Solid electrolyte interphases in lithium metal batteries

Lithium metal batteries (LMBs) have recently received enormous interest as a higher energy density alternative to conventional lithium-ion batteries (LIBs). However, the commercialization of LMBs is currently impeded by poor cycle life due to inhomogeneous lithium deposition and active lithium loss. These are controlled by the solid electrolyte interphase (SEI) that forms on the anode surface, and there have been numerous reported strategies to produce SEIs with desired properties. However, these have not been sufficient to achieve the high cycling stabilities necessary for widespread LMB commercialization, requiring additional understanding of the SEI. In this perspective, we highlight recent progress in characterizing the SEI that forms in LMBs and outline the need to consider SEI nanostructure, transport, and mechanical properties together. We conclude by prescribing several key research fronts necessary for an accurate, systematic study of the SEI that will guide future electrolyte design and enable the development of safe and stable LMBs.