Advances in multi-functional flexible interlayers for Li-S batteries and metal-based batteries

In the last two decades, tremendous attention has been devoted to the development of novel electrodes for boosting the electrochemical performance of batteries. Despite the high energy densities, the novel battery systems, such as Li-S batteries, have encountered several issues that cannot be overcome by the electrode design alone. Therefore, a multi-functional freestanding film is inserted between the anode and cathode, which is normally named as "interlayer", to enhance the electrochemical performance. Thanks to the wide selection of materials, various straightforward, flexible films with controllable physical and chemical properties are rapidly developed and employed as interlayer in rechargeable batteries. This review introduces the design strategies for producing interlayers and describes the essential roles of interlayers in polysulfide adsorption for Li-S batteries and anode protection for alkali-metal batteries and multivalent-ion batteries. Exceptional functionalities of interlayers in providing polysulfide adsorption through physical interaction, H-bond interaction, and chemical interaction in Li-S batteries are summarized. The ameliorating effect of interlayers on metal batteries in suppressing the formation and growth of anode dendrites via mechanically barricading the continuous growth of dendrite, lowering local current density of anodes, reducing the overpotential of nucleation, and offering active sites for uniform ion deposition are reviewed. Recent advances of interlayers on structural design, component optimization, and function mechanism are critically assessed to elucidate the fundamental understandings in various rechargeable battery systems. Potential opportunities of interlayers for novel energy storage applications are revealed to offer new insights towards the development of advanced batteries.