Recent advances in rechargeable battery technology have intensified the search for next-generation systems with higher energy densities and longer cycle lives. Lithium metal batteries (LMBs) have emerged as a promising candidate, offering a substantially greater theoretical capacity than conventional lithium-ion batteries. However, practical implementation of LMBs is hampered by issues such as dendrite growth and the instability of the Li plating–stripping interface. In our previous work, we demonstrated that depositing a thin SnO₂ layer onto Cu current collectors via atomic layer deposition (ALD) can effectively facilitate uniform Li nucleation and suppress dendrite formation, thus enhancing both the efficiency and cycle life of LMBs [1]. Nevertheless, the intrinsic volume expansion of SnO₂ during repeated charge–discharge cycles exerts mechanical stress, which may lead to cracks or ongoing solid electrolyte interphase (SEI) consumption, ultimately causing gradual capacity loss. To address these challenges, we introduced an Al₂O₃ artificial SEI layer on top of the SnO₂-coated Cu. By depositing Al₂O₃ layers ranging from 0.5 nm to 20 nm and performing electrochemical analyses in Li metal half cells, we found that a 2 nm Al₂O₃ coating provided the highest efficiency and longest cycle life. We suggest thatthis performance to the Al₂O₃ film’s ability to mitigate mechanical degradation while protecting the current collector from oxidation. Consequently, this study offers new insights into the design of artificial SEI layers, underscoring their critical role in achieving sustained operational reliability in LMBs. Figure 1