The Role of Carrier Ion-Ligand Interaction on Intercalation Potentials and Phase Evolution of Berlin Green Cathodes in Rechargeable Batteries

Intercalation and deintercalation are fundamental processes in battery electrodes that involve the reversible addition and extraction of carrier ions such as lithium (Li) and sodium (Na) into a host framework made of transition metal (TM) ions and ligands. Although TM–ligand interactions are known to primarily determine intercalation potentials, a comprehensive understanding of their interactions involving the carrier ions still remains elusive. This study investigates the complex interactions between carrier ions (Li and Na) and the ligand in Berlin green (BG, Fe³⁺[Fe³⁺(CN)₆]) cathodes to elucidate their effects on intercalation potentials and phase evolution. Comprehensive X-ray characterizations and electrochemical analyses reveal that the strong carrier ion–ligand interaction, influenced by the type and concentration of carrier ions, can modify the electronic structure of Fe ions and affect the intercalated structure of BG cathodes, thereby tuning the intercalation potentials. Specifically, the enhanced covalent interaction of the Na–CN units compared with that of the Li–CN units induces rhombohedral distortion in the fully sodiated state and increases the working potential of high-spin Fe ions. The findings highlight the critical role of carrier ion–ligand interactions in tuning the electrochemical properties of battery electrodes for the design of advanced battery systems.