A 3D hollow Sn@C-graphene hybrid material (HSCG) with high capacity and excellent cyclic and rate performance is fabricated by a one-pot assembly method. Due to the fast electron and ion transfer as well as the efficient carbon buffer structure, the hybrid material is promising in high-performance lithium-ion battery.
Metallic Sn has long been considered as a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical specific capacity [
Besides aforementioned routes, many researches also indicate that dispersing Sn NSs into carbon matrix is another effective approach to improve and stabilize the cyclability, where the carbon matrix restricts the volume expansion of Sn and moreover acts as an electron conductor to increase the conductivity [
Aggregation, which hinders the fast Li+ transportation, is another problem for nanomaterials in real applications. Many latest results have shown that graphene nanosheets (GNs) are ideal substrates to well disperse NSs, which can also construct a flexible network through a “plane-to-point” mode to bridge the active material particles and form effective ion and electron transfer networks [
Herein, we integrate the above concerns into one hybrid structure, and a three-dimensional hollow Sn@carbon-graphene hybrid structure (abbreviated as HSCG) is obtained. In such an HSCG, hollow Sn NS was encapsulated in a carbon shell to form a core-shell sphere structure (hollow Sn@C) and these NSs are uniformly anchored onto the flexible GNs. On one hand, hollow Sn NSs are formed to decrease the intrinsic volume variation, and the carbon shell acts as a buffer layer to restrict the volume changes of Sn NS, which also improves the conductivity and helps the formation of stable SEI film. On the other hand, GNs are used as the substrate to fix and isolate the NSs to avoid the aggregation and the presence of NSs prevents the restacking of GNs, which ensure the open ion transportation channels retention. The interlacing of GNs in the hybrid forms an interconnected conducting network, and the NSs are in a good contact with the GNs, which guarantee the electron transfer between the two components and the fabricated electrode. Furthermore, the GNs endure the volume change of the NSs at some extent. Therefore, the hybrid structure has combined the three above discussed characters, defined as fast ion transfer, interconnected conducting network, and efficient volume variation control in electrochemical performance improvement, and is a promising anode candidate for high performance LIBs [
In a typical preparation process, 20 mL ammonia (26 wt%) was added into 20 vol% ethanol aqueous solution, and 2.4 g hexadecyltrimethylammonium bromide (CTAB) was then dissolved into the above solution with continuous stirring for 20 min. After that, 10 mL ethanol and 10 mL tetraethyl orthosilicate (TEOS) containing 5 g SnCl2 were dropwise introduced into the above solution under stirring for 48 h and the obtained mixture solution was denoted as Mixture A. Mixture B was obtained by sonicating 1 g glucose and 400 mg graphene oxide (GO) in 400 mL DI water with a probe sonicator (JY92-N, China, 300 W) for 1 h. Mixtures A and B were mixed under continuous stirring for 5 h followed by an overnight storage at room temperature. After dried at 120°C, the obtained mixture was calcined at 800°C for 4 h under N2 and then treated with HF followed by washing with ethanol and DI water. Finally, the HSCG was obtained after dried at 120°C (Figure
Scheme for the assembly process of hollow Sn@C-graphene hybrid nanostructure (HSCG).
XRD measurements were conducted at room temperature using a specular reflection mode (Bruker D-8, Cu K
First, the as-prepared sample as active material was ground into fine powders. 80 wt% active materials, 10 wt% Super P, and 10 wt% PTFE were well mixed in ethanol solution to make uniform mixture slurry under sonication. Then, the mixture slurry was spread uniformly on nickel foam and dried at 120°C for 12 h under vacuum to obtain a loading density of active material in the range of 2.2–2.5 mg cm−2. Finally, coin cells (CR2032) were assembled in Ar filled glove box and the lithium foils were used as the anode, and 1 M LiPF6 mixture solution (1 : 1 (v/v) of ethylene carbonate (EC) and dimethyl carbonate (DMC)) as the electrolyte.
The coin cells were tested at room temperature using battery tester (Lixing, China) and electrochemistry workstation (Gamry Instrument, USA). Note that the charge and discharge processes were conducted at the same current density with the voltage range of 0.006 V–2.5 V.
X-ray diffraction (XRD) analysis was performed to indicate the reduction process from Sn2+ to metallic Sn. As shown in Figure
(a) XRD patterns of HSCG and crystalline Sn; (b) typical FESEM and (c and d) TEM images of HSCG. Inset of (d) shows eggs with an egg support, which is similar to HSCG nanostructure where a core-shell Sn@C is supported by planar graphene sheets.
The morphologies of the HSCG were examined with field-emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). Figure
The TEM images (Figures
To reveal the advantages of HSCG, its lithium insertion/extraction process is characterized with cyclic voltammetry in the range of 0.005 to 2.5 V at a scan rate of 0.1 mV s−1 [
Electrochemical performance of HSCG. (a) CV profiles of the first and second cycles; (b) galvanostatic profiles at a current density of 50 mA g−1 with a cut-off potential between 5 mV and 2.5 V; (c) cycling profiles; (d) rate performance profile.
Figures
The HSCG also shows a good rate capability and stability. As shown in Figure
In summary, a novel 3D hollow Sn@carbon-graphene structure was designed and prepared by a one-pot assembly method. Such a unique hybridized nanostructure, which is similar to non-yolk eggs within a rigid egg support, is proved a highly active hollow metal@carbon structure attached on flexible and high-strength conductive GNs plane. The synergic effects, resulting from the combination of three kinds of selected infrastructures, contribute to a high reversible specific capacity of 922.7 mAh g−1 and excellent cyclic performance, while an outstanding rate capability is also achieved simultaneously due to the fast ion and electron transfer characteristic. This hybrid structure provides us a promising model for the design of high-performance anode material for LIBs.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors appreciate support from NSAF (no. U1330123), National Natural Science Foundation of China (nos. 51372167 and 51302146), Shenzhen Basic Research Project (nos. JC201104210152A and CYJ20130402145002430), and China Postdoctoral Science Foundation (2012M520012 and 2013T60111).