Sulfur is a promising cathode material for lithium–sulfur batteries because of its high theoretical capacity (1,675 mA h g−1); however, its low electrical conductivity and the instability of sulfur-based electrodes limit its practical application. Here we report a facile in situ method for preparing three-dimensional porous graphitic carbon composites containing sulfur nanoparticles (3D S@PGC). With this strategy, the sulfur content of the composites can be tuned to a high level (up to 90 wt%). Because of the high sulfur content, the nanoscale distribution of the sulfur particles, and the covalent bonding between the sulfur and the PGC, the developed 3D S@PGC cathodes exhibit excellent performance, with a high sulfur utilization, high specific capacity (1,382, 1,242 and 1,115 mA h g−1 at 0.5, 1 and 2 C, respectively), long cycling life (small capacity decay of 0.039% per cycle over 1,000 cycles at 2 C) and excellent rate capability at a high charge/discharge current.

For the first time, a composite of fluorine-doped SnO 2 and reduced graphene oxide (F-SnO 2 @RGO) was synthesized using a cheap F-containing Sn source, Sn(BF 4 ) 2 , through a hydrothermal process. X-ray photoelectron spectroscopy and X-ray diffraction results identified that F was doped in the unit cells of the SnO 2 nanocrystals, instead of only on the surfaces of the nanoparticles. F doping of SnO 2 led to more uniform and higher loading of the F-SnO 2 nanoparticles on the surfaces of RGO sheets, as well as enhanced electron transportation and Li ion diffusion in the composite. As a result, the F-SnO 2 @RGO composite exhibited a remarkably high specific capacity (1277 mA h g -1 after 100 cycles), a long-term cycling stability, and excellent high-rate capacity at large charge/discharge current densities as anode material for lithium ion batteries. The outstanding performance of the F-SnO 2 @RGO composite electrode could be ascribed to the combined features of the composite electrode that dealt with both the electrode dynamics (enhanced electron transportation and Li ion diffusion due to F doping) and the electrode structure (uniform decoration of the F-SnO 2 nanoparticles on the surfaces of RGO sheets and the three-dimensional porous structures of the F-SnO 2 @RGO composite).

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