Lithium-sulfur batteries have been significantly studied owing to a high theoretical specific capacity of 1672 mAh g^-1. However, the fast capacity fading issue which mainly stems from both polysulfide dissolution into the bulk electrolyte followed by notorious ‘shuttle reaction’ and insulating nature of sulfur are hindering commercialization of lithium-sulfur batteries. Different types of porous materials were investigated to encapsulate the soluble polysulfide species into the pore by physically and/or chemically adsorption, thus the polysulfide species are able to stay in the cathode side. In addition, electrolyte additives such as lithium nitrate and separator coating with various materials were also attempted. Among them, separator coating strategies seemed to be very effective in inhibiting polysulfide diffusion when the effect is evaluated on the basis of the areal unit scale. Furthermore it can be easily scaled up. For example, roll-to-roll process can be utilized in fabricating polysulfide-trapping material-coated separators.

In the aspect of separator oriented strategy, diverse carbon materials such as carbon black [1], carbon nanotubes [2], reduced graphene oxides [3], and microporous carbons [4] were investigated as a polysulfide trapping layer. These attempts were based on the attractive interaction to anchor the polysulfide species in the cathode side. We realized that repulsive force might be also effective in restricting polysulfide species in the cathode side during the electrochemical reactions as illustrated in Figure 1.

Here, we report a facile and effective way to suppress the polysulfide diffusion by introducing a repulsion interaction rather than an attractive interaction on the cathode side of separator. Fluorine functionalized graphene layers were examined as a potential candidate for repulsing polysulfide species from the separator. In addition to the chemically repulsive interaction, 2D shapes of fluorinated graphene layers enhance the effect of polysulfide blocking layers by increasing the diffusion length through the separator. The synergistic effect of chemical and physical nature of the functionalized carbon material led to the improved electrochemical performance. To understand the improved performance, RAMAN, XPS, SEM, AFM and EIS characterizations were conducted.

[1] Adv. Funct. Mater. 2014, 24, 5299

[2] J. Phys. Chem. Lett. 2014, 5, 1978

[3] J. Power Sources 2013, 242, 65

[4] Adv. Mater. 2014, 26, 7352