Investigation of SEI Layer Formation in Conversion Iron Fluoride Cathodes by Combined STEM/EELS and XPS

Abstract

Li-ion cathodes based on conversion reactions such as iron fluoride (FeF2) can achieve in principle high specific capacity. However, significant capacity fading is observed upon cycling. This has been attributed in part to the formation and continuous growth of a solid electrolyte interphase (SEI) layer at the cathode/electrolyte interface. In this work, scanning transmission electron microscopy, electron energy loss spectroscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy were used in combination to study both the structural changes of the FeF2/C active material and the growth and evolution of the SEI layer upon cycling. Two main sources of capacity loss have been found. An increasing amount of Fe2+ appeared trapped inside the SEI layer with increasing cycle number, thus resulting in the loss of active material. In addition, reconversion is strongly impeded with increasing cycle number, leaving untransformed LiF and Fe0 upon delithiation. This correlates with the irreversible growth of a SEI layer that limits electronic and ionic transport.