Analysis of the structural evolution of the SEI layer in FeF2 thin-film lithium-ion batteries upon cycling using HRTEM and EELS

Conversion compound electrodes such as FeF2 allow a reversible change of more than one Li ion per 3d metal cation, which provides a pathway for overcoming the intrinsic capacity bottleneck associated with traditional intercalation electrodes. However, many of the basic mechanisms involving structural changes to the electrode with cycling, and the correlation with capacity changes must be better understood for future implementation in batteries. Understanding the SEI layer is of particular importance as it is where ion exchange occurs, and the kinetics of the battery is largely determined. We note in this context that thin-films are binder free, and therefore provide a model platform to obtain insight to the structural and electrochemical behavior of uncontaminated phases of the FeF2 and other conversion materials. FeF2/LiPON thin-film batteries were fabricated and subjected to Galvanostatic cycling, and the evolution of the SEI analyzed using HRTEM and EELS spectroscopy. Samples were subjected to single and 15cycles, and compared in their lithiated and delithiated states to understand the relationship between microstructural evolution and capacity. The SEI formation is essentially complete after the first cycle, and is characterized by incomplete reconversion of LiF which is resistive to both electron and ion transport. With subsequent cycling the L3/L2 ratios (compared to the pristine and singly cycled films) show that the phase fractions of unconverted Fe and LiF increase, making the SEI more resistive to ion exchange. The large capacity drop measured after the first cycle is due to the formation of the SEI and the presence of LiF which degrades Li and electron mobility, and in turn adversely impacts capacity.