LITHIUM INTERCALATION INTO CrO3-GIC AND ITS
DERIVATIVES
Jan M. Skowroński1, Mariusz Walkowiak2
1
Poznań University of Technology, Institute of Chemistry and Technical Electrochemistry, ul.
Piotrowo 3, 60-965 Poznań, Poland, email: [email protected]
2
Central Laboratory of Batteries and Cells, ul. Forteczna 12/14, 61-362 Poznań, Poland,
email: [email protected]
1. Introduction
Exfoliation of graphite intercalation compounds creates new graphitic materials having
substantially different properties compared to the original graphite. Upon expansion the flake
surface area increases markedly which is accompanied by a destruction of the material crystal
structure. This phenomenon can be considered as a peculiar type of grinding of the graphite
flakes. It was shown that exfoliated graphite may exhibit larger reversible capacity of lithium
intercalation compared to the original graphite [1-2]. It was also proved in a number of works
that mechanical grinding of graphite flakes has a strong influence on both the reversible and
irreversible capacities [3-5]. Both phenomena can be explained in terms of facilitated
transport of lithium ions within the graphite structure. Unfortunately, also the irreversible
capacity may rise after exfoliation due to the increase in surface area.
In a former work [6] we examined composition and structure of some graphite-based
materials obtained by thermal exfoliation of CrO3-GIC. This compound was exfoliated in air
in the temperature 800°C, and the material obtained this way was subsequently heat-treated at
1000°C in the flow of hydrogen. The present paper aims at analyzing the lithium
insertion/deinsertion behaviour of a wide range of materials derived from CrO3-GIC.
2. Experimental
CrO3-graphite intercalation compound (CrO3-GIC) was synthesized by the so called
solvent method described in [7] using natural graphite (99.9 wt.% C, flakes of 180 µm in
diameter, Graphitwerk Kropfmőhl, Germany). Thermal exfoliation of CrO3-GIC was carried
out by placing a ceramic vessel containing the GIC sample for ten minutes in a laboratory
owen heated to appropriate temperature. Samples subjected to hydrogen treatment were
heated slowly in a quartz tube using a pipe owen until the desired temperature was reached
and then kept in this temperature for a given period of time. The XRD measurements were
carried out using X-ray diffractometer (Phillips PW-1710, 35kV, 20mA) with FeKα radiation
(λ = 0.1937 nm). The lithium insertion/deinsertion behaviour of samples were examined in a
two electrode coin cell (CR 2430-type ) with a lithium foil playing simultaneously role of both
the counter and the reference electrode. The electrolyte was 1M LiClO4 - EC/DEC (1:1 by
weight). The working electrodes were prepared by mixing the sample (80 wt.%) with
acetylene black (10 wt.%) and PVDF (10 wt.%) dissolved in cyclopentanone. After spreading
the slurry on a nickel grid current collector the electrodes were dried under vacuum at 140°C
for 4 hours. The cells mounted in glove box filled with dry argon were galvanostatically
cycled between 0 V and 2 V vs. Li/Li+ at a rate of 30 mA/g of active substance.
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3. Results and discussion
Table 1 shows the sequence of experimental procedures starting from a natural graphite
denoted GK180 with the average flake diameter of 180 µm, which was subsequently
intercalated with chromium trioxide.
TABLE 1 Samples descriptions and main characteristics
sample
description
Q1ch
GK180
CrO3-GIC
CR800
CR800/H1000
CR800/H1000/G1/HCL
natural graphite
intercalation of CrO3 into GK180
exfoliation of CrO3-GIC at 800°C
treatment of CR800 in H2 at 1000°C
removing Cr3C2 from CR800/H1000
[mAh/g]
400
635
709
469
442
Q1dis
[mAh/g]
322
263
446
185
354
Q1dis/Q1ch FWHM of
002 peak
*100
[%]
[deg 2Θ]
81
0.112
41
63
0.258
39
0.125
80
0.122
2,0
2,0
1,5
1,5
E [V]
E [V ]
The resulting graphite intercalation compound (CrO3-GIC) can be characterised by the
third stage of intercalation and shows the so-called "ash tray" effect, which means that
intercalate (CrO3) is located mainly in the outer regions of the graphite flakes. Figure 1 shows
galvanostatic charge/discharge curves of both materials. As compared to its precursor, CrO3GIC is characterised by a significantly smaller reversible (discharge) capacity (see Table 1),
what can be easily explained by a smaller number of interlayer spacings accessible for lithium
intercalation. Another striking feature on the galvanostatic curve is a huge irreversible
capacity. Such a great retention of charge cannot be attributed only to the solid electrolyte
interface (SEI) formation at the flakes surface, but also to lithium ion cointercalation into the
graphite galleries occupied by CrO3. There is no indication of CrO3 reduction between the
graphene layers, what is in agreement with earlier report in this matter [8].
1,0
0,5
1,0
0,5
0,0
0,0
0
100
200
300
400
500
Q [m Ah /g ]
600
700
a
0
100
200
300
400
Q [m A h /g ]
500
600
700
b
Fig. 1 Galvanostatic charge/discharge cycles of samples GK180 (a), and CrO3-GIC (b)
26-2
40
C R800
In te n s ity
35
30
25
20
30
40
50
60
70
80
2 T h e ta [d e g ]
Fig. 2 XRD pattern for sample CR800 with marked reflections of Cr2O3
E [V]
Thermal treatment of CrO3-GIC at 800°C makes the intercalate decompose violently to
give gaseous products (O2, CO2) and lower chromium oxide Cr2O3 (for crystallografic
evidence see Fig. 2), which is ejected outside the graphite galleries. Cr2O3 thus obtained
makes a composite material together with the expanded graphite created during the thermal
shock. Exfoliation has a destructive effect on the graphite structure. The XRD pattern
provides evidence for this in a significant broadening of the diffraction peaks (see Tab. 1).
Electrochemical tests show that
2,0
lithium is intercalated independently into
both components, the potential regions are
clearly separated on the discharge curve
1,5
(Fig. 3). In the case of Cr2O3 the term
“insertion” should be used rather then
1,0
“intercalation", since the later is
appropriate for layered structures. After
0,5
heat-treatment of CR800 at 1000 °C in the
flow of hydrogen Cr2O3 disappears from
the expanded graphite particles and
0,0
0
100
200
300
400
500
600
700
80 instead a new phase of chromium carbide
Q [m Ah /g ]
Cr3C2 is created (see Fig. 4). The new
material is denoted as CR800/H1000. In
Fig. 3
Galvanostatic charge/discharge of our previous work we proposed the
sample CR800
mechanism for the Cr3C2 formation [6].
26-3
50
C R800/H1000
45
In te n s ity
40
35
30
25
20
20
30
40
50
60
70
80
2 T h e ta [d e g ]
Fig. 4 XRD pattern for sample CR800/H1000 with marked reflections of Cr3C2.
2,0
2,0
1,5
1,5
E [V ]
E [V ]
A very interesting phenomenon occurring during heat-treatment in hydrogen is an
improvement of the graphite crystal structure. As can be seen in Table 1 the peak broadening
returned to the value very close to that observed before exfoliation. This surprising restoration
of the graphite structural perfection after shock of exfoliation we attribute to the catalytic
action of chromium species. Presence of carbide greatly affects the electrochemical behaviour
of the composite material. Since Cr3C2 is not capable of intercalating lithium ions, it can be
considered as a kind of inactive ballast, hence low discharge capacity (Tab. 1 and Fig. 5).
1,0
1,0
0,5
0,5
0,0
0,0
0
100
200
300
400
500
Q [m Ah /g ]
600
700
Fig. 5 Galvanostatic charge/discharge of
sample CR800/H1000
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0
100
200
300
400
500
Q [m Ah/g]
600
Fig. 6 Galvanostatic charge/discharge of
sample CR800/H1000/G1/HCL
700
300
250
Inte ns ity
200
150
100
50
0
20
30
40
50
60
70
80
2T h e t a [d e g ]
Fig. 7 XRD pattern for sample CR800/H1000/G1/HCL .
However the presence of Cr3C2 makes the assessment of the electrochemical behaviour
of the "restored" expanded graphite difficult. Thus in the last step of the preparation way the
appropriate chemical procedures were applied to remove thecarbide from sample
CR800/H1000, which allowed us to release the graphite component from the influence of the
other (the sample thus obtained is labelled CR800/H1000/G1/HCL). The XRD pattern
confirms that chemically treated material is pure graphite (see Fig. 7). Fig. 6 shows the
respective charge/discharge curve. Its shape suggests well ordered structure (well defined
stages of intercalation), which confirms crystallographic data. Morover, the reversible capacity
of this graphite (354 mAh/g - see Tab. 1) is close to the theoretical capacity of lithium
intercalation into graphite (372 mAh/g) and is ca. 10% higher than that of the precursor
GK180. At the same time the relative irreversible capacity remained at the same level.
4. Conclusions
Upon heat-treatment at 1000°C in hydrogen the composite graphite-Cr2O3 (obtained by
thermal exfoliation of CrO3-GIC) changes to another composite graphite-Cr3C2. The side
effect of this transformation is restoration of the graphite structure, which was previously
destroyed due to exfoliation. The present paper gives a strong electrochemical evidence for
this restoration. The new graphite can reversibly accomodate 10% lithium more than the
original graphite, and the shape of the charge/discharge curve reveals a very well defined stage
structure.
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