Lithium metal is a promising anode to provide high energy density for next-generation batteries. However, it has not been implemented due to its low cycling efficiency, which results from the formation of an unstable solid electrolyte interphase (SEI). The SEIs formed with traditional liquid electrolytes are heterogeneous and easy to crack during cycling, thus resulting in the formation of dendritic and dead Li, and further devastating the electrode performance. To solve these issues, efforts have been made to replace natural SEIs with artificial SEIs (ASEIs). Here, we discuss critical design principles of ASEIs based on the understanding of SEI failure mechanisms. Three key principles for a successful ASEI are identified: (1) mechanical stability, which can be either high strength or adaptivity, (2) spatially uniform Li+ transport with moderate conductivity and even single-ion conduction, and (3) chemical passivation to mitigate Li-electrolyte parasitic reactions. Selected examples of recently developed ASEIs are categorized and elaborated. Finally, future directions are given for ASEI designs.
