# Graphene-Based Materials for the Separator Functionalization of Lithium-Ion/Metal/Sulfur Batteries
## 2 Preparation Methods of Graphene-Based Materials
### The Use of Graphene-Based Materials for the Separator of a Lithium-Sulfur Battery
proposed a flexible integrated structure of sulfur and graphene on a PP separator (S-G@PP) for Li-S batteries in which the graphene membrane was coated directly on the PP separator as an internal current collector to support electrode materials with 70 wt% pure sulfur. After 30 cycles at 0.75 A g-1, the flexible Li-S battery with the S-G@PP separator retained a capacity of 722 mA h g-1 with a Coulombic efficiency of 98% [145]. Weak adsorption of non-polar bare graphene on polar polysulfide hinders the achievement of higher performance by the separator [146; 147]. To address this issue, heteroatoms, such as N, P, and S, can be doped onto the graphene surface, forming polar bonds and strengthening the interaction force with polysulfide to inhibit the shuttle effect [147], while the introduction of abundant defects effectively amplified the specific capacity and improved the ion transfer kinetics [148; 149; 150; 151]. [PERSON] et al. used H\({}_{3}\)BO\({}_{3}\)/NH\({}_{2}\)CN for GO reduction to obtain B-doped and N-doped graphene and coated them on the separator, respectively, for comparison. The battery with a nitrogen-doped graphene composite separator maintained a specific capacity of 430 mAh g\({}^{-1}\) after 400 cycles at 0.2 C, delivering better cycling stability [148]. [PERSON] et al. synthesized a two-dimensional layered nitrogen-doped graphene (NG) composite coating on the separator of a Li-S battery. The N atoms mainly distributed at the edge and inside of the carbon plane showed outstanding absorption of polysulfide through dipole-dipole interaction; meanwhile, a catalyzed redox reaction converted soluble polysulfide intermediate into insoluble Li\({}_{2}\)S. At a rate of 1 C, the initial discharge capacity was 794.6 mAh g\({}^{-1}\) and remained at 575 mAh g\({}^{-1}\) after 400 cycles [149]. O-containing functional groups, such as carboxyl, aldehyde, and hydroxyl groups, were demonstrated with a stronger anchor effect on polysulfides than on N-containing functional groups and a superb catalytic effect on the conversion of polysulfides. [PERSON] used the unique excimer ultraviolet light (EUV) technique to induce the generation of O-containing functional groups on graphene material, which was coated onto the glass fiber separator (EUV/graphene separator) for Li-S batteries. The battery with the EUV/graphene separator had an initial specific capacity of up to 1164 mAh g\({}^{-1}\) at 0.2 C and remained at 640.5 mAh g\({}^{-1}\) after 400 cycles [150]. [PERSON] et al. coated the S electrode with a high-concentration GO solution as an interlayer between the sulfur electrode and the separator. The study found that GO presented as a columnar liquid crystal state at high concentrations. During the coating process, GO was arranged in an ordered structure in the direction of shear stress, presenting a layered and ordered structure that not only suppressed polysulfide ions effectively but also reduced the bending degree and roughness between layers. Thus, it successfully decreased the transport resistance of lithium ions across the GO layer. The carbon composite separator modified with the GO interlayer in the Li-S battery delivered an initial discharge-specific capacity of 1182 mAh g\({}^{-1}\). It also maintained 835 mAh g\({}^{-1}\) after 100 cycles at 0.5 C, showing a satisfying sulfur utilization rate [151]. Expected for the combination of advantages from different materials to build up high-performance separators in Li-S batteries, multilayer composites comprising graphene and other materials have been extensively studied. [PERSON] et al. employed an electrospinning method to prepare a PAN/GO nanofiber membrane separator, which exhibited excellent performance due to the synergistic effect of the N groups of PAN and the O groups of GO.