Competition for Ion Intercalation in Prussian Blue Analogues as Cathode Materials for Calcium Ion Batteries

With multivalent ion batteries being considered viable post‐lithium energy storage technologies, the last few years have seen increasing interest in intercalation type cathode materials for Ca‐ion batteries (CIBs). Prussian blue analogues (PBAs) display many positive attributes such as open structural framework, large interstitial sites, and versatile transition metal redox activity. The PBA cathodes reported for CIBs often contain K‐ion or Na‐ion due to the syntheses involving the use of potassium or sodium hexacyanoferrate. It has been long overlooked whether the presence of K‐ion or Na‐ion in pristine PBAs could affect the Ca‐ion storage in the PBAs when assessing their CIB performance. Through a combined experimental and computational investigation, this work reports that when K+/Na+ is present in the pristine PBAs, K/Na intercalation is preferred by the PBA structure over Ca intercalation even in the Ca cells. Ca‐ion intercalation can be increased when the K+/Na+ content in the pristine PBAs is reduced, shifting from a K or Na‐dominated intercalation process to a Ca/K or Ca/Na co‐intercalation process. This work consolidates that PBAs are an interesting and versatile intercalation host for several ions, but investigating PBAs as intercalation cathodes requires careful consideration of all ions present in the battery system. Competition for ion intercalation in Prussian blue analogue (PBA) cathodes is investigated for the application of Ca‐ion batteries. K and Na intercalation are strongly favored over Ca intercalation. As a result, using K‐ or Na‐containing PBA cathodes in the Ca electrolyte results in minimal Ca intercalation, and reducing K or Na amount in Ca cells can increase Ca intercalation.