The study of interfacial properties, especially of their change upon lithiation, is a fundamentally significant and challenging topic in designing heterogeneous nanostructured electrodes for lithium ion batteries. This issue becomes more intriguing for Si electrodes, whose ultrahigh capacity is accompanied by large volume expansion and mechanical stress, threatening with delamination of silicon from the metal current collector and failure of the electrode. Instead of inferring interfacial properties from experiments, in this work, we have combined density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations with time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements of the lithium depth profile, to study the effect of lithiation on the a-Si/Cu interface. Our results clearly demonstrate Li segregation at the lithiated a-Si/Cu interface (more than 20\% compared to the bulk concentration). The segregation of Li is responsible for a small decrease (up to 16\%) of the adhesion strength and a dramatic reduction (by one order of magnitude) of the sliding resistance of the fully lithiated a-Si/Cu interface. Our results suggest that this almost frictionless sliding stems from the change of the bonding nature at the interface with increasing lithium content, from directional covalent bonding to uniform metallic. These findings are an essential first step toward an in-depth understanding of the role of lithiation on the a-Si/Cu interface, which may contribute in the development of quantitative electrochemical mechanical models and the design of nonfracture-and-always-connected heterogeneous nanostructured Si electrodes. © 2013 American Chemical Society.