Document [original]

The Behavior of the Intercalant AlCl

Anion during the

Formation of Graphite Intercalation Compound: An X-ray

Absorption Fine Structure Study

Giorgia Greco,* Giuseppe Antonio Elia,* Yves Kayser, Burkhard Beckhoff,

Marko Perestjuk, Simone Raoux, and Rober Hahn

1. Introduction

Understanding the processes behind the phase transitions,

particularly the property of some solids to host different species,

is of extreme interest. Indeed this phenomenon has a practical

application in energy storage and conversion systems such as

high-energy-density batteries and hydrogen storage, which are

very important topics due to the need to store and convert energy

from renewable sources.

[1]

The ﬁrst step for material design and

engineering to reinforce our energy storage

capacity is a comprehensive structural

characterization which is a challenging

research activity.

In our previous studies, we have com-

prehensively characterized the graphite

intercalation compound (GIC) formation

in aluminum graphite dual-ion cell

(AGDIC) by combining operando small-

and wide-angle X-ray scattering

[2]

and

tomography with X-ray diffraction

(XRD).

[3]

A detailed electrochemical charac-

terization of the system is reported in our previous studies.

[2–4]

We have directly observed the graphite structural deformation

induced by the electrochemical intercalation process and its

intermediate states, which result in two-phase transitions from

graphite to GIC with 0.05 mole of anion intercalated for 1 mol

of graphite or stage 3.

[2,3]

The intercalation stage is deﬁned as

the numbers of free graphene layers between two layers of inter-

calant species; stage 3 means 3 graphene layers between two

layers of AlCl

4anion. We also highlighted the differences

G. Greco,

[+]

M. Perestjuk,

[++]

S. Raoux

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Hahn-Meitner-Platz 1, D-14109 Berlin, Germany

E-mail: giorgia.greco@uniroma1.it

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/pssa.202300776.

[+]

Present address: Department of Fusion and Technology for Nuclear

Safety and Security, ENEA Centro Ricerche Casaccia, Via Anguillarese

301, 00123 Rome, Italy

[++]

Present address: Université de Lyon, Institut des Nanotechnologies

de Lyon (INL) UMR CNRS 5270, Ecole Centrale Lyon, 69131 Ecully,

France

[+++]

Present address: Department of Applied Science and Technology

(DISAT), Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129

Torino, Italy

[++++]

Present address: Max Planck Institute for Chemical Energy

Conversion (MPI CEC), Stiftstr. 34-36, 45470 Mülheim an der Ruhr,

Germany

science published by Wiley-VCH GmbH. This is an open access article

under the terms of the Creative Commons Attribution License, which

permits use, distribution and reproduction in any medium, provided

the original work is properly cited.

DOI: 10.1002/pssa.202300776

G. Greco,

Chemistry Department

Sapienza University of Rome

P.le Aldo Moro 5, 00185 Roma, Italy

G. A. Elia,

[+++]

R. Hahn

Technical University of Berlin

Research Center of Microperipheric Technologies

Gustav-Meyer-Allee 25, D-13355 Berlin, Germany

E-mail: [email protected]

Y. Kayser,

[++++]

B. Beckhoff

Physikalisch Technische Bundesanstalt (PTB)

Abbestr. 2-12, 10587 Berlin, Germany

M. Perestjuk,

School of Engineering

RMIT University

Melbourne, VIC 3001, Australia

R. Hahn

Fraunhofer IZM Institut für Zuverlässigkeit und Mikrointegration

Gustav-Meyer-Allee 25, D-13355 Berlin, Germany

This work aims to study the insertion of AlCl

4anion in the crystalline structure

of oriented pyrolytic graphite (PG) at the point of view of the anion itself. The

electronic and atomic structures of the anion at different intercalation stages are

studied. In particular double-edge (bicolor) X-ray absorption spectroscopy at the

Al and Cl

-edges is carried out, highlighting a contraction of the anion bonding

at the highest intercalation degree obtained electrochemically (stage 3), while the

electronic population changes for both the edges upon cycle.

RESEARCH ARTICLE

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between two types of graphite, natural (NG) and pyrolytic (PG),

showing how the difference in the elasticity of the crystalline

structure and the mesoscopic porosity inﬂuences the reversibility

of the intercalation process.

[3,5]

Nevertheless, some aspects, such

as this system’s limited capacity, are still unclear. Theoretically,

each graphene layer could host a layer of intercalant species

reaching stage 1, but there is an empirical limitation to stage 3.

In this article we study the behavior of AlCl

4anions during

the intercalation process in PG layers by X-ray absorption ﬁne

structure (XAFS) spectroscopy at both the Cl and Al absorption

K-edges. This bicolor technique being selective to two elements

gives a new insight into the intercalation process from the point

of view of the intercalant species, clarifying some aspects that

have so far remained questionable. The article is structured as

follows.

The ﬁrst part shows the electronic structure of the anion at Al

and Cl K-edges for different intercalation stages. The second part

focuses on quantifying the anion uptake at the different interca-

lation stages, while the third one aims to highlight the structural

changes due to the strain and the compression of the graphite

lattice on the anion.

2. Results and Discussion

2.1. Samples

The electrochemical processes of the AGDIC function are the

following.

4Al2Cl

7þ3e⇌Al þ7AlCl

4(1)

CnþAlCl

4⇌CnAlCl4

½þe(2)

During the charging process at the negative electrode, Al2Cl

anions in the ionic liquid electrolyte react at the Al anode side

forming AlCl

4anions and Al metal. Concurrently at the positive

electrode, AlCl

4anions intercalate into the PG graphitic struc-

ture. Figure 1 shows the typical voltage proﬁle for this system.

The arrows in the ﬁgure mark the state of charge selected for

the ex situ investigation, that is, 25 Ch. (25 mAh g

1

charged),

50 Ch. (50 mAh g

1

charged), fully Ch. (fully charged), and

the same for discharging. As already described in our previous

work,

[3]

PG shows an irreversible capacity of about 30% due to

partial trapping of a certain amount of anions in the PG struc-

ture. This phenomenon is most likely associated with the PG

structure’s low elasticity and poor porosity.

[5]

Ex situ XAFS spec-

troscopy at both the Al and Cl absorption K-edges experiments

was carried out on different PG electrodes at different intercala-

tion degrees

[5]

(see Table 1) to follow the intercalant behavior in

the host at different states of charge. Moreover, samples obtained

at the 5th, 500th, and 1000th cycles have been investigated to

evaluate the long-term cycling effect. A more detailed description

of sample preparation can be found in the Experimental Section.

2.2. XANES

In order to better understand the molecular composition of the

intercalated species, ﬁrst we compared the X-ray absorption near-

edge spectra (XANES) jump (absorption discontinuity due to the

ionization cross section at the K-edge) at the Al and Cl K-edges, as

shown in Figure 2 for the fully discharged sample. As already

described in our previous work,

[3]

the electrodes expanded notice-

ably up to 300% from the initial thickness of ≈20 μm at the fully

charged state (fully intercalated). Due to incorporated material,

we could not collect at Al K-edge (1559.6 eV) in transmission

mode, but only in ﬂuorescence mode. For this reason, we reveal

information of the evaluated molecule stoichiometry for a total of

ﬁve samples (see Table 1). The XANES jump (absorption discon-

tinuity) in transmission mode is proportional to the actual areal

mass density (mass per unit area as integrated into the depth

direction) of an element in the sample.

[6,7]

Thus, the AlCl

intercalated stoichiometry in PG could be obtained from the

jump ratio between Al and Cl K-edges, knowing the cross section

of the elements for the incident photon energies used around the

respective ionization thresholds.

[6]

The results are shown in

Table 1. The expected stoichiometry 4:1 Cl:Al atomic ratio is con-

ﬁrmed for all the samples except for the sample after 500 cycles,

most likely associated with Cl accumulation upon cycling with

the formation of chlorine species on the electrode surface con-

sistent with the presence of CClxand AlClxspecies.

[4]

This phe-

nomenon has also been evidenced in our previous study by X-ray

photoemission spectroscopy (XPS).

[4]

As conﬁrmed by the actual

stoichiometry of the anion, we can calculate with high accuracy

the mass uptake of the molecule in GIC by the formula

mAlCl

4¼NAlCl

4mat;Al þ4mat;Cl

 (3)

where NAlCl

4is the number of uptake molecules, which can be

evaluated from XAS jump and the cross section of the element

and

[6]

mat;m(where m¼Al, Cl) is the elemental atomic mass.

Results in Table 1 are cross-checked at Al and Cl K-edges and

are in good agreement with the estimation of the AlCl

4uptake

from the electrochemical results.

[3]

Going forward, we want to study the transformations of the

electronic structure upon cycling. Figure 3 shows the normalized

near-edge XANES spectra at the Al and Cl K-edges collected in

ﬂuorescence and transmission mode, respectively, for a set of

electrodes in a subsequent intercalation stage. Differences in

Figure 1. Voltage proﬁle of the PG.

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the intensity of the spectral features (labeled in Figure 3 as A–D

for the Al K-edge data and A0–C0for the Cl K-edge data) are

clearly visible for both edges. The normalized spectra are com-

pared with AlCl3powder as reference and with Al metal at the Al

K-edge. Looking at Figure 3a (Al K-edge), an edge shift of

ΔE=1.32 eV and strong decrease of the peak B–C of the signal

of the electrodes compared to the AlCl3powder one is observed.

At Cl K-edge from AlCl3to AlCl

4, a different behavior (Figure 3)

is evident; while the white line increases (B’peak), the peaks A’

and C’slightly decrease. This effect is due to a change in the

coordination number of the Al─Cl and Cl─Cl bonds from three-

to fourfold and from two to threefold, respectively.

[8]

The oxida-

tion state of Al and Cl is always þ3 and 1 respectively, for both

structures.

Each peak of the XANES signal is associated with an electronic

transition as reported in Table 2 for both the Al and Cl

K-edges.

[8–11]

Figure 3a’s inset compares normalized XANES signals at the

Al K-edge of electrodes at subsequential intercalation stages (see

also Figure 1 and Table 1). The Al K-edge signal evidences the

presence of four main peaks, labeled A, B, C, D. The main peaks

A and B decrease from 25 Ch. to 25 Disch. and again increase at

Fully Disch. state. The Cl K-edge signal evidences the presence of

four main peaks, labeled A’,B’, and C’. Similarly, the Cl K-edge

peak B’related to the main transition (see Table 1 and 2)

decreases from 25 Ch. to Fully Ch. and increases again at

Fully Disch. state. It has to be noted that the intensities observed

for the Fully DisCh sample do not reach the initial state condition

as expected,

[4]

due to the partial retention of AlCl

4in the PG

structure, as shown in Figure 1.

2.3. Double-Edge Analysis EXAFS

In order to comprehensively analyze the AlCl

4atomic anion

structure during the intercalation process, a double-edge analysis

(Al and Cl K-edges) of the extended X-ray absorption ﬁne struc-

ture (EXAFS) part of the XAFS data is presented. An advanced

technique based on multiple scattering (MS) calculations of

the absorption cross section in the framework of GNXAS

program

[12,13]

was used. This method has already been applied

to different functional materials for energy conversion and stor-

age.

[7,14,15]

Figure 4a,c shows the best ﬁt and its Fourier trans-

form (FT) obtained for the Fully Disch. sample as an example.

As shown in Figure 4, three bodies Al–Cl, Cl–Al, and Cl–Cl sig-

nal were used to ﬁt the EXAFS signal for both Al and Cl K-edges.

Figure 4b shows the theoretical structure of AlCl

4calculated by

Table 1. In the columns, sample names and the related speciﬁc capacity in mAh g

1

, the intercalation stage number, the calculated atomic ratio between

Al and Cl, and the calculated anion mass uptake in mg obtained from Al and Cl XAS jump ratio (absorption discontinuity due to the ionization cross

section at the K-edge), respectively, are reported.

Sample Speciﬁc Capacity Stage Al:Cl [atm] AlCl

4Mass from

Al K-edge [mg]

AlCl

4Mass from

Cl K-edge [mg]

[mAh g

1

]

1 cycle, 25 mAh g

1

charged ~25 6 (4.2 0.7):1 1.6 0.3 1.7 0.1

1 cycle, 50 mAh g

1

charged ~50 5, 4

–3.0 0.1

1 cycle, fully charged ~100 3 –3.4 0.1

1 cycle, 25 mAh g

1

discharged ~75 3, 4

–3.6 0.1

1 cycle, 50 mAh g

1

discharged ~50 4, 5

(4.0 0.7):1 3.0 0.3 3.3 0.1

1 cycle, fully discharged ~30 6 (4.0 0.5):1 2.3 0.3 2.3 0.1

5 cycle, 25 mAh g

1

charged ~55 5 (4.0 0.4):1 3.0 0.3 3.0 0.1

5 cycles, 50 mAh g

1

charged ~80 4 –3.4 0.1

5 cylcles, fully charged ~100 3 –3.4 0.1

5 cycles, 25 mAh g

1

discharged ~75 3, 4

3.0 0.3 –

5 cycles, 50 mAh g

1

discharged ~50 4, 5

–2.6 0.1

5 cycles, fully discharged ~30 6 (3.5 0.4):1 2.3 0.3 2.0 0.1

500 cycles, fully discharged ~30 6 (5.4 0.9):1 1.6 0.3 2.2 0.1

1000 cycles, fully discharged ~30 6 –2.1 0.1

Coexistence of two stages.

Figure 2. Comparison between near-edge XAS spectra at Al and Cl K-edge

of one electrode fully discharged after the ﬁrst cycle. E0is the edge energy.

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the Avogadro

[16]

program. The ﬁtting procedure was applied to all

the electrodes at different intercalation stages, in Table 3 the

main results are shown. The ﬁrst consideration is the signiﬁcant

difference between the AlCl3theoretical and intercalated one of

Al─Cl/Cl─Al bond distance (ΔRAlCl=ClAl 0.07 Å). Moreover

the interatomic distance between Cl–Cl is shrunk with respect

to theory (ΔRClCl 0.07 Å). Upon cycling, the interatomic dis-

tances remain unchanged up to the fully charged state (stage 3).

Here we have a clear contraction of ΔRAlCl=ClAl 0.01 and

0.02 Å for the electrode fully charged for the ﬁrst cycle and after

ﬁve cycles, respectively (see Table 3). Figure 5 compares the

EXAFS signals and its FT of the 25 Ch. with the Fully Ch. after

ﬁve cycles, for both edges. It is possible to observe a contraction

of the interatomic distances in particular at the Al K-edge and a

decrease in the FT intensities, showing an increase in the struc-

tural disorder. The EXAFS study shows a contraction of AlCl

bonding for both edges at the fully charged state, which corre-

sponds to stage 3. The FT of the EXAFS signal also shows an

intensity decrease at Cl and Al K-edges (Figure 5). This reveals

an increase in the disorder of AlCl

4. This is probably due to the

high level of strain of graphite structure measured in our previ-

ous work.

[3]

The anions ﬁrst get a preferential orientation to

occupy less space and then start to contract the bonding length

up to a maximum level corresponding to stage 3.

[2,3]

3. Conclusion

An XAFS study of the AlCl

4anion intercalated in PG at different

intercalation stages has been studied at Al and Cl K-edges. The

XANES structure shows differences in the features and intensi-

ties of both edges revealing an active part of the Cl in the elec-

tronic exchange with PG upon cycling. The high level of strain in

the PG structure causes an increase in the disorder level in the

local structure detected at both edges. From the double-edges

analysis, AlCl

4, the last intercalation stage 3 shrunk its inter-

atomic distances probably due to the high level of strain in

the PG structure.

[3]

The obtained information gives further

insight into the mechanism of anion intercalation in graphite,

thus potentially leading to a more rational design of cell compo-

nents for high-performance batteries.

Figure 3. Normalized a) Al and b) Cl K-edge XANES collected in ﬂuorescence (Al-K-edge) and transmission mode (Cl K-edge) of a set of electrodes

subsequently charged and discharged compared to the signal obtained for AlCl3powder and Al metal.

Table 2. Al K-edge and Cl K-edge: Peaks label depicted in Figure 2, its

position (energy) and the electronic transition related to.

Electrodes

Edge E0=1563.46 eV

Peak Energy Related transition

[eV]

A 1565.15 1 s !3p(t2)

B 1568.80 MS

C 1576.36 1 s !3d(e)

D 1587.00 1 s !3d(t2)

AlCl3powder

Edge E0=1564.84 eV

Peak Energy Related transition

[eV]

A 1568.80 1 s !3p(t1u)

Electrodes

Edge E0=2823.53 eV

A’2824.40 1 s !3d/3p

B’2825.73 1s !4p

C’2828.20 1 s !higher states in Cl ion

AlCl3powder

Edge E0=2823.53 eV

A’2824.40 1 s !3d/3p

C’2828.20 1 s !higher states in Cl ion

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Figure 4. Double-edge best ﬁt of the fully discharged electrode. EXAFS signal and its FT, a) and c) panel, respectively. The theoretical molecular structure

of the AlCl

4anion in panel (b) is shown.

Table 3. Best ﬁt results of the electrodes at a different intercalation stage. Elect. (mid. stages) indicates the electrodes except for the fully charged states

electrodes.

Sample NAlCl=ClAl RAlCl=ClAl σAlCl=ClAl NClCl RClCl σClCl

[Å] ½Å2[Å] ½Å2

AlCl

4Theor. 4/1 2.17 –3 3.54 –

Elect. (mid. stages) “2.10 0.001/0.011 ”3.47 0.020

Fully Ch. I “2.09 0.010/0.010 ”3.46 0.024

Fully Ch. V “2.08 0.004/0.011 ”3.44 0.022

Fully Disch. “2.10 0.001/0.010 ”3.48 0.023

Figure 5. FT of the EXAFS signal at Al and Cl K-edges of the electrodes 25 mAh g

1

charged I cycle compared to the fully charged 5th cycle.

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4. Experimental Section

Sample Preparation: The electrolyte 1-ethyl-3-methylimidazolium chlo-

ride:aluminum trichloride EMIMCl:AlCl3 in a 1:1.5 mole ratio was provided

by IOLITEC. The water content of the electrolyte was lower than 100 ppm.

The electrochemical tests were performed using high-purity aluminum (Al

99.99% Alfa Aesar) as anode and PG with a thickness of 25 μm and a load-

ing of 4.71 mg cm

2

as the cathode material.

[17,18]

The electrochemical

measurements were performed using Teﬂon Swagelok-type T cells. The

cycling tests of Al/EMIMCl:AlCl3/PG cells were carried out applying

increasing speciﬁc currents of 25 mA g

1

) in the voltage range

0.4–2.4 V. The upper cutoff was selected within the electrochemical stabil-

ity of the electrolyte, evaluated by LSV to be 2.45 V versus Al quasirefer-

ence.

[4]

All galvanostatic cycling tests were carried out at 25 °C in a

thermostatic climatic chamber (with a possible deviation of 1 °C), using

a Maccor 4000 Battery Test System. Prior to the ex situ analyses, the stud-

ied electrodes were rinsed in order to remove residual electrolyte using

dimethyl carbonate, and rigorous anhydrous and water content was

detected by the Karl Fischer titration method. The preparation of the

electrode was performed in the glovebox, with oxygen and water content

below 1 ppm to avoid the electrode’s degradation.

Al K-Edge XAFS: The XAFS were realized in the near-edge region of the

Al K-edge (i.e., XANES measurements ranging from ≈1550 to 1600 eV for

Al) and the extended regime (i.e., EXAFS measurements ranging from

≈1570 to 1840 eV for Al) by varying the incident photon energy from

1490 to 1837.5 eV in variable-energy steps at the plane-grating monochro-

mator beamline of PTB for undulator radiation.

[18]

In the near-edge X-ray

absorption ﬁne structure (NEXAFS) range, a constant increment in the

incident photon energy of 0.3 eV was selected, in the EXAFS regime,

the incident photon energy was varied such that the change in k-space

was constant (0.05 A

1

). At each incident photon energy, the X-ray spec-

trum recorded by the silicon drift detector (SDD) was integrated over 20 s

and the signal of the diode downstream of the sample was averaged over

the same time interval. The samples were oriented at 45° with respect and

with the normal vector to the sample surface plane contained within the

polarization plane of the incident X-ray photons. The SDD was positioned

in the polarization plane and perpendicular to the propagation direction of

the linearly polarized incident X-ray beam in order to minimize scattered

radiation.

Cl K-Edge XAFS: The XAFS measurements on Cl were carried out at

PTB’s four-crystal-monochromator beamline.

[17]

It provides monochrom-

atized radiation between 1.75 and 10.5 keV by means of either four

InSb(111) or Si(111) crystals. The use of four monochromator crystals

allowed for the provision of X-ray radiation with a high spectral resolving

power of 10

[17]

Furthermore, the design of the monochromator unit

allows for a ﬁxed beam position. The incident photon energy during

the XAFS measurements around the Cl K-edge was varied between

2800 and 2950 eV in variable-energy steps, as described in the section

before, using the Si(111) crystals for monochromatizing X-ray radiation

originating from a bending magnet.

Acknowledgements

This project has received funding from the European Unions Horizon

2020 research and innovation programme under the Marie Sklodowska

Curie grant agreement no. 101029608. Furthermore, the European

Commission funded this research within the H2020 ALION project under

contract 646286 and the German Federal Ministry of Education and

Research in the AlSiBat project under contract 03SF0486, and the project

ALIBATT under contract 03XP0128E. The project 21GRD01 (OpMetBat)

received funding from the European Partnership on Metrology, coﬁnanced

by the European Union’s Horizon Europe Research and Innovation

Programme and by the Participating States.

Conﬂict of Interest

The authors declare no conﬂict of interest.

Data Availability Statement

The data that support the ﬁndings of this study are available from the

corresponding author upon reasonable request.

Keywords

electrochemically induced phase transitions, electronic structures,

graphite intercalation compounds, superlattices, thermodynamics, X-ray

absorption ﬁne structure

Received: October 5, 2023

Revised: December 4, 2023

Published online: February 18, 2024

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