Gradient doping-induced triphasic intergrowth hexacyanoferrate cathode for high-performance sodium-ion batteries

As a high-capacity, high-voltage, and low-cost cathode material for sodium-ion batteries (SIBs), Na/Mn-based hexacyanoferrates (NaxMn[Fe(CN)(6)](1-y)center dot square(y)center dot nH(2)O) typically exhibit poor structural reversibility and interfacial stability during cycling. In this study, we report the in-situ construction of a triphasic intergrowth Na/Mn/Zn-based hexacyanoferrate (mcr-Zn0.2Mn0.8) with concentration-gradient Zn distribution by an anion-controlled fractional precipitation route. Density functional theory calculations verify that the unique heterostructure arises from the different binding energies between cations and anions. A small amount of Zn2+ doping in the inner cubic/monoclinic phases enhances their structural reversibility during cycling by impeding lattice deformation induced by the Jahn-Teller effect of Mn3+. Meanwhile, the rhombohedral Zn-based hexacyanoferrate epitaxially grown on the surface diminishes the particle size of mcr-Zn0.2Mn0.8 resulting in short Na+ diffusion distance, and provides interface protection that suppresses detrimental electrolyte decomposition. Consequently, mcrZn(0.2)Mn(0.8) exhibits a high discharge capacity of 121.1 mAh/g at 15 mA g(-1), improved cycling stability at 750 mA g(-1) with a capacity retention of 65.0 % after 2000 cycles, and enhanced rate capability with a discharge capacity of 71.4 mAh/g at 6000 mA g(-1). The successful implementation of simultaneous surface and bulk modifications through a one-step reaction opens new possibilities for developing high-performance hexacyanoferrate cathode materials for SIBs.