# 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  
With high theoretical specific capacity (1675 mAh g-1) and energy density (2600 Wh kg-1) [135], lithium-sulfur (Li-S) batteries are considered one of the most promising next-generation batteries. However, several factors are hindering the practical application of Li-S batteries, including large volume change and the low conductivity of sulfur/lithiated products (Li2 Sn, \(n\geq 1\)), especially the shuttle effect of polysulfide (Li2 Sn, \(n\) > 1) intermediates generated from the incomplete reduction reaction of the cathode, which moves to react with the Li-metal anode causing irreversible loss of active material of anode. The polysulfide's back to the S cathode tends to form a sluggish interface on the cathode surface, which further hinders the reduction of polysulfides. From an external view beyond the electrochemical performance of Li-S batteries, the shuttle effect reduces conductivity and Li+ transfer kinetics, causing serious self-discharge, a sharp decline of capacity, and poor cycle/rate performance. To overcome these issues, it is necessary to limit the sulfur in certain channels and inhibit the diffusion of polysulfides through physical adsorption and/or chemical combination [136; 137; 138]. Over the past decades, many works have been reported that tackle the problems mentioned above by changing the electrode structure, adjusting the composition of electrolytes, and modifying the separator [139; 140; 141]. Among these, separator modification is particularly economical. A suitable separator may prevent the transfer of polysulfides and ensure the normal diffusion of other important ions. A traditional PP separator cannot be used in a Li-S battery system because of its large pore size (~100 nm), which is insufficient to prevent the transportation of polysulfides, and the poor wettability to the electrolyte brings about sluggish ion transfer kinetics through the separator. The graphene-based interlayer with a large specific surface area, abundant adsorption sites, and excellent mechanical properties facilitates the adsorption/capture of polysulfides and buffers the volume change of the S electrode. In addition, constructing a suitable pore size and large pore volume and modifying the graphene material with abundant functional groups on the surface are critical for superior wettability and deposition of polysulfides. In recent studies, [PERSON] et al. produced porous graphene (PG) by the CVD method and coated it on the PP separator. This composite layer effectively captured the polysulfide production due to the high specific surface area of PG and the chemical adsorption of PVP, while the high conductivity of PG facilitated the activation of the sluggish interface between the separator and cathode. The Li-S battery with the PG separator showed an initial discharge capacity of 1165 mAh g-1 (equivalent to 69.7% sulfur utilization) and retained a reversible capacity of 877 mAh g-1 after 150 cycles, with a current density of 0.5 C (Figure 7A) [142]. [PERSON] et al. coated a Celgard 2320 separator with rGO through vacuum filtration. The rGO straddled on the surface of the separator improved the strength of the modification layer, while others inserted into the coating layer increased the conductivity. At 0.2 C after 200 cycles, the reversible specific capacity of the Li-S battery with the rGO-modified separator reached 762.4 mAh g-1 with an average coulombic efficiency of 82.7% [143]. [PERSON] et al. reported a Janus separator with an asymmetrical two-face structure constructed by a mesoporous cellular graphene framework (CGF)/polpropylene separator for a Li-S battery. The CGF-modified separator achieved a reversible capacity of up to 800 mAh g-1 after 250 cycles at 0.2 C (1.0 C = 1675 mA g-1) [144]. [PERSON] et al.