Ball Milling-Enabled Fe^2.4+ to Fe³⁺ Redox Reaction in Prussian Blue Materials for Long-Life Aqueous Sodium-Ion Batteries

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 Fe³⁺ redox reaction of Prussian blue electrodes cycled in a 17 m NaClO₄ water-in-salt electrolyte. The cubic phase c-Na_1.17Fe[Fe(CN)₆]·0.35H₂O) derived from monoclinic Prussian blue (m-Na_1.88Fe[Fe(CN)₆]·0.7H₂O) 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 X-ray 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.