Monoclinic Phase Na₃Fe₂(PO₄)₃: Synthesis, Structure, and Electrochemical Performance as Cathode Material in Sodium-Ion Batteries

Sodium iron phosphate (Na 3 Fe 2 (PO 4 ) 3 ) as cathode material for sodium-ion batteries has been synthesized through a simple method of a solid state reaction. It crystallizes in a monoclinic structure in the space group C 2/ c . The morphology of the as-prepared sample has been investigated by scanning electron microscopy and transmission electron microscopy. The charge/discharge curves show a very flat plateau at about 2.5 V (vs Na/Na + ). The initial specific discharge capacity is 61 mAh g –1 and remains at 57 mAh g –1 after 500 cycles at a current rate of 1 C. X-ray photoelectron spectroscopy measurements indicate that not all of the Fe 3+ of Na 3 Fe 2 (PO 4 ) 3 is reduced during the electrochemical process. The ex-situ X-ray diffraction measurements were applied to research the mechanism of sodium-ion storage; the results indicated that the Na 3 Fe 2 (PO 4 ) 3 compound partly transformed into Na 4 Fe 2 (PO 4 ) 3 and Na 3+ x Fe 2 (PO 4 ) 3 compound. These results testify to the potential of monoclinic Na 3 Fe 2 (PO 4 ) 3 as a cathode material in sodium-ion batteries.