# 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
SEM characterization results showed that Li\({}^{+}\) was deposited in a smoother way, leaving a flat surface without large Li protuberances or sharp Li dendrites when compared to the bare PP film, where the massive Li protuberances (>30 \(\upmu\)m) are indicated with red arrows (Figure 6B) [128]. [PERSON] et al. proposed an environment-friendly transition metal nitride/nitrogen-doped reduced graphene oxide (tMN/N-rGO) multifunctional layer strategy, which prepared the PP separator modified by a VN@N-rGO multifunctional layer. The synergistic effect of N-rGO and lithophilic VN notably improved the electrolyte wettability and mechanical stability of the PP separator for lithium-metal batteries, which maintained 94.9% of their initial discharge capacity after 100 cycles [129]. Afterward, they prepared a protective shield layer consisting of Fe\({}_{3}\)N nanoparticles enfolded by N-rGO (Fe\({}_{3}\)N@NG) for the modification of the PP separator. In the Fe\({}_{3}\)N@NG protective layer, N-rGO was able to buffer the volume change of the lithium-metal electrode, improve the reaction kinetics and cycle life of lithium-metal batteries, and maintain the shape of the separator at different temperatures, owing to the high mechanical strength, excellent thermal stability, and high nitrogen doping content of N-rGO. At the same time, the chemically stable Fe\({}_{3}\)N nanoparticles provided abundant deposition sites for uniform lithium deposition, reduced the local current density applied to the anode electrode, and generated a durable solid electrolyte film. The Fe\({}_{3}\)N@NG/PP separator testified a maximum electrolyte uptake of 114.3% and excellent thermal stability (almost without deformation, even at 150 \({}^{\circ}\)C). Li \(\mid\) \(\mid\) Li symmetric cells constructed by the Fe\({}_{3}\)N@NG/PP separator revealed an ultra-long cycle stability of more than 2300 h at a current density of 5 mA cm\({}^{-2}\)[130]. In addition, a scheme using W\({}_{2}\)N\({}_{3}\) as the transition metal nitride was also investigated [131]. [PERSON] et al. reported a reduced GO/Li-Al layered bimetallic hydroxide interlayer prepared by a hydrothermal method and coated on a PP separator (rGO/Li-Al-LDH@PP). This separator displayed an exceptionally high electrolyte uptake of 232.64%. Furthermore, the rGO/Li-Al-LDH@PP separator maintained its shape with only slight shrinkage at 180 \({}^{\circ}\)C, while the PP separator melted almost completely at 160 \({}^{\circ}\)C. The polarization voltage of the Li \(\mid\) \(\mid\) Li symmetric cell with the rGO/Li-Al-LDH@PP separator was nearly constant after cycling for 1100 h at a current density of 1.0 mA cm\({}^{-2}\) and cut-off capacity of 0.5 mAh cm\({}^{-2}\)[132]. [PERSON] et al. reported a hole graphene oxide@electrospun polyacrylonitrile (HGO-PAN) composite separator for a lithium-metal battery that maintained a discharge capacity of 135.7 mAh g\({}^{-1}\) at 2 C after 900 cycles, which is 22.3% higher than that of a commercial separator-based cell [133]. [PERSON] et al. reported porous polyacrylamide-grafted graphene oxide bilayer composite (GO-g-PAM) separators for Li metal batteries, which showed an average coulombic efficiency of 98% over 150 cycles under 1 mA cm\({}^{-2}\) and more than 1900 h cycling with a cycling capacity of 5 mAh cm\({}^{-2}\) at 20 mA cm\({}^{-2}\) in symmetric Li \(\mid\) \(\mid\) Li cells (Figure 6C) [134]. The above relevant literature information is summarized in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Separator** & **Electrolyte** & \begin{tabular}{c} **Anode** \\ **/Cathode** \\ \end{tabular}  & **Rate Performance** & **Cycle Performance** & **Main Functions** & **Ref. ** \\ \hline \multirow{3}{*}{PP} & 1 M LiPF\({}_{6}\) in & & & & & \\  & DOL/DME (1:1, \(v\)) with 25 LNO\({}_{3}\) & & & & & \\  & & & & &  
Figure 6: Graphene-based materials for the separator of lithium-metal batteries.