Local Structure and Stability of SEI in Graphite and ZFO Electrodes

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Local Structure and Stability of SEI in Graphite and ZFO Electrodes Probed by As K-edge Absorption Spectroscopy Seyed Javad Rezvani, Matteo Ciambezi, Roberto Gunnella, Marco Minicucci, Miguel A. Muñoz-Márquez, Francesco Nobili, Marta Pasqualini, Stefano Passerini, Christian Schreiner, Angela Trapananti, Agnieszka Witkowska, and Andrea Di Cicco J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11798 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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The Journal of Physical Chemistry

Local Structure and Stability of SEI in Graphite and ZFO Electrodes Probed by As K-edge Absorption Spectroscopy S.J. Rezvani,

Nobili,

¶

∗, †

M. Ciambezi,

M. Pasqualini,

¶

†

R. Gunnella,

S.Passerini,

Witkowska,

@

§

†

M. Minicucci,

C. Schreiner,

and A. Di Cicco

⊥

†

M. A. Muñoz,

A. Trapananti,

#

‡

F.

A.

†

Physics Division, School of Science and Technology, University of Camerino, via madonna delle Carceri, 62032 Camerino, Italy ‡ CIC Energigune, Parque Tecnólogico de Álava, Albert Einstein 48, 01510 Miñano, Spain ¶Chemistry Division, School of Science and Technology, University of Camerino, via madonna delle Carceri, 62032 Camerino, Italy §Helmholtz Institute Ulm (HIU), Albert-Einstein-Allee 11, 89081 Ulm, Germany kKarlsruhe Institute of Technology (KIT), Karlsruhe, Germany ⊥Technology and Innovation, SGL Carbon, Werner-von-Siemens-Straÿe 18, 86405 Meitingen, Germany #CNR- Istituto Ocina dei Materiali, c/o Dipartimento di Fisica e Geologia, Via A. Pascoli, I-06123 Perugia, Italy @Department of Solid State Physics, Gdansk University of Technology, Gabriela Narutowicza 11/12, 80-233 Gdansk, Poland

†

E-mail: [email protected] Phone: +39 0737 40 2523

1

ACS Paragon Plus Environment


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Abstract

The evolution of the Solid Electrolyte Interphase (SEI) during the rst Li uptake in advanced Li-ion electrodes is studied by x-ray absorption spectroscopy (XAS). The As atoms present in the electrolyte solution were used as a local probe for monitoring the SEI growth on dierent electrodes. High quality As K-edge spectra were collected in uorescence mode for a set of graphite and carbon-coated ZnFe2 O4 electrodes. XAS measurements have been preceded and corroborated by electrochemical characterizations. SEI phase evolution was analyzed by distinct As valence states in the subsequent stage of SEI formation while x-ray uorescence (XRF) was used to estimate the As content. Detailed structural results are presented for dierent Li contents in dierent electrodes including the estimated thickness of the SEI layer, contribution of dierent As oxidation states and As local structure. The formation of AsFN complexes with dierent local coordination N is clearly observed and measured at various SEI evolution stages. Evidence of partially reversed redox process, taking place within the SEI by charge-discharge cycling was also obtained.

Keywords: X-ray absorption spectroscopy, Solid Electrolyte Interphase, Lithium Ion Battery, ZnFe2O4,Graphite

Introduction In lithium ion batteries (LIBs), the necessary condition of the electrolytes to be inert in contact with both anode and cathode materials is usually realized by the kinetic rather than thermodynamic stability of the electrolyte against reduction and oxidation.

Due to

the strong reducing and oxidizing potency of the electrode materials employed in batteries, the possibility of a thermodynamically stable electrolyte is very limited.

Therefore it is

the chemical passivation of these reducing or oxidizing surfaces that ensures the inertness of the bulk electrolytes during cell operation.

On the anode, by passivation, electrolyte

2


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salt and solvents are initially decomposed and reduced, forming a dense and protective lm that covers up the anode surface forming the electrode/electrolyte interface that prevents any sustained decomposition. The electrolyte components undergoing the passivation layer formation have a determining eect on its physicochemical properties, such as the chemical stability and Li

+

ion conductivity. Peled et al. were the rst to report the formation of the

passivation layer over alkali metals in non-aqueous battery systems, for which they gave the name of Solid Electrolyte Interphase (SEI).

1

Due to the high electro-negativity of lithium, a

free contact between the electrode and the electrolyte never actually exists since the reaction between them occurs instantaneously.

2

The decomposition ceases when this passivation layer

reaches a certain thickness. The resulting SEI should be permeable to Li

+

ions while non

conductive for electrons. Once this layer is formed, it cannot be completely removed by a reverse decomposition. Thus the performance of the LIB is strongly dependent on the SEI layer formed on the surface of the electrodes. In recent decades the endeavor to explore novel electrode and electrolyte materials has been pursued. Carbonaceous anodes was introduced by Sony in 1991 as a commercial carbon insertion host anode

3

while, Graphite quickly replaced hard carbon due to its superior

capacity and a SEI formation induced by reducing C at electrode/electrolyte interface was demonstrated as well. Li

+

1

The presence of such a layer over graphite electrode, acting as a

ion conductor and as an e

−

insulator, disclosed the use of highly-reversible graphite

intercalation anodes and determined the market uptake by Li-ion cells.

4,5

More recently,

ZnFe2 O4 encapsulated in a carbonaceous matrix (ZFO-C) has been proposed as a mixed alloying-conversion anode material,

68

showing quite high charge/discharge capacity and cy-

cling eciency, the latter being, once again, related to the formation of a stable and ecient SEI. The electrolyte salt commonly used in Li-ion cells is LiPF 6 , even with potential issues coming from possible HF formation because of PF 6

−

decomposition in presence of moisture.

As discussed below, our x-ray experiments have been performed using a mixture of LiPF 6

3



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and LiAsF6 electrolyte salts to exploit the penetration depth of the x-ray beam at the As K-edge within the composite electrodes.In fact, while LiAsF 6 has been discarded for commercial applications because of its toxicity, its lithiation reactions are similar to that of LiPF6 in which for both electrolytes the LiF and Li(As/P)F x phases are expected to be the components of the inorganic, inner layer of the SEI. Detailed reaction pathways, involving possible moisture traces in electrolyte mixtures and cells, can be found in Refs [ 9,10]. The clear similarity of SEI phases in both electrolytes makes

As

a suitable probe to

study the lithiation process and local structure of the formed phases on the SEI layer also for conventional LiPF 6 electrolyte salt. X-ray absorption spectroscopy (XAS) is a powerful technique to investigate the evolution of the short range local structure. Moreover, XAS is element specic that allows targeting the evolution of the structure in the desired chemical species in complex systems.

These

properties make XAS a suitable technique to study phase evolution and stability of the passivation layer formed on LIBs electrodes. Monitoring the evolution of the phases, formed by direct reaction with the electrolyte salt can serve as an eective probe to identify the SEI local structure evolution and stability during the Li-ion cells charge-discharge cycles. Attaining this information by using XAS can lead to a better understanding of the SEI formation process during the rst charging cycles and hence result in a more reliable battery design. In the present work, we have used for the rst time hard X-ray absorption spectroscopy to study the SEI phase evolution and stability in reaction with the electrolyte composed by mixed LiPF6 /LiAsF6 salts in carbonate solvents, taking As atoms as a local probe. High quality As K-edge spectra were collected to monitor the SEI formation on graphite and ZFO-C electrodes. Extended x-ray absorption ne structure (EXAFS) analysis was used to identify the local structure of the inorganic fraction of the SEI. X-ray absorption near-edge spectroscopy (XANES) and X-ray uorescence spectroscopy (XRF) were used for quantitative analysis of the SEI evolution. XAS measurements have been preceded and corroborated

4


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by electrochemical characterizations including galvanostatic intermittent titration technique (GITT) with the aim of correlating each XAS experiment with half-cell open circuit potential (OCV) and capacity of the cells.

Experimental Electrodes preparation Graphite electrodes were prepared by using Na-Carboxymethylcellulose (Na-CMC, Walocel CRT 2000) dissolved in deionized water and left under magnetic stirring overnight. Subsequently Super C65, Graphite (carbon-coated syntethic graphite prepared by SLG Carbon) and the Na-CMC solution were mixed in an Electric Helical Blade Mixer at 2000 rpm for 3 hours under vacuum. The resulting slurry was then casted using a battery-line on 10 µm thick Cu foil by using the Doctor Blade technique, with a wet lm thickness of 165

µm.

The

◦ slurry was immediately dried using a 80 C air stream inside the battery line. ZnFe2 O4 carbon-coated (ZFO-C) electrodes were prepared by using Sodium Carboxymethylcellulose (Na-CMC, Sigma-Aldrich) dissolved in deionized water (5:95 w/w).

ZFO-C and

Super-P carbon (MMM-Carbon), previously mixed and ground in an agate mortar, were added to the binder solution and the resulting slurry was stirred for 5 h with magnetic stirrer. The mixture was stratied on Cu foil (whose thickness is 10

µm;

the surface was rst

wetted with acetone and scraped o with sandpaper) through Doctor Blade technique and the thickness was set to 100

µm.

The obtained layer was dried on at room temperature for

◦ 30 minutes and in oven at 55 C overnight. Then the layer was pressed by Roll Press until getting a uniform thickness. Circular electrodes with a diameter of 9 mm and a surface area of 0.636 cm and dried overnight at 120 als.

◦

2

were cut

C under vacuum, for both ZFO-C and graphite active materi-

The capacity of graphite

11

and ZFO-C electrodes

specic theoretical capacity of 372 mAh g

−1

6

has been calculated considering a

and 1000 mAhg

−1

5


, respectively. Electrochemical


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measurements have been carried out by using three electrode T cells (Swagelock type), by using graphite or ZFO-C electrodes as working electrode and metallic lithium as counter and reference electrodes. A glass ber (Whatman GF/A) has been used as separator and a solution of LiPF 6 / LiAsF6 (0.5M/0.5M) in EC:DMC 1:1 v/v has been used as electrolyte. All the cells have been assembled in a dry-box lled with Ar.

Electrodes electrochemical characterization Galvanostatic intermittent titration technique (GITT) was performed, as a series of galvanostatic steps (10 minutes each, at current rate C/20, corresponding to 50 mAg mAg

−1

−1

and 18.6

for ZFO-C and graphite electrodes, respectively), followed by open-circuit relaxation

periods (60 minutes each). Open circuit potential OCV (E) vs. equilibrium capacity (Q) prole as acquired by GITT measurements are shown for both electrodes in Fig. 1. Based on previous electrochemical results of the SEI evolution,

6,12,13

samples for ex-situ XAS analysis

have been prepared by submitting ZFO-C and graphite electrodes to selected Li uptakes by GITT, specied in Fig.

1.

These specic points correspond to capacity and potential

values of the electrodes that underwent ex-situ XAS characterization (see XAS experiment ).

Point 10S (ZFO-C) obtained at E=1.02 V and Q=83 mAhg

−1

is related to the begin-

ning of Li uptake by ZFO-C and limited SEI formation is expected. (ZFO-C), obtained respectively at E=0.87 V, Q=167 mAhg mAhg

−1

−1

Points 20S and 50S

and at E=0.79 V, Q=417

, correspond to the beginning and ending of a potential region where most of SEI

formation commonly occurs. Q=15.5 mAhg

−1

For graphite electrode, point 5S obtained at E=0.88 V and

is related to the beginning of Li intercalation and limited SEI formation

is expected. Points 10S and 15S, obtained respectively at E=0.51 V, Q=31 mAhg E=0.32 V, Q=46.5 mAhg

−1

−1

and at

, correspond to a potential region where most of SEI formation

commonly occurs.

6


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Figure 1: Open circuit potential (E) vs. specic capacity (Q) of the graphite and ZFO-C electrodes obtained during rst Li uptake.

For graphite, potential and capacity values of

the investigated electrodes are marked as 5S, 10S and 15S corresponding to E=0.88 V and −1 −1 −1 Q=15.5 mAhg , E=0.51 V, Q=31 mAhg and E=0.32 V, Q=46.5 mAhg respectively. Potential and capacity values of the investigated ZFO-C electrodes are marked as 10S, 20S −1 −1 and 50S corresponding to E=1.02 V and Q=83 mAhg , E=0.87 V, Q=167 mAhg and −1 E=0.79 V, Q=417 mAhg , respectively.

XAS samples and experiments Performance of high-quality XAS experiments on the electrodes under consideration required special care in the preparation of samples. XAS measurements had to be mostly performed in uorescence mode, due to the small amount of active materials (As atoms) embedded in the electrodes.

Studied electrodes were installed on a Cu foil (current collector), as

described above, therefore an intense XRF signal of the metal foil was expected. the primary XAS experiments the removal of Cu foil was necessary.

From

Hence, for better

isolating the x-ray absorption signal of the active material (As), after suitable electrochemical characterizations as described in electrochemical characterization section, the cell has been disconnected.

Then, it has been disassembled and each electrode has been washed with

dimethyl carbonate (DMC). The electrode powder, removed from the Cu foil, was ground and mixed under Ar atmosphere with ultra-pure cellulose and pressed in form of pellets, then sealed in plastic leak-proof boxes suitable for x-ray experiments.

7



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As K-edge XAS spectra of dierent electrodes at selected stages of SEI formation (as described inelectrochemical characterization section) were measured in scanning energy mode using a sagittally focusing double Si(311) crystal monochromator at the GILDA beamline (BM08)

14,15

of the European Synchrotron Radiation Facility (ESRF). Two Pd-coated mirrors

working at an incidence angle of 3.6 at the sample was about 2 mm

mrad were used for harmonics rejection.

The beam size

× 0.3 mm FWHM. XAS spectra were collected in uorescence

mode using a 12-elements high purity Ge solid-state detector

15

and normalized by measuring

the incident beam intensity with an ion chamber lled with Ar gas.

The samples were

installed in the standard BM08 experimental chamber, on a manipulator rotated by

45◦

with respect to the X-ray beam direction. XAS spectra in uorescence mode were obtained by selecting a suitable photon energy window in the solid-state detector, enclosing the K α As emission line related to the relaxation of the photo-excited As atoms in the sample. XAS spectra of pure As placed in a second experimental chamber after the sample were acquired simultaneously with each sample scan for continuous monitoring of the energy scale against possible monochromator instabilities. Samples have been also tested for possible xray degradation phenomena, by collecting multiple near-edge x-ray absorption scans without changing the spot position.

We have found that some samples can be subject to a slight

degradation eects within timescales of the order of 20-30 minutes. Final XAS spectra were thus collected by changing the spot position (typical size 300 collected XAS spectra.

µm) on samples and averaging

Several reference XAS spectra were also collected in transmission

mode using two Ar-lled ionization chambers as detectors, including the As K-edge XAS of the LiAsF6 electrolyte salt.

8


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Results and discussion Near-edge x-ray absorption spectra and As content results The As K-edge XANES spectra were collected on the set of electrodes under consideration are shown in Fig.

2.

These experimental data allowed us to detect a clear trend: 1) the

intensity of the As-related uorescence signal was found to increase with the charging level increase (capacity); 2) the shape of the As K-edge XANES was found to change drastically in the dierent electrodes. The intensity of the As uorescence signal was used to estimate the amount of As atoms embedded in the SEI of the electrodes. The total counting rate is proportional to the number of excited As atoms for a given photon ux, acquisition time and amount of active material for unit surface. In Fig. 3 we show the trend of the As

Kα

uorescence emission intensity,

as obtained by a set of XRF experiments carried out with a home laboratory source with photon energies up to 30 KeV and samples at 45

◦

with respect to the detector. The As peak

intensity was normalized to the total emission ux including the background of each sample due to incoherent inelastic scattering, to take into account small uctuations (below 10% in present samples) in the sample mass and composition.

X-ray emission data were used to

estimate the thickness of the inorganic SEI layer in both types of electrodes. A reasonable assumption is that the inclusion of As in the SEI layers proceeds with the growth of that interphase, therefore the intensity of the As signal can be used to monitor directly the average thickness of the inorganic SEI over the electrodes. On the other hand, the thickness evolution of the SEI layer over ZFO-C electrodes has been studied in a totally dierent way by soft x-ray absorption experiments, using the limited probing depth of the yield techniques.

13

The

limiting SEI thickness for ZFO-C samples was estimated to be 40 nm with a typical 15% uncertainty,

13

and this value was taken as a reference for present thickness evaluation (100%

intensity in Fig. 3). Our results showing the trend of the SEI thickness for ZFO-C and C electrodes are

9



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reported in Fig. 3. We observed similar trends for the SEI thickness in graphite and ZFO-C electrodes, using suitable scales for the capacity in the two electrodes. good agreement with our previous ZFO-C results with LiPF 6 , thickness above a typical capacity Q=46 mAhg (50S) for ZFO-C. This study

13

1

13

The curves are in

showing saturation of the SEI

(15S) for the graphite and Q=417 mAhg

1

further conrms the compatibility of the results obtained by

two similar electrolyte salts. The Li 2 CO3 which is considered one of the SEI main elements formed using LiAsF 6

16,17

was also observed by conventional LiPF 6 . Li2 O and Li2 O2 forming

in layers close to the electrodes are also reported in both cases formation of POF 3 due to HF contamination is also expected.

13,16,18

9,16

while, in LiPF 6 the

External layers, in both

cases, are mainly formed by organic decomposition products of the non-aqueous solvents. However, the main phases formed directly by the redox of the electrolyte salts in both LiAsF 6 and LiPF6 are LiF, LiAsF x and LiPFx .

2,9,16,18

The shape evolution of the XANES spectra at dierent charging state of the ZFO-C and graphite electrodes are shown in Fig. 2, along with the electrolyte salt reference (see dashed line in Fig.

4, left panel, for LiAsF 6 in solution).

It is clear that visible changes of the

peaks are obtained within the initial charging steps, associated with new As valence states in addition to As As

2+

5+

of the electrolyte anion. The peaks are identied to be the As

as reported in Fig 2.

3+

and

1921

Based on the previously discussed SEI phases, the evolution of the XANES peaks can be explained considering the chemical reactions suggested for the redox reaction of the electrolyte salt (LiAsF 6 ) in LIB as:

LiAsF6 + 2Li+ + 2e− −→ 3LiF ↓ +AsF3

(1)

AsF3 + 2xLi+ + 2xe− −→ xLiF ↓ +Lix AsF3−x

(2)

Equations above acknowledge the chemical reaction leading to two building blocks (LiF and Lix AsFx ) of the inorganic SEI components, previously observed in LIBs.

10


10,22,23

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The XANES spectra clearly shows an evolution of the electrolyte (LiAsF 6 ) compatible with the reactions described in Eqs. 1 and 2 within the initial charging steps. This observation conrms the rapid onset in the formation of the SEI.

22

By increasing the charging steps

the reactions proceed to further reductions of As-F and formation of LiF and LiAsF x in the SEI structure. This is thus compatible with the formation of the Li x AsF3−x phase (see Eq. 2) in which the higher the value of the ( x), the higher is the

xLiF

content. Within 10 charging

steps the conversion of electrolyte to form LiF based on equation 2 is clearly visible by the appearance of a strong As

2+

(Lix AsF3−x ) peak. The reaction proceeds to reach a relative

maximum of Li x AsF3−x phase observed at 15S (E=0.32 V, Q=20 mAhg

−1

) in graphite elec-

trode (see Fig. 2) that corresponds to a high amount of reduced electrolyte i.e., a relatively saturated SEI formation.

Similar XANES peak evolutions observed in ZFO-C electrodes

suggest a similar reduction mechanism related to the reactions described in Eqs. 1 and 2. In these electrodes a maximum LiF formation on SEI is detected at 50S corresponding to E=0.79 V and Q=417 mAhg The prominent As

5+

−1

(see Figs. 1 and 2).

peak in ZFO-C electrode even at 20S (E=0.87 V, Q=167 mAhg

−1

)

is in agreement with the mild capacity slope detected in GITT measurements for ZFO-C electrodes. This implies a larger capacity requirement of ZFO-C electrodes to reach a similar SEI state similar to the graphite electrodes, due to concurrent Li-Fe formation processes, occurring at higher potentials than Li-C intercalation in graphite electrodes and involving reversible utilization of relevant amounts of Li

+

.

XANES results are also in a good agreement with the previously reported results for SEI saturation in charging cycle in Fig.

1.

6,12,13

and are compatible with our GITT measurements shown

Based on these results it is expected that the SEI formation is substantially

completed in a range of capacity between Q=167 mAhg

−1

and Q=417 mAhg

−1

in ZFO-C

electrodes, while in graphite electrodes a stabilization of the SEI thickness is expected to take place between Q=31 mAhg

−1

and Q=46.5 mAhg

The prominent and increasing As

5+

−1

(Fig. 1).

peak detected in graphite electrodes after 15S sample

11



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Page 12 of 27

Figure 2: XANES spectra of the graphite (left) and ZFO-C (right) electrodes for dierent charging levels. The intensity increase is associated with a corresponding increase of the SEI +5 +3 +2 thickness. Three components in the XANES spectra, assigned to As , As and As , are clearly observable.

i.e, Q=31 mAhg

−1

(Fig. 2, left) can be assigned to residual electrolyte salt adsorbed on the

surface of the porous structure of the electrodes.

This suggests the formation of a quasi

stabilized and saturated SEI for which the LiF conversion (reaction of Eq. 1) does not take place. As

5+

peak is also observable at lower charging steps although its weakness is assigned

to a lower SEI thickness and a non saturated SEI. This component is less noticeable in charged ZFO-C electrodes (Fig. 2, right) and may be due to a combination of two eects. First, a larger surface to volume ratio in nanosized ZFO-C particles compared to microsized graphite may lead to a higher reduction rate in ZFO-C electrodes. ZFO-C electrodes also explains the higher As

3+

and As

2+

The large surface of

ratio even for initial charging

levels, corresponding to a higher LiF/Li x AsF3−x concentration in the SEI layer. Second, the high internal surface of ZFO-C electrodes also results in a large reactivity of the amorphous carbon coating compared with graphite electrodes that can result in extensive interactions of the electrolyte salt with the amorphous carbon shell. In both type of electrodes, after several cycling of the cell (20 cycles), a relative reduction of As

+2

compared with As

+3

was observed, as shown in Fig. 2. This eect can be assigned

to a relative instability of the phases formed in the SEI during the charging process.

12


It

Page 13 of 27

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Table 1: Distribution of As valence states following the SEI evolution as determined by LCF XANES analysis. The weight of dierent As valence states and the estimated uncertainty are indicated for each sample in the last three columns of the table. Sample

Capacity (mAhg−1 )

As5+

As3+

As2+

C 5S C 10S C 15S C 20C

12 17 20 -

0.70±0.03 0.01±0.02 0.17±0.02 0.10±0.01

0.03±0.04 0.11±0.05 0.13±0.05 0.34±0.02

0.27±0.07 0.88±0.04 0.70±0.05 0.56±0.02

ZFO-C 10S ZFO-C 20S ZFO-C 50S ZFO-C 20C

83 167 417 -

0.57±0.03 0.69±0.03 0.02±0.01 0.03±0.015

0.37±0.06 0.30±0.05 0.16±0.02 0.23±0.02

0.06±0.02 0.01±0.03 0.82±0.03 0.74±0.03

implies a re-oxidation within the SEI layer occurring via the reversible reaction of Eq. 2 or introduction of oxygen atoms into the structure. We have performed a linear combination tting (LCF) of the ZFO-C and graphite As K-edge XANES spectra, in order to evaluate the weight of each component. A typical LCF analysis is shown in Fig. 4 where both the individual components and the total tting curve are compared with the measured XANES spectra (results are shown for the 20 times cycled graphite sample-20C). The LCF analysis was carried out using three reference spectra for As

5+

(LiAsF6 ), As

3+

(As2 O3 ) and As

11867.5 eV, respectively.

20,24

2+

(AsS) with energies of 11874.8 eV, 11871.6 eV and

Limited energy shifts (below 1 eV for both As

3+

and As

2+

) due

to the possible mismatch of the energy scales in dierent experiments were also considered in the tting procedure. The results of the LCF analysis are reported in Tab. 1. The weight of each component shows trends in agreement with the SEI formation in the redox process (see Eqs.

1 and 2).

The SEI appears to be relatively more stable in ZFO-C electrodes.

In particular, changes in As valence states reported in Tab. 1 indicate the occurrence of a re-oxidation process as an eect of the electrode cycling. In fact, the weight of the As is decreased (while As

3+

2+

ions

is increased) by about 14% and 8% in graphite and ZFO-C anodes

13



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Page 14 of 27

respectively, in the cycled electrodes with respect to the C 15S and ZFO-C 50S samples.

Figure 3: As K α uorescence emission intensity obtained by XAS experiments for graphite and ZFO-C electrodes. The estimated As eective (within the SEI) thickness (see text) is also shown for both systems (right axis) as evaluated by matching the As

Kα

XRF intensity

with the maximum SEI thickness found by soft x-ray measurements in ZFO-C electrodes .

Extended X-ray absorption ne structure analysis EXAFS signals of the electrodes under consideration were analyzed to study the evolution of the local structure around As within the SEI. EXAFS data-analysis was carried out using well-established methods (GNXAS

2527

based on ab-initio multiple-scattering calculations).

Within this approach, we have used accurate simulations of the associated with the

2−body

distribution functions,

g2 ,

γ (2)

XAS

2−body

signals,

describing the local structure around

photoabsorbing atoms (As). Dynamical and statical distance uctuactions (associated with vibrations or congurational disorder) have been accounted for averaging the calculated signals either with Gaussian or with asymmetric

Γ

functions (see for example

γ (2)

27,28 and

refs. therein). In most cases, excellent simulations were obtained including only the rst

g2

peak (nearest neighbors of As atoms, typically F atoms in the present case). The structural renement has been performed by minimizing the dierence of the raw absorption spectra with the simulation including the structural oscillation,

χ(κ) and a suitable

background factor. The chosen As K-edge atomic background includes also contribution of

14


Page 15 of 27

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Figure 4: Left: Normalized XANES spectra of the graphite electrodes along with the electrolyte salt solution LiAsF 6 spectra shown as the reference. All the spectra are normalized to one with a constant oset for better visibility while the LiAsF 6 spectrum is multiplied by a constant (×2). Right: linear combination tting (LCF) of the XANES spectrum (graphite 5+ 3+ 2+ electrode 20C). Three reference spectra of As (LiAsF6 ), As (As2 O3 ) and As (AsS), with typical peak energies of 11874.8 eV, 11871.6 eV and 11867.5 eV respectively, were used.

multi-electron channels amounting up to a few percent of the absorption cross section, more evident at low amplitude signals. We have specically considered the contributions of the As double-electron excitation channels ( 1s3p) and (1s3s) about 150-160 eV and 600 eV above the absorption edge. The structural renement was applied to EXAFS spectra of both graphite and ZFOC electrodes at dierent charging levels along with the LiAsF 6 electrolyte reference.

The

quality of the renement was rst tested against the data of the LiAsF 6 electrolyte measured in transmission mode. In that case, we have found that an excellent tting of the EXAFS data can be obtained using a single rst-shell As-F contribution (lower curve of Fig. 5) within a simple Gaussian approximation. were found to be: 10% on

σ 2 ).

R=

1.727 Å,

σ2

Distance =0.002 Å

2

The rst-neighbor As-F distance

bond length range ( 1.6

− 1.7

R

and mean-square relative displacement

σ2

(typical uncertainty 0.005 Å on distances and

R

is in an excellent agreement with the As-F

Å) reported in the literature for AsF 6 .

29

Due to the obvious

correlation among amplitude parameters, the coordination number N As−F =6 was kept xed

15



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Page 16 of 27

for the structural renement of the electrolyte. An overall best-t amplitude factor

S02 =0.9

(corresponding to N As−F =6) was obtained for the reference spectrum of the electrolyte. The EXAFS spectra of the electrodes reported in Fig. 5 show that the EXAFS signals of the electrodes are very similar to that of the LiAsF 6 electrolyte in solution, but the amplitude of the oscillations are reduced. This is also conrmed by simple Fourier-transform (FT) analysis of the As K-edge EXAFS spectra of the electrodes showing a single peak associated with the rst-neighbor distribution around the As atoms.

Dierent models for

the rst-neighbor distribution around As in the electrodes were put to a test, in view of the complexity of the SEI structure, using several possible combinations of chemical species. As an example, we show in Fig. 6 the result of the EXAFS structural renements using carbon or uorine atoms as rst neighbors (15s sample). In those ts, rst-neighbor distances were oated in a range compatible with the known As-C and As-F bond distributions ( RAs−F 1.7 Å and

RAs−C ∼

∼

1.9 Å respectively). The renement obtained using F atoms as rst-

neighbors matches closely amplitudes and frequency of the EXAFS oscillation while that including

C

atoms results in a poor t with residual values 3-4 times higher (see gure 6).

The subsequent analysis of the SEI local structure was then carried out xing of the reference and rening range

0 ≤ NAs−F ≤ 6.

R, σ 2

S02

to that

and NAs−F , while NAs−F was also constrained to be in the

Our XAS structural renements of the short-range structure of the

electrodes are shown in Fig. 5 while structural results are reported in Tab. 2. Fitting range was 3.6-12 Å

−1

corresponding to the

∆k

region reported in the gure. Looking at Fig. 5 it

is clear that the amplitude of the EXAFS signal related to the electrodes is strongly damped as compared with that of the electrolyte, also shown in the gures as a reference (bottom). However, the oscillations show roughly the same frequency and decay trend. Hence, changes appear to be mainly related to the modications of the local As coordination within the SEI under development. The evolution of SEI can be thus analyzed by EXAFS looking at the modication of the polyhedral structure of the salt, associated with local LiAsF N fragments. For graphite electrodes we observed a specic trend for the average coordination number

16


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Figure 5:

As K-edge experimental (dots) and best-t calculated (solid) EXAFS

structural signals for each electrode under consideration.

χ(k)k 2

Graphite and ZFO-C electrodes

at dierent charging states are shown in the left and right-hand panels, respectively. The EXAFS signal of the LiAsF 6 electrolyte solution is shown as a reference (bottom curves).

NAs−F

measured by EXAFS, as reported in Tab. 2. The best-t rst-neighbor structural

coordination number for C 5S sample can be assigned to a mixture of LiAsF 6 with other As-F phases produced through reactions described in Eqs. 1 and 2. Taking into account our XANES results, this reduction can be assigned to the presence of AsF 3 /Lix AsF3−x . 10S and C 15S samples, the coordination number

NAs−F

In C

reaches a minimum which conveys

a maximum reduction (domination of Li x AsF3−x phase) indicating a quasi-saturation of the SEI. The slight increase of

NAs−F

in the 15S electrode can be explained by taking into

account two eects discussed before.

First, the residual electrolyte salt remaining in the

porous electrode and SEI structure, when the SEI is approaching saturation. Second, the reoxidation of the SEI during the oxidation state of the charging cycle due to its instability. The slight increase in bond length also observed in As-O bonds.

R is also in agreement with the change of the oxidation state as 24

The bond length reported here is associated with fragments

AsFN with dierent coordination N, embedded in the SEI, and represents an average As-F

17



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Page 18 of 27

distance. Since several AsF N complexes are present in each sample, it is reasonable that the average

R

is substantially unchanged in all samples within the estimated uncertainty.

Table 2: Best-t structural parameters obtained by As K-edge EXAFS dataanalysis for each electrode under consideration. Average coordination number N , distance R and mean-square relative displacement σ2 of the As-F rst-neighbor shell are reported in the table. The estimated uncertainty is about 10% on N and σ2 values, and 0.01 Å on distance R. Sample

N

C 5S

4.0

C 10S

2.2

C 15S

2.7

ZFO-C 10S

4.3

ZFO-C 20S

5.0

ZFO-C 50S

1.9

σ2

R (Å)

(Å

2

± ± ±

0.4

1.73

0.002

0.2

1.74

0.002

0.2

1.74

0.002

± ± ±

0.4

1.71

0.002

0.5

1.72

0.002

0.2

1.75

0.002

)

Figure 6: Comparison of As-F (solid line) and As-C (dashed line) rst shell renements of 2 the EXAFS χ(k)k structural signals (15S sample). An excellent agreement obtained using a rst-neighbor shell of F atoms while presence of rst-neighbor C atoms is ruled out.

For ZFO-C electrodes, the best-t coordination numbers

NAs−F

reported in Tab.

2

are substantially higher and compatible with a predominance of the AsF 6 structure up to the charging levels of the 20S electrode, as also indicated by the capacity slope in GITT measurement discussed before.

The coordination number

NAs−F

reaches its minimum at

50S in agreement with the XANES results, conrming the substantial saturation of the SEI.

18


Page 19 of 27

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The smaller coordination numbers of ZFO-C electrodes compared to the graphite ones at high charging steps are also consistent with the higher weight of As

2+

states observed in

ZFO-C electrodes in XANES spectra. This is a result of a higher reduction rate due to the larger surface of ZFO-C electrodes. Additionally, the need of more charging steps in ZFO-C for similar potentials (see Fig. 1) as compared to graphite, causes further reduction processes resulting in a lower average coordination number assigned to a LiAsF ∼2 meta-phase. After 20 charge-discharge cycles a new structural signal corresponding to a second coordination shells is clearly visible in the XAS spectra of both electrodes. This second shell signal can be nicely reproduced introducing As atoms as back-scatterers. An accurate structural renement is obtained adding As atoms at a distance

R ∼ 2.48Å (Figure 8) in both graphite

and ZFO-C cases. Average coordinations for ZFO-C are found to be N As−F

∼ 0.65, while for graphite N As−F ∼ 2 and NAs−As ∼ 0.8.

∼2

, NAs−As

The presence of As atoms as second

neighbors is compatible with the re-oxidation process mentioned in previous section and can also imply the introduction of further oxygen atoms into the SEI structure. However, due to the similar backscattering properties and average bonding distances for F and O atoms, we are not able to assess the presence of oxygen in the structure. Oxide formation in the SEI structure can occur by electron pair donation based on the Lewis model while re-oxidation can occur via sharing a F atom between two As atoms that possibly leads to formation of the structures indicated in Fig.7 based on the N As−F calculated in our analysis.

Figure 7: Sketch of As-based complexes possibly formed within the SEI layer. Local coordination and short-range are compatible with present XAS data. Typical interatomic distances are reported in gure.

19



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This new rearrangement can also explain the increase in As

3+

Page 20 of 27

observed in XANES spectra

of both graphite and ZFO-C electrodes. On the other hand the detected structure could be also partially associated with the presence of Li atoms between dierent As-F structures or reduction of the defect-induced lattice displacements and crystallization tendency in As-F structures after many charging cycles. Considering contribution of a second As atom in the structure of lower charging level samples only enhances slightly the SEI saturated samples (15S for graphite and 50S for ZFO-C) without alteration of As-F coordination number. This may indicate that the reordering initiates shortly after the SEI saturation and is observable after many cycles. However, this structure reformation should be studied at large chargingdischarging cycles to clear out the precise mechanism of the structure evolution in saturated and stabilized SEI. As a nal remark, we notice that EXAFS structural renements were also attempted allowing for the presence of other As bonding mechanisms in the SEI structure.

In par-

ticular we veried whether the addition of carbon in the rst-neighbor shell could improve agreement with the experimental data. Our analysis was done adding a specic As-C contribution with the constraint of keeping constant the number of nearest neighbors. The As-C renements slightly improved the EXAFS renement only for ZFO-C electrodes. Due to the intrinsic noise of present As K-edge XAS uorescence data (small amount of As embedded in the electrodes) the uncertainty in As-C structural results was found to be relatively high. However, the occurrence of As-C bonding possibly related to As-C-Li structures is in line with previous results

13

indicating the presence of LiC x in ZFO-C electrodes, and may be

either due to limited Li surface storage through carbon or to interactions of the inorganic SEI fraction with the organic components formed by solvent reduction and decomposition.

20


Page 21 of 27

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Figure 8:

As K-edge Experimental (dots) and best-t calculated (solid) EXAFS

structural signals for each electrode under consideration.

χ(k)k 2

ZFO-C and graphite electrodes

after 20 charging-discharging cycles are shown at top and bottom respectively. The best t revealed presence of As second-neighbor in the structure.

Conclusions In conclusion, for the rst time, the evolution and stability of the SEI layer in advanced LIB electrodes using hard X-ray absorption spectroscopy at the As K-edge was studied. The As atoms embedded in the SEI, produced by the electrolyte decomposition were used as the local probe.

The XAS element selectivity and local structure sensitivity is shown to

be a suitable method for observations of the SEI structural evolution. Our results indicate that SEI formation commence within the initial charging levels and a SEI of nearly-constant thickness is formed after specic charging levels in both graphite and ZFO-C electrodes in agreement to GITT measurements.

We have found that SEI formation process occurs

with a higher reduction rate in ZFO-C electrodes due to their large surface to volume ratio. Our detailed analysis of XANES and EXAFS spectra shows that important changes in As valence state and local coordination occur during the SEI formation that are correlated to

21



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its structural evolution. The formation of AsF N complexes with dierent local coordination

N

is clearly observed and measured at various SEI evolution stages, in agreement with a

redox reaction mechanism leading to the formation of two building blocks of the SEI (LiF, Lix AsFx ).

We also observed a SEI instability in both electrodes after several charging-

discharging cycles and our analysis evidenced that SEI formed on ZFO-C electrodes is more stable compared with graphite.

EXAFS analysis of the cycled electrodes has shown the

presence of a well-dened shell of As second neighbors conrming the re-oxidation of the SEI structure after prolonged charge-discharge cycling. Present results show the potential of the XAS technique for investigating the evolution of the SEI with an embedded local probe and stimulate further investigations for improving our understanding on these important phenomena aecting the performances of LIBs.

Acknowledgement The support of the European Commission under the Project Stable Interfaces for Rechargeable Batteries (SIRBATT) (FP7-ENERGY-2013, grant agreement No. 608502) is gratefully acknowledged. The authors thank F. Capodaglio for his contribution in an initial stage of this work.

We acknowledge the European Synchrotron Radiation Facility for provision of

beamtime (MA-2351) and we would like to thank F. d'Acapito for assistance in using beamline BM08.

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Graphical TOC Entry

27


Local Structure and Stability of SEI in Graphite and ZFO Electrodes

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