Fabrication of carbon nanofiber-driven electrodes from electrospun polyacrylonitrile/polypyrrole bicomponents for high-performance rechargeable lithium-ion batteries

doi:10.1016/j.jpowsour.2009.10.021

Journal of Power Sources

Volume 195, Issue 7, 2 April 2010, Pages 2050-2056

https://doi.org/10.1016/j.jpowsour.2009.10.021 Get rights and content

Abstract

Carbon nanofibers were prepared through electrospinning a blend solution of polyacrylonitrile and polypyrrole, followed by carbonization at 700 °C. Structural features of electrospun polyacrylonitrile/polypyrrole bicomponent nanofibers and their corresponding carbon nanofibers were characterized using scanning electron microscopy, differential scanning calorimeter, thermo-gravimetric analysis, wide-angle X-ray diffraction, and Raman spectroscopy. It was found that intermolecular interactions are formed between two different polymers, which influence the thermal properties of electrospun bicomponent nanofibers. In addition, with the increase of polypyrrole concentration, the resultant carbon nanofibers exhibit increasing disordered structure. These carbon nanofibers were used as anodes for rechargeable lithium-ion batteries without adding any polymer binder or conductive material and they display high reversible capacity, improved cycle performance, relatively good rate capability, and clear fibrous morphology even after 50 charge/discharge cycles. The improved electrochemical performance of these carbon nanofibers can be attributed to their unusual surface properties and unique structural features, which amplify both surface area and extensive intermingling between electrode and electrolyte phases over small length scales, thereby leading to fast kinetics and short pathways for both Li ions and electrons.

Introduction

Among various energy storage and conversation systems, high-performance rechargeable lithium-ion batteries (LIBs), with their high-energy density, long cycle life, and flexible design, are considered as effective solution to the increasing need for high-energy density electrochemical power sources [1], [2], [3], [4], [5], [6], [7]. Currently, LIBs are commercially available mainly as portable power sources for consumer electronic devices; however, there is an ever-increasing demand for higher capacity and higher power, especially for emerging large-scale applications including electric and hybrid vehicles, advanced wireless communication devices and storage systems for future power grids. This demand has promoted widespread research efforts toward developing high-capacity alternative electrode materials with long cycle life, improved safety, reduced environmental impact, and low cost [1], [2], [3], [4], [5], [6], [7], [8], [9].

Various types of carbon materials with different micro/macrostructures have been investigated in order to improve the electrochemical performance of LIBs [10], [11]. In particular, the development of continuous carbon nanofibers (CNFs) through a combination of polymer electrospinning and subsequent thermal treatments has demonstrated advantages in terms of easy process, low cost, and environmentally benignity [12], [13], [14]. These fabricated CNFs present unique structures, such as extremely long fiber length, high surface area, and complex porous structure, creating large amount of active sites and reduced charge transport pathways. As a result, they are charming anode materials for high-performance rechargeable LIBs [12], [13], [14].

Polypyrrole (PPy) is one important conductive polymer and can be used as a precursor to produce carbon materials [15], [16], [17]. Improved electrochemical performance could be obtained if PPy-based carbon can be transformed to continuous nanofibers. However, it is difficult to obtain pure PPy-based CNFs via electrospinning because of the poor solubility and high conductivity of this polymer. Therefore, a polymer carrier must be used during electrospinning to produce conductive PPy precursor nanofibers [18]. Polyacrylonitrile (PAN) is easy to spin and can be used as a carrier to disperse conductive polymers to obtain electrospun multi-phase nanofibers [19]. In addition, PAN can also be directly heat-treated to form carbon [12], [13], [14], [20], and hence electrospinning PAN/PPy biocomponent nanofibers will be a promising approach to produce high-performance CNF anodes for LIBs.

We, therefore, present here one relatively novel route to prepare CNFs that rely on the electrospinning of PAN/PPy bicomponent solutions. After carbonization, the resultant CNFs have high surface area and fast lithium charge/discharge kinetics, and they can be directly used as anodes for LIBs without adding any polymer binder or conducting additive. In this work, we incorporated these electrodes into laboratory-scale cells, which showed improved overall electrochemical performance when compared with graphite, which is presently being used in commercial LIBs. It is envisaged that this class of materials are promising electrode candidates for LIBs to meet the stringent demands of a society constantly seeking more advanced device lifetimes and energy densities.

Section snippets

Experimental

PAN, PPy and solvent N,N-dimethylformamide (DMF) were purchased from Aldrich. All these reagents were used without further purification. DMF solutions of PAN (8 wt%) containing different amounts of PPy (15, 30, and 50 wt%) were prepared at 60 °C with mechanical stirring for at least 72 h.

A variable high voltage power supply (Gamma ES40P-20W/DAM) was used to provide a high voltage (around 14 kV) for electrospinning with 0.5 ml h⁻¹ flow rate and 15 cm needle-to-collector distance. The electrospun PAN/PPy

Results and discussion

Fig. 1 shows SEM images of electrospun PAN/PPy bicomponent nanofibers with different PPy contents (15, 30, and 50 wt%). It is seen that all nanofibers have regular and straight fibrous morphology. With the increase of PPy content, irregularities, such as so-called ‘beads on a string’ morphology, begin to appear [21], [22]. This may be a result of the increased conductivity and viscosity of the electrospinning solutions at higher PPy contents [22], [23].

Thermal properties of electrospun PAN/PPy

Conclusion

PAN/PPy bicomponent nanofibers containing different amounts of PPy were prepared through electrospinning. SEM results show that with the increase of PPy content, the surface morphology of the fibers becomes more irregular, most likely due to the increasing solution conductivity and viscosity induced by the PPy phase. The DSC and TGA results indicate that there are some interactions between PAN and PPy phases, which influence the complex chemical reactions of PAN phase upon thermal treatment

Acknowledgements

This work was supported by the US National Science Foundation (Nos. 0555959 and 0833837), the ERC Program of the National Science Foundation under Award Number EEC-08212121, and ACS Petroleum Research Fund 47863-G10. The authors would like to thank Dr. Dale Balchelor, Mr. Chuck Mooney and Mr. Roberto Garcia in Analytical Instrumentation Facility at North Carolina State University and Dr. Mark D. Walters in the Shared Materials Instrumentation Facility at Duke University for their help in sample

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Fabrication of carbon nanofiber-driven electrodes from electrospun polyacrylonitrile/polypyrrole bicomponents for high-performance rechargeable lithium-ion batteries

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

Carbon

Electrochem. Commun.

Polymer

Polymer

Polymer

Polymer

Polymer

Eur. Polym. J.

Electrochim. Acta

Carbon

Carbon

Electrochem. Commun.

J. Power sources

Nature

Energy Environ. Sci.

Energy Environ. Sci.

Angew. Chem. Int. Ed.

Chem. Mater.

Adv. Mater.

Adv. Funct. Mater.