Instability of Zinc Hexacyanoferrate Electrode in an Aqueous Environment: Redox-Induced Phase Transition, Compound Dissolution, and Inhibition

The structural stability of electrode materials and their compatibility with electrolytes are the important properties for ion-in-tercalative electrochemical energy-storage devices. In the present work, we employed zinc hexacyanoferrates (ZnHCFs), which occurs as cubic or rhombic phases, as the probe to tailor the mechanism of capacity decay upon electrochemical cycling and the corresponding mitigating strategy. Capacity fading results from the loss of active materials, which is highly correlated to the phase states; this has been identified for both phases, where the cubic phase is demonstrated to be the dominant source of ZnHCF dissolution. In 1 M KNO3 electrolyte, rhombic ZnHCF behaves evidently more stable than the cubic phase for long-term galvanostatic charge/discharge cycling. Even when simply immersed in an aqueous environment, the rhombic-cubic phase transition can spontaneously occur, which, in particular, can be accelerated considerably by electrochemical redox processes in the potential window of 0.8-1.1 V. Utilizing the common-ion effect, specifically by incorporating Zn-II into aqueous electrolytes, could considerably enhance the capacity retention of ZnHCF. Our results suggest that, if electrode materials are soluble at certain electrochemical stages, introducing electrochemically inert common ions into the electrolyte should be an efficient approach to improve the electrode-electrolyte compatibility for pursuing enhanced cycling performances.