Electrodeposited Na2Ni[Fe(CN)6] Thin-Film Cathodes Exposed to Simulated Aqueous Na-Ion Battery Conditions

Na-ion batteries have recently attracted great attention regarding their application in large-scale energy storage systems. Among different types of electrode materials for those classes of batteries, so-called Prussian blue analogues (PBAs) are among the very attractive ones due to their comparatively simple and low-cost methods of synthesis coupled with a promising cycle performance. In this study, one of the state-of-the-art PBA battery materials, namely electrodeposited Na2Ni[Fe(CN)6] (NiHCF) thin films, were tested under simulated battery conditions in aqueous and mixed (H2O/organic) electrolytes. Prolonged stability tests in aqueous electrolytes were performed together with in-operando electrochemical AFM monitoring. It is demonstrated that degradation of this material is not associated with noticeable morphological changes (mechanical stress) but is likely caused by changes in the chemical composition of the films. Intercalation and deintercalation reversibility of Na+ and thin film stability in aqueous electrolytes appear to be unaffected negatively by changes in the pH to values below 7. However, the films showed unstable behavior in basic media (pH > 10). The increase of the content of acetonitrile, which was used as an additive to simulate the influence of antifreezes in aqueous electrolytes, appears to primarily affect the deintercalation of Na ions in Na2SO4-based aqueous electrolytes.