Anode-free lithium metal batteries (AFLMBs) are emerging as a promising next-generation energy storage system due to theirhigh energy density and simplified manufacturing process. However, the absence of excess lithium makes them highly vulnerableto dendritic growth and irreversible lithium loss, leading to rapid capacity decay. Despite extensive efforts in current collectormodification and electrolyte optimization, the interfacial chemistry between the electrolyte and current collector remains poorlyunderstood. Here, we reveal that a pre-solid electrolyte interphase (pre-SEI) can spontaneously form on the Cu surface during theresting stage, governed by the electrolyte composition. Aggregated anion–cation complexes (AGGs) with low lowest unoccupiedmolecular orbital (LUMO) levels undergo preferential reduction at the Cu surface, forming a mechanically protective layer. Thispre-SEI effectively guides uniform Li nucleation, suppresses dendritic growth, and dramatically improves cycling stability. Incontrast, when no pre-SEI is present, dendritic Li and dead Li accumulation occur even with LiF- and Li 3 N-rich SEI layers, resultingin rapid capacity fading. Our findings uncover the hidden interfacial chemistry that governs the reversibility of AFLMBs andestablish pre-SEI formation as a critical design principle for achieving durable, high-performance anode-free batteries.