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Plasma-assisted thermal chemical vapor deposition (CVD) was carried out to synthesize high-quality graphene film at a low temperature of 600°C. Monolayer graphene films were thus synthesized on Cu foil using various ratios of hydrogen and methane in a gaseous mixture. The in situ plasma emission spectrum was measured to elucidate the mechanism of graphene growth in a plasma-assisted thermal CVD system. According to this process, a distance must be maintained between the plasma initial stage and the deposition stage to allow the plasma to diffuse to the substrate. Raman spectra revealed that a higher hydrogen concentration promoted the synthesis of a high-quality graphene film. The results demonstrate that plasma-assisted thermal CVD is a low-cost and effective way to synthesis high-quality graphene films at low temperature for graphene-based applications.

In this study, the separator coated with reduced graphene oxide (rGO) layer is prepared via doctor blading. The lithium-sulfur (Li-S) battery with rGO-coated separator exhibits much smaller impedance and much better electrochemical performance. After coated rGO, the initial discharge capacity can be as large as 1067 mAh g −1 at 0.2 C which can retain 878 mAh g −1 after 100 cycles, and the discharge capacity can reach 710 mAh g −1 even at 2 C. The significant performance enhancement can be attributed to the bi-functionality of rGO; the rGO coating layer has unique porous structure, high conductivity and various kinds of functional groups, which can not only effectively prevent the diffusion of polysulfide through the separator, but also significantly increase the conducting surface between cathode and separator. It is promising to use rGO-based slurry to continuously produce large-scale, low-cost, and bi-functional rGO coated separator for high-performance Li-S batteries.Lithium-sulfur (Li-S) battery has attracted much attention due to its high theoretical capacity (1675 mAh g −1 sulfur) and energy density (2600 Wh kg −1 sulfur). 1-6 In addition, sulfur is inexpensive, non-toxic, abundant and environmentally benign 7 which makes the Li-S battery becomes one of the most promising rechargeable batteries. However, there are still several challenges hinder the practical application of Li-S batteries: 7-12 the insulating nature of sulfur and its discharge end products Li 2 S and Li 2 S 2 causes poor utilization of active material and decrease of discharge capacity; the high solubility of lithium polysulfides into electrolyte leads to loss of active materials and rapid capacity fading during cycling; the huge volume change during discharge/charge processes results in poor contacts between active material and the conductive substrate and even collapse of cathode structure. To address the above challenges, various strategies have been explored, such as developing conductive polymer-sulfur composites, 7,13,14 imbedding sulfur into various conductive carbon materials, 2,4-6 exploring new kinds of Li-S battery configurations, 11,15,16 using metal oxide as polysulfide absorbents, 17,18 modifying Li anode 3,19 and designing novel electrolyte. 20,21 The dissolved polysulfide intermediates can freely diffuse through the separator and then shuttle between the anode and cathode, resulting in low discharge/charge efficiency and loss of active material. Generally, it is effective and convenient to prevent the diffusion of the polysulfides and localize them in the cathode side by using a functional coating layer on separator. Recently, two kinds of functional separator coating were developed. One is to use electrostatic shield layer Nafion 22,23 or oxide absorbent 24 to trap and deposit dissolved sulfur-containing active materials to improve the performance of Li-S batteries; however, the Nafion is too expensive, and the trapping polysulfides by oxide absorbent is not so effective. Another one is to use conductive carbon materials co...

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