Hollow SnO2@Co3O4 core–shell spheres encapsulated in three-dimensional graphene foams for high performance supercapacitors and lithium-ion batteries
Graphical abstract
Hollow SnO2@Co3O4 spheres encapsulated in three-dimensional graphene foams demonstrate excellent electrochemical performance for supercapacitors and lithium-ion batteries.
Introduction
Electrochemical energy storage devices, such as lithium-ion batteries (LIBs) and supercapacitors, are the most popular power supplies in portable electronic devices and have been intensively developed for renewable energy due to their high energy storage density and conversion efficiency [1], [2], [3], [4], [5], [6], [7], [8]. Carbon materials are widely used not only directly for electrode active materials of LIBs and supercapacitors but also as conductive network materials for electrodes [9], [10], [11], [12], [13]. However, the low storage capability of traditional carbon materials limits their further wide applications as electrode active materials.
In recent years, transition metal oxides have been widely explored as alternative electrode materials for supercapacitors and LIBs owing to their high theoretical specific capacities compared with the commercially used graphite and active carbon [14], [15], [16], [17], [18]. Especially, complex oxides of transition metals [19], such as ZnMn2O4[20], NiCo2O4[21], ZnCo2O4[22], Fe2O3–Co3O4[23], SnO2–Co3O4[24], Co3O4@MnO2[25], have attracted much attention for their synergistic effect in enhancement of reversible capacity, structural stability, and electrical conductivity. However, most transition metal oxides and complex oxides show poor rate capability and cycling performances because of their low conductivity and large volume expansion/contraction during the long-term charge/discharge cycling.
Various strategies have been developed to solve these problems. Such as fabrication of hollow metal oxide nanostructures which could provide short electron-transporting paths and a high surface-to-volume ratio [26], [27], [28]. Modification of metal oxide nanomaterials with carbon (especially graphene in recent years) conductive materials is another very promising method to improve electronic conductivity and accommodate volume change of metal oxide active materials during cycling [29], [30], [31], [32], [33], [34].
Herein, we report hybrid structure of hollow SnO2@Co3O4 core–shell spheres (h-SnO2@Co3O4) encapsulated in three-dimensional (3D) graphene foams as electrode materials for high performance supercapacitors and LIBs. The 3D graphene foams encapsulated h-SnO2@Co3O4 architectures (3D h-SnO2@Co3O4@GF) illustrates much higher capacity, cycling stability and rate capability relative to the single or two components. The 3D graphene foams could enhance the electrical conductivity of the overall electrode and protect against the volume changes during electrochemical processes. The hollow core–shell structure also could shorten the electron/ion diffusion path and avoid volume change.
Section snippets
Synthesis of hollow SnO2@Co3O4 spheres
Hollow SnO2 spheres were prepared by using spherical SiO2 particles as template. Firstly, 1.8 g CO(NH2)2 and 0.266 g Na2SnO3·3H2O was dissolved in 34 mL DI water, then 18 mL ethanol was added in the solution under mildly stir for about 30 min, 4 mL colloidal SiO2 solution (60 mg mL−1) prepared according to the literature [35] was added into the above solution. The mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave at 170 °C for 36 h. After the autoclave cool down, the
Materials characterization
The morphology structures of the products during the 3D h-SnO2@Co3O4@GF architecture fabrication process are examined by SEM as shown in Fig. 1. The diameter of SiO2 spheres template is about 300 nm (Fig. 1a). SnO2 coated SiO2 spheres are obtained by a deposition procedure. After removing the SiO2 cores the remained products are hollow SnO2 spheres (Fig. 1b). The surface of these h-SnO2@Co3O4 spheres become relative rough after Co3O4 coating (Fig. 1c). The morphology of Co3O4 coating is like
Conclusions
Hollow SnO2@Co3O4 spheres encapsulated in 3D graphene foams are successfully fabricated as electrode materials for supercapacitors and lithium ion batteries. The capacity, cycling stability and rate capability of hollow SnO2@Co3O4 spheres could be considerably improved by the 3D graphene foams encapsulated architecture. For supercapacitors, the capacitance of 3D h-SnO2@Co3O4@GF is 523.2 F g−1 at 2 A g−1 after 1000 cycles, whereas only 223 F g−1 for h-SnO2@Co3O4 and 110 F g−1 for 3D graphene
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 21203236), Guangdong and Shenzhen Innovative Research Team Program (No. 2011D052, KYPT20121228160843692), Shenzhen Electronic Packaging Materials Engineering Laboratory (2012-372), Shenzhen High Density Electronic Packaging and Device Assembly Key Laboratory (ZDSYS20140509174237196).
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