Synthesis of orthorhombic LiMnO2 as a high capacity cathode for Li-ion battery by emulsion drying method

Article abstract of DOI:10.1246/cl.2001.574

Orthorhombic LiMnO₂ was readily synthesized by calcination of an emulsion-dried powder precursor. The optimum synthesis condition to crystallize into zigzag layered β-NaMnO₂ system was to calcine at 925 °C for 12 h in an Ar atmosphere. According to TEM observation, the prepared material from the emulsion-dried precursor consisted of highly ordered single crystalline particle. Li/LiMnO₂ cell showed the capacity of about 173 mAh (g-oxide)^-1 and excellent capacity retention upon cycling with help of cycle-induced spinel like phase, more than 155 mAh g^-1 over 300 cycles at 25 °C.

Full text of DOI:10.1246/cl.2001.574

5
74
Chemistry Letters 2001
Synthesis of Orthorhombic LiMnO as a High Capacity Cathode for Li-Ion Battery
2
by Emulsion Drying Method
Seung-Taek Myung, Shinichi Komaba,* and Naoaki Kumagai
Department of Chemical Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551
(Received March 30, 2001; CL-010291)
Orthorhombic LiMnO was readily synthesized by calcination
of an emulsion-dried powder precursor. The optimum synthesis
fractometry (XRD; Rigaku Rint 2200) was employed to identify
crystal structure of the prepared powder. The powder morphology
was observed using transmission electron microscopy (TEM; H-800,
Hitachi) with an acceleration voltage of 200 kV.
2
condition to crystallize into zigzag layered β-NaMnO system was
2
to calcine at 925 °C for 12 h in an Ar atmosphere. According to
TEM observation, the prepared material from the emulsion-dried
precursor consisted of highly ordered single crystalline particle.
For electrochemical testing, cathode was fabricated by blend-
ing the prepared powder, acetylene black and polyvinylidene fluo-
ride (75:20:5) in N-methylpyrolidinon. The slurry was pasted on
–1
Li/LiMnO cell showed the capacity of about 173 mAh (g-oxide)
2
2
and excellent capacity retention upon cycling with help of cycle-
nickel ex-met (1 cm ). The cell consisted of the oxide cathode as a
–1
induced spinel like phase, more than 155 mAh g over 300 cycles
at 25 °C.
working electrode and lithium ribbon as a counter electrode was
assembled in an Ar-filled glove box. The cells were charged and
+
discharged between 2.0 and 4.3 V
cm at 25 °C.
at current density of 0.1 mA
Li/Li
–2
Intercalation compounds of the layered rock salt structure such
as LiCoO and LiNiO , and spinel such as LiMn O , are widely
studied and used as electrodes in advanced lithium batteries. In par-
Figure 1 shows phase evolution step of o-LiMnO as a func-
2
tion of calcination temperature. As-received powder was firstly cal-
2
2
2 4
2+
3+
4+
cined at 400 °C in air to oxidize from Mn to Mn and/or Mn .
The resultant (hereafter called precursor) was confirmed as amor-
phous manganese and Li MnO compounds. It is likely that the
ticular, LiMnO is a promising cathode material because man-
2
ganese is cheaper and more abundant than cobalt or nickel.
2
3
Orthorhombic layered LiMnO (space group of Pmnm, here-
average oxidation state of Mn is in more than 3+ in the compounds.
2
after referred as to o-LiMnO ) which has a zigzag layered β-
When the precursor calcined at 600 °C in Ar atmosphere, it began
2
3+
4+
NaMnO structure is expected to work as 3-V class cathode materi-
to convert to spinel LiMn O which consists of Mn and Mn ,
2
2 4
al for Li-ion secondary battery. Most of the synthesis routes in pre-
indicating that partial reduction of Li MnO phase in the precursor
2 3
vious studies concerning o-LiMnO powders have been developed
is in progress. From 700 °C, the o-LiMnO appeared as a minor
2
2
by solid-state reaction, that is, prolonged calcination with intermit-
tent grinding, caused relatively larger particle size. It was reported
phase, meaning that Li MnO phase is partially reduced in each par-
2
3
ticle. After calcination at 925 °C, o-LiMnO phase was formed as a
2
that electrochemical behavior of o-LiMnO is highly dependent on
major phase, deducing that the oxidation state of manganese
reached nearly 3. In the case of calcination in the range of 700 to
950 °C in air, we would obtain mixture phase of Li MnO and
2
its crystallite size.1 So, wet-process has advantages to synthesize
homogeneous oxide powders. Low-temperature solution technique
seemed to be necessary to obtain high capacity upon cycling, but
obvious capacity fading is usually observed during cycling, though
2
3
LiMn O where the average oxidation state of Mn is between 3.5
2
4
2,3
the capacity fading is more moderate than LiMn O .
2
4
Recently, the capacity fading was improved by using an well-
4
ordered o-LiMnO oxide. Jang et al. reported that highly ordered
2
o-LiMnO prepared by calcination of freeze-dried precursor at 950
2
°
C showed a high capacity retention upon extensive cycling.
Though the o-LiMnO exhibited good electrochemical properties,
2
its powder preparation process is too difficult, that is, only sensitive-
–6
ly controlled oxygen atmosphere (p = 10 atm).
o2
To overcome the difficulty of o-LiMnO synthesis and
2
enhance electrochemical properties, we previously reported
5
hydrothermal synthesis of o-LiMnO2. To develop powder and
electrochemical properties more and more, we employed the emul-
sion drying method which can intermix cations homogeneously in
atomic scale.6 In the present work, we investigated the prepara-
–9
tion condition and electrochemical properties of o-LiMnO synthe-
2
sized by an emulsion drying method.
Reagent-grade LiNO and Mn(NO ) ·6H O (Kanto) with a ratio
3
3 2
2
of Li/Mn = 1 were used as starting materials. Precursor powder was
6–9
prepared by an emulsion drying method, as reported previously.
The obtained powder precursor was firstly treated at 400 °C in air for
h, and then it was calcined at various temperatures in an Ar atmos-
phere, and cooled to room temperature in a tube furnace. X-ray dif-
3
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
575
and 4, while the calcination in Ar atmosphere gave us a well-crystal-
lized o-LiMnO , of which the average oxidation state of Mn is nearly
2
3
. Crystallographic parameters of the synthesized powder is in a
5,10,11
good agreement with literatures.
As can be seen in the XRD
pattern at 950 °C of Figure 1, there are two secondary phases,
6
LiMn O and Li MnO . In our previous report, we confirmed that
2
4
2
3
Li could be evaporated as an oxide form of Li O at around 940 °C. At
2
higher temperature about 950 °C in Figure 1, lithium can evaporates
from particles. On that way, the formation of Li MnO and LiMn O
2
3
2 4
12
is due to after lithium evaporation. And, the amount of Li MnO
Mn ) is relatively small, comparing to o-LiMnO .
2
3
4+
(
2
LiMnO structure was gradually changed to cycle-induced spinel
2
like phase during cycling. After activation from about the 40th
cycle, the material begins to show a capacity of more than 160 mAh
–
1
g
reversibly. The higher capacity was maintained upon 300
cycles. The excellent cyclability is due probably to the fine single
crystalline particle oxide.
From these results, it was found that o-LiMnO was easily
2
formed from reduction of Li MnO prepared by the emulsion dry-
2
3
ing method. This method is quite effective to synthesize fine single
crystalline oxide electrode material. The product calcined from
emulsion dried powder precursor would show enhanced battery per-
1,2
formance than other powders prepared by other synthetic routes.
We are going to develop the physical and electrochemical proper-
Figure 2 shows TEM bright-field image and selected-area elec-
ties of the o-LiMnO2, and report elsewhere in near future.
tron diffraction (SAD) pattern obtained from o-LiMnO calcined at
2
925 °C for 12 h in an Ar atmosphere. As can be seen in SAD pat-
The authors would like to thank Ms. Nobuko Kumagai for her
helpful assistance in the experimental work. This study was sup-
ported by Industrial Technology Research Grant Program in ’00
from the New Energy and Industrial Technology Development
Organization (NEDO) of Japan, the Iwatani Naoji Foundation’s
Research Grant, and Yazaki Memorial Foundation for Science and
Technology.
tern of Figure 2, the product consists of single crystalline particle
oxide. Most of particles have smoothly developed edges and cor-
ners. The particle size distribution was 0.5–1.5 µm in diameter
from TEM observation. It is interesting to note that the colors of
calcined powder changed from a tone of brown to olive green with
increasing calcination temperature from 700 to 925 °C. During the
heat-treatment, there must happen changes in band gap between
conduction and valence bands. From XRD and TEM experiments,
the emulsion drying method is how effective to intermix the cations
homogeneously in atomic scale, leading to highly fine single crystal
particle oxide.
References and Notes
1
2
3
4
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Electrochem. Soc., 146, 3217 (1999).
Figure 3 shows charge–discharge curves and their correspon-
ding cyclability of the o-LiMnO calcined at 925 °C for 12 h in Ar
2
atmosphere. The cell showed two potential plateaus (3.7 and 4.1 V)
in the initial charge process. It was thought that phase transforma-
tion to spinel might occur during electrochemical lithium deinterca-
lation, leading to much faster phase transformation to reach highest
capacity upon cycling. As cycle goes by, 4-V subplateaus and 3-V
plateau are getting longer, meaning that activation of the oxide
derived from the phase transformation is in progress to show high
5
6
7
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8
9
S.-T. Myung, N. Kumagai, S. Komaba, and H.-T. Chung, Solid State
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4,13
capacity, as reported previously.
During the activation to pro-
10 Joint Committee on Powder Diffraction Standards, File no. 24-0734.
11 V. R. Hoppe, G. Brachtel, and M. Jansen, Z. Anorg. Allg. Chem., 417,
duce high capacity derived from phase transition from orthorhom-
bic to cycled-induced spinel like phase, the charge–discharge curves
confirm that structural reordering from orthorhombic to spinel
LiMn O occurs to result in higher capacity. Indeed, ex-situ XRD
1
(1975).
1
2
M. M. Thackeray, M. F. Mansuetto, and J. G. Bates, J. Power
Sources, 68, 153 (1997).
2
4
13 I. M. Kötschau and J. R. Dahn, J. Electrochem. Soc., 145, 2672
experiment showed close result to refs 4 and 13 that the original o-
(1998).