Fiber separators: optimizing bulk and interface behaviors in high energy batteries.

High energy density batteries represent a pivotal focus in the advancement of next-generation energy storage technologies, necessitating rigorous specifications for separators, which are critical inactive components. A significant technical strategy for enhancing battery energy density involves reducing separator thickness. However, the inherent mechanical properties of separators present significant challenges for thinner designs, particularly in mitigating the risk of lithium dendrite penetration and managing dynamic interfacial stress within high energy density batteries. Consequently, this limitation has emerged as a critical bottleneck constraining the performance of high energy density batteries. The meticulous design of fiber separators can improve performance of high energy density batteries across multiple dimensions. In this review, we systematically summarize the interaction mechanisms between fiber separators and the bulk and interface within the battery, elucidating unique roles of fiber separators in tuning ion solvation structures, promoting the formation of a stable solid electrolyte interphase, and mitigating dynamic interfacial stress. Furthermore, we highlight the essential role of advanced characterization and simulation applied across various spatial scales. These methods are vital for addressing existing problems in separator mechanisms and overcoming application challenges in high energy density batteries.