Structural insights into solid electrolyte interphase (SEI) on lithium metal anode: From design strategies to the stability evaluation

Solid electrolyte interphase (SEI), known as an ionic conductor but electronic insulator, plays a dominant role in cycling stability and safety of rechargeable lithium metal batteries (RLMBs). Although tremendous endeavors have been devoted to identifying the chemical component and spatial distribution of SEI, the linkage between exact nanostructure and its stability still remains to be clarified. In this highly focused research area, plenty of profound papers have been published. However, structural insights, designing guidelines and corresponding stability evaluation approaches of SEI film are still not comprehensively summarized. In this review, we extract major breakthroughs of SEI structure, and then elucidate the mechanism insights of SEI film, including formation mechanism and transportation mechanism of Li+ in the SEI film. Subsequently, we systematically summarize the design principles and strategies for a stable SEI film. Afterwards, we propose general evaluation standards for the stability of SEI film, including chemical/electrochemical methods, atomic-level characterization techniques, and theoretical calculations. Finally, personal perspectives on the construction of stable SEI film are provided at the end of the manuscript. We envision that this timely review can be used as a handy reference for designing stable SEI films with a unique nanostructure and motivate researchers to build better RLMBs.