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·<i>z</i>H₂O (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