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2nd International Conference on Electrical, Electronics and Civil Engineering (ICEECE'2012) Singapore April 28-29, 2012
Preparation of Li-ion conductive ceramics
through a sol-gel route
Masashi Kotobuki
electrolyte currently used in commercial Li ion batteries.
Therefore, the LATP solid electrolyte has been much attention
to fabricate all-solid-state Li ion battery.
LATP preparation has been performed by a melt-quenching
and solid solution methods so far [15,16]. However, these
methods have been required high temperature process more
than 1200 oC. From view of energy saving, low temperature
synthesis has been strongly required. A sol-gel method allows
us to prepare inorganic materials at low temperature [17].
Takada et. al. successfully prepared LiTi 2 (PO 4 ) 3 (LTP) solid
electrolyte by the sol-gel method from LiOC 2 H 5 , Ti(OC 3 H 7 ) 4 ,
and PO(OC 2 H 5 ) 3-x (OH) x [18]. However, they used costly
reagents. This point may become a shortcoming for mass
production.
In this study, it is reported that the LATP preparation through
the sol-gel process using inexpensive reagents like CH 3 COOLi
and (NH 4 ) 2 HPO 4 .
Abstract— Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP) solid electrolyte is
recognized one of the promising solid electrolytes for
all-solid-state Li ion battery to construct CO 2 emission-free
world. In this study, LATP preparation through the sol-gel
process using inexpensive reagents like CH 3 COOLi and
(NH 4 ) 2 HPO 4 was attempted. Prepared LATP revealed high
bulk conductivity of 1.5×10-3 S cm-1 which is enough high for
all-solid-state battery application, however, grain boundary
conductivity was insufficient. Although some modifications
are required, this finding provides us a new picture for the
LATP solid electrolyte preparation.
Keywords—Li battery, Sol-gel method, Solid electrolyte
I. INTRODUCTION
R
ECHARGEABLE Li ion batteries are a key component of
present information-rich world [1]. They have been used
as energy sources for mobile phones, laptop computers, and so
on due to their high energy density. Therefore, the rechargeable
Li ion battery has been expected as a power source for electric
vehicle and storage device for electric power generated by
natural resource such as wind power, and solar power, and
expected to promote fabrication of CO 2 emission-free world.
Origin of the high energy density of the Li ion battery results
from high operation voltages. Thus, flammable organic
electrolyte which can tolerate such high voltage has been
exclusively used and sometimes caused serious safety problems
such as fire hazards and electrolyte leakage. When the Li ion
battery is applied to the electric vehicle and electric power
storage device, large scale Li ion battery is required, therefore,
dangerousness also increase.
To solve these problems, polymer [2-6] and ceramic [7-13]
solid electrolytes have been studied to eliminate the flammable
components in the rechargeable Li ion battery. Particularly,
all-solid-state battery with ceramics solid electrolyte is
recognized as an ultimate safe battery.
Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP) solid electrolyte is a member of
Li ion conductive ceramics with NASICON (Na super ion
conductor) type structure. Its Li ion conductivity was reported
to ~ 10-3 S cm-1 [14] which is quite similar to the organic
II. MATERIALS AND METHODS
A precursor sol for LATP was prepared by mixing
CH 3 COOLi, Al(C 3 H 7 O) 3 , Ti(C 3 H 7 O) 4 , (NH 4 ) 2 HPO 4 ,
C 3 H 7 OH, and H 2 O in a molarration of 1.5:0.5:1.5:3:40:800,
respectively. The precursor sol was converted to gel at 100 oC.
Obtained gel was ground in a motar and then calcined at 450 oC
to obtain amorphous LATP powder.
The powder was ground by a planetary ball-mill to reduce
particle size to obatin well-sintered pellet. To check the particle
size of the amorphous LATP powder, a scanning electron
microscope (SEM, JEOL JSM-6300LA) was used.
The ground powder was pressed into a pellet at a pressure of
130 MPa followed by calcination at 1000 oC for 6h to obtain
sintered pellet. Crystal structure of the pellet was identified
with X-ray diffraction (XRD, RIGAKU Ultima-IV)
Li ion conductivity of the LATP pellet was measured by AC
impedance method (HIOKI Chemical impedance meter
3532-80). Prior to measure, Au was sputtered on both side of
the pellet to ensure electrical contact. Data was obtained at ±10
mV voltage signal in a frequency range of 4 ~ 1MHz using
as-prepared cell at OCV (open circuit voltage) at 30 oC.
Correspounding Author: Masashi Kotobuki, Hakodate National College of
Technology, 14-1 Tokura-cho, Hakodate, Hokkaido, 042-8501, Japan,
TEL&FAX +81-138-59-6466, E-mail: [email protected]
III. RESULTS
Figure 1 shows SEM images of amorphous LATP particles
before and after ball-milling. Before milling, particles with a
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2nd International Conference on Electrical, Electronics and Civil Engineering (ICEECE'2012) Singapore April 28-29, 2012
frequency regions, respectively. This tail at low frequencies is a
usual behavior of ionically blocking electrodes with ionically
conductive nature [19]. A similar behavior has been observed
another ceramic conductor [20]. Bulk (σ bulk ) and Total (σ total )
conductivities estimated from intercepts of semicircle at high
and low frequency sides, respectively, were 1.5×10-3 and 1.0
×10-4 S cm-1.
Fig. 1 SEM images of amorphous LATP
(a)before and (b)after ball-milling
diameter of 1 ~ 2 μm were observed. A surface of the particles
seems to be amorphous, no facet, which usually appears in
crystalline material, was observed. After milling, the particle
size was obviously reduced to below 100 nm.
XRD pattern of the LATP pellet is revealed in fig. 2. All
diffraction peaks were coincident with standard peaks of LTP
(PDF 35-0754) with NASICON structure. It is confirmed that
LATP solid electrolyte with NASICON type structure was
successfully prepared. Peak intensities of the pellet were
relatively sharp, implying that high crystallinity pellet was
obtained.
Fig. 3 A complex impedance spectrum of the
LATP pellet calcined at 1000 oC for 6h.
IV. DISCUSSION
Herein, LATP preparation through the sol-gel process using
inexpensive reagents like CH 3 COOLi and (NH 4 ) 2 HPO 4 was
reported.
It is obvious that LATP could be prepared through this
sol-gel process as confirmed in XRD measurement. The bulk
conductivity of the LATP pellet was comparable with reported
value of ~10-3 S cm-1 [14], implying that successful preparation
of LATP was achieved.
However, total conductivity was 1.0×10-4 S cm-1. This value
is not enough high to construct all-solid-state battery. The
Fig. 2 XRD pattern of the LATP pellet calcined
at 1000 oC for 6h.
Figure 3 displays a complex impedance plot of the LATP
pellet using Au blocking electrode. A semicircle and
Warburg-type impedance were appeared at high and low
Fig. 4 Li ion conduction mechanism in solid
electrolyte
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2nd International Conference on Electrical, Electronics and Civil Engineering (ICEECE'2012) Singapore April 28-29, 2012
[17] M. Kotobuki, K. Hoshina, Y. Isshiki, and K. Kanamura, “Preparation of
Li 1.5 Al 0.5 Ge 1.5 (PO 4 ) 3 solid electrolyte by sol-gel method“, Phs.
Research. Bull., vol. 24, p.61, 2010.
[18] K. Takada, K. Fujimoto, T. Inada, A. Kajiyama, M. Kouguchi, S. Kondo,
and M. Watanabe, “Sol-gel preparation of Li+ ion conductive thin film“,
Applied Surface Science, Vol.189, p.300, 2002.
[19] M. Kotobuki, K. Kanamura, Y. Sato, and T. Yoshida, “Fabrication of
all-solid-state battery with lithium metal anode using Al 2 O 3 -added
Li 7 La 3 Zr 2 O 12 solid electrolyte“ J. Power Sources, vol.196, p.7750, 2011.
[20] M. Kotobuki, K. Kanamura, Y. Sato, K. Yamamoto, and T. Yoshida,
“Electrochemical properties of Li 7 La 3 Zr 2 O 12 solid electrolyte prepared
in argon atomosphere“ J. Power Sources, vol.199, p.346, 2012.
resistance of the solid electrolyte is separated to bulk and grain
boundary resistances (fig. 4). The bulk conductivity (inverse of
bulk resistance) of LATP prepared in this study was enough
high to apply for the all-solid-state battery, thus, the grain
boundary resistance was a reason for insufficient total
conductivity. It is concluded that lowering the grain boundary
resistance is a key of the LATP preparation by sol-gel process.
A reduction of the grain boundary resistance would be
achieved by obtaining well-sintered pellet because number of
grain boundary was decreased. It is expected that the
grain-boundary resistance could decrease by optimization of
sol-gel process, calcination procedure, and addition of sintering
promotion. These attempts are now under way. The results will
report in due course.
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