Elsevier

Electrochimica Acta

Volume 194, 10 March 2016, Pages 151-160
Electrochimica Acta

MoS2 anchored free-standing three dimensional vertical graphene foam based binder-free electrodes for enhanced lithium-ion storage

https://doi.org/10.1016/j.electacta.2016.02.120Get rights and content

Abstract

The vertical graphene with hierarchical three-dimensional network architecture is a promising substrate for high energy and power density Li-ion battery due to its large surface area, inherent three-dimensional network and excellent ion transport property. Three dimensional vertical graphene (3DVG) is synthesized via plasma enhanced chemical vapor deposition (PECVD) using cost-effective and environment-friendly natural oil of M. alternifolia as precursor. The MoS2 nanosheets are then anchored on free-standing 3DVG by hydrothermal method to make the binder free MoS2@3DVG anode of a Li-ion battery. The MoS2@3DVG electrodes deliver an enhanced capacity of 670 mAh g−1 with the capacity retention of 99% after 30 cycles at 100 mA g−1, much better than that of the reference sample of MoS2@3DG (550 mAh g−1 at 100 mA g−1) which uses 3D planar graphene. Superior performance of the vertical graphene based electrode is attributed to the unique hierarchical structure and densely packed reactive edges of the as-synthesized 3DVG. The versatility of plasma-assisted natural precursor based vertical graphene as functional nano-structured substrate for MoS2, as active material, for advanced energy storage devices is demonstrated.

Introduction

Rechargeable lithium-ion batteries (LIBs) have received considerable attention in a wide number of applications because of their high energy density and extraordinary flexibility [1], [2]. They have been proven as one of the most popular power supplies for electric vehicles, which require operation under higher power and energy densities than other mobile instruments. However, for practical application of LIBs in electric vehicles, several significant drawbacks cannot be neglected, which include high level polarization of bulk electrode and degradation of electrochemical performance at high current during charging and discharging processes [3]. This is mainly associated with poor electrical conductivity and large volume change occurring during cycling, which lowers the overall performance of LIBs [4]. One of the approaches to enhance the cyclic stability is to coat the layer of conducting polymer on the outer surface of electrode materials [5]. Among many materials of choice for this conductive layer, the graphene, a two-dimensional carbon nanosheet, is expected to play a crucial role in high-energy LIBs, not only due to its high electrical conductivities; but also due to its unusual mechanical strength and ultra-large specific surface area.

From the last decade, various graphene-based composites are serving as the promising candidates for LIBs electrode material. For instance, the metal oxide particles wrapped by the reduced graphene oxide (RGO) exhibit enhanced electrochemical performances as electrode materials for LIBs [6]. Nevertheless, further improvement of the as-prepared composite is hindered due to relatively lower conductivity of RGO compared with that of the pristine graphene [7]. In comparison, the pristine graphene serves as an excellent substrate for anchoring nano-architecture, which enhances the electronic conductivity of the overall electrode and promotes the high dispersity of anchored nano-structured active material [8]. However, the exposed nano-materials on the outer surface of graphene are still prone to random aggregation, resulting in the volume expansion and decreased electrochemical performance [9]. Therefore, it is essential to develop a hybrid with unique structure that can effectively prevent self-aggregation of exposed nano-structure and fully retain the advantages of active nanostructured materials.

Recently, the vertical graphene (VGs) have attracted considerable attention because of their promising electrical conductivity, significant surface area, and the inherent three dimensional netlike architecture of graphene flakes which are oriented perpendicularly to the substrate surface [10]. Such hierarchical structures and morphologies are considered to remarkably increase the ion diffusivity and accessibility, leading to enhanced performance of lithium-ion intercalation/extraction. Notably, previous researches have demonstrated that VGs-based electrodes retain exceptionally high power density and stable properties and hence are promising for applications in supercapacitors and miniaturized electronic devices [10], [11]. For instance, the MnO2 film supported on VGs has delivered a specific capacitance of 1170 F g−1 with capacitive retention of 110% after 2000 cycles [12]. VG, however, so far are usually produced by hazardous, expensive purified hydrocarbon and fluorocarbon gases, such as CH4, C2F6 etc., which extensively impedes their further utilization for various applications [13], [14]. During the VGs fabrication via fluorocarbon, non-conductive fluorocarbon layer is formed on the substrate, which inevitably results in the increased electrical resistance and lower capacitance of electrode materials [15]. Therefore, the scalable, environmentally sustainable and economically viable processes to produce VGs is still recognized as an issue unless the precursor is derived from renewable sources and are environmentally-friendly. We have recently reported the catalyst free plasma enhanced growth of graphene from sustainable source, the tea tree oil (Melaleuca alternifoia extract), on silicon substrates and demonstrated its application in resistive random access memory (RRAM) device. Our interest is to further develop this simple and sustainable pathway of using easily renewable monomer of Melaleuca alternifolia extract in radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) system to synthesize high quality vertical graphene in relatively short time and demonstrate their suitability for various applications including energy storage.

In this paper, we report the a simple approach to synthesize 3DVG using a low-cost, non-toxic, renewable and environment-friendly natural organic material, M. alternifolia essential oil, instead of hazardous and costly hydrocarbon gases in a RF-PECVD system [16]. Compared to 3DG, 3DVG grown by the PECVD strategy retains hierarchical structure, resulting in many edges and ensuring direct contact and superior ion/electron transportation between active materials and the conductive substrate. To support this claim, the MoS2 nanoarchitectures are anchored on the surface of the free-standing 3DVG by the hydrothermal method (MoS2@3DVG). In comparison, the MoS2 anchored three dimensional planar graphene foam (3DG) is used as the reference sample, designated as MoS2@3DG. The as-synthesized MoS2@3DVG composites demonstrate better electrochemical performance not only in charge/discharge measurements but also in cyclic stability than that of the reference MoS2@3DG sample.

Section snippets

Growth of 3DG foam via chemical vapor deposition (CVD)

The growth of 3DG foam was achieved using the CVD approach described by Cao et al. [17]. The nickel foams with a size of 1 × 1 cm directly served as the scaffold templates and were placed in the center of a quartz tube furnace and annealed at 950 °C for 15 min under the gas flow of argon (100 sccm) and hydrogen (100 sccm), in order to reduce the oxide layer of Ni foam. Ethanol was then bubbled into the tube along with argon (100 sccm) at 950 °C. The content of ethanol was controlled by the flow rate of

Results and discussion

Fig. 2a-c present the typical SEM images of free-standing 3DVG at different magnifications. The free-standing 3DVG can be seen to follow nickel foam profile in Fig. 2a. The uniform and dense structure of VG is evident in Fig. 2b. The SEM image at higher magnification in Fig. 2c shows VGs growing on top of the graphitic-type flake (which can be seen at the bottom). It also shows the typical extent of VG to be the order of two to three hundred nanometers and hence has large amount of edges. The

Conclusion

To concluded, we successfully extended our previous work of synthesis of vertical graphene using renewable and sustainable natural precursor of Melaleuca Alternifolia extract in RF plasma enchanced CVD based system to directly fabricate 3D vertical graphene (3DVG) on porous nickel foam. The 3DVG substrates were also anchored with MoS2 to demonstrate their suitability for applications in lithium-ion storage. The as-synthesized 3DVG sample exhibits much better electrochemical performance than

Acknowledgement

This study was supported by NIE AcRF research grant No. RI 7/11 RSR and RI 6/14 RSR provided by National Institute of Education, Nanyang Technological University, Singapore. Authors thank Dr. T. Yu, Mr. Y. Wang and Mr. W. Ai from School of Physical and Mathematical Sciences of Nanyang Technological University in Singapore for the kindly help and fruitful discussions.

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