Li⁺ Ion Transport at the Lithium Metal Anode: A Fundamental Picture and Future Perspectives

The unsurpassed energy density of lithium metal batteries (LMBs) offers a promising route to meet increasing clean energy demand. Nonetheless, their instability creates critical safety and performance concerns that prevent their effective implementation. Careful design of the solid-electrolyte interphase (SEI) that passivates the lithium metal anode could allow for stable, efficient cycling, unlocking the commercialization of LMBs. However, the complexity of the SEI, both formed in situ through electrolyte decomposition and fabricated ex situ, introduces significant ambiguity. Here, we present several mechanisms that define ionic transport through the SEI, grounded in studies that concretely relate structure to ionic conductivity. We build on these mechanisms with examples of their application in real battery systems. Overall, we seek to lay a foundation of structure–property relationships that will guide the rational design of stable SEIs that efficiently transport Li + ions.