illinois institute of technology phys-570 introduction to...

Structural investigations of LiFePO4 electrodes and in situ studies by Fe X-ray absorption spectroscopy including

EXAFS and XANES region measurements

Final Presentation: Kamil KUCUK-A20306046 Prof. Dr. Carlo U. Segre

Illinois Institute of Technology PHYS-570

Introduction to Synchrotron Radiation

Spring-2015 Chicago

Outline of My Presentation:

A. Short information about XAS Scynhrotron Techniques:

• What is XAS, XAFS, EXAFS and XANES techniques briefly? • Which disciplines are these techniques being used in science?

B. The aim of the study C. Experimental Setup and Process D. Data, Analysis and Discussion E. Results and Conclusions

Kamil KUCUK (IIT) PHYS-570 - Spring May 5, 2015 1/25

A.1 What is XAS ?


For example:

3d orbital of Fe

X-ray High energy

photons eject core e-s (1s, 2s, etc.)

Absorption Spectroscopy Scan over a spectrum, measure absorbance of

photons that eject e-s or fluorescence of photons emitted when valence e-s relax into hole

Ligand 1s

Metal

d manifold

hn hn

Sample Backside

of Detector

Inside the sample chamber

5d5 metal complex, D4h symmetry

A.1 What is XAFS, EXAFS and XANES techniques?

X-ray Absorption Fine Structure (XAFS) spectroscopy is a very useful tool at

the atomic and molecular scale, if we are studying on the local structure

around selected elements that are contained within a material.

XAFS can be widely applied to many materials not only to crystals, but also to

materials that possess little or no long-range translational order: amorphous

systems, glasses, quasicrystals, disordered films, membranes, solutions,

liquids, metalloproteins – even molecular gases.

As a basic physical quantity, we measure in XAFS is μ(E). (𝜇 𝐸 ~ 1𝐸3 ~𝜎)

Although the X-rays become more penetrating. at some specific energies that

are characteristic of the atoms in the material, sudden increases, called X-ray

absorption edges, are observed “Bunker G., 2010” [2].


A.2 Which disciplines are these techniques being used in science?

It can be said that “XAFS” is a broad one that comprises several different

techniques: EXAFS, XANES, NEXAFS (Near Edge XAFS); and SEXAFS (Surface

EXAFS).

Although the basic physics of these techniques is fundamentally the same,

different approximations, techniques, terminology, and theoretical approaches

may be employed in different situations, particularly in the low-energy (soft X-

ray) and high-energy (hard X ray) regimes, “Bunker G., 2010” [2].

Because of the versatility of XAFS, it and all others are used in a wide variety of

disciplines: physics, chemistry, biology, biophysics, medicine, engineering,

environmental science, materials science, and geology.


A. Short information about XAS Scynhrotron Techniques:

X-ray properties of elements


Electron Binding Energies (eV)

• L3: 99.8

• L2: 100.4

• L1: 149.7

• K: 1838.9

Electron Level Widths (eV)

• L3: 0.014

• L2: 0.015

• L1: 1.030

• K: 0.480

K (1s)

L1 (2s)

L3, L2 (2p3/2,1/2)

EFermi

E ·t ≤ ħ/2

Lifetime of the core hole

Eo: threshold BE: Binding Energy

Silicon (Si) Z = 14 20eV EXAFS Region

XANES Region

B. The aim of the study

In this study, 1- Determining the local atomic and electronic structure and also the stability in electrochemical cycling of the electrodes containing LiFePO4 by using XANES and EXAFS techniques for the K-edge absorption of Fe. 2- Construction of a versatile electrochemical in situ cell for the structural investigation of battery electrodes during the lithium-insertion/extraction processes so that long-term soft and hard X-ray experiments can be used. Note: In this study, the electrode was composed of about 7.7 mg of LiFePO4 on a 20𝜇𝑚 thick Al-Foil.


B. The aim of the study

During the lithium-insertion/extraction processes;

Determining which state the initial state A(0.0 mAh) or the final

charged state E(3.0 mAh) between the lithiated state(LiFePO4

with Fe2+ state) and the delithiated state (FePO4 with Fe3+)?

Some other different stages in the charging process at which the

X-ray measurements were performed is represented by B (0.75

mAh), C (1.5 mAh), D (2.25 mAh) respectively.

For spin configuration, XANES region of XAS spectra will be

investigated for the Fe (II), d6 and Fe (III), d5 states


C. Experimental Setup and Process

XAFS measurements, which included both XANES and EXAFS techniques, were performed on BM beam line DND-CAT of APS at ANL, with 100mA at the top of fill.

For energy selection, a Si(1 1 1) double crystal monochromator was used and the energy resolution of the monochromatic beam was determined to be about 1 eV as measured by the rocking curve width of the second crystal.

By employing a platinum-coated mirror. The large harmonic content in the beam was rejected

All the measurements were recorded in the transmission mode using three ion chambers to measure the intensities of the incident beam I0, the beam transmitted by the sample It and the beam subsequently transmitted by the reference foil Iref

The intensities of the incident and the transmitted X-rays were monitored by nitrogen and helium filled ionization chambers.

The monochromator was scanned in energy from 312 below to 1050 eV above the Fe K absorption edge (7112 eV).

Again Transmission ion chambers were used to measure the incident (I0), transmitted (It), and reference (Iref) signals.


C. Experimental Setup and Process

A 9μm thick Fe foil was used as a reference, and both foil and Iref detectors were positioned behind the sample along the direction:

[X-ray beam/ Io -detector/sample/ Isample -detector/Fe-foil/ Iref -detector]. Data were collected using a 0.1 eV step size through the edge region and a

variable step size, giving ∆𝒌𝒎𝒂𝒙 = 𝟎. 𝟎𝟓 𝑨°−𝟏 for the EXAFS region. To normalize the XANES, a quartic spline fit was used through the background. The χ(k) function was weighted with k3 to account for damping of oscillations

with increasing k. The radial structure function was obtained by Fourier transform of k3χ(k)

using a k-range of 1.5–16.4 𝑨°−𝟏 The k range was adjusted until the radial structure function exhibited no

unexpected peaks below the anticipated Fe-O distance. The EXAFSPAK was used to fit the EXAFS spectrum with theoretical scattering

paths, which were generated with FEFF 8 code, “Streltsov V.A., et al.”


D. Data, Analysis and Discussion

The crystal structure of LiFePO4 , which has been investigated by "Streltsov V.A., et al.", shows the unit cell containing four LiFePO4 units.


Fig-1. "Streltsov V.A., et al." Streltsov, Victor A., et al. "Multipole analysis of the electron density in triphylite, LiFePO4, using X-ray diffraction data." Acta Crystallographica Section B: Structural Science 49.2 (1993): 147-153.

The O-sites form a nearly tetrahedral arrangement around P-stites and approximately octahedral arrangement around each Fe-sites

Even if Li+ ions are removed electrochemically the remaining FePO4 framework has the same Pnma (space-group) symmetry with a 7% reduced volume [4,8,9].

It has 3 different positions and 4 different interatomic Fe-O distances (2×2.2506 A° equatorial, 2×2.0639 A° equatorial, 1×2.2034 A° axial, 1×2.1077 A° axial).

The electrode area of 3.14 cm2, and was charged at a current density of 1 mA/cm2, which corresponds to a 3 h charge rate.

D. Data, Analysis and Discussion EXAFS measurements were recorded at different states of charge and results which was observed at states of charge A (0.0 mAh), B (0.75 mAh), C (1.5 mAh), D (2.25 mAh) and E (3.0 mAh), respectively, was showed in this study.


Fig-2. Calibrated and normalized XANES data at the Fe K edge during charge. The A (0 mAh), B (0.75 mAh), C (1.50 mAh), D (2.25 mAh), and E (3.0 mAh) in the figure represent the different stages in the charge process at which the X-ray measurements were performed. Deb A., et al.

* For Fe 1s-4p or 1s to continuum-transition, there is strong shift at main edge at the different charging states due to the change in the valance state of Fe (in aggrement with 4ev) * The reason of shifthing from A to E because of Fe (III) > Fe(II) * Pre-Edge Trns are different from Main edge wrt structurally

4ev


Fig. 3 shows the dipole forbidden pre-edge transitions which is attributed to Fe 1s→3d bands. The ligand field resulting from the phosphate oxygen atoms, which are octahedrally co-ordinated to the iron, splits the 3d states into t2g and eg states "Pouchard M., et al.".


Fig-3. Pre-edge region of the XAS spectra, which show the t2g and eg absorption bands at the stages A (LiFePO4) and E (FePO4)Deb A., et

al..

* Deviation of O atoms * Fe-O dist ance changes by 0.2eV because of CFS *Doublet state has Higher-transition probability than triplet state due to overlapping better with 2p orbitals

Crystal Field Splitting (CFS)

2 eV


We can say from "Yamada A., et al." that the 3d electrons of Fe in FePO4 and LiFePO4 are high spin. Since the Fe sites are in approximately octahedral environment, the O2− ions split the 3d states into t2g and eg states and the Fermi level lies in the Fe 3d states.

Therefore, with deviation from the octahedral symmetry, interaction of the O

2p and Fe 3d orbitals leads to further splitting of these states.

The mixing of the O 2p and the Fe 3d orbitals increases the transition probability of the 1s→3d transition (which is dipole forbidden or pre-edge transitions given at Fig-3).

Now if we turn the pre-edge feature, the intensity of the pre-edge for the oxidized state E is similar for the sample at A state, that is O atoms retain the octahedral cordination.


D. Data, Analysis and Discussion The XANES spectra of the four model compounds are shown in Fig. 4a and 4b. FePO4 and Fe2O3(Tetrahdrly), Fe(acac)3(octahdrly) as a sample of Fe+3 FeCp2 was chosen as an example of a Fe+2 compound.


Fig-4 (a). XANES spectra of the model compounds: Fe2O3 (short dash), Fe(acac)3 (dash-dot), FeCp2 (dash-dot-dot) and FePO4 (solid line). The inset shows the

comparison of the XANES spectrum recorded at the stage E (solid line), with respect to the other Fe3+ model compounds, in octahedral symmetry [𝛼-Fe2O3

(short dash) and Fe(acac)3 dash dot)]. The axes in the inset are the same as in (a). (b) Magnified 1s→3d pre-edge of XANES spectra for the model

compounds. The different line codes are the same as in Fig. 4a. The four spectra have been vertically shifted for clarity. The arrows represent the position of

the pre-edge peaks Deb A., et al.

D. Data, Analysis and Discussion By electrochemical delithiation during charging of the cell, the stoichiometry

changes from LixFePO4 to FePO4, and consequently, iron is oxidized from Fe2+ (stage A) to Fe3+ (stage E).

The three compounds to exhibit a K-edge have higher energy (7122.3–7123.2 eV) with respect to FeCp2 (7119.3 eV).

The main underlying factor which determines the energy required to induce the photoelectric effect on a 1s electron is the oxidation state of the absorbing atom.

Therefore, more energy is required to ionize the metal ion and this explains the resulting consistent edge shift in the Fe2+ compounds with respect of the Fe3+ compounds. This interpretation is also correct for our observed consistent edge shift in the charging process of the electrode from the initial state A(Fe2+ ) to the final state E (Fe3+ ) [Fig. 2].

Turning our attention to the 1s→3d pre-edge feature [Fig. 4(b)], it is seen that the pre-edge is at slightly higher energy for Fe3+ compounds than the Fe2+ compound. Furthermore, the pre-edge intensity of FePO4 (in tetrahedral symmetry) is relatively intense (at 7114.1) with respect to the Fe2O3 and Fe(acac)3 (in octahedral symmetry), where in the case of the latter two compounds the pre-edge is split into two components of much lower intensity. Kamil KUCUK (IIT) PHYS-570 - Spring May 5, 2015 15/25

D. Data, Analysis and Discussion For the Fe3+ compounds in octahedral symmetry, the relatively higher

intensity observed for Fe2O3 results from the higher degree of distortion from the ideal octahedral symmetry with respect to Fe(acac)3 . Therefore, the pre-edge feature represents the degree of mixing of the 3p and the 3d atomic orbitals to form molecular orbitals.

In case of a perfect Oh symmetry the degree of mixing is zero, and it increases as the distortion increases in the Oh symmetry and is higher in case of Td

symmetry, hence explaining the change in the pre-edge features in these model compounds.

Now comparing the pre-edge region of themodel compound FePO4 (tetrahedral symmetry) which exhibits relatively a higher intensity compared to our oxidized sample at the state of charge E (i.e. FePO4), once again confirming that the Fe (III) state at the final charged state E retains the octahedral coordination of the oxygen atoms.

Another interesting feature of the XANES spectra reported here (Figs. 2 and 4) is the intensity of the white line, which is proportional to the coordination of the absorbing atom.




Table-1. Position and normalized intensity of the observed 1s→3d pre-edge peak and of the white line of the XANES for the different

model compounds and for different states of charge of LixFePO4 (A, B, C, D and E) during charging of the in situ cell Deb A., et al.

The quantitative results obtained from the XANES spectra for the model compounds and also from the different states of charge for the LixFePO4 electrode are summarized in Table 1.

Much lower than others

D. Now we will consider absorbtion data


Fig. 5. Observed k3-weighted [k3 χ(k)] Fe EXAFS at stages (a) A, (b) B, (c) C, (d) D and (e) E during charging. Comparison of the

experimental and the FEFF fit theoretical analysis of the k3-weighted Fe EXAFS spectrum are shown for Fe(II) recorded at the state A and

Fe(III) recorded at the state E, for LixFePO4. For both (a) and (e): circles symbol indicate experimental data and broken lines indicate FEFF

fit theoretical results Deb A., et al.

This Fig. 4 shows that the k3χ(k)-function of the Fe K-edge EXAFS spectrum as a function of k(A°−1) after performing standard corrections, which included background subtraction, energy calibration, normalization, and weighting of the data with k3

for the different states of charge.



Fig. 6. Comparison of the radial distribution function obtained after Fourier transformation of k3χ(k) observed at the stages (a) A, (b) B,

(c) C, (d) D and (e) E, respectively. A comparison of the theoretical analysis for the initial state A and final state E are shown here, where

circles (filled) symbol indicate experimental data and solid lines indicate FEFF fit theoretical results. Deb A., et al.

Fig. 6 shows the corresponding radial structure function as a function of the inter atomic distance, R (A° ), which we obtain by Fourier transformation of k3χ(k) over the limited k-space range between 1.5 and 16.4 A°−1. The radial structure function shows one strong peak at the start followed by two weaker peaks at higher distances.

Quantative analyses between R=0.5 and 4.1A°



Table-2 Structural parameters resulting from the FEFF fit for LixFePO4 during the initial stage A (0 mAh), derived from fitting the Fe-edge

EXAFS spectra, Table-3 Structural parameters resulting from FEFF fit for LixFePO4 during fully charged stage E (3 mAh), derived from fitting

the Fe-edge EXAFS spectra Deb A., et al.

The structural parameters shown in Tables 2 and 3 are obtained from the FEFF fit analysis of LixFePO4 using all possible scattering paths. The coordination atom of the first shell is oxygen, represent the Debye–Waller disorder parameter. and that of the second and the third shells are phosphorous and iron respectively.

Table-2 Table-3

D. For a better understanding of bond distances


Fig.-7. First shell average metal oxygen and second shell average metal phosphorus bond length changes during Li/LixFePO4 cell cycling.

The filled symbols represent the Fe-O and Fe-P bond distances as observed during different states of charge and discharge Deb A., et al.

As mentioned before, no single Fe-O distance varies more than by 0.2A°. This again shows the basis for the truly excellent cycling properties for this system. The results obtained for both stages A and E are in agreement with the crystallographic data. (retains the same Pnma structure, but 7% reduction in Vol.)

E. Results and Conclusions

XAFS has provided us with an excellent tool for analyzing the changes that take place when Li is cycled out of and into LixFePO4 in a Li-ion cell.

The XAS analyses indicate that during charging of LixFePO4 to 3.9V the

oxidation state of Fe changes from +2 to +3. In conclusion, all results in this study confirm that this material has very

desirable characteristics for an electrode, including small volumetric changes and retention of the Fe-O octahedral symmetry on charging, which makes it an excellent choice for Li-ion cell applications.

A comparison with the 1s→3d pre-edge characteristics of the model

compounds also confirms that the Fe and O are octahedrally arranged, as is evidenced by a weak pre-edge intensity, unlike FePO4 where a strong pre-edge intensity is observed because of the tetrahedral co-ordination.

The pre-edge intensities at the different states of charge of LixFePO4 show the

octahedral arrangement of the phosphate oxygen atoms around each Fe-site is retained.


E. Results and Conclusions


The XANES studies revealed that the LiFePO4 and FePO4 are both in the high

spin state and exhibit a crystal field splitting of about 2 eV. The EXAFS data show one strong peak for the first co-ordination shell

corresponding to Fe-O scattering contributions and two weaker peaks at higher distances comprise of contributions from Fe-P and Fe-Fe, respectively (Tables 2 and 3).

The change in the RFe-O from the initial stage A (0.0 mAh) to the fully charged

state E (3mAh) varies at the most by 0.2A°.

The all results presented in this study confirm that only minor changes occur in the structure of the electrode during electrochemical cycling.

Thanks for your attention...

References:

[1] Deb, A., Bergmann, U., Cramer, S. P., & Cairns, E. J. (2005). Structural investigations of LiFePO 4 electrodes and in situ studies by Fe X-ray absorption spectroscopy. Electrochimica acta, 50(25), 5200-5207.

[2] Grant Bunker, “Introduction to XAFS- A Practical Guide to X-ray Absorption Fine Structure Spectroscopy”, 2010 [3] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 54 (1998) 7565. [4] V.A. Streltsov, E.L. Belokoneva, V.G. Tsirelson, N.K. Hansen, Acta Cryst. B 49 (1993) 147. [5] M. Pouchard, A. Villesuzanne, J.-P. Doumerec, J. Solid State Chem.162 (2001) 282. [6] A. Yamada, S.C. Chung, J. Electrochem. Soc. 148 (2001) 960. [7] P.F. Eiland, R. Pepinsky, J. Am. Chem. Soc. 74 (1952) 4971. [8] A.S. Andersson, J.O. Thomas, J. Power Sources 97-98 (2001) 498. [9] P. Tang, N.A.W. Holzwarth, Phys. Rev. B 68 (2003) 165107.


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