Coulomb trapping of ions in a solid electrolyte interphase by deprotonation-induced surface charges.

Ion transport through the solid electrolyte interphase (SEI) is a limiting factor for achieving battery cells with high power density. Furthermore, highly ionically resistive SEIs in aged cells reduce the cell capacity, which is available at practical rates of (dis)charge. Electrolyte-filled pores in the particulate SEI layer have been discussed to play a significant role in its transport properties and may feature fixed negative surface charges. Here, we compare ion and molecule transport in the SEI before and after its deprotonation with a strong base. Our results give strong indication that negative charges created by deprotonation lead to a drop in the ionic conductivity by several orders of magnitude, while the transport of neutral molecules is only slightly impeded. Since existing models contrarily predict an increase in the ionic conductivity in porous systems due to fixed charges, we propose a new model based on Coulomb trapping. Interphases are not limited to batteries, but exist in many electrochemical systems, such as electrocatalysts based on non-noble metals, and their transport properties are highly relevant for performance of these systems. Therefore, our observations should be of overarching importance for various types of electrochemical applications.