Microstructural Changes of Prussian Blue Derivatives during Cycling in Zinc-Containing Electrolytes

The reversible electrochemical insertion of zinc into host materials has been shown to be very promising for large-scale energy storage applications. In particular, copper hexacyanoferrate (CuHCF) and its derivatives from the Prussian Blue family enable a fast and reversible (de-)insertion of zinc ions when operated in a zinc-based aqueous electrolyte. In this work, the effect of the concentration of a zinc sulfate-containing aqueous neutral solution on the aging of CuHCF and copper-zinc hexacyanoferrate mixture (CuZnHCF) was studied. The electrochemical and morphological analysis revealed the occurrence of a phase change during the materials aging. Such phase transformation induced morphological changes, different from the ones formed when the materials were cycled in lower electrolyte concentration. In the case of CuZnHCF, the occurrence of a spike-shaped morphology upon cycling, which appears to be electrochemically inactive, caused a faster capacity fading.