Mechanochemical synthesis of fine-particle γ-LiAlO2

Article abstract of DOI:10.1134/S0020168507060179

We describe a new approach for the synthesis of fine-particle gamma lithium aluminate, γ-LiAlO₂, which involves mechanical activation of mixtures of Al(OH)₃ and Li₂CO₃ and heat treatment of the products. The formation of γ-LiAlO₂ was followed using x-ray diffraction, chemical analysis, thermal analysis, IR spectroscopy, and BET specific surface measurements. The specific surface of the synthesized γ-LiAlO₂, 10 to 33 m²/g, is large enough for this material to be used as an electrolyte matrix support for molten carbonate fuel cells.

Full text of DOI:10.1134/S0020168507060179

ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 6, pp. 645–650. © Pleiades Publishing, Inc., 2007.
Original Russian Text © O.A. Kharlamova, R.P. Mitrofanova, V.P. Isupov, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 6, pp. 732–737.
Mechanochemical Synthesis of Fine-Particle g-LiAlO2
O. A. Kharlamova, R. P. Mitrofanova, and V. P. Isupov
Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences,
ul. Kutateladze 18, Novosibirsk, 630128 Russia
e-mail: isupov@solid.nsc.ru
Received September 18, 2006
Abstract—We describe a new approach for the synthesis of fine-particle gamma lithium aluminate, γ-LiAlO2,
which involves mechanical activation of mixtures of Al(OH)3 and Li2CO3 and heat treatment of the products.
The formation of γ-LiAlO2 was followed using x-ray diffraction, chemical analysis, thermal analysis, IR spec-
troscopy, and BET specific surface measurements. The specific surface of the synthesized γ-LiAlO2, 10 to
33 m2/g, is large enough for this material to be used as an electrolyte matrix support for molten carbonate fuel
cells.
DOI: 10.1134/S0020168507060179
INTRODUCTION
mechanical activation (MA) of mixtures of Al(OH)3
and various lithium salts, such as LiCl, LiF, LiBr,
Li2SO4, and Li2CO3.
Fine-particle gamma lithium aluminate, γ-LiAlO2,
finds application in various areas of electrochemical
energy conversion and storage, in particular as an elec-
trolyte support material for molten carbonate fuel cells
[1]. The ability to produce material with tailored prop-
erties, such as phase purity and high specific surface, is
critical for this application. For example, the specific
surface of γ-LiAlO2 electrolyte support material must
In this paper, we describe the synthesis of γ-LiAlO2
through MA of Al(OH)3 + Li2CO3 mixtures, followed
by heat treatment. It is reasonable to expect that prelim-
inary MA of Al(OH)3 + Li2CO3 mixtures will reduce the
synthesis temperature of LiAlO2 in comparison with
conventional ceramic processing techniques.
be at least 20 m2/g.
γ-LiAlO2 is commonly synthesized via solid-state
reactions or sol–gel processing. These approaches,
however, have a number of drawbacks which limit their
utility for large-scale application. In particular, the
solid-state synthesis of γ-LiAlO2 from a mixture of
Al2O3 and Li2CO3 requires temperatures above 1000°ë
EXPERIMENTAL
The starting materials used were commercial
Al(OH)3 (gibbsite) manufactured at the Achinsk Alu-
mina Refinery (RUSAL) and commercial Li2CO3. A 2 : 1
and a calcination time of at least 10 h [2, 3]. The result- mixture of 2Al(OH)3 and Li2CO3 was mechanically
ing material has a rather low specific surface, <5 m2/g,
because of the sintering of the reactants. In the sol–gel
method, lithium aluminate is prepared from gel pro-
duced by hydrolyzing a mixture of aluminum alkoxides
and lithium salts in an organic solvent [4, 5]. The draw-
backs to this approach are the use of organic reagents
and the large amount of liquid organic waste. In this
context, there is currently considerable interest in com-
mercially viable and environmentally friendly pro-
cesses for the synthesis of fine-particle γ-LiAlO2.
activated in an AGO-2 planetary mill at 40 g (steel
vials, 5-mm-diameter steel balls, powder-to-ball weight
ratio of 1 : 20). The milled material was placed in
corundum crucibles and heated in a muffle furnace in
air at a rate of about 6°C/min to temperatures from 300
to 1000°ë at 100°ë intervals.
The phase composition of the reaction products was
determined by x-ray diffraction (XRD) analysis in air
(DRON-4 powder diffractometer, CuKα radiation, scan
rate of 2°/min) and IR spectroscopy (Infralum FT-801
spectrometer, KBr disk method). In addition, γ-LiAlO2
formation was followed using thermal analysis (TG +
DSC) with a Netzsch STA 449C in air (corundum cru-
As shown earlier [6], lithium aluminum layered
double hydroxides (Li–Al LDHs) with the general for-
mula [LiAl2(OH)6]n+ Xn– · mH2O can be used as precur-
sors for γ-LiAlO2 synthesis via calcination of mixtures cibles, heating rate of 6°C/min, 10- to 15-mg samples).
The specific surface area of the synthesized powders
was determined by BET analysis of argon sorption/des-
of such compounds and lithium carbonate at tempera-
tures above 900°ë. On the other hand, Menzheres et al.
[7] reported the formation of an Li–Al LDH during orption isotherms.
645

646
KHARLAMOVA et al.
4
3
7.69 Å
2
1
H
C
H
C C
C
C
C
H
C
10
15
20
25
30
35
40
2θ, deg
Fig. 1. XRD patterns of 2Al(OH) + Li CO mixtures milled for (1) 3, (2) 5, (3) 10, and (4) 15 min; C = Li CO , H = Al(OH) .
3
2
3
2
3
3
RESULTS AND DISCUSSION
reflection from the Li–Al LDH [LiAl2(OH)6]2CO3 ·
3H2O (2θ = 11.5°, d = 7.69 Å), indicating that the MA
of the mixture of Al(OH)3 and Li2CO3 leads to the for-
mation of an LDH according to the scheme
Figure 1 shows XRD patterns of milled 2Al(OH)3 +
Li2CO3 mixtures. Milling for 10 min or a longer time
causes the reflections from Al(OH)3 to disappear,
which implies that the hydroxide becomes x-ray amor-
phous, while the reflections from Li2CO3 persist. After
milling for 5 min, the XRD pattern shows a broad
4Al(OH)3 + Li2CO3 + 3H2O
[LiAl2(OH)6]2CO3 · 3H2O.
The interplanar spacing d = 7.69 Å suggests that the
forming LDH contains three water molecules per for-
mula unit. Water can be incorporated into the interlayer
spaces of the LDH from air or through partial dehy-
droxylation of Al(OH)3 during milling [8]. As the mill-
ing time increases to 10 min or longer, the reflection in
question broadens further, which is probably associated
with amorphization of the forming LDH.
4
3
650
Figure 2 shows the IR spectra of a 2Al(OH)3 +
Li2CO3 mixture after milling for 10 min and calcina-
tion, in comparison with the spectrum of the uncalcined
LDH. The IR spectra of the milled and unmilled LDH
810
2
show a strong band at 1380 cm–1, due to the CO32–
1
1505
860
stretching mode of the carbonate ions residing in the
interlayer spaces of the LDH. In the spectrum of
1380
Li2CO3, the symmetric stretching mode of CO23– is rep-
resented by a doublet at 1437 and 1503 cm–1. The IR
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber, cm–1
spectrum of the milled 2Al(OH)3 + Li2CO3 mixture
shows a shoulder at 1380 cm–1, characteristic of the
stretching mode of the carbonate ions in the interlayer
spaces of the LDH. There are also bands arising from
lithium carbonate, at 860, 1440, and 1505 cm–1.
Fig. 2. IR spectra of (1) the as-milled Li–Al LDH, (2) as-
milled Al(OH) + Li CO mixture, and (3, 4) Al(OH) +
3
2
3
3
Li CO mixtures milled for 10 min and then calcined at 650
2
3
and 800°ë, respectively.
INORGANIC MATERIALS Vol. 43 No. 6 2007

MECHANOCHEMICAL SYNTHESIS OF FINE-PARTICLE γ-LiAlO2
647
4.04 Å
G
(a)
G
G
4.85 Å
Ä
G
H
G
5
4
H
3
2
C
C
C
H
C
H
H
H
H
H
1
10
15
20
25
30
35
40
G
G
(b)
G
G
G
Ä
4
3
2
C
C
C
C
1
10
15
20
25
30
35
40
2θ, deg
Fig. 3. XRD patterns of materials obtained by calcining a 2Al(OH) + Li CO mixture at (2) 650, (3) 700, (4) 800, and (5) 900°C ;
3
2
3
(1) uncalcined mixture; (a) without MA, (b) after milling for 10 min; H = Al(OH) , C = Li CO , G = γ-LiAlO , A = α-LiAlO .
3
2
3
2
2
According to Isupov [9], Al(OH)3 has no absorption 10 min. Heating the unmilled mixture to 650°ë led to
bands in the range 1350–1500 cm–1. Therefore, the IR
spectroscopy data confirm the formation of an Li–Al
LDH during milling of a mixture of Al(OH)3 and
Li2CO3.
the formation of the α-phase of lithium aluminate, but
some of the Li2CO3 remained unreacted. After heating
to 800°ë, the sample consisted of both α-LiAlO2 (pre-
dominant phase) and γ-LiAlO2. At 900°ë, we also did
not obtain phase-pure γ-LiAlO2. At the same time, after
MA we obtained a mixture of α- and γ-LiAlO2, in
Figure 3 displays XRD patterns of the materials
obtained by 2-h calcination of a 2Al(OH)3 + Li2CO3
mixture without MA and after milling at 40 g for which the γ-phase prevailed, at a temperature as low as
INORGANIC MATERIALS Vol. 43 No. 6 2007

648
∆m/m, %
KHARLAMOVA et al.
molecular water resulting from partial Al(OH)3 dehy-
100
95
90
85
80
75
70
65
60
droxylation during milling and the water accommo-
dated in the interlayer spaces of the forming LDH. The
higher temperature endotherm is attributable to the
thermal decomposition of the activated Al(OH)3 and
the decomposition of the metal–hydroxide layers in the
LDH [8, 10]. On heating to 400°ë, the weight loss is
32%. Since the removal of all the water from the system
leads to a weight loss of about 23%, we conclude that
the endothermic peak near ꢀ220°ë is due not only to
further water removal but also to partial CO2 release
from the system. The same is evidenced by chemical
analysis data: after calcination at 300°ë, the CO2 con-
tent of the product is about 14%, whereas that of the
starting mixture is ꢀ19%. This provides additional evi-
dence for the formation of a double hydroxide during
milling, since Li2CO3 decomposes at higher tempera-
tures, and the carbonate ions are removed from the
interlayer spaces in the structure of the LDH at temper-
atures from 200 to 300°ë.
230
550
730
560
2
120
310
220
1
0
100 200 300 400 500 600 700 800 900
t, °C
Fig. 4. Thermal analysis data for 2Al(OH) + Li CO mix-
3
2
3
Further heating results in an endothermic peak at
ꢀ560°ë. This endotherm, accompanied by a weight
loss, is attributable to lithium carbonate decomposition
and LiAlO2 formation. Note that, during heating of the
milled mixtures, the TG curve plateaus at a tempera-
ture as low as 700°ë. The net weight loss is 58%,
which corresponds to the removal of all the water
and CO2 from the system and the formation of lith-
ium aluminate.
tures without MA (dashed lines) and after milling for
10 min (solid lines): (1) TG, (2) DSC.
650°ë. Calcination at 800°ë yielded phase-pure
γ-LiAlO2. The IR spectrum of the mixture calcined at
650°ë showed bands characteristic of γ-LiAlO2, at 810
and 650 cm–1, due to the stretching mode of AlO4 tetra-
hedra [10]. Heating to 800°ë increased the intensity of
all the bands due to γ-LiAlO2.
The disappearance of the reflections from Al(OH)3
after milling for 10 min or a longer time may be due to
the full conversion of Al(OH)3 to an Li–Al LDH or to
partial amorphization of the hydroxide. A different
mechanism of γ-LiAlO2 formation is then possible. The
x-ray amorphous phase resulting from Al(OH)3 milling
is known to possess increased reactivity [8, 10]. Its
reaction with Li2CO3 during heating may lead to
γ-LiAlO2 formation. The synthesis of γ-LiAlO2 can
The appearance of an LDH during milling of a
2Al(OH)3 + Li2CO3 mixture suggests that the formation
of lithium aluminates during heat treatment of the
milled mixtures is due in part to the thermal decompo-
sition of the forming LDH and reactions between the
decomposition products and Li2CO3. The same is evi-
denced by the thermal analysis data in Fig. 4. The DSC
curve of the milled sample shows two thermal events at
temperatures from 50 to 400°ë. It seems likely that the
endothermic peak at 120°ë is due to the removal of the then be represented by the following scheme:
2nAl(OH)3 + nLi2CO3 + 3H2O
MA
x[LiAl2(OH)6]2CO3 · 3H2O + 2(n – 2x)Al(OH)3(am.) + (n – x)Li2CO3
<600°C
x(Li2O · 2Al2O3)(‡m) + (n – 2x)Al2O3 + (n – x)Li2CO3
(thermolysis
product)
600–800°C
LiAlO2
INORGANIC MATERIALS Vol. 43 No. 6 2007

MECHANOCHEMICAL SYNTHESIS OF FINE-PARTICLE γ-LiAlO2
649
A
G
G
G
A
3
G
G
G
2
1
10
15
20
25
30
35
40
2θ, deg
Fig. 5. XRD patterns obtained after calcination of mixtures at 800°ë for 2 h: (1) 2Al(OH) (åÄ) + Li CO (MA), (2) 2Al(OH)
3
3
2
3
(åÄ) + Li CO (no MA), (3) 2Al(OH) (no MA) + Li CO (MA); milling time, 10 min; G = γ-LiAlO , A = α-LiAlO .
2
3
3
2
3
2
2
To ascertain whether this mechanism of γ-LiAlO2
Phase-pure γ-LiAlO2 was also obtained by calcining
formation is indeed possible, Al(OH)3 and Li2CO3 were a 2 : 1 mixture of milled Al(OH)3 and unmilled Li2CO3,
milled separately, and then a mixture of the milled com- whereas calcining milled Li2CO3 with unmilled
pounds was calcined. Figure 5 shows XRD patterns of
the samples obtained by 2-h calcination. As seen, the
calcination of the separately milled powders at 800°ë
leads to the formation of γ-LiAlO2. Thus, γ-LiAlO2 may
form by two mechanisms:
(1) LDH formation during milling of a 2Al(OH)3 +
Li2CO3 mixture, followed by thermal decomposition of
the LDH and reaction of the decomposition products
with Li2CO3 on heating;
Al(OH)3 we obtained a mixture of α- and γ-LiAlO2
(Fig. 5). Thus, we are led to conclude that the MA of
Al(OH)3, as well as LDH formation, plays a central role
in reducing the synthesis temperature and enhancing
the reactivity of the mixture, whereas the milling of
Li2CO3 has an insignificant effect.
The specific surfaces of the calcined MA products
are listed in Table 1. The specific surface of the samples
prepared at 900°ë is about 10 m2/g and is a weak func-
tion of the milling time in the range 3–10 min. The spe-
cific surface of the samples synthesized at lower tem-
(2) reaction between the x-ray amorphous product
of Al(OH)3 milling and lithium carbonate on heating.
Table 1. Phase composition and specific surface of samples obtained by milling 2Al(OH)3 + Li2CO3 mixtures at 40g, fol-
lowed by calcination for 2 h at different temperatures
Phase composition
Milling time, min
no calcination
650°C
700°C
800°C
900°C
No milling
Al(OH)3 + Li2CO3 (0.8) α + Li2CO3 (41)
Al(OH)3 + Li2CO3 (10) α + γ (29)
Al(OH)3 + Li2CO3 (4.9) α + γ (33)
α + γ (20)
α + γ (22)
α + γ (24)
γ + α (<5%) (33)
γ + α (<5%) (28)
α + γ (4.8)
γ + α (16)
γ + α (24)
γ (25)
γ + α (<5%) (1.6)
γ (9.3)
γ (10.2)
γ (9.1)
γ (8.8)
3
5
10
15
Li2CO3 (5.3)
γ + α (40)
γ + α (37)
Li2CO3 (7.6)
γ (18)
2
Note: α = α-LiAlO , γ = γ-LiAlO ; the first indicated phase prevails. Numbers in parentheses specify S, m /g.
2
2
INORGANIC MATERIALS Vol. 43 No. 6 2007

650
KHARLAMOVA et al.
Table 2. Comparison of processes for LiAlO2 synthesis
Heat treatment
Process
Phase
S, m2/g
τ, h
t, °C
Solid-state reaction
α
γ
γ
86
10
–
600
1000
<5
<2
Sol–gel
700
2–10
10–30
Mechanochemical
γ
2
700–800
4. Hirano, S., Hayahi, T., and Kageyama, T., Synthesis of
LiAlO2 Powder by Hydrolysis of Metal Alkoxides,
J. Am. Ceram. Soc., 1987, vol. 70, no. 3, pp. 171–174.
5. Kwon, S.W. and Kim, E.H., The Formation of Phase
Pure Lithium Aluminate from the Bulky Lithium and
Aluminium Alkoxides, J. Mater. Sci. Lett., 1999, no. 18,
pp. 931–933.
6. Isupov, V.P., Mitrofanova, R.P., Chupakhina, L.E., and
Rogachev, A.Yu., Application of Aluminum Hydroxide
Intercalation Compounds with Lithium Salts in Waste-
Free Synthesis of Lithium Aluminates, Khim. Interesakh
Ustoich. Razvit., 1996, vol. 4, pp. 215–219.
peratures varies with milling time. At a calcination tem-
perature of 800°ë, the largest specific surface (25 m2/g)
was achieved at a milling time of 10 min, and the result-
ant material was phase-pure γ-LiAlO2. Increasing the
milling time to 15 min reduces the specific surface to
18 m2/g, presumably because of the particle agglomer-
ation during milling. The specific surface of phase-pure
γ-LiAlO2 synthesized at 800–900°ë ranges from 10 to
33 m2/g. Table 2 compares the specific surfaces of
LiAlO2 synthesized by different methods.
7. Menzheres, L.T., Kotsupalo, N.P., and Mamylova, E.V.,
Solid-State Interaction of Aluminium Hydroxide with
Lithium Salts, J. Mater. Synth. Process., 1999, vol. 7,
no. 4, pp. 239–244.
8. Zolotovskii, B.P., Scientific Principles behind the Mech-
anochemical and ThermochemicalActivation of Crystal-
line Hydroxides in the Fabrication of Supports and Cat-
alysts, Doctoral (Chem.) Dissertation, Novosibirsk:
Inst. of Catalysis, Sib. Div., Russ. Acad. Sci., 1992.
CONCLUSIONS
The mechanochemical synthesis of fine-particle
γ-LiAlO2 offers a number of advantages over conven-
tional processes and makes it possible to markedly
reduce the synthesis temperature and time.
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1967, vol. 23, pp. 2127–2143.
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INORGANIC MATERIALS Vol. 43 No. 6 2007