Towards separator safety of lithium-ion batteries: a review

The safety problem of lithium-ion batteries (LIBs) has restricted their further large-scale application, especially in electrical vehicles. As a key component of LIBs, separators are commonly used as an inert component to provide a migration path for the ions and prevent direct contact of the cathodes with the anodes. However, the separator damage caused by external impact puncture, local overheating, and internal and external short circuit leads to the thermal runaway and explosion accidents of LIBs. Therefore, how to enhance separator safety has been a challenge in the development of high-performance LIBs. In this review, the recent advance of high-safety separators with high mechanical strength, high thermal stability and good lithium dendritic resistance is the main focus. Various factors affecting the separator's safety are discussed, including the species of the polymer substrate, structure, synthesis and modification processes. More importantly, the designed strategies towards high safety separator are comprehensively addressed in terms of various aspects: (i) to composite ceramic materials or organic polymers to improve thermal stability, (ii) to modify the polymer and composite structure to increase mechanical strength, (iii) to design a functional separator to regulate ion transport and lithium deposition for mitigating lithium dendrites, and (iv) to intelligently control the thermal runaway of LIBs by flame retardant and thermal shutdown function. Furthermore, the component-structure-performance relationship of separators is summarized, and the impact of separator compositions and structures on the safety of LIBs is emphasized. In addition, the future challenges and perspectives of separators are provided for building high safety rechargeable lithium batteries. The recent advance of high-safety separators with high mechanical strength, high thermal stability and good lithium dendritic resistance is the main focus in this review. The future challenges and perspectives of separators are provided for building high safety rechargeable lithium batteries.