The battery separator is one of the most essential components that highly affect the electrochemical stability and performance in lithium-ion batteries. In order to keep up with a nationwide trend and needs in the battery society, the role of battery separators starts to change from passive to active. Many efforts have been devoted to developing new types of battery separators by tailoring the separator chemistry. In this article, the overall characteristics of batter. The battery separator is one of the most essential components that highly affect the electrochemical stability and performance in lithium-ion batteries. In order to keep up with a nationwide trend and needs in the battery society, the role of battery separators starts to change from passive to active. Many efforts have been devoted to developing new types of battery separators by tailoring the separator chemistry. In this article, the overall characteristics of battery separators with different structures and compositions are reviewed. In addition, the research directions and prospects of separator engineering are suggested to provide a solid guideline for developing a safe and reliable battery system.••Battery separatorNext-generation batteriesBattery safetyPolyolefin separatorSurface-modified separatorCeramic-coated separatorWith the increasing demand for high-performing electronic devices and a global mission to reduce greenhouse gases created by fossil fuels, tremendous attention has been paid to the development of rechargeable energy storage systems, especially for lithium-ion batteries (LIBs) [1, 2, 3, 4]. Since the advent of practical LIBs in our everyday life, numerous researches have been performed by replacing each of the battery components with new types of materials in order to improve the energy/power density, electrochemical stability, and cyclability of LIBs [5, 6, 7, 8, 9]. Among the essential components, a battery separator is the main component responsible for the overall safety of batteries [10, 11, 12]. The major role of the battery separator is to physically isolate the anode from the cathode while allowing mobile Li-ions to transport back and forth. Unfortunately, two technical challenges associated with separator puncture and significant thermal shrinkage of polymer separators threaten the overall safety of batteries. In addition, most researches have mainly focused on reducing the total thickness of the battery separator and increasing the size of battery packs, i.e. putting large amount of active materials into the battery [14,15]. This approach also raises serious safety concerns with respect to short circuits [14,15].In order to keep up with the recent needs from industries and improve the safe. Figure 1 illustrates how each phase of the battery separators plays a role in affecting the morphology of the deposited Li on the electrode and thus protecting the battery from safety hazards. Polyolefin separators (termed 'first-phase membrane') have high porosity and insulative properties. But these are easily deformed by thermal or mechanic. Despite the remarkable advancement of separator technology, it is still not able to eliminate the root cause of unexpected threats such as dendritic Li formation. The adoption of solid-state electrolytes would be the feasible solution to circumvent most of the safety hazards with regard to thermal runaway and short-circuit. However, the relatively low ionic conductivity and high interfacial resistance of solid-state electrolytes are the two biggest obstacles for facilitating the operation of high-power electronic devices. Considering these points, it would be necessary to sort each type of the batteries in terms of specific purposes of use. For instance, solid-state batteries would be more suitable for applications requiring high energy density and long-term durability. In contrast, liquid-electrolyte LIBs with an advanced separator would be more appropriate for applications requiring high-power densities. In this regard, extensive researches on both surface-modified separator (second-phase membrane) and solid-state electrolyte (third-phase membrane) should be conducted in parallel to meet each of the requirements from the different applications.In the future, the most important part of the advanced separator research is how to reduce the thickness of the coating-layer on the separator without sacrificing the electrochemical performance and stability. The studies on scaling-up the solid-state electrolyte from laboratory to industrial level, establishing a cost-eff.
