Growth of the solid-electrolyte interphase: Electron diffusion versus solvent diffusion

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.