No abstract

Aqueous Na-ion batteries using Prussian blue materials have inherent advantages in safety, material sustainability, and economic cost. However, it is challenging to obtain long-term cycling stability because many redox reactions have poor intrinsic stability in water. Here, we demonstrate reversible Fe 2.4+ to Fe 3+ redox reaction of Prussian blue electrodes cycled in a 17 m NaClO 4 water-in-salt electrolyte. The cubic phase c-Na 1.17 Fe[Fe(CN) 6 ]•0.35H 2 O) derived from monoclinic Prussian blue (m-Na 1.88 Fe[Fe(CN) 6 ]•0.7H 2 O) through ball milling delivers excellent cycling stability of >18,000 cycles with >90% capacity retention at the 10C rate. The specific capacity is ∼75 and ∼67 mAh/g at 1C and 10C rates, respectively. Systematic characterizations including electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Xray absorption spectroscopy have verified the phase transition and iron oxidation state evolution, revealing the mechanism that enables the material's high rate and long durability as the battery cathode.