High‐Performance Manganese Hexacyanoferrate with Cubic Structure as Superior Cathode Material for Sodium‐Ion Batteries

Abstract Sodium manganese hexacyanoferrate (Na x MnFe(CN) 6 ) is one of the most promising cathode materials for sodium‐ion batteries (SIBs) due to the high voltage and low cost. However, its cycling performance is limited by the multiple phase transitions during Na + insertion/extraction. In this work, a facile strategy is developed to synthesize cubic and monoclinic structured Na x MnFe(CN) 6 , and their structure evolutions are investigated through in situ X‐ray diffraction (XRD), ex situ Raman, and X‐ray photoelectron spectroscopy (XPS) characterizations. It is revealed that the monoclinic phase undergoes undesirable multiple two‐phase reactions (monoclinic ↔ cubic ↔ tetragonal) due to the large lattice distortions caused by the Jahn–Teller effects of Mn 3+ , resulting in poor cycling performances with 38% capacity retention. The cubic Na x MnFe(CN) 6 with high structural symmetry maintains the structural stability during the repeated Na + insertion/extraction process, demonstrating impressive electrochemical performances with specific capacity of ≈120 mAh g −1 at 3.5 V (vs Na/Na + ), capacity retention of ≈70% over 500 cycles at 200 mA g −1 . In addition, the TiO 2 //C‐MnHCF full battery is fabricated with an energy density of 111 Wh kg −1 , suggesting the great potential of cubic Na x MnFe(CN) 6 for practical energy storage applications.