The morphologies of the samples were observed by scanning electron microscope (SEM, Hitachi S-4700, Hitachi, Ltd, Chiyoda-ku, Japan). The information of functional groups was

measured by Fourier transform infrared spectroscopy instrument (FTIR, Nicolet Nexus 670, Thermo Fisher Scientific, RepSox Shanghai, China). The electrochemical performances of the HGSs as anode materials for lithium-ion batteries were measured with the coin-type cells. The lithium sheets were used as both reference and counter electrodes, and composite electrodes comprising active mass (HGSs, 85 wt%), carbonaceous additive (acetylene black, 5 wt%), and poly(vinylidene difluoride) (PVDF, 10 wt%) binder were used as working electrodes. The thickness and density of electrode are 50 μm and 1.95 mg cm-2, check details respectively. One molar LiPF6 solution in a selleckchem 1:1 (volume) mixture

of ethylene carbonate (EC) and dimethyl carbonate (DMC) from Merck & Co., Inc. (Whitehouse Station, NJ, USA) was used as electrolyte. The Celgard 2400 microporous polypropylene film provided by Jimitek Electronic (Shenzhen, China) Co. Ltd was used as separator. The coin-type cells were galvanostatically discharged (Li insertion) and charged (Li extraction) in the voltage range from 0.01 to 3.50 V vs. Li/Li+ at the different current densities. Electrochemical impedance spectroscopy measurements of the electrodes were carried out on an electrochemical workstation (Princeton VersaSTAT3-200, Princeton Applied Research, Oak Ridge, TN, USA) using the frequency response analysis. The impedance spectra were obtained by applying a sine wave with amplitude of 5.0 mV over the frequency range from 100 kHz to 0.01 Hz. Results and discussion The morphology and structure of HGOSs and Protirelin HGSs were characterized by SEM, and their images are shown in Figure 1. SEM images in Figure 1 exhibit the hollow structures of HGOSs

and HGSs. In particular, some spheres collapse after heat treatment as shown in Figure 1d. The SEM images in Figure 1c,d show that HGSs hold a compact and hollow microstructure, distinct from the laminar structure of bulk graphite oxide and paper-like texture of graphene nanosheets. From Figure 1a, it is observed that some small holes and protuberances emerge on the surface of microspheres, which is assigned to the removal of water and will be discussed in detail later. An unambiguously broken sphere reveals that the interior is hollow, and the thickness of the wall is approximately 1 μm (Figure 1d). The continuous and smooth cross section implies that the adjacent graphene nanosheets possess a close connection. Figure 1 SEM images of HGOSs (a and b) and HGSs (c and d). The structural changes from GO to HGSs were investigated by XRD measurement, and the patterns are shown in Figure 2a. After oxidation, the (002) peak of graphite disappears, and an additional peak at 11.56° is observed, which is corresponding to the (001) diffraction peak of GO. The d-spacing of GO increased to 0.765 nm from 0.