# 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-Metal Battery
The introduction of heteroatoms into the graphene layer is an effective way to provide abundant reactive sites, improve the reaction kinetics, and strengthen the interaction with Li\({}^{+}\), which contributes to higher specific capacity and cycle stability for Li-metal batteries with a graphene-based separator. [PERSON] et al. prepared nitrogen-sulfur co-doped graphene nanosheet (NSG) dispersions through a GO reduction method, which was coated on one side of a polyethylene separator by the vacuum infiltration method. The NSG on the separator was demonstrated to be effective in inhibiting dendrite growth, promoting the uniform distribution of Li\({}^{+}\) above the lithium-metal surface, and further avoiding massive, localized lithium precipitation at the electrode. The ultra-thin layer of NSG nanosheets also bolstered the dimensional stability of the polymer separator at high temperatures. In addition, the electrostatic attraction between the lone pair electrons of N/S atoms and the lithium metal enhanced the interfacial interaction between the separator and Li anode, releasing the surface tension of lithium metal. Meanwhile, the NSG was used as a physical barrier and relied on excellent mechanical properties, which also inhibited the initiation of lithium dendrite growth. At a 0.5 C rate, the Li/LiNi\({}_{0.8}\)Co\({}_{0.15}\)Al\({}_{0.05}\)O\({}_{2}\) cell with PE/NSG as separator delivered an initial discharge specific capacity of 200 mAh g\({}^{-1}\) and retained 85% of that after 240 cycles, and the impedance kept a low value during 200 cycles, indicating remarkable electrochemical stability and reaction kinetics properties (Figure 6A) [124]. [PERSON] et al. reported a novel type of metal/graphene oxide (M/GO) battery in which M was used as the anode while GO acted as both the cathode and separator. The insulating nature of GO avoided occurring a short circuit of the battery. The discharging capacities of the Li/GO battery reached up to 245.6 mAh g\({}^{-1}\), 239.7 mAh g\({}^{-1}\), 227.5 mAh g\({}^{-1}\), and 210.5 mAh g\({}^{-1}\) at 0.5 C, 1 C, 2 C, and 5 C respectively (1 C = 175 mA g\({}^{-1}\)) [125]. [PERSON] et al. reported an F-doped reduced graphene oxide fiber (rGOF) attached to an aramid paper separator (rGOF-A). During solid electrolyte interphase (SEI) layer formation, the decomposition of the LiPF\({}_{6}\) electrolyte produced F\({}^{-}\) anions, which formed C-F bonds on the rGOF surface, thereby facilitating F doping and generating LiF-stabilized SEI between the Li-metal anode and separator. After 1000 cycles, Li-metal batteries with the rGOF-A reached 98.06% (5 C-rate), 89.80% (10 C-rate), and 79.91% (20 C-rate) of the initial specific capacity [126]. Moreover, composite interlayers incorporating graphene with/without heteroatom doping and other materials have also been reported to realize the multi-functionalization and higher performance of graphene-based separator materials, offering more possibilities for designation. [PERSON] et al. reported a multifunctional trilayer film of polydopamine-graphene-carboxymethylcellulose (PDA/Gr-CMC) deposited on a PP separator. The PDA coating conferred hydrophilicity to the PE separator, which enhanced its wettability with the electrolyte and promoted the uniform distribution of Li-ion flux on the lithium-metal surface. Meanwhile, the carboxymethyl cellulose (CMC) aqueous binder also improved the hydrophilicity of the graphene layer. The Li/LiFePO\({}_{4}\) (LFP) full cell with PDA/Gr-CMC as the separator revealed a specific capacity close to 100% of the initial discharge capacity after 500 cycles [127]. [PERSON] et al. designed a BN/graphene bilayer separator (BN\({}_{\mathrm{x}}\)Gr\({}_{\mathrm{y}}\)/PP) which exhibited notably higher wettability with the electrolyte than the bare PP. The BN/Gr layer reduced the local current density of the PP separator during Li plating/stripping, improved the thermal conductivity and mechanical stability, and reduced the impedance of the cell, which resulted in better electrochemical performance.