Sodium ion batteries (SIBs) are considered one of the most promising alternatives for large-scale energy storage due largely to the abundance and low cost of sodium. However, the lack of high-performance cathode materials at low cost represents a major obstacle toward broad commercialization of SIB technology. In this work, we report a green route strategy that allows cost-effective fabrication of carbon-coated NaFePOF cathode for SIBs. By using vitamin C as a green organic carbon source and environmentally friendly water-based polyacrylic latex as the binder, we have demonstrated that the NaFePOF phase in the as-derived NaFePOF/C electrode shows a high reversible capacity of 117 mAh g at a cycling rate of 0.1 C. More attractively, excellent rate capability is achieved while retaining outstanding cycling stability (∼85% capacity retention after 1000 charge-discharge cycles at a rate of 4 C). Further, in operando X-ray diffraction has been used to probe the evolution of phase structures during the charge-discharge process, confirming the structural robustness of the NaFePOF/C cathode (even when charged to 4.5 V). Accordingly, the poor initial Coulombic efficiency of some anode materials may be compensated by extracting more sodium ions from NaFePOF/C cathode at higher potentials (up to 4.5 V).

To achieve high energy/power output, long serving life, and low cost of carbon-based electrodes for energy storage, we have developed a unique synthesis method for the fabrication of hierarchically porous carbon of high graphitization (HPCHG), derived from pyrolysis of an iron-containing organometallic precursor in a molten ZnCl media at relatively low temperatures. The as-prepared HPCHG has a large specific surface area (>1200 m g), abundant micro/mesopores, and plenty of surface defects. When tested in a supercapacitor (SC), the HPCHG electrode delivers 248 F g at 0.5 A g and a high capacitance retention of 52.4% (130 F g) at 50 A g. When tested in a sodium-ion battery (SIB), the HPCHG electrode exhibits a reversible capacity of 322 mA h g at 100 mA g while maintaining ∼75% of the initial stable capacity after 2000 cycles with the applied current density as high as 5000 mA g, implying that the HPCHG electrode is very promising for energy storage.