# 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  
Compared with the previously discussed basic lithium-ion batteries, lithium-metal batteries have garnered significant attention from researchers for their exceptional energy density and high voltage. However, the formation of metal dendrites during the electrode process can react with the electrolyte, leading to a rapid decay in battery cycle life, a substantial reduction in coulomb efficiency and energy density, as well as deformation and melting of the separator due to the puncturing and thermal effects of lithium dendrites. These phenomena can ultimately cause irreversible capacity loss, internal short circuits, and even explosions [106; 107; 108]. In order to solve these problems, research on electrodes [109; 110; 111], electrolytes [112; 113; 114; 115; 116], and interfaces [117; 118; 119] has been reported extensively. Importantly, separator materials have also been studied recently, which mainly focus on the suppression effect of lithium dendrite growth and the realization of higher conductivity, better mechanical strength, and thermal stability [120; 121; 122]. However, commercial polyethylene (PE), polypropylene (PP), and glass fiber separators face significant challenges in meeting  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Separator**} & \multirow{2}{*}{ \begin{tabular}{c} **Thermal** \\ **Shrinkage (\%)** \\ \end{tabular}  } & \ \begin{tabular}{c} **Electrolyte** \\ **Uptake** \\ **(\%)** \\ \end{tabular}  & \begin{tabular}{c} **Anode** \\ **/Cathode** \\ \end{tabular}  & \multirow{2}{*}{**Rate Performance**} & \multirow{2}{*}{**Cycle Performance**} & \multirow{2}{*}{**Main Functions**} & \multirow{2}{*}{**Ref. **} \\ \cline{5-6}  & & & & & 135, 113, 83, & \\ \cline{5-6} \cline{5-6} \cline{5-6} \cline{5-6} \cline{5-6} \cline{  
these requirements. Consequently, numerous modification methods have been proposed to overcome these bottlenecks. Attributed to excellent electrical and mechanical properties, graphene or graphene-based composite interlayers have garnered significant attention as coating materials for conventional separators in lithium-metal batteries to provide additional electron transfer paths, increase electron storage and conductivity, reduce the local current density of the positive electrode, and buffer the volume change of the electrode material during the discharge/charge process. [PERSON] et al. established an electrochemical lithium deposition model matching the heat transfer model and explored the time-dependent variation of heat generation powers under different deposition overpotential conditions. They demonstrated the existence of local hot spots at the tip of the protuberance area of inhomogeneous Li growth, where the nonuniform distribution of electric field and large potential difference were detected, which concentrated Li-ion flux and caused further growth of Li dendrites (also called \"tip effect\"). They also produced a polypropylene separator (graphene@PP) covered by a graphene stack for lithium-metal batteries. The graphene side, as in situ thermal dispersion medium, was faced with the lithium metal, which eliminated local heat accumulation around irregular lithium deposition and inhibited further dendrite growth. The Li-Cu cell with graphene@PP as a separator achieved a coulomb efficiency of more than 95% after 250 cycles at 1 mA cm\({}^{-2}\)[100]. [PERSON] et al. demonstrated that a graphene-coated separator (G@PP) was able to obstruct Mn ions by adsorption for lithium protection in lithium-metal batteries with an Mn-based (Ni, Co, Mn, or their combination, denoted as NCM) cathode. There were no obvious Li dendrites in Li | NCM523 batteries with a G@PP separator after 50 cycles [123].