Suppressing Mn Dissolution and Phase Transition in Na 2 MnFe(CN) 6 via Fe‐Substitution for Ultralong‐Life Sodium‐Ion Batteries

Mn‐based Prussian blue analogs are attracting great attention as cathode materials for sodium‐ion batteries (SIBs) due to their remarkable features, such as high theoretical capacity, superior rate capability, and low cost. However, it faces the issues of unstable cycling performance due to the dissolution of Mn 2+ and a cubic‐to‐tetragonal phase transition. Herein, Fe‐substitution was employed to tailor the structure and electrochemical behavior of Na 2 MnFe(CN) 6 . Na 2 MnFe(CN) 6 with Fe‐substitution (MFHCF) adopts a Na‐rich rhombohedral phase to achieve a high capacity delivery, and Fe‐substitution averts the stress‐induced deformation by Mn 2+ dissolution and cubic‐tetragonal phase transition during cycling. As a result, the MFHCF electrode with 20% Fe‐substitution (MFHCF‐20) delivers a high discharge capacity (139.1 mAh g −1 ) and excellent cycle life over 1000 cycles. In situ XRD confirms that MFHCF‐20 effectively suppresses Mn 2+ dissolution and generates minimal structural stress during charge/discharge cycling. We believe that this work can serve as an indispensable reference to tailor the structure of Prussian blue analogs to obtain superior electrochemical performance.