Chemical Vapor Transformation of Lithium Metal: Mechanism and Enhanced Stability

Constructing a stable solid electrolyte interface (SEI) with high productivity and scalability is essential for the practical application of thin Li metal anodes. Here, we report a chemical vapor transformation (CVT) strategy in which Li metal is exposed to trimethylaluminum (TMA), inducing a rapid and spontaneous surface reaction that forms a robust, multilayered SEI. <i>In situ</i> quartz crystal microbalance (QCM) and quadrupole mass spectrometry (QMS), combined with <i>ex situ</i> X-ray photoelectron spectroscopy (XPS), UV Raman spectroscopy, and density functional theory (DFT) calculations, reveal that TMA removes the native passivation layer, reacts with Li metal, and drives a coupled bulk-surface transformation involving Li-Al interdiffusion. The resulting SEI exhibits a chemically graded structure consisting of an inner Li-Al alloy and an outer amorphous carbon layer formed via demethylation and ligand-exchange pathways. This modified surface exhibits significantly enhanced stability compared to that of bare Li in electrochemical cycling using liquid and solid-state electrolytes. This work unveils a unique surface-mediated bulk transformation mechanism for lithium metal and establishes CVT as a scalable and fundamentally distinct approach for the interfacial engineering of reactive metals.