Stress Distortion Restraint to Boost the Sodium Ion Storage Performance of a Novel Binary Hexacyanoferrate

Mn‐based hexacyanoferrate Na x MnFe(CN) 6 (NMHFC) has been attracting more attention as a promising cathode material for sodium ion storage owing to its low cost, environmental friendliness, and its high voltage plateau of 3.6 V, which comes from the Mn 2+ /Mn 3+ redox couple. In particular, the Na‐rich NMHFC ( x > 1.40) with trigonal phase is considered an attractive candidate due to its large capacity of ≈130 mAh g −1 , delivering high energy density. Its unstable cycle life, however, is holding back its practical application due to the dissolution of Mn 2+ and the trigonal‐cubic phase transition during the charge–discharge process. Here, a novel hexacyanoferrate (Na 1.60 Mn 0.833 Fe 0.167 [Fe(CN) 6 ], NMFHFC‐1) with Na‐rich cubic structure and dual‐metal active redox couples is developed for the first time. Through multiple structural modulation, the stress distortion is minimized by restraining Mn 2+ dissolution and the trigonal‐cubic phase transition, which are common issues in manganese‐based hexacyanoferrate. Moreover, NMFHFC‐1 simultaneously retains an abundance of Na ions in the framework. As a result, Na 1.60 Mn 0.833 Fe 0.167 [Fe(CN) 6 ] electrode delivers high energy density (436 Wh kg −1 ) and excellent cycle life (80.2% capacity retention over 300 cycles), paving the way for the development of novel commercial cathode materials for sodium ion storage.