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 (FeF 2 ) 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 FeF 2 /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 Fe 2+ 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 Fe 0 upon delithiation. This correlates with the irreversible growth of a SEI layer that limits electronic and ionic transport.

Microstructural Evolution Of Iron Oxyfluoride/Carbon Nanocomposites Upon Electrochemical Cycling

“…9,40 These EELS results are identical to our previous report on SEI evolution in FeF 2 upon cycling, suggesting the dissolution of active materials in the electrolyte. 37 However, these features appear only after a few cycles in FeF 2 (after 10 cycles) while they start to appear later after more than 20 cycles in FeOF.…”

“…37 However, FeO 0.7 F 1.3 /C, as opposed to FeF 2 , contains rocksalt-type phase throughout the lithiation−delithiation range, and it is most likely that the presence of this rocksalt phase that surrounds the Fe nanoparticles minimizes the dissolution rate and loss of Fe, resulting in an initial higher cycling performance. For the cycled FeO 0.7 F 1.3 /C, the presence of rocksalt phase reduces the possibility of Fe immediate contact with the electrode/electrolyte interface, thereby offering some protection against Fe nanoparticle-induced catalytic reactions with electrolyte.…”

Improved Electrochemical Performance of FeF3 by Inlaying in a Nitrogen‐Doped Carbon Matrix

“…[20,76] That is to say no electrochemical reaction occurred in core region of FeF 3 (proved by TEM in Figure S5). [14,[80][81][82] To gain further insight into the improved electrochemical performance of FeF 3 @N-doped carbon composites, EIS measurements were conducted at two different states, one is the open circuit voltage after the batteries were assembled, the other is the charge state after the 200 th cycle at 2 C. All the Nyquist plots are similar with a semicircle at high frequency and an inclined straight line at low frequency (Figure 6a-b). And the rapid insertion/deintercalation of Li + would cause the volume of FeF 3 to change constantly, resulting in the collapse of crystal structure.…”

PEDOT Encapsulated FeOF Nanorod Cathodes for High Energy Lithium-Ion Batteries

“…[37][38][39][40] Compared to the classical carbon coating, the in situ PEDOT coating does not require high temperature, and can therefore avoid the possible reduction of fast electronic path in the electrode, which allows faster charge transport. [39][40][41][42] Moreover, PEDOT conformal coating could avoid the nanoparticle aggregation upon cycling, maintain the structural stability against volume variation, and prevent the side reactions between the in situ formed nano-Fe 0 species and electrolyte during cycling, 34 which would benefit the cycling stability.…”