Unveiling the Hidden Interface: Pre‐SEI Governs Li Morphology in Anode‐Free Li Metal Batteries

Anode-free lithium metal batteries (AFLMBs) are emerging as a promising next-generation energy storage system due to their high energy density and simplified manufacturing process. However, the absence of excess lithium makes them highly vulnerable to dendritic growth and irreversible lithium loss, leading to rapid capacity decay. Despite extensive efforts in current collector modification and electrolyte optimization, the interfacial chemistry between the electrolyte and current collector remains poorly understood. Here, we reveal that a pre-solid electrolyte interphase (pre-SEI) can spontaneously form on the Cu surface during the resting stage, governed by the electrolyte composition. Aggregated anion-cation complexes (AGGs) with low lowest unoccupied molecular orbital (LUMO) levels undergo preferential reduction at the Cu surface, forming a mechanically protective layer. This pre-SEI effectively guides uniform Li nucleation, suppresses dendritic growth, and dramatically improves cycling stability. In contrast, when no pre-SEI is present, dendritic Li and dead Li accumulation occur even with LiF- and Li₃N-rich SEI layers, resulting in rapid capacity fading. Our findings uncover the hidden interfacial chemistry that governs the reversibility of AFLMBs and establish pre-SEI formation as a critical design principle for achieving durable, high-performance anode-free batteries.