# Graphene-Based Materials for the Separator Functionalization of Lithium-Ion/Metal/Sulfur Batteries
## 1 Introduction  
Given the escalating concerns surrounding pollution and climate change as a result of the extensive utilization of non-renewable resources, it has become crucial to prioritize the development of energy storage technologies that are sustainable, efficient, and environmentally friendly. With the decreasing cost of renewable energy sources, such as wind and solar power, rechargeable batteries have emerged as a superior option to traditional mechanical and hydraulic energy conservation due to their higher energy density and accessibility. In particular, lithium-ion batteries have gained immense popularity in various industries, including the automotive, electronic, and aerospace sectors, owing to their remarkable operating voltage, substantial energy density, minor memory effect, and eco-friendliness. Additionally, lithium-metal and -sulfur batteries, which boast higher theoretical specific capacity and lower costs, are widely regarded as one of the most promising candidates for next-generation energy storage systems. In recent decades, the separator has developed significantly as a crucial component of lithium-ion/metal/sulfur batteries through extensive research efforts. It has been established that the separator must exhibit desirable insulation and mechanical strength to prevent short-circuiting of electrodes and the appropriate pore size and porosity to facilitate the uniform transmission of lithium ions. Furthermore, excellent thermal stability, adequate wettability, and electrolyte retention are critical to enhancing battery capacity and performance. Hence, selecting a separator with comprehensive performance is essential to improve battery performance [1; 2; 3; 4]. Commercial battery separators typically use semi-crystalline polyolefin-based materials due to their good insulation, high corrosion resistance, low cost, and easy processing. The Celgard separator is a widely used example, consisting of a polyethylene (PS) and polypropylene (PP) composite prepared on a large scale via the dry stretching method [5; 6; 7]. However, polymer separators have drawbacks, including high crystallinity, low polarity, low surface energy, and poor wettability with the electrolyte, leading to electrolyte leakage. Furthermore, the stretching process results in low porosity, reducing the absorption rate of the electrolyte, impeding lithium-ion transport, and negatively impacting the battery's electrochemical performance [8]. Additionally, the thermal stability of polymer separators is limited. In the event of battery overcharge or abuse resulting in thermal runaway, the separator may fail to shut down the reaction in time, leading to extensive contact between positive and negative materials and a significant threat to battery safety. The functionalized composite separator, which modifies the polymer separator with advanced material as an interlayer between the conventional separator and electrode, has been developed as a promising strategy to provide excellent insulation, superior thermal/mechanical strength, and an appropriate self-shutdown function. Meanwhile, the higher wettability offers high-speed ion transfer channels, and the active material and functional groups loaded on the surface of the separator promote electron storage, which improves the specific capacity of batteries. Particularly, a rationally designed functional separator can also inhibit the severe lithium dendrite growth in the lithium-metal battery and the shuttle effect of polysulfides in the lithium-sulfur battery. As the star material over recent years, graphene-based materials (such as graphene, graphene oxide (GO), reduced graphene oxide (rGO), and graphene-related composite) are widely used in the field of electrode materials, and the application of them to the separator has received various attempts [9; 10].