Recent progress in thin separators for upgraded lithium ion batteries

Lithium-based batteries are promising and encouraging energy storage devices in different fields such as portable electronic equipment and new-energy vehicles. Separator, which serves as a physical blockade between electrodes as well as a reliable bridge for ion transport, plays a vital role in maintaining the sustainability of batteries. Unfortunately, most studies in the field of lithium-based batteries have only focused on separators between 20-25 mu m so as to achieve a balance between battery safety and performance. However, such thick separators come at the expense of less free space for accommodating active materials inside the battery, thus impeding further development of next-generation lithium-based batteries with high energy density. Thin separators with robust mechanical strength are undoubtedly prime choice to make lithium-based batteries more reliable and safer. This review focuses mainly on recent developments in thin separators for lithium-based batteries, lithium-ion batteries (LIBs) and lithium-sulfur (Li-S) batteries in particular, with a detailed introduction of thin separator preparation methodologies and an analysis of new progress in separators owning the thickness less than 15 mu m or an ultrathin functional layer less than 1 mu m. Effects of separator thickness on energy density are explored through practical evaluation. Furthermore, a brief outlook for realizing an `infinitely-thin' separator with excellent physical performances and negligible resistance is provided.