High-stability monoclinic nickel hexacyanoferrate cathode materials for ultrafast aqueous sodium ion battery

Prussian blue and its analogues are regarded as superior cathode materials for sodium ion batteries (SIBs) owing to low cost, open framework and large interstitial spaces for the insertion/extraction of sodium ion. Herein, a highly stable monoclinic sodium rich nickel hexacyanoferrate (II) nanocube (m-NiHCF) has been synthesized via a facile coprecipitation method with the aid of chelating agent and surfactant. It delivers a high specific capacity of 70.1 mAh g−1 and maintains 97.1% capacity retention after 8000 cycles. Even at a high current density of 2000 mA g−1, an impressive capacity of 53.2 mAh g−1 is obtained. The fast kinetics of m-NiHCF is mainly benefited from the capacitive-controlled domination under the charge storage process. Meanwhile, ex-situ X-ray diffraction together with ex-situ X-ray photoelectron spectroscope and ex-situ Raman and ex-situ Fourier transform infrared analysis have revealed the reversible phase transition between monoclinic and cubic phases with the reaction of carbon coordinated FeII/FeIII redox-active site during extraction and insertion of sodium ion in m-NiHCF framework. Furthermore, a high voltage aqueous SIB full cell assembled with NaTi2(PO4)3@C anode achieves a high energy density of 86 Wh kg−1 with capacity retention of 83% after 600 cycles, showing great prospects in the grid-scale energy storage application.