Facile Synthesis of Necessary Amorphous Structure FePO4 Nanospheres as Superior Sodium-Ion Battery Cathodes

Uniform amorphous FePO4 nanospheres are successfully synthesized by a simple hydrothermal process. The phase and chemical composition of the sample are, respectively, obtained from the X-ray diffraction (XRD) data of the original sample and the XRD data of the substance after annealing. The as-prepared sample is composed of uniform nanospheres with a diameter of 220 nm. The necessity of an amorphous structure for the cathode application of this material is analyzed through the crystal-phase structure diagram. The transformation between the amorphous structure and the microcrystalline structure can be realized in the battery during the charging and discharging process. Electrochemical results show that amorphous iron phosphate shows good sodium-ion storage performances. The amorphous structure of iron phosphate is very necessary in the sodium-ion battery system with sodium iron phosphate as the positive electrode.