Solvent-free mechanochemical synthesis of Na-rich Prussian white cathodes for high-performance Na-ion batteries

Prussian blue analogs (PBAs) with rigid open framework are promising low-cost and easily prepared cathodes for Na-ion batteries. However, their electrochemical performances are hindered from the crystal vacancies and interstitial water in a traditional aqueous co-precipitation process. Herein, we explore a solvent-free mechanochemical protocol to prepare a monoclinic Na1.94Mn[Fe0.99(CN)6]0.95.0.05.1.92H2O via regulating the crystal water in precursors. The interstitial water of NaMHCF can be further reduced through increasing the drying temperature. Ex-situ XRD confirms that the monoclinic phase transformed to the rhombohedral structure during the first cycle, and a highly reversible multi-phase evolution among rhombohedral, cubic, and Na-poor phase upon Na+ (de)intercalations occurred from the second cycle on. Finally, it delivers a specific capacity of 168.8 mA h g-1 with a stable average voltage of 3.44 V at 10 mA g-1, showing ultra-high rate capability (127 mA h g-1 at 2000 mA g-1) and cycling stability (87.6% capacity retention after 100 cycles at 100 mA g-1) for half cells. For the full cell of NaMHCF/NaTi2(PO4)3, it can deliver an ultra-stable cycle performance, retaining 84% capacity after 500 cycles at 100 mA g-1. Our work provides a facile avenue to prepare monoclinic NaMHCF with low water and vacancies for high-performance Na-ion batteries.