Galvanic Corrosion and Electric Field in Lithium Anode Passivation Films: Effects on Self-Discharge

Battery interfaces help govern rate capability, safety/stability, cycle life, and self-discharge, but significant gaps remain in our understanding at atomic length scales that can be exploited to improve the interfacial properties. In particular, Li partially plated on copper current collectors, relevant to the anodeless, lithium metal cell, which is a holy grail of high-density-energy battery research, has recently been reported to undergo galvanic corrosion and exhibit short shelf lives. We apply large-scale density functional theory (DFT) calculations and X-ray photoelectron spectroscopy to examine the reaction between the electrolyte and Li| Cu junctions coated with uniform, thin electrolyte interphase (SEI) passivating films at two applied voltages. The DFT galvanic corrosion calculations show that electrolyte degradation preferentially occurs on Li-plated regions and should lead to thicker SEI films. We find similarities but also fundamental differences between traditional metal localized pitting and Li-corrosion mechanisms due to overpotential and ionic diffusion rate disparities in the two cases. Furthermore, using the recently proposed, highly reactive lithium hydride (LiH) component SEI as an example, we distinguish between electrochemical and chemical degradation pathways that are partially responsible for self-discharge, with the chemical pathway found to exhibit slow kinetics. We also predict that electric fields should, in general, exist across natural SEI components like LiH and across artificial SEI films like LiI and LiAlO2 often applied to improve battery cycling. Underlying and unifying these predictions is a framework of DFT voltage/overpotential definitions, which we have derived from electrochemistry disciplines like structural metal corrosion studies; our analysis can only be made using the correct electronic voltage definitions.