A self-assembled monolayer regulates solid electrolyte interphase formation in anode-free lithium metal batteries

Controlling solid electrolyte interphase (SEI) formation is critical for anode-free lithium-metal batteries (AFLMBs). However, plain copper (Cu) current collectors fail to form a reliable SEI during long Li plating and stripping cycles. Here, we demonstrate that a self-assembled monolayer (SAM) on Cu can direct SEI chemistry by simultaneously modulating the electrolyte accessibility and interfacial electronic structure. We investigated carboxylate-terminated SAMs of varying chain length, specifically thioglycolic acid (TGA) and mercaptoundecanoic acid (MUA), as modifiers for copper foil. The polar carboxylate terminal group enhances the accessibility of electrolytes to the Cu surface, with TGA-modified Cu (TGA@Cu) showing complete wetting by a carbonate electrolyte solution. Density functional theory shows that the TGA and PF₆^- interaction lowers the anion's lowest unoccupied molecular orbital energy from -0.02 to -0.73 eV for Cu and TGA@Cu, respectively. This shift facilitates the anion decomposition, leading to an SEI enriched with LiF and Li₂O. When paired with an NMC622 cathode, the TGA@Cu enhances the full-cell AFLMB performance, achieving 82% capacity retention over 30 cycles, compared to 55% with a plain Cu current collector. This work shows that SEI formation can be tuned by tuning the molecular-level electronic structure and interfacial accessibility, providing guidance for current collector interfacial design for AFLMBs.