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

Sodium manganese hexacyanoferrate (NaxMnFe(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 NaxMnFe(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 Mn3+, resulting in poor cycling performances with 38% capacity retention. The cubic NaxMnFe(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 approximate to 120 mAh g(-1) at 3.5 V (vs Na/Na+), capacity retention of approximate to 70% over 500 cycles at 200 mA g(-1). In addition, the TiO2//C-MnHCF full battery is fabricated with an energy density of 111 Wh kg(-1), suggesting the great potential of cubic NaxMnFe(CN)(6) for practical energy storage applications.