  The solid-electrolyte interphase (SEI) substantially influences the lifetime
of lithium-ion batteries. Nevertheless, the transport mechanism responsible for
the long-term growth of the SEI remains controversial. This study aims at
discussing the characteristic time and state-of-charge dependence of SEI growth
mediated by electron diffusion versus solvent diffusion. We describe both
transport mechanisms with continuum models and compare them to experimental
results. We show that electron diffusion can explain both the observed
state-of-charge dependence and the time dependence. In contrast, we demonstrate
that solvent diffusion can reproduce either the state-of-charge dependence or
the time dependence of capacity fade. There is no intermediate regime where
solvent diffusion can explain both dependencies simultaneously. Furthermore, we
emphasize the crucial role of anode voltage and state-of-charge on SEI growth
in general. Due to self-discharge, this dependence can explain deviations from
the typical square-root behavior in the time domain. We conclude that electron
diffusion is the relevant process leading to the state-of-charge dependent SEI
growth. Further experiments are needed to investigate the reason for
contributions to the capacity fade that are independent of the state-of-charge.
