Highly Porous Electrospun ZnCo₂O₄ Nanofibers for Lithium Ion Battery Electrodes

Abstract

Lithium ion batteries have been widely used in a variety of fields such as electric vehicles, portable electronic devides, and large scale energy storage systems. In order to fullfill the stringent industrial requirements in these fields, novel electrode materials should be developed providing high electrochemical performances (high energy density and high power density). Transition metal oxides (TMO) are possible candidate materials to satisfy requirements of next generation anodes due to their high theoretical capacities. Among TMO materials, spinel Co 3 O 4 which is a typical Li-conversion reaction material, represents the high specific capacity and reversibility. In addition, binary TMO such as NiCo 2 O 4 , CoMn 2 O 4 , MnCo 2 O 4 , and ZnCo 2 O 4 have been suggested as novel high-capacity anode materials. Especially, ZnCo 2 O 4 has the high theoretical capcity of 975 mAh/g based on both Li-alloying and –conversion reactions. The nanostructuring of electrode materials is a key enabling factor to improve their electrochemical properties. An electrospinning process is the simple and versatile way to fabricate one dimensional fibrous nanostrcutured electrodes. In this study, we fabricated highly porous ZnCo 2 O 4 nanofibers (P-ZnCo 2 O 4 NFs) using simple electrospinning of ZnCo 2 O 4 /ZnO nanofibers by controlling the ratio of Zn to Co and selevtive etching of ZnO. Firstly, ZnCo 2 O 4 /ZnO nanofibers were fabricated by electrospinning and subsequent two-step heating processes. Zn and Co nitrate precursors with different Zn/Co ratios were completely dissolved in dimethyl formamide (DMF) containing Polyacrylronitrile. The mixed solution was loaded into a plastic syringe and metal-containing polymeric fibers were produced by appling a voltage of 15 kV to the nozzle. Then, the as-spun ZnCo 2 O 4 /ZnO nanofibers were stabilized in air to 280 ° C for 6 h and heated in Ar at 600 ° C for 2 h. For selective etching the excess ZnO, the ZnCo 2 O 4 /ZnO nanofibers were soaked in KOH solution. Finally, highly porous ZnCo 2 O 4 nanofibers were obtained. The fabricated P-ZnCo 2 O 4 NFs with the diameter of ~120 nm provided large surface area and porosity by the selective etching of ZnO phase. Furthermore, P-ZnCo 2 O 4 NFs exhibited enhanced electrochemical properties due to porous nature offering large contact area with electrolyte, efficient electron pathway, and short Li ion diffusion length.