In order to increase the penetration of renewable energy sources in the power grid, it is necessary to develop economically feasible energy storage systems. Although lithium-ion batteries have the potential to fulfill the requirements for grid power storage in terms of power density and cycle life, their capital cost remains high. In this frame, zinc-ion batteries (ZIB) have the potential to reach the wished performances at a lower cost with respect to lithium-ion batteries. ZIBs are based on metallic zinc negative electrode and a zinc-intercalating positive electrode and use neutral or acid zinc electrolytes. The initially reported performances of ZIBs based on Prussian Blue Analogues could reach a specific energy equal to 46 Wh kg^-1 at 1C, a specific power equal to 480 W kg^-1 at 10C, with a charge efficiency of more than 98%, and 100 cycles of life by using copper hexacyanoferrate (CuHCF). With the aim of reaching a longer cycle life, ideally 10000 cycles, the aging mechanism of CuHCF was investigated in details by combining the information from electrochemical measurements, post mortem XRD, SEM and EDX.

We have observed that the aging mechanism of CuHCF is not related to the dissolution of the active material, but rather to a phase transformation of the host structure, probably caused by the intercalation-induced stress. After 1000 cycles, the XRD pattern shows the appearance of new peaks , which suggest the formation of a new phase with different composition than the original one. The new peaks could not be assigned to other known hexacyanoferrate phases containing zinc or copper. SEM images showed that after aging the initial nanoparticles of the active material with diameter lower than 100 nm were transformed in large cubes with 3-10 μm length. The formation of the cubes is also accompanied by the formation of other irregular structures and wires, which were not present in the initial active material. EDX performed in different part of the electrode showed that the newly formed cubes contain a large amount of zinc and iron, but are almost completely empty from copper. So, this suggests that the aging mechanism is correlated to the formation of distorted zinc hexacyanoferrate, while copper is leaking from the electrode during the cycling. ICP-MS of the electrolyte solution shows the presence of a small amount of copper in solution, which does not justify the loss in performances. Based on these results, the aging mechanism of copper hexacyanoferrate during zinc ion intercalation will be discussed in details.