In recent decades, research on energy storage materials, including carbon, metal oxides/sulfides/phosphides, and conducting polymers, has greatly promoted the development of energy storage technologies that could contribute to the practical application of clean and renewable energy [1] ; [2]. Due to the shortcomings of an individual component, such as poor conductivity, low capacity, or cycling stability problems, the design and synthesis of composites with high electrochemical energy storage performance are essential. For instance, composite electrode materials, synthesized through various strategies including in-situ composites, designing hierarchical structures, and surface coating, and modification, exhibited improved performance compared with the single component [3] ; [4]. However, these composite structures usually suffer from low electronic conductivity and mechanical stability, leading to inferior performance or an unsatisfied cycling life. Thus, strategies for fabricating composite electrode materials with highly stable hierarchical structures and good conductivity are essential for energy conversion and storage.
Graphene has attracted tremendous attention since the first report in 2004, because of its unique 2D atomic structure and outstanding properties, making it among the most promising materials for a variety of applications such as energy storage, flexible electronics, biosensors, water purification, and gas and ion separation [5] ; [6]. Graphene can be directly used as the electrode material for supercapacitors and batteries by itself, and it also holds great potential as the substrate to fabricate various nanocomposites. First, graphene coupled with electro-active materials can be easily integrated into 1D fibers, 2D films, or 3D foams to meet the light and flexible demand of wearable electronic devices and energy storage devices. Second, graphene has an ultra-high theoretical specific surface area of 2620?m2?g−1 and excellent intrinsic electrical conductivity and mechanical stability. Therefore, graphene not only is a great choice as the skeleton in composites to support nanoparticles but also can facilitate the electro-active materials/electrolyte contact and shorten ion transportation length [7]. Third, graphene can be manufactured in ton quantities, thus effectively reducing the cost of graphene-based composites [7] ; [8]. Recently, Cui et al. fabricated a sandwiched phosphorene–graphene hybrid material that exhibits a high Na-ion storage capacity of 2440?mAh?g−1, a high-rate capability, and a good capacity retention as compared to pure phosphorene. The enhanced Na-ion charge properties of the phosphorene–graphene hybrid material can be attributed to the graphene, which increases the electrical conductivity of the material and provides a preferential pathway to the electrons generated by the redox reaction of phosphorene [9]. Yan et al. reported the fabrication of a 3D VN-reduced graphene oxide(VN-RGO)composite that exhibited much better capacity and rate capability compared with pure VN [10]. Chen et al. developed a one-step, continuous synthesis of spherical lithium titanate (Li4Ti5O12)/graphene composites, in which Li4Ti5O12 nanocrystals were in-situ grown on the graphene surface [11]. The enhanced Li-ion charge storage performance indicated that the fabricated composite effectively overcame the existing challenges of Li4Ti5O12-based anodes. Despite the notable progress of graphene-based composite electrode materials, challenges still remain.
Transition metal chalcogenides have proven to be exceptionally promising and can be used in various fields, such as batteries, supercapacitors, photocatalysis, nanotribology, and hydrogen evolution reaction (HER) catalysis [12]. Among these, amorphous MoS3has generated much attention due to its great potential applications in energy storage. Lu and co-workers investigated the Na-ion storage performance of amorphous MoS3nanoparticle-CNT composite, and the corresponding results showed that amorphous MoS3-CNT composites exhibited high gravimetric specific capacity, as well as impressive volumetric and areal capacities, greatly superior to the analogous MoS2-CNT composite [13]. DFT calculations demonstrated that the CNTs’ introduction resulted in abundant open-sites that facilitate the diffusion of electrolyte ions and relatively low activation energy barriers. However, systematic studies on MoS3 are still insufficient and the negative effect on performance by low conductivity and surface area need to be improved.
This issue’s cover of Materials Today shows a composite structure of ultra-small MoS3nanoparticles uniformly distributed on the corrugated reduced graphene oxide (RGO) synthesized using graphene oxide (GO) suspension and ammonium tetrathiomolybdate ((NH)4MoS4) through a one-step hydrothermal method. The novelty of the present image is that MoS3 nanoparticles are locally deposited and in-situ grown on the surface of the corrugated RGO sheet, effectively preventing the agglomeration of RGO and MoS3nanoparticles. The image was taken using a scanning electron microscope (FESEM, JSM7800F). This type of architecture extraordinarily improves the conductivity of the MoS3 and facilitates the electron/ion transportation, that is, the open spaces on/in the surface of the composite sheet benefits the electrolyte penetration and diffusion of ions, for high-efficiency utilization of the electro-active materials. This nanostructured composite, providing plenty of active sites, may find a variety of applications such as HER, catalysis, sensors, supercapacitors, and batteries.
Acknowledgments
The authors acknowledge support from the National Natural Science Foundation of China (No. 51772131, 51772127).
Further reading
[1] W. Lu, et al.
Chem. Soc. Rev., 46 (2017), p. 2199
[2] X. Wu, S. Yao
Nano Energy, 42 (2017), p. 143
[3] X. Lu, et al.
Adv. Mater., 24 (2012), p. 938
[4] H. Kim, et al.
Mater. Today, 7 (2014), p. 285
[5] M.F. El-Kady, et al.
Nat. Rev. Mater., 1 (2016), p. 16033
[6] J. Chen, et al.
J. Phys. Chem. Lett., 4 (2013), p. 1244
[7] Y. Zhu, et al.
Science, 332 (2011), p. 1537
[8] M. Segal, et al.
Nat. Nanotechnol., 4 (2009), p. 612
[9] J. Sun, et al.
Nat. Nanotechnol., 10 (2015), p. 980
[10] R. Wang, et al.
Adv. Funct. Mater., 25 (2015), p. 2270
[11] S. Mao, et al.
NPG Asia Mater., 7 (2015), p. e224
[12] J. Kibsgaard, et al.
Nat. Mater., 11 (2012), p. 963
[13] H. Ye, et al.
Adv. Energy Mater., 7 (2016), p. 1601602