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

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Mn-based hexacyanoferrate NaxMnFe(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 Mn2+/Mn3+ 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 Mn2+ and the trigonal-cubic phase transition during the charge–discharge process. Here, a novel hexacyanoferrate (Na1.60Mn0.833Fe0.167[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 Mn2+ 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, Na1.60Mn0.833Fe0.167[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.