The research on graphene has become one of the most attractive scientific topics at present, because this new carbon material possesses unique physicochemical properties which impart it with great potentials in various application areas. The application of graphene in energy storage materials, such as electrode active materials in Li ion battery, is believed to be a promising field with a huge market. The ultra-thin graphene layers with graphitic basal structure may favor a higher accommodation sites and a faster migration rate of Li ions during charge-discharge processes while keeping high electron conductivity, comparing with conventional graphite anodes. Therefore, graphene offers a new option to carbon-based anode materials in Li ion batteries. Since 2008, several research groups have studied the electrochemical properties of graphene anode materials. It was found that graphene powders could deliver a higher specific capacity than graphite, which was suitable for high-capacity Li ion batteries. On the contrary, graphene paper with a much denser stacking of graphene sheets comparing with powders shows apparently lower storage capacity. Accordingly, it is speculated that the morphology and the 3D construction manner of graphene is crucial to its charge-discharge characteristics. It is also well known that novel electrode materials with low cost, high capacity and easy to be produced at large scale is needed in order to meet the increasing energy storage demand. Metallic structures such as Sn, Ge, Si, Al, Sb and etc. have higher specific capacity than commercial graphite anode electrodes. Among these electrodes tin anodes have attracted much attention because it delivers a capacity up to three times higher than that of graphite. Theoretically, one tin atom can maximally react with 4.4 lithium atoms to form Li4.4Sn alloy, reaching a capacity of 993 mAh/g. However, the large amount of lithium insertion/extraction into/from Sn causes a large volume change (about 300%), which causes pulverization of tin particles and loss of contact with current collector, resulting in poor electrochemical performance.

In this study, a “yolk-shell” structure for a stabilized and scalable tin anode is designed. Tin nanoparticles (∼8-20 nm) as the “yolk” were produced through a facile chemical reduction synthesis method. The surfaces of the tin nanoparticles firstly coated with a SiO₂ sacrificial layer and the obtained composite nano tin/SiO₂ particles were subjected to microwave hydrothermal carburization in order to obtain the shell structure. The as-synthesized nanocomposite particles were then subsequently treated with hydrofluoric acid in order to selectively remove the SiO₂sacrificial layer and the tin/C yolk shell structure is obtained. As synthesized graphene oxide and carbon coated Sn was dispersed in 50 mL bidistilled water by the aid of 80 mg of SDS (Sodium dodecyl sulfate) surfactant and sonicated to form a well-dispersed suspension. In order to produce Sn/graphene paper, the as-synthesized graphene oxide paper was chemically reduced immediately after filtration by hydrazine solution. 2.0 M, 50 mL hydrazine solution slowly poured on to membrane supported graphene oxide paper and filtered via vacuum technique.

The surface and cross-section morphologies of the produced sample electrodes were observed by scanning electron microscopy (SEM, Jeol 6060 LV). The phase structures of the samples were investigated by X-ray diffraction (XRD) (Rigaku D/MAX 2000 with thin film attachment) with CuKa radiation. Coin type CR2016 cells were assembled in an argon-filled glove box. The electrolyte solution was 1 M LiPF₆  in EC/DMC  (1:1 by volume). The electrochemical performance of the tin-C/Graphene nanocomposites was evaluated by galvanostatic discharge–charge measurement using a computer-controlled battery tester between 0.02 and 2.5 V using metallic lithium as the counter electrode. The cells were cyclically tested on a MTI Model BST8-MA electrochemical analyzer using 1C (18 mA/dm²) current density over a voltage range of 0.02–2.5 V. After being cycled for 50 cycles, electrochemical impedance spectroscopy (EIS) was conducted on coin cells using an electrochemical workstation (Gamry Instruments Reference 3000) over a frequency range from 100 kHz to 0.001 Hz with an ac amplitude of 5 mV. The measured voltage was about 0.2V after the cells were relaxed for 1 h. The data has been normalized and referred per unit of mass for the purpose of comparison. Cyclic voltammograms (CVs) were recorded on an electrochemical workstation (Gamry Instruments Reference 3000) at a scan rate of 0.5 mVs⁻¹ between 0.02 and 2.5 V. All the potentials indicated here were referred to the Li/Li⁺ electrode potential. All electrochemistry tests were carried out at room temperature (25 °C).

Keywords: Yolk-Shell, Tin/C/Graphene, Free-standing, Anode Electrode, Li-ion.