A systematic study of Prussian blue analogs as cathode materials for calcium ion batteries and interstitial water...

Prussian blue analogs (PBAs), known for their open three-dimensional framework and adjustable redox activity, have emerged as promising cathode materials for multivalent ion batteries. This study provides theoretical insights into Ca_xMFe(CN)₆ (M = Mn, Fe, and Co) as cathode materials of calcium ion batteries (CIBs), elucidating the regulatory mechanism of the interstitial water through first-principles calculations. Ca_xMnFe(CN)₆ and Ca_xFeFe(CN)₆ undergo significant structural distortion and substantial volume changes during the charging process, which can be effectively alleviated by the insertion of the interstitial water, whereas Ca_xCoFe(CN)₆ maintains a relatively stable structure. Furthermore, the low average voltage of Ca_xMFe(CN)₆ can be increased by incorporating the interstitial water. Surprisingly, the diffusion barriers of Ca²⁺ are extremely low (0.24-0.31 eV) in PBAs, which directly accounts for their superior kinetic performance. It is revealed that there is a sequential redox mechanism with M²⁺/M³⁺ first, and then low-spin Fe²⁺/Fe³⁺ during the decalcification in Ca_xMFe(CN)₆. This study not only explores the electrochemical performance of PBAs for CIBs, but also establishes a theoretical foundation for designing high-performance CIBs through the interstitial water.