Zinc ferrite nanorods coated with polydopamine-derived carbon for high-rate lithium ion batteries

Zinc ferrite (ZnFe₂O₄) nanorods are synthesized using a facile and scalable method, i.e., decomposition of oxalate precursors which are obtained through a polyvinyl alcohol-assisted aqueous precipitation reaction. The ZnFe₂O₄ nanorods are further coated with carbon by self-polymerization of dopamine on the surfaces of the ZnFe₂O₄ nanorods followed by carbonization. The carbon layer on the ZnFe₂O₄ nanorods is homogeneous with the thickness of 3 to 5 nm. The polydopamine-derived carbon coating greatly improves the electrochemical performances of the ZnFe₂O₄ nanorods, especially rate capability and cycling stability. When galvanostatic diacharging/charging at 0.5 and 2 A/g, the ZnFe₂O₄/carbon nanorod anode can deliver reversible capacities of 805 and 504 mAh/g, respectively, whereas the ZnFe₂O₄ nanorod anode only delivers the capacities of 616 and 154 mAh/g, respectively. Besides, the ZnFe₂O₄/carbon nanorod anode shows excellent cycling stability; no obvious capacity fading is observed at the current density of 1 A/g for 100 cycles. The improved electrochemical performances could be attributed to the enhanced electron conductivity of the ZnFe₂O₄ nanorods with the carbon layer, which is confirmed by electrochemical impedance spectroscopy measurements and scanning electron microscopic studies of the cycled electrodes.