Structural phase transformation and Fe valence evolution in FeOxF2−x/C nanocomposite electrodes during lithiation and de-lithiation processes

Abstract

In this study, the structural changes of FeO x F 2Àx /C during the first discharge and recharge cycles were studied by ex situ electron microscopy techniques including annular dark field scanning transmission electron microscopy (DF-STEM) imaging, selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) as well as by in situ X-ray absorption spectroscopy (XAS). The evolution of the valence state of Fe was determined by combined EELS using the Fe-L edge and XAS using the Fe-K edge. The results of this investigation show that the conversion reaction path during 1 st lithiation is very different from the re-conversion path during 1 st delithiation. During lithiation, intercalation is first observed followed by conversion into a lithiated rocksalt (Li-Fe-O-F) structure, and metallic Fe and LiF phases. During delithiation, the rocksalt phase does not disappear, but co-exists with an amorphous (rutile type) phase formed initially by the reaction of LiF and Fe. However, a de-intercalation stage is still observed at the end of reconversion similar to a single phase process despite the coexistence of these two (rocksalt and amorphous) phases.

Microstructural Evolution Of Iron Oxyfluoride/Carbon Nanocomposites Upon Electrochemical Cycling

“…The SAED pattern of a delithiated sample after three cycles (Figure 2a) contains diffuse rings corresponding to the presence of amorphous rutile and rocksalt-type phases similar to what has been reported after the first cycle. 15 However, more intense rings are observed for delithiated samples after 60 cycles ( Figure 2b) associated with the presence of crystalline LiF and Fe phases. Despite the overlap between the reflections, the presence of crystalline LiF phase can be asserted from the appearance of 111 and 311 reflections.…”

“…15 At full delithiation, the Fe molar fraction, as determined by PDF, is 30% in the rocksalt phase, with the remaining 70% in the amorphous rutile phase. 14 This result seems to indicate that Fe in a high valence state is required for the occurrence of electron beam crystallization to rutile phase and such a transformation is more pronounced with increasing fraction of Fe 3+ , that is, at the end of delithiation.…”

“…5,6,13−15 Among the iron fluoride systems, FeOF nanocomposite shows the most promise as a high capacity cathode material because of its higher capacity retention upon cycling and lowest intrinsic charge−discharge voltage hysteresis of 0.7 V (measured by reverse step PITT at C/1000) as compared to 1.3 V for FeF 3 and FeF 2 . 16 In addition, observations made by X-ray diffraction, 6 pair distribution function (PDF), 14 and electron diffraction 15 reveal a reduction in crystallinity from the original rutile structure transforming upon delithiation into a complex multiphase microstructure consisting of a (Li−Fe−O-F) rocksalt and an amorphous phase with rutile type nearest neighbors. 14, 15 Although capacity fading has been reported upon electrochemical cycling, relatively little is known on the microstructural changes that are occurring upon cycling.…”

“…15 It is now well documented that the electron beam can lead to irradiation effects such as heating, electrostatic charging, ionization (radiolysis), and knock-on atomic displacement. 17 These irradiation effects are generally associated with material degradation such as decomposition, material loss by sputtering, and phase transformations.…”

Advanced analytical electron microscopy for lithium-ion batteries

“…For example, because the cycling of conversion materials typically leads to the formation of metallic nanoparticles and lithium oxide/fluoride, which differ significantly in their average atomic number, the phase conversion process can be directly visualized using Z-contrast HAADF-STEM imaging and the associated elemental mapping (Figures 3c and d). [40][41][42] In addition, the intermediate and final reaction products and volume change of Si, which is the most important alloying anode material, can also be determined by a combination of STEM/EELS and electron diffraction. The results provide valuable insight into the structural and morphological optimization of Si-based materials.…”