Copper hexacyanoferrate (CuHCF) is a compound belonging to the Prussian Blue Analogue (PBA) family, known for its open structure with cubic symmetry and large cavities capable of accommodating various cations, such as alkali-metal ions and divalent cations, making it an interesting candidate for ion-insertion batteries.
CuHCF has been increasingly attracting attention for innovative approaches in rechargeable aqueous batteries which represent an emerging alternative technology for realizing sustainable and efficient stationary storage of intermittent electricity produced via renewable sources1.
CuHCF stands out as an environmentally friendly cathode material in rechargeable aqueous Zn-ion batteries (ZIBs), enabling the realization of cost-effective and safe cells that support elevated charge/discharge rates and thus provide high power delivery2, 3. Relying only on abundant, inexpensive and non-toxic materials, combined with non-flammable and mildly acidic electrolytes, Zn/CuHCF cells are an attractive electrochemical system for addressing the challenges of developing energy storage devices for power grids due to their quick response and contained fabrication costs, although their energy density is intrinsically limited4 (< 100 Wh·kg-1).
CuHCF displays one of the highest operating potentials (≈1.7 V vs. Zn2+/Zn) among cathode materials in aqueous ZIBs2, 3 and a modest capacity of ≈60 mAhg-1. Yet, what sets CuHCF apart is its ability to undergo ion insertion/de-insertion with negligible structural variations or volume changes5, making these electrodes apt for extensive cycling under heavy-duty conditions without risking induced mechanical or electrical damages causing early cell failure.
However, a series of obstacles are present along the way of Zn/CuHCF cell development. Aside from issues on the Zinc side (e.g. uncontrolled Zn deposition/stripping), notoriously challenging in terms of interface stabilization6 and effective utilization4, significant limitations arise from the CuHCF ageing mechanisms during cycling. Such mechanisms have remained elusive so far, mostly due to the complexity of this compound and a simultaneous interaction of various species in the electrochemical processes. To date, there have been only speculative attempts to explain the capacity fading in CuHCF and, consequently, only a few scenarios have been suggested for it. Possible phase segregation and formation of spurious, non-stoichiometric Zn-rich phases (e.g. ZnxCu1-xHCF) have been argued7-9, while also signs of Cu dissolution were previously detected10, yet thought to be minimal and not affecting the CuHCF performances.
Here, we will shed light on the limitations of CuHCF during cycling in ZIBs and explore its ageing mechanism by conducting a thorough study employing X-ray techniques11. Our investigation reveals that CuHCF is less stable than previously assumed, and that its debated Cu dissolution is linked to the ageing process11, forming Cu vacancies that can accept Zn2+. This Cu dissolution also results in a gradual activation of the Fe3+/Fe2+ couple, which has unambiguously been assigned to the appearance of a plateau on charge at ≈1.8 V vs. Zn2+/Zn (Figure 1). This is believed as the main contributor to the subsequent capacity loss. These insights into the CuHCF ageing in ZIBs may help in devising strategies to mitigate this critical aspect and improve battery performances in terms of stability and cycle life.

Figure 1. (a) Scheme of an aqueous Zn/CuHCF cell. (b) Galvanostatic charge-discharge profiles of a Zn/CuHCF cell between 1.00 and 2.15 V at 0.6 mAcm-2. Note the emergence of a sloping plateau around 1.8 V and a slight capacity loss after 200 cycles.

Acknowledgements

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

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Figure 1