# 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
The initial discharge capacities of the Li-S battery with the PAN/GO separator were 699, 591, and 448 mAh g\({}^{-1}\) at 0.2, 0.5, and 1 C, respectively, while the open-circuit voltage of the battery was as high as 2.73 \(V\), indicating an exceptional anti-self-discharge capability [152]. [PERSON] et al. reported a high-performance ultrathin light GO layer loaded with Cobalt phthalocyanine (CoPc) on a polypropylene (PP) separator. The initial specific capacity of the battery with the CoPc@GO-PP separator was 1092 mAh g\({}^{-1}\) and retained 757 mAh g\({}^{-1}\) at 1 C after 400 cycles and 919 mAh g\({}^{-1}\) after 250 cycles at 0.5 C (Figure 7C) [153]. [PERSON] et al. prepared a sandwich structure modified separator with nano-scale flower strontium fluoride and graphene (SrF\({}_{2}\)-G), which was filtered onto the PP separator. The graphene layer at the upper and lower layers not only acted as an effective physical barrier to block the shuttle of polysulfides but also enhanced electron transport to improve the reactivity. SrF\({}_{2}\) in the middle layer strongly adsorbed polysulfides and catalyzed the transformation of polysulfides. At a current density of 0.2 C, the initial specific discharge capacity separator of the Li-S battery assembled with the SrF\({}_{2}\)-G separator was 1188 mAh g\({}^{-1}\) and maintained at 931 mAh g\({}^{-1}\) after 200 cycles [154]. [PERSON] et al. synthesized a composite constructed by an N-doped graphene sheet and N-doped carbon nanotube filled with Ninanoparticles (Ni@NG-CNTs), which was coated on the PP separator for a Li-S battery. The N-doped nanocage structure integrated with a 1D carbon nanotube and 2D graphene was demonstrated to block polysulfide diffusion, providing abundant Li\({}^{+}\) transfer channels and catalytic sites for redox reactions. The Li-S batteries with the Ni@NG-CNTs-PP separator kept high reversible specific capacities of 309.1 and 265.0 mAh g\({}^{-1}\) at 5 and 10 C rates after 500 cycles, respectively. At 0.2, 0.5, 1, and 2 C, the discharge-specific capacities were 1036.7, 934.8, 821.9, and 710.9 mAh g\({}^{-1}\) for the Ni@NG-CNTs-PP separator, respectively [155]. [PERSON] et al. used a hydrothermal reaction to synthesize a Cu5/graphene composite as an efficient polysulfide immobilizer onto the pristine separator. The separator also reactivated the accumulated necrotic sulfur-containing species. The Li-S battery with the Cu5/graphene-coated separator exhibited an initial discharge capacity of 1302 mAh g\({}^{-1}\) at 0.2 C, maintaining at 760 mAh g\({}^{-1}\) after 100 cycles [156]. [PERSON] et al. coated graphene and sulfur directly onto the PP separator (S-G@PP) for a Li-S battery, which strengthened the adhesion between the sulfur and graphene, lowered the interfacial contact resistance, and improved the energy density and rate performance with the lightweight graphene layer as a current collector and barrier for polysulfides. The Li-S batteries with the S-G@PP separator exhibited 1128, 980, 833, 670, 586, and 512 mAh g\({}^{-1}\) at 0.75, 1.5, 3, 6, 9, and 12 A g\({}^{-1}\), respectively [145]. [PERSON] et al. prepared hierarchical oxygen-doped carbon on the surface of reduced graphene oxide (ODC/rGO) for Li-S battery separator modification. The combination of physical adsorption by hierarchical structure and chemical adsorption by oxygen-containing groups exhibited effective adsorption of the diffusing polysulfides. The ODC/rGO maintained a discharge capacity of 456.5 mA h g\({}^{-1}\) at 5 C, which is much higher than that (53.6 mAh g\({}^{-1}\)) of the battery with the bare glass fiber separator [157]. [PERSON] et al. coated an rGO/sodium lignosulfonate composite layer directly onto the standard PP separator (rGO@SL/PP) for a Li-S battery; abundant negatively charged sulfonic and dendritic groups effectively hindered PS shuttling while ensuring excellent Li\({}^{+}\) transport kinetics. The Li-S battery with rGO@SL/PP showed an initial specific capacity of 707 mA h g\({}^{-1}\) at 2 C, which retained 74% over 1000 cycles [158].