Abstract
Aqueous Na-ion batteries using Prussian blue materials have intrinsic advantages on 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 enable Fe+2.5 to Fe+3 redox reaction in Prussian blue materials by converting the monoclinic sodium iron hexacyanoferrate (m-Na1.81Fe[Fe(CN)6]·0.7H2O) to a cubic phase (c- Na1.17Fe[Fe(CN)6]·0.7H2O) and demonstrated high performance by cycling in a water-in-salt electrolyte. Systematic characterization including electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy have verified the phase transition and iron oxidization state evolution. The cubic phase Na1.17Fe[Fe(CN)6]·0.7H2O delivers excellent cycling stability of >18,000 cycles with >90% capacity retention. The specific capacity is ~75 mAh/g and ~67 mAh/g at 1C and 10C rate, respectively.
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