Removal of interstitial H₂O in hexacyanometallates for a superior cathode of a sodium-ion battery

Sodium is globally available, which makes a sodium-ion rechargeable battery preferable to a lithium-ion battery for large-scale storage of electrical energy, provided a host cathode for Na can be found that provides the necessary capacity, voltage, and cycle life at the prescribed charge/discharge rate. Low-cost hexacyanometallates are promising cathodes because of their ease of synthesis and rigid open framework that enables fast Na⁺ insertion and extraction. Here we report an intriguing effect of interstitial H₂O on the structure and electrochemical properties of sodium manganese(II) hexacyanoferrates(II) with the nominal composition Na₂MnFe(CN)₆·zH₂O (Na_2-δMnHFC). The newly discovered dehydrated Na_2-δMnHFC phase exhibits superior electrochemical performance compared to other reported Na-ion cathode materials; it delivers at 3.5 V a reversible capacity of 150 mAh g^-1 in a sodium half cell and 140 mAh g^-1 in a full cell with a hard-carbon anode. At a charge/discharge rate of 20 C, the half-cell capacity is 120 mAh g^-1, and at 0.7 C, the cell exhibits 75% capacity retention after 500 cycles.