NASICON-type Na₃Fe₂(PO₄)₃ as a low-cost and high-rate anode material for aqueous sodium-ion batteries

Aqueous sodium-ion batteries are attractive battery alternatives for stationary energy storage due to their inherently low cost and high safety. The development of advanced electrode materials with excellent performance and low cost is crucial for the success of aqueous Na-ion batteries. Albeit the high capacity and stable cycling of the leading anode of NaTi₂(PO₄)₃, the high price of titanium element and the low Na-insertion potential close to the hydrogen evolution reaction may plague its application. Here we introduced a NASICON-type Na₃Fe₂(PO₄)₃ as a promising anode alternative for aqueous Na-ion batteries. Synchrotron X-ray diffraction and spectroscopy confirm its phase and atomic structure. Electrochemical characterization and X-ray photoelectron spectroscopy reveal the reversible Na⁺ insertion in Na₃Fe₂(PO₄)₃ due to the Fe³⁺/Fe²⁺ redox couple, which renders a specific capacity of ∼60 mAh g⁻¹ and excellent cycling of 1000 cycles at 10C rate. A Super high rate of 100C was also achieved for this anode with a capacity retention of 61% after 1000 cycles. In addition, the Na₃Fe₂(PO₄)₃ anode is paired with a Prussian white analogue of Na₂Mn[Fe(CN)₆] into a full cell, exhibiting promising cell performance and material sustainability. Our work underlines the importance of fabricating aqueous batteries on the basis of the Earth-abundant, cost-effective, and non-toxic elements.