3D RGO frameworks wrapped hollow spherical SnO2-Fe2O3 mesoporous nano-shells: fabrication, characterization and lithium storage properties
Introduction
Lithium-ion batteries (LIBs) are the predominant power source for portable electronics and expectedly for electric vehicles in the near future because of their advantages of high energy density, long lifespan, no memory effect, and environmental benignity [1], [2]. The anode in current commercial LIBs is usually composed of carbonaceous materials with long lifespan and low cost, but their lithium storage capacity (<372mAh g−1) and the safety are the major concerns for large-scale energy storage applications [3], [4]. Transition metal oxides have been intensively studied as alternative anode materials due to the high capacity, widespread availability, and enhanced safety [5], [6]. Among them, SnO2 and Fe2O3 have been proved to be possible candidates as the dominant anode materials for commercial scale lithium storage because of their high theoretical capacity (790 mAh g−1 for SnO2, 1007 mAhg−1 for Fe2O3), low toxicity and cost, and environmental benignity, high availability [7], [8]. Currently, the employment of these metal oxides in LIBs is still largely hampered by their poor long-term cycling stability and intrinsic low charge/ionic conductivity [9].
One of effective methods to mitigate these problems is to fabricate hollow nano-shelled active materials with high surface area and short diffusion paths [10], [11], [12], [13], [14]. The large surface area endows the metal oxide with more lithium storage sites and large electrode-electrolyte contact area for high Li+ flux across the interface; the permeable thin nano-shells provide significantly reduced paths for both Li+ and electrons diffusion, leading to a better rate capability; the hollow interior provide extra free space for alleviating the structural strain and accommodating the large volume variation associated with repeated Li+ insertion/extraction processes, giving rise to improved cycling stability. As a result of significantly mitigated electrode pulverization and polarization, exceptional electrochemical performance is thus highly anticipated for metal oxide hollow structures. For instance, hollow SnO2 nanospheres were reported with high initial reversible charge capacity and improved cycling performance [15]. The ultrahigh lithium storage capacity is speculated to result from the ability of the mesopores in the nano-shells and interior microcavities of hollow nanospheres to enhance Li storage. Multishelled Fe2O3 [16], Co3O4 [17] and SnO2 [18] hollow microspheres were prepared using carbonaceous microspheres, which exhibit higher capacity and improved cycling performance compared with single-shelled Fe2O3, Co3O4 and SnO2 hollow spheres. Moreover, this strategy can be readily extended to the fabrication of more complex materials such as Fe2O3@SnO2 [19], SnO2@Co3O4 [20] and Fe2O3@TiO2 [21]. In virtue of the synergetic effect of two distinct compositions, the resultant structures exhibit higher capacity and much lower initial capacity loss for lithium storage than bare component hollow spheres.
Another effective way to resolve the crumbling and pulverization of metal oxides is to fabricate a hybrid of metal oxides and carbonaceous materials, represented by graphene [22], [23], [24], [25]. Graphene holds fascinating characteristics such as excellent electrical conductivity, high surface area, extraordinary elasticity and ultra-light weight. The integration of graphene sheets with metal oxides can efficiently enhance the electrical conductivity and improve the stability of these metal oxide lithium storage materials. Recently, 3D reduced graphene oxide (RGO) frameworks with porous architectures have been extensively investigated as excellent building blocks for electrochemically active metal oxides particles [24], [26], [27]. Generally, most nanoparticles homogeneously dispersed in the aqueous suspension could be embedded into the graphene network to form a 3D graphene-nanoparticles gel in a simple one-step process during the network formation by hydrothermal method [28]. These freestanding 3D graphene-based frameworks couple a low density with the high flexibility and electrical conductivity of graphene nanosheets. Meanwhile, the porous graphene frameworks give a large contact area between electrolyte and active materials, providing multidimensional electron transport pathways to enhance the electrochemical performance [29].
Herein, we report novel hybrid architectures of 3D RGO frameworks confined hollow spherical SnO2-Fe2O3@RGO mesoporous nano-shells (3D h-SnO2-Fe2O3@RGO), which exhibit excellent electrochemical performance for lithium storage. The unique 3D hierarchically porous metal oxides-RGO core-shell nanostructures could enhance mass transport and electron transfer in the electrochemical process due to the within continuous graphene porous networks and hierarchically mesoporous nano-shells. The 3D graphene frameworks encapsulated nanolayers also could provide protection against the volume changes and nanosized active materials aggregation during electrochemical processes.
Section snippets
Synthesis of hollow SnO2 spheres
Hollow SnO2 spheres (h-SnO2) were prepared by depositing SnO2 layer on ∼300 nm spherical SiO2 template then etching the SiO2 core [30]. In this procedure, 1.8 g urea and 0.266 g Na2SnO3·3H2O were firstly dissolved in 34 mL deionized water, and then 18 mL ethanol was added into the above solution to form a milky suspension under mildly stir for about 20 min. 4 mL SiO2 spheres colloid (60mg mL−1) was added into the above solution and transferred into a 100 mL Teflon-lined stainless steel autoclave for
Fabrication and characterizations
The fabrication process of 3D RGO frameworks confined hollow spherical metal oxides nano-shells (3D h-SnO2-Fe2O3@RGO) is illustrated in Scheme 1. SiO2 spheres with diameter of about 300 nm (Fig. 1a1–a3) are prepared by a modified Stöber method as sacrifice template. After deposition of SnO2 through hydrothermal procedure, the surfaces of SiO2@SnO2 spheres (Fig. 1b1–b3) become rather roughness than bare SiO2, indicating the successful formation of SnO2 shells. Hollow spherical SnO2 nano-shells (
Conclusions
3D RGO frameworks encapsulated hollow spherical h-SnO2-Fe2O3@RGO mesoporous nano-shells are successfully fabricated as anode materials for lithium ion batteries. The 3D h-SnO2-Fe2O3@RGO hieratical architectures display great improvement in revisable capacity (∼830 mAh g−1 after 100 cycles at 200 mA g−1), cycling performance and rate capability (∼550 mAh g−1 at 1000 mA g−1) over the h-SnO2 nano-shells, 3D h-SnO2@RGO and h-SnO2-Fe2O3 nano-shells electrodes. The unique core-shell 3D porous structures
Acknowledgements
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 High Density Electronic Packaging and Device Assembly Key Laboratory (ZDSYS20140509174237196), Shenzhen Peacock plan (KQCX2015033117354154) and Shenzhen basic research plan (JCYJ20150521144320990).
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