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Aqueous Na-ion batteries have the competitive advantages of being low costing andp ossessingi nherent safety,a nd they are particularly suitable for large-scale energy-storage applications. However,t he development of this new battery system is hindered by the lack of suitable Na hosts with sufficiently high capacity and cycle life. Herein, we report vacancy-free Na 2 CoFe(CN) 6 nanocubes synthesized by ac ontrolled crystallization reactiona nd reveal their use as an ew aqueous cathode with reversible Na-storage behavior.O wing to its perfect lattice framework with two redox-active sites, this material exhibits ah igh reversible capacity of 130 mA hg À1 ,astrong rate capability at 20 C, and superior cyclability with 90 %c apacity retention over 800 cycles,a ll of which indicatet hat this materialm ay possibly serve as ah igh-performance and long-life cathode for aqueous Na-ion batteries. Also, the synthetic strategy described in thisw ork may be extended to aw ide range of Prussian blue compounds for the fabrication of well-shaped nanocubes with few defects for energy storage, catalysis, and other applications.[a] X.

Aqueous rechargeable sodium-ion batteries have the potential to meet growing demand for grid-scale electric energy storage because of the widespread availability and low cost of sodium resources. In this study, we synthesized a Na-rich copper hexacyanoferrate(II) Na2 CuFe(CN)6 as a high potential cathode and used NaTi2 (PO4 )3 as a Na-deficient anode to assemble an aqueous sodium ion battery. This battery works very well with a high average discharge voltage of 1.4 V, a specific energy of 48 Wh kg(-1) , and an excellent high-rate cycle stability with approximately 90 % capacity retention over 1000 cycles, achieving a new record in the electrochemical performance of aqueous Na-ion batteries. Moreover, all the anode, cathode, and electrolyte materials are low cost and naturally abundant and are affordable for widespread applications.

A Na-host cathode is developed by grafting the polypyrrole chains with ionizable sodium sulfonate. Due to the immobile p-doping of organic anions, the self-doped polymer can act as a Na-host for reversible Na insertion-extraction reaction, thus offering a low cost and renewable organic cathode for Na ion battery applications.

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