  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 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 thin, uniform
electrolyte interphase (SEI) passivating films at two applied voltages. These
novel DFT galvanic corrosion simulations show that electrolyte degradation
preferentially occurs on Li-plated regions and should lead to thicker SEI
films. Our simulations reveal similarities but also fundamental differences
between traditional metal localized pitting and Li-corrosion mechanisms.
Furthermore, using the recently proposed, highly reactive lithium hydride (LiH)
component SEI as example, we distinguish between electrochemical and chemical
degradation pathways which 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 LiAlO(2) 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.
