Low-temperature synthesis of Li7La3Zr 2O12 with cubic garnet-type structure

Article abstract of DOI:10.1016/j.materresbull.2012.01.027

In this paper, we report the direct synthesis of Li₇La ₃Zr₂O₁₂ with the cubic garnet-type structure at low temperature with a lattice constant of 13.0035 . The synthesis condition is optimized to be at 750 °C for 8 h with 30 wt% excess lithium salt. No intermediate grinding was involved in this straightforward route. Without the adventitious of Al³⁺, the cubic Li₇La₃Zr ₂O₁₂ is unstable above 800 °C and has an ionic conductivity of the order of 10^-6 S cm^-1.

Full text of DOI:10.1016/j.materresbull.2012.01.027

Materials Research Bulletin 47 (2012) 1229–1232
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Low-temperature synthesis of Li La Zr O with cubic garnet-type structure
7
3
2 12
a
a,b
a,
Hui Xie , Yutao Li , John B. Goodenough *
a
Texas Materials Institute, ETC 9.184, University of Texas at Austin, Austin, TX 78712, United States
b
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
In this paper, we report the direct synthesis of Li La Zr O with the cubic garnet-type structure at low
7
3
2 12
Received 9 December 2011
Accepted 25 January 2012
Available online 5 February 2012
˚
temperature with a lattice constant of 13.0035 A. The synthesis condition is optimized to be at 750 8C for
h with 30 wt% excess lithium salt. No intermediate grinding was involved in this straightforward route.
8
3+
Without the adventitious of Al , the cubic Li
7
La
3
Zr
2
O
12 is unstable above 800 8C and has an ionic
À6
À1
conductivity of the order of 10 S cm
.
Keywords:
ß 2012 Elsevier Ltd. All rights reserved.
Oxide
Chemical synthesis
Ionic conductivity
1
. Introduction
were carefully weighed and then dissolved in acetic acid. More
than 20 wt% of excess LiAc was added to compensate for the loss of
Li during the heat treatment. The mixture was magnetically stirred
and heated until a fine powder was precipitated.
x 3 2
Compounds with a composition of Li La M O12 (x = 5 or 7,
M = Ta, Nb, Zr) have been extensively investigated recently as a
novel family of fast Li -ion conductors [1–5]. The nominal
compound ‘‘Li
reported to have a
stable on contact with metallic lithium [7]. However, the solid-
state synthesis process has involved three steps that are time and
energy consuming. The synthesis temperature is too high for
+
Thermogravimetric analysis (TGA 7, Perkin-Elmer) was
employed to narrow the proper temperature range for the
synthesis. The decomposition behaviors of the starting materials
were also obtained by TGA. The measurements were performed in
flowing air from room temperature to 1000 8C at a heating rate of
7 3 2
La Zr O12’’ having a garnet-type structure has been
À4 À1
s
Li > 10 S cm [3,6]. It is also chemically
À1
1 8C min . A sample weight of 25–30 mg was used.
retention of the Li; the pyrochlore phase La
formed as a competitive phase [3]. A lower synthesis temperature
2
Zr
2
O
7
is commonly
The obtained powder was spread on an alumina or platinum
crucible, first heated slowly to 400 8C, then sintered at a designed
temperature (650–800 8C) and maintained for 4–20 h.
À7
À1
resulted in a tetragonal phase having a
previous paper [9], we reported the neutron diffraction study of the
aluminum-free Li La Zr 12 with cubic garnet structure, which
sLi ꢀ 10 S cm [8]. In our
Powder X-ray diffraction (Philips X-ray diffractometer, Model:
PW 1830) was used to examine the phase formation and crystal
structure of the product. To calculate the lattice parameter, silicon
powderwasaddedas theinternal standard. The Rietveld method and
the FullProf software package were applied to refine the structure.
Scanning electron microscopy (SEM, JEOL JSM-5610 with 20 kV
accelerating voltage) and high-resolution transmission electron
microscopy (HRTEM, JEOL 2010F working at 200 kV) were applied to
investigate the surface morphology and microstructure of the
synthesized product. For HRTEM, the powder was ground carefully
and then ultrasonically dispersed in ethanol. A few drops of the
resulting suspension weredeposited on a carbon-coated copper grid.
The obtained powder was pressed into ꢁ1 mm-thick pellets
with a diameter of 12 mm and reheated at 750 8C for 4 h. The
density of the sintered pellets was determined by the Archimedes’
method. A layer of gold was sputtered on both parallel surfaces of
the pellets. A Solartron Impedance Analyzer (model 1287) was
used to measure the ionic conductivity. The applied frequency
7
3
2
O
leads to a conclusion that the maximum upper limit of Li content in
the garnet structure is 7.5 Li per formula unit. In this paper, we
report the detailed synthesis and its optimization of the cubic
phase at low temperature via a one-step solution method, along
with the measurement of the ionic conductivity. The synthesis
conditions were optimized in terms of the amount of excess
lithium source, synthesis temperature, and time.
2
. Experimental
Acetate salts were used as starting materials. LiCH
3
COOÁ2H
2
O
O
(
(
GFS Chemicals, hereafter denoted as LiAc), La(CH
Alfa Aesar, as LaAc) and Zr(CH COO)
3 3
COO)
Á1.5H
2
3
4
(MP Biomedicals, as ZrAc)
*
Corresponding author. Tel.: +1 512 471 1646; fax: +1 512 471 7681.
E-mail address: jgoodenough@mail.utexas.edu (J.B. Goodenough).
6
range was from 10 Hz to 1 Hz with a 10 mV amplitude. The
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2012.01.027

1
230
H. Xie et al. / Materials Research Bulletin 47 (2012) 1229–1232
+
a
activation energy (E ) of the Li -ion conduction was calculated
•
Li
2
ZrO
3
* La
2
Zr
2
O
7
from the Arrhenius plot of the relationship of the total ionic
conductivity to temperature.
5
4
0%
•
•
•
0%
3
. Results
3.1. Thermal analysis
30%
The TGA curves for the starting acetate salts and the
*
precipitated powder are presented in Fig. 1a and b, respectively.
From Fig. 1a we can see that ZrAc and LiAc decompose below
*
2
0%
20
*
4
00 8C. Another weight loss for LiAc is found at about 700 8C, which
indicates the volatilization of Li O. The decomposition of LaAc is a
more complicated multi-step process [10]. The main weight loss
happens before 400 8C to form La (CO ); the final decomposition
occurs at about 700 8C to form La . Accordingly, two weight-loss
2
30
40
50
2
θ / °
2
O
2
3
Fig. 2. XRD patterns with different amounts of excess LiAc (by weight) after heating
at 750 8C for 8 h.
2 3
O
steps are found in Fig. 1b. The first step is the loss of adsorbed water
and then the initial decomposition of acetate salts. The second
stage is the final decomposition of lanthanum acetate with a
800 °C
750 °C
La
2
Zr
2
O
7
2
certain amount of the volatilization loss of Li O. At this stage, all
the acetates are completely decomposed to oxides and are ready to
react. Therefore, the synthesis temperature was optimized in the
range from 650 8C to 800 8C. Before the final synthesis, the material
was heated at 400 8C and maintained for 2 h.
3.2. Optimization of the synthesis
6
50 °C
To find the best condition for the synthesis, three factors were
considered and optimized: the amount of excess lithium source,
the synthesis temperature, and the time.
For conventional synthesis of materials containing lithium,
20
30
40
50
60
Li
Li
2
2
CO
CO
3
is most widely used as the lithium source; 10 wt% of excess
is normally enough due to its relatively high decomposition
2θ / °
3
Fig. 3. XRD patterns after different synthesis temperatures with 30 wt% excess LiAc.
temperature. LiAc decomposes below 350 8C and the lithium is
easily vaporized at temperatures above 700 8C, as the TGA curve of
LiAc suggests. Therefore more than 10 wt% excess LiAc is needed
for this synthesis.
5
2 3
0 wt% excess lithium, Li ZrO emerges, which indicates that too
much lithium remains after the heat treatment and Li
2 3
ZrO is
readily formed with the existing excess lithium.
Different amounts of excess LiAc from 20 to 50 wt% were tested.
The obtained powders were sintered at 750 8C for 8 h; their XRD
patterns are shown in Fig. 2. It should be noted that in most of the
XRD patterns in this paper, there are some impurity peaks with
In summary, the optimized amount of excess lithium is 30–
0 wt% at 750 8C. Less lithium results in La Zr , while more
lithium gives Li ZrO . It is noteworthy that the synthesis results are
the same with both Al and Pt crucibles.
4
2
2 7
O
2
3
2 3
O
2 3 2
very weak intensities that can be indexed to La O or ZrO . The
Fig. 3 shows the XRD patterns sintered at different tempera-
tures from 650 8C to 900 8C for 8 h with 30 wt% excess LiAc. It is
main reason for the presence of the impurity is that the acetates are
highly hydrophilic, so it is difficult to precisely control the molar
ratio of the starting materials.
obvious that the Li
significant amount of La
the 750 8C product shows a quite pure phase of cubic Li
No extra peak indexed to La Zr or Li ZrO exists.
At 800 8C, the XRD pattern is quite similar to that obtained at
50 8C; both Li La Zr 12 and La Zr phases exist. It is known
7
La
3
Zr
2
O
12 phase can be formed at 650 8C, but a
is also observed. The XRD pattern of
La Zr
Zr
2 7
O
It is clear that the Li
excess lithium. However, a significant amount of La
present. With 30 wt% excess lithium, La Zr is nearly eliminated
and Li La Zr 12 becomes the only dominant phase. The peaks are
7
3 2
La Zr O
12 phase is formed with 20 wt%
2
2 12
O .
2
2 7
Zr O is also
7
3
O
7
2
3
2
2 7
O
2
2
7
3
2
O
6
7
3
2
O
2
2 7
O
indexed to a cubic symmetry. No big difference is observed when
the excess lithium increases from 30 wt% to 40 wt%; but with
from the TGA curves in Fig. 1 that there is an obvious weight loss of
1
00
(a)
(b)
1
00
8
6
4
2
0
0
0
0
8
6
0
0
Zirconium acetate
Lanthanum acetate
Lithium acetate
40
0
200
400
600
800
1000
0
200
400
600
800
1000
Temperature / °C
Temperature / °C
Fig. 1. (a) TGA curves for the starting acetate salts. (b) The TGA curve for the mixture of starting material.

H. Xie et al. / Materials Research Bulletin 47 (2012) 1229–1232
1231
Fig. 4 compares the XRD patterns synthesized at 750 8C with
3
0 wt% excess lithium from 4 to 20 h. No significant difference can
2
0h
5h
be observed between these patterns, but the pattern qualities,
especially the peaks at high 2 angle, are slightly different. Also, the
relative intensities of the peaks in these patterns are different. The
synthesis time does not appear to have a significant effect on the
phase formation. However, taking both the efficiency and pattern
quality into consideration, 8 h appears to be optimal.
u
1
8
h
The refinement result of the XRD pattern indicates a cubic
symmetry with the space group Ia 3¯ d. The lattice parameter is
4
h
˚
calculated to be 13.0035 A, slightly higher than that obtained by
high-temperature synthesis [3,6].
The high-temperature stability of this low-temperature cubic
phase was investigated. The XRD patterns (not shown here) show
2
0
30
40
50
2θ / °
2 2 7
that it can easily decompose to La Zr O above 800 8C.
Fig. 4. XRD patterns after different synthesis times at 750 8C with 30 wt% excess
LiAc.
3.3. Microstructure
SEM and HRTEM images of the product sintered at optimized
conditions are shown in Fig. 5a and b, respectively. The SEM image
lithium at temperatures up to 800 8C. When heated at 800 8C, not
shows that the obtained particles are of the order of 10 mm in size.
enough lithium remains to form only the Li
significant amount of La Zr phase is formed.
Therefore, the optimized temperature for synthesis of the
La Zr 12 phase is 750 8C. Temperatures lower than that lead to
incomplete reaction, while too much lithium will be lost at higher
temperature. In both cases, the La Zr phase is formed.
7
La
3
Zr
2
O
12 phase, and a
These particles are actually the aggregation of small grains. This
can be shown clearly in the HRTEM image. After being ultrasoni-
cally dispersed in ethanol, the aggregates are broken up into grains
of about 20 nm in size, and a clear lattice fringe can be observed,
which suggests good crystallinity.
2
2 7
O
Li
7
3
2
O
2
2 7
O
The synthesis time is another important factor to effect the
phase formation and synthesis quality. A long synthesis time will
allow the atoms to diffuse to their lattice position, but it also results
3.4. Ionic conductivity
+
The density of the sintered pellet used to investigate the Li -ion
in more Li
2
O loss.
conductivity was 89.2% of theoretical density (the theoretical
Fig. 5. (a) SEM and (b) HRTEM images of the product synthesized at optimized condition.
-
-
-
200.0k
150.0k
100.0k
-
3.5 (b)
(
a)
Experimental data
Linear fitting
experimental plot
fitted curve
-4.0
4.5
5.0
5.5
-
-
-
50.0k
-
0
.0
2.4
2.6
2.8
3.0
3.2
3.4
0
.0
50.0k
100.0k
150.0k
200.0k
-1
-3
1
000/T /K 10
Z' / Ω
7 3 2
Fig. 6. (a) The impedance plot and the fitted result (solid line) of Li La Zr O12 synthesized at optimized condition. (b) Arrhenius plot of the total ionic conductivity.

1
232
H. Xie et al. / Materials Research Bulletin 47 (2012) 1229–1232
À3
density is calculated to be 5.066 g cm ). The impedance plot at
room temperature is displayed in Fig. 6a with the fitted result
Acknowledgement
(
solid line). In the high-frequency range, only one pressed
This work was supported by the Assistant Secretary for Energy
Efficiency and Renewable Energy, Office of Vehicle Technologies of
the US Department of Energy under Contract No. DE-AC02-
05CH11231, under the Batteries for Advanced Transportation
Technologies (BATT) Program subcontract #6805919.
semicircle is present. So the total resistance was determined
from the plot and the ionic conductivity was calculated to
be about 2.85 Â 10 S cm . An activation energy with the
value of 0.36 eV was obtained from the Arrhenius plot shown in
Fig. 6b.
À6
À1
References
4
. Conclusions
[1] V. Thangadurai, H. Kaack, W.J.F. Weppner, J. Am. Ceram. Soc. 86 (2003) 437–440.
Cubic Li La Zr O12 with garnet-type structure has been
7
3 2
[2] V. Thangadurai, W. Weppner, J. Solid State Chem. 179 (2006) 974–984.
[
[
[
3] R. Murugan, V. Thangadurai,W. Weppner, Angew. Chem. Int. Ed. 46 (2007) 7778–7781.
4] E.J. Cussen, J. Mater. Chem. 20 (2010) 5167–5173.
5] E.J. Cussen, Chem. Commun. (2006) 412–413.
synthesized at low temperature. The optimized synthesis
condition is 750 8C for 8 h with 30 wt% excess lithium salt.
The lattice parameter of the obtained product is slightly larger
than that of the phase obtained by high-temperature solid-state
synthesis. However, the resulting ionic conductivity is about
two orders of magnitude lower than that of the cubic Al-
containing phase obtained at higher temperature. Moreover, this
low-temperature phase is unstable and decomposes above
[6] C.A. Geiger, E. Alekseev, B. Lazic, M. Fisch, T. Armbruster, R. Langner, M. Fechtelk-
ord, N. Kim, T. Pettke, W. Weppner, Inorg. Chem. 50 (2010) 1089–1097.
[
7] M. Kotobuki, H. Munakata, K. Kanamura, Y. Sato, T. Yoshida, J. Electrochem. Soc.
57 (2010) A1076–A1079.
[8] J. Awaka, N. Kijima, H. Hayakawa, J. Akimoto, J. Solid State Chem. 182 (2009)
046–2052.
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[10] G.A.M. Hussein, J. Therm. Anal. 42 (1994) 1091–1102.
1
2
[
(
8
00 8C.