Mahsa Sina

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

In this study, the structural changes of FeOxF2−x/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 1st lithiation is very different from the re-conversion path during 1st 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.

STEM/EELS Analysis of Conversion Reactions in Cycled FeOF/C

Transition metal fluoride/carbon nanocomposites have been under extensive investigation in the past ten years due to their theoretical high capacity in the range of 500 to 800 mAh/g [1]. In this study, the structural changes of FeOF/C positive electrode during lithiation/delithiation have been studied as a function of number of cycles (up to 20) under constant cycling current of 50 mA/g and at 60°C. ADF-STEM imaging technique combined with electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED) were used to track the phase evolution of FeOF/C and chemical composition with bonding characteristics of phases present after cycling.

EELS Determination of Li Distribution and Fe Valence Mapping in Lithiated FeOF/C Nanocomposite Battery Materials

Amongst the techniques to investigate Li-ion battery materials, electron energy loss spectroscopy (EELS) play a unique role as Li distribution, chemical state and valence of transition metals (charge transfer) can all be determined with nanometer scale spatial resolution. In this work, we use EELS to investigate new positive electrodes for Li-ion batteries based on transition metal fluoride (FeF3, FeOF, FeF2, CuF2....)/C nanocomposites [1]. The high specific capacity in these new electrodes is obtained by using all the oxidation states of Fe from Fe to Fe during discharge cycles via a complete conversion process. In this study, we used Scanning Transmission Electron Microscopy (STEM) combined with EELS to determine the Li spatial distribution, its chemical state and the Fe valence state in FeOF/C nanocomposite electrodes during charge and discharge processes. This STEM-EELS analysis was done using a JEOL 2010F equipped with a Gatan GIF 200 spectrometer and with a Hitachi 2700 STEM equipped with an Enfina spectrometer. In order to minimize electron beam damage and F loss, the samples were cooled to LN2 temperatures and imaged with a total electron dose not exceeding 10 C/cm. Both lithiated (discharged) and delithiated (re-charged) FeOF/C nanocomposites electrodes were analyzed by EELS. The Fe valence state was obtained by measuring the Fe L3/L2 intensity ratio [2,3]. The L line intensities were obtained using either a 4.5 eV window or by taking the positive component of the EELS spectra second derivative. An ADF STEM image of a FeOF/C cathode material discharged to 1.5V is shown in Fig.1a with the corresponding low energy EELS signal (c.f. Fig. 1b) taken from area marked A revealing the superposition of the Li-K and Fe-M edges. The extracted Li-K edge has two prominent peaks whose energies are separated by 6.6 eV. In addition to the two prominent Li peaks, there is a third one located at a distance of 4.2 eV from the first peak. The existence of these peaks is indicative of the presence of two Li-base compounds (LiF) and a new Li-Fe-O-F cubic phase. The Li-K/Fe-M intensity map shown in Fig.1c from the area depicted in Fig.1a reveals the presence of Li and Fe rich phases with a spatial distribution in the 3-5 nm range. At this voltage the expected phases are LiF+Fe+LixFeOyFz [3]. At the surface, a 10-20 nm thick Li rich phase (c.f. Fig 1d) is observed corresponding to a mixed LiF-Li2CO3 solid electrolyte interface (SEI) surface layer. Upon lithiation, the Fe valence state decreases as represented by a decrease in the Fe-L3/L2 intensity ratio. At the lowest voltage of 0.8V, all Fe is in the metallic state. At the intermediate discharge voltage of 1.5 V, the Fe valence state is not uniform and the microstructure is composed of a mixture of high and low valence state phases as depicted in Fig. 2b. The O-K concentration map shown in Fig.2a has a similar distribution as the valence map of Fig. 2b which indicates that the oxygen rich phase is also the phase with highest valence state. A quantitative analysis of the Fe L3/L2 intensity ratios using standard model compounds (Fe, FeF2 and FeOF) as reference indicate a Fe valence state of 2.3 for this cubic LixFeOyFz phase. Upon recharge to 4.5 V, all the Fe in the electrode returns to its initial Fe valence state with the electrode material converting back to its initial rutile FeOF phase. [4].