Sodium iron hexacyanoferrate with high Na content as a Na-rich cathode material for Na-ion batteries

Owing to the worldwide abundance and low-cost of Na, room-temperature Na-ion batteries are emerging as attractive energy storage systems for large-scale grids. Increasing the Na content in cathode materials is one of the effective ways to achieve high energy density. Prussian blue and its analogues (PBAs) are promising Na-rich cathode materials since they can theoretically store two Na+ ions per formula unit. However, increasing the Na content in PBAs cathode materials remains a major challenge. Here we show that sodium iron hexacyanoferrate with high Na content can be obtained by simply controlling the reducing agent and reaction atmosphere during synthesis. The Na content can reach as high as 1.63 per formula, which is the highest value for sodium iron hexacyanoferrate. This Na-rich sodium iron hexacyanoferrate demonstrates a high specific capacity of 150 mAh·g^-1 and remarkable cycling performance with 90% capacity retention after 200 cycles. Furthermore, the Na intercalation/de-intercalation mechanism has been systematically studied by in situ Raman spectroscopy, X-ray diffraction and X-ray absorption spectroscopy analysis for the first time. The Na-rich sodium iron hexacyanoferrate can function as a plenteous Na reservoir and has great potential as a cathode material for practical Na-ion batteries.