  Continued growth of the solid electrolyte interphase (SEI) is the major
reason for capacity fade in modern lithium-ion batteries. This growth is made
possible by a yet unidentified transport mechanism that limits the passivating
ability of the SEI towards electrolyte reduction. We, for the first time,
differentiate the proposed mechanisms by analyzing their dependence on the
electrode potential. Our calculations are compared to recent experimental
capacity fade data. We show that the potential dependence of SEI growth
facilitated by solvent diffusion, electron conduction, or electron tunneling
qualitatively disagrees with the experimental observations. Only diffusion of
Li-interstitials results in a potential dependence matching to the experiments.
Therefore, we identify Li-interstitial diffusion as the cause of long-term SEI
growth.
