Removal of Interstitial H₂O in Hexacyanometallates for a Superior Cathode of a Sodium-Ion Battery

TLDR

Sodium‑ion batteries are attractive for large‑scale energy storage because sodium is abundant, and low‑cost hexacyanometallates provide a rigid open framework that facilitates fast Na⁺ insertion and extraction. The study investigates how interstitial water influences the structure and electrochemical performance of sodium manganese(II) hexacyanoferrates(II). The authors characterize the Na₂₋δMnHFC phase, examining its crystal structure and electrochemical behavior. The dehydrated Na₂₋δMnHFC delivers a reversible capacity of 150 mAh g⁻¹ at 3.5 V in a half cell and 140 mAh g⁻¹ in a full cell, retains 75 % capacity after 500 cycles at 0.7 C, and maintains 120 mAh g⁻¹ at 20 C.

Abstract

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 (Na2−δMnHFC). The newly discovered dehydrated Na2−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.