Precisely deciphering solid electrolyte interphase

Solid electrolyte interphase (SEI) plays a critical role in the cycling stability and safety issues of rechargeable batteries. To provide valuable suggestions for customized SEI regulation, a precise SEI understanding is essential and advanced detection techniques are indispensable. In this review, SEI formation, structure, ion transport, and failure mechanisms were first elucidated from the fundamental perspectives. Emerging detection techniques were briefly introduced, according to the high demands of SEI deciphering in high sensitivity, visualization, quantification, and simulation. Then, emphasis was given to the current advances of SEI study, to provide a systematic cognition of SEI in the aspects of component identification, structural distribution, physical-chemical properties, SEI functionalities (including ion conductivity and electronic insulation), and SEI chemistry-structure-property relationship. In the future, more efforts are suggested to penetrate into basic scientific issues, dynamic processes, multi-technique integration, and simulational techniques to provide more reliable understanding and guidance of high-quality SEI. Advanced alkali metal (such as Na, Li, and K) battery systems, which play a very important role in the large-scale energy storage field, have put forward high demands for long lifespan. To ensure stable operation, a passivation film called the solid electrolyte interphase (SEI) is very critical, and it acts as the bridge between the electrolyte and the electrode, becoming the rate-determining step to determine the overall alkali metal ion delivery. Besides, a series of battery deterioration and safety problems, such as continuous electrolyte consumption, increased battery polarization, and drastic thermal eruption, are supposed to be initiated by SEI failure. However, due to the complex, versatile, and fragile nature of SEI, people’s understanding of SEI remains rather limited. Considering its critical role, this review provides a comprehensive overview of the SEI, including the fundamental mechanisms, the emerging detection techniques, and the recent cognitive advances. Additionally, it also puts forward the challenges and perspectives of SEI study in the future. This review summarizes the current understanding of the solid electrolyte interphase (SEI) layer, from its fundamental mechanisms, advanced detections, and research progress. Challenges and perspectives are discussed to encourage more efforts dedicated to the remaining issues with SEI. This review aims to provide valuable guidance for SEI beginners to quickly attain current SEI advances, inspire more endeavors to search for the last piece of the puzzle in battery systems, and contribute SEI understanding from vagueness to clearness.