Aqueous metal-ion batteries are attractive battery alternatives for stationary energy storage due to their inherently low cost and high safety. The development of advanced electrode materials with excellent performance and low cost is crucial for the success of aqueous batteries. We recently have studied Prussian-blue analogous based electrodes as promising cathodes for aqueous sodium-ion and magnesium-ion batteries. We found that such electrodes not only exhibited fast ionic diffusion that enables the fast-charging rate (> 100C), super long cycle life (> 17,000 cycles), high-capacity retention and rate capability, but also could hosted monovalent and multivalent cations. In particular, ball-milling of Prussian-blue analogous electrodes can further improve the efficiency of the cation intercalation, resulting in much improved capacity and better utilization of the wider stability window of the water-in-salt electrolyte. In combination of electrochemical characterization and multimodal synchrotron X-ray scattering and spectroscopy methods, we attributed the superior performance to the cation deficiency or vacancy and new phase induced through the ball-milling processes. Our work also underlines the importance of fabricating aqueous batteries on the basis of the Earth-abundant, cost-effective, and non-toxic elements.
