# 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-Ion Battery  
Due to high energy density and long cycle life, lithium-ion batteries are regarded as the most favorable choice among all secondary batteries. The separator plays a critical role in enhancing electrochemical performance and avoiding some accidents, such as shortcuts, combustion, and even explosion. Graphene-based materials with higher conductivity, better mechanical strength, large specific area, thermal/chemical stability, and suitable pore structure are widely used for separator modification to provide low impedance, large buffer area for electrode volume change, and abundant active sites, which improve the specific capacity, cycle number, and safety. A suitable pore structure and hydrophilic surface functional group offer excellent wettability, which remarkably promotes ion transfer kinetics. Oxygen-containing functional groups on the surface of GO material are usually used as the catalyst for a redox reaction, while excellent insulation makes GO-based material a promising interlayer between the separator and electrode. In addition, hydrophilic sites on the surface of GO, such as -OH, -O-, and -COOH functional groups, effectively improve the wettability of the separator to the electrolyte, which promotes Li+ transfer [90]. [PERSON] et al. found that the addition of porous GO nanoflakes significantly improved the thermal stability and electrochemical performance of the electrospun polyurethane (PU) separator. Under 170 \({}^{\circ}\)C for 1 h, the Celgard film showed large shrinkage, while the shrinkage of the PU@GO separator was only \(\sim\)20%. The high porosity of the PU@GO separator enabled an extraordinary electrolyte absorption capacity of up to 733%, far beyond that of traditional Celgard separators (\(\sim\)77%). The high electrolyte retention further increased ionic conductivity to 3.73 mS cm\({}^{-1}\). The reversible discharge capacity of the PU@GO separator after 100 cycles was 154 mAh g\({}^{-1}\), indicative of 95.6% of the first cycle [101]. [PERSON] et al. used an electrospinning technique to prepare a polyvinylde@graphene oxide (PI@GO) separator in situ with a graphene oxide (GO) nanosheet as the reinforcing agent, which improved the thermal stability, mechanical properties, and ionic conductivity of the PI membrane. When the working temperature rose to 180 \({}^{\circ}\)C, the Celgard separator showed a shrinkage rate of up to 90%, while the composite material PI@GO remained unchanged. The Li/LiFePO\({}_{4}\) battery with the PI@GO separator had a discharge capacity of 160 mAh g\({}^{-1}\) at 0.2 C and 50 mAh g\({}^{-1}\) at 5 C [102]. Using polystyrene (PS) nanoparticles as hard templates, [PERSON] et al. designed and prepared a novel kind of GO grafted hyperbranched polyether (GO-g-HBPE) macroporous membrane without any polymer binder. The GO-g-HBPE separator proved to have good wettability with an electrolyte absorption rate as high as 158% and thermal dimensional stability without dimensional change at 200 \({}^{\circ}\)C for 0.5 h. The cell using the GO-g-HBPE-based separator finally showed an initial discharge capacity of 160 mAh g\({}^{-1}\) and kept 84% of that after 200 cycles (Figure 5A) [103]. Figure 5: Morphology and electrochemical performance of Graphene-based materials for the separator of lithium-ion batteries. (**A**) (**i**) Mechanism diagram of independent GO-g-HBPE macroporous membrane; the SEM images of (**ii**) high-magnified cross-section of GO-g-HBPE free-standing macroporous membrane and (**iii**) GO-g-HBPE; Electrochemical performance of the LiFePO4 half cells assembled with PP and GO-g-HBPE separators; Lithium foil served as both the counter and reference electrodes: (**iv**) Rate performance; (**v**) 1 st cycle AC impedance spectra of the GO-g-HBPE separator and the PP separator (**vi**) Cycling performance; (**vii**) Charge–discharge performance at an elevated temperature of 80 \({}^{\circ}\)C [103]. Reproduced with permission: Copyright 2017, Royal Society of Chemistry.