Free-standing N-doped carbon nanofibers/carbon nanotubes hybrid film for flexible, robust half and full lithium-ion batteries

doi:10.1016/j.cej.2017.10.030

Chemical Engineering Journal

Volume 334, 15 February 2018, Pages 682-690

https://doi.org/10.1016/j.cej.2017.10.030 Get rights and content

Highlights

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N-doped carbon films with nanofibers and nanotube composites are prepared.
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The CNFs/CNTs hybrid electrode can achieve a reversible capacity of 1099.5 mAh g⁻¹.
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Carbon films work as flexible, robust electrodes for half and full Li-ion batteries.

Abstract

The development of flexible electronics requires the power sources with matchable flexibility, robust mechanic property, as well as high power and energy density that would maintain the durability of flexible electronics during use. Herein, newly designed free-standing N-doped carbon films with hierarchical structure of carbon nanofibers and carbon nanotubes (CNFs/CNTs hybrids) are prepared via a simple electrospinning process. This interconnected hybrid film displays high conductivity and excellent flexibility. As a free-standing and binder-free anode material, the CNFs/CNTs hybrid electrode can achieve a reversible capacity of 1099.5 mAh g⁻¹ at a current density of 0.05 A g⁻¹ with superior rate capabilities and excellent cyclic stability. What's more, utilizing the CNFs/CNTs hybrid film as the electrode, this assembled highly flexible half and full batteries also show attractive performances and can easily light up a group of 12 light emission diodes at flat and various bending positions, making it possible to be applied in the flexible electronic devices.

Graphical abstract

Introduction

Flexible electronics, with the properties of portable, bendable or even wearable and implantable technology, are now taking us into a new era that will revolutionize our daily life. Examples range from soft, portable electronic devices, roll-up displays, wearable devices and touch screen, to implantable biomedical devices, and etc [1], [2], [3], [4], [5], [6]. Increasing demands for flexible electronics have simultaneously accelerated the development of matchable flexible power sources, i.e., solar cells, lithium-ion (Li-ion) batteries, supercapacitors, and fuel cells. However, the fabrication of such flexible power sources is still in the infancy stage of development due to the difficult in designing and fabricating ultrathin/lightweight materials with good ion diffusion and electron conductivity as well as reliable flexibility that can operate normally under various conditions such as bending, twisting or other deformation modes. Recently, amounts of attentions have been focused on flexible Li-ion batteries in particular because of their high power density, superior energy storage, and long-term stability [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Nevertheless, traditional electrodes in Li-ion batteries, consisting of active materials, binders and conductive additives, are not only heavy and rigid but also easy to detach from the metal current collectors such as Al or Cu foils. In addition, the low energy density and power capability of graphite, the current commercial anode material, remains a barrier for the applications of flexible Li-ion batteries. While a lot of efforts have been devoted to seeking new electrode materials and novel designations and fabrications that are suitable for flexible Li-ion batteries, they usually suffer from limited capacities and insufficient cycle stabilities. Therefore, it is still highly desirable to construct well flexible, robust Li-ion batteries with superior conductivity, high energy and power density.

One dimensional (1D) carbon nanomaterials, especially carbon nanofibers (CNFs) and carbon nanotubes (CNTs), have offered new opportunities for flexible energy storage devices [18], [19], [20], [21]. Recent progresses in flexible Li-ion batteries demonstrate that CNTs and CNFs are promising conductive skeletons for loading active electrode materials because of their convenient ion and electron diffusion, excellent mechanical property, structure integrity and good conductivity [22], [23], [24]. For example, Sn nanoparticles embedded in CNFs have been reported as self-supported anode electrodes for Li-ion battery [25]. Parallelly, V₂O₅ encapsulated in CNTs as free-standing anode electrode has also been fabricated for the same purpose [26]. These materials have shown attractive electrochemical performance and acceptable flexibility. However, issues affecting these materials are the huge volume expansions, ca. 300% for Si and even over 300% for Sn during the lithiation and delithiation processes, which may result in structure destruction in harsh environment like bending or twisting for flexible Li-ion batteries. CNFs have a one-dimensional carbon structure with large surface area and large specific surface to volume, which provide more active sites and good electrolyte penetration capability. However, specific capacities and cycle stabilities of these fibers for flexible batteries are not satisfactory due to limited structure stability and mechanical properties. CNTs are well known as good materials with excellent mechanical flexibility and high conductive capabilities. However, the preparation of CNTs usually needs the methane or ethylene flammable gases and the CNTs can deliver a limited electrochemical capacity. More recently, the N-doping has been investigated as a new strategy to enhance the Li-ion storage capacity of the carbon materials because of the modification of electronic configuration and structural defect from the introduced N-doping.

In this work, we here combine CNFs and CNTs to fabricate free-standing N-doped carbon films with hierarchical structure of carbon nanofibers and carbon nanotubes (CNFs/CNTs hybrids) via a simple electrospinning process as all-carbon lightweight and free-standing anode materials for flexible batteries. The hierarchical structures of nanofibers and nanotube hybrids of N-doped carbon hybrid film could not only possess excellent electrical and mechanical properties by combining the advantage of the CNFs and the CNTs, but provide more lithium active sites and structural defects, which would enhance the flexibility of the hybrid film and benefit their electrochemical properties at the same time. The free-standing CNFs/CNTs hybrid film exhibited an enhanced electric conductivity, better mechanical property and good flexibility, superior to those of pure CNFs. As anode materials for flexible half batteries, they show a highly reversible specific capacity of over 1099.5 mAh g⁻¹ at a current density of 0.05 A g⁻¹, good cycling stability and improved rate capability. More importantly, the half Li battery could withstand the bending at different angles and release a stable discharge capacity of 463.6 mAh g⁻¹ after 60 cycles at various bending positions (0°, 45°, 90°, and 180°) at a current density of 0.5 A g⁻¹. Meanwhile, flexible full Li-ion battery based on CNFs/CNTs// LiNi_0.5Co_0.2Mn_0.3O₂ can also maintain excellent flexibility and be able to light up a “C” pattern lighting emitting diodes (LEDs) in the flat, 90°, 180°, and 360° bending positions, respectively, being a potential candidate for flexible energy storage devices.

Section snippets

Preparation of precursor solutions for electrospinning

Polyacrylonitrile (PAN, Mw ∼150,000, Sigma-Aldrich) was used as received, MWCNTs (denoted as CNTs, Chengdu Organic Chemistry depart, 99.95%, OD 20–30 nm, 30–100 μm) were modified by a simple hydrothermal method before using. To prepare the electrospinning solution, 0.4 g PAN was dissolved in N,N –dimethylformamide (DMF, Chengdu Kelong Chemical Inc. Co.) to make a 10 wt% solution, then 20 mg CNTs were dispersed into the solution and sonicated for 4 h, with following magnetic stirring for another

Results and discussion

A typical electrospinning and carbonization process is employed to prepare CNFs/CNTs hybrid film. Fig. 1a schematically illustrates the fabrication procedure of flexible CNFs/CNTs hybrid film. This precursor solution of PAN/DMF with well-dispersed CNTs is firstly electrospun into polymer fibers, followed by stabilization and carbonization to yield carbon nanofibers. Photographs of the as-spun fibers and carbonized fibers are displayed in Fig. 1b and c, respectively. The gray fiber disc (left in

Conclusion

In summary, free-standing CNFs/CNTs hybrid films have been synthesized via a simple electrospinning process. The presence of embedded as well as protruding CNTs in the CNFs network with the increased N-doped content could enhance the electronic conductivity and shorten the lithium ion diffusion way. Excellent electrochemical performance with a capacity of 1099.5 mAh g⁻¹ was obtained after 100 cycles at a current density of 0.05 A g⁻¹. The good mechanical property of the CNFs/CNTs flexible film

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 21401177, 51741305 and 21501160), the “1000plan” from the Chinese Government, the Science Foundation for Distinguished Young Scholars of Sichuan Province (2016JQ0025 and 2017JQ0036), the “QianYingBaiTuan” Plan of China Mianyang Science City, and the Science Foundation of Institute of Chemical Materials (No. 011100301).

Notes

The authors declare no competing financial interest.

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Free-standing N-doped carbon nanofibers/carbon nanotubes hybrid film for flexible, robust half and full lithium-ion batteries

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of precursor solutions for electrospinning

Results and discussion

Conclusion

Acknowledgments

Notes

J. Power Sour.

Electrochem. Commun.

Electrochim. Acta

Nano Today

Nano Energy

Nano Energy

Carbon

Nano Energy

Polymer

Electrochim. Acta

Carbon

J. Colloid Interf. Sci.

Carbon

Adv. Energy Mater.

Nano Energy

Adv. Mater.

Sci. Rep.

Science

Adv. Mater.

Adv. Mater.

Nano Lett.

Adv. Mater.

Nano Lett.

Adv. Funct. Mater.

Nanotechnology

Chem.-A Eur. J.