nanomaterials
Article
Synthesis of Lithium Metal Oxide Nanoparticles by
Induction Thermal Plasmas
Manabu Tanaka 1 , Takuya Kageyama 2 , Hirotaka Sone 2 , Shuhei Yoshida 3 , Daisuke Okamoto 2
and Takayuki Watanabe 1, *
1
2
3
*
Department of Chemical Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku,
Fukuoka 819-0395, Japan; [email protected]
Department of Chemical Systems and Engineering, Graduate School of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; [email protected] (T.K.);
[email protected] (H.S.); [email protected] (D.O.)
Chemical Engineering Course, School of Engineering, Kyushu University, 744 Motooka, Nishi-ku,
Fukuoka 819-0395, Japan; [email protected]
Correspondence: [email protected]; Tel./Fax: +81-92-802-2745
Academic Editors: Krasimir Vasilev and Melanie Ramiasa
Received: 25 January 2016; Accepted: 29 March 2016; Published: 6 April 2016
Abstract: Lithium metal oxide nanoparticles were synthesized by induction thermal plasma.
Four different systems—Li–Mn, Li–Cr, Li–Co, and Li–Ni—were compared to understand formation
mechanism of Li–Me oxide nanoparticles in thermal plasma process. Analyses of X-ray diffractometry
and electron microscopy showed that Li–Me oxide nanoparticles were successfully synthesized in
Li–Mn, Li–Cr, and Li–Co systems. Spinel structured LiMn2 O4 with truncated octahedral shape
was formed. Layer structured LiCrO2 or LiCoO2 nanoparticles with polyhedral shapes were also
synthesized in Li–Cr or Li–Co systems. By contrast, Li–Ni oxide nanoparticles were not synthesized
in the Li–Ni system. Nucleation temperatures of each metal in the considered system were evaluated.
The relationship between the nucleation temperature and melting and boiling points suggests that
the melting points of metal oxides have a strong influence on the formation of lithium metal oxide
nanoparticles. A lower melting temperature leads to a longer reaction time, resulting in a higher
fraction of the lithium metal oxide nanoparticles in the prepared nanoparticles.
Keywords: thermal plasmas; lithium metal oxide; nanoparticle formation mechanism
1. Introduction
Nanoparticles have become widely utilized due to their enhanced and unique properties relative
to bulk materials. The preparation of nanoparticles can be classified into physical and chemical
methods. The physical methods include mechanical milling [1,2], laser ablation [3–5], and other aerosol
processes with energy sources to provide a high temperature. Among these methods, attractive material
processing with thermal plasmas have been proposed for the nanoparticles production. This is because
thermal plasmas offer unique advantages; high enthalpy to enhance reaction kinetics, high chemical
reactivity, rapid quenching rate in the range of 103–6 K/s, and selectivity of atmosphere in accordance
with the required chemical reactions. Thermal plasmas are capable of evaporating large amount
of raw materials, even with high melting and boiling temperatures [6–9]. Furthermore, high-purity
nanoparticles can be synthesized in an induction thermal plasma because thermal plasma can be
generated in a plasma torch without internal electrodes [10,11]. These advantages of thermal plasmas
have brought about advances in plasma chemistry and plasma processing [12–15].
Lithium metal oxides have attracted many researchers because of their unique properties as cathode
for lithium-ion batteries [16–18], CO2 sorption material [19], and other magnetic, electrochemical
materials. Layer structured LiCoO2 is widely employed as cathodes in commercial battery applications,
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Lithium metal oxides have attracted many researchers because of their unique properties as
cathode for lithium-ion batteries [16–18], CO2 sorption material [19], and other magnetic,
electrochemical
Layer structured LiCoO2 is widely employed as cathodes in commercial
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battery applications, in spite of its toxicity and high cost. To solve the economic and environmental
problems, alternatives of LiCoO2 have been intensively explored. One of the alternatives is spinelin spite of its toxicity and high cost. To solve the economic and environmental problems, alternatives
structured LiMn2O4, which provides a promising high-voltage cathode material for lithium-ion
of LiCoO2 have been intensively explored. One of the alternatives is spinel-structured LiMn2 O4 , which
batteries owing to their high theoretical energy density, low cost, and eco-friendliness [18,20–22].
provides a promising high-voltage cathode material for lithium-ion batteries owing to their high
LiNiO2 is also a candidate due to its excellent cycle life, with negligible capacity fading etc. [17,23].
theoretical energy density, low cost, and eco-friendliness [18,20–22]. LiNiO2 is also a candidate due to
The liquid phase method is generally used in the synthesis of lithium metal oxide nanoparticles;
its excellent cycle life, with negligible capacity fading etc. [17,23]. The liquid phase method is generally
however, productivity of the nanoparticles in the liquid phase method is insufficient for industrial
used in the synthesis of lithium metal oxide nanoparticles; however, productivity of the nanoparticles
application. Therefore, the synthesis method of lithium oxide nanoparticles with a high productivity
in the liquid phase method is insufficient for industrial application. Therefore, the synthesis method of
is strongly demanded.
lithium oxide nanoparticles with a high productivity is strongly demanded.
Thermal plasmas are expected to be promising energy sources to fabricate nanoparticles at high
Thermal plasmas are expected to be promising energy sources to fabricate nanoparticles at high
productivity from micron-sized powder as raw materials. Here, one-step synthesis of lithium-metal
productivity from micron-sized powder as raw materials. Here, one-step synthesis of lithium-metal
oxide nanoparticles with induction thermal plasma is studied. The purpose of the present study is to
oxide nanoparticles with induction thermal plasma is studied. The purpose of the present study
synthesize the lithium metal oxide nanoparticles via the induction thermal plasma and to investigate
is to synthesize the lithium metal oxide nanoparticles via the induction thermal plasma and to
the formation mechanism of lithium-metal oxide nanoparticles. Different lithium metal systems—Li–
investigate the formation mechanism of lithium-metal oxide nanoparticles. Different lithium metal
Mn, Li–Cr, Li–Co, and Li–Ni—were compared to understand the formation mechanism.
systems—Li–Mn, Li–Cr, Li–Co, and Li–Ni—were compared to understand the formation mechanism.
2. Results and Discussion
2. Results and Discussion
2.1. Experimental
2.1.
Experimental Results
Results
Figure 11 shows
shows X-ray
X-ray diffraction
diffraction spectra
spectra of
of the
the prepared
prepared nanoparticles
nanoparticles by
by the
the induction
induction thermal
thermal
Figure
plasma in
Li–Cr,
and
Li–Ni.
In the
of theofLi–Mn
system,
main
plasma
in different
differentsystems—Li–Mn,
systems—Li–Mn,Li–Co,
Li–Co,
Li–Cr,
and
Li–Ni.
In case
the case
the Li–Mn
system,
diffraction
peaks
correspond
to
spinel-structured
LiMn
2O4, while diffraction peaks of Mn3O4 are also
main diffraction peaks correspond to spinel-structured LiMn2 O4 , while diffraction peaks of Mn3 O4
found.
cases ofIn
Li–Co
LiCoO2 and LiCrO
2 were found as well as their
are
alsoInfound.
casesand
of Li–Cr,
Li–Co layer-structured
and Li–Cr, layer-structured
LiCoO
2 and LiCrO2 were found
oxides.
In
the
Li–Ni
case,
Li
0.4Ni1.6O2 and unreacted Li2CO3 were confirmed. These results clearly
as well as their oxides. In the Li–Ni case, Li0.4 Ni1.6 O2 and unreacted Li2 CO3 were confirmed.
show the
strong
influence
constituent
metals
onconstituent
the formation
of lithium
oxide nanoparticles.
These
results
clearly
show of
thethe
strong
influence
of the
metals
on the formation
of lithium
Moreover,
Figure
2
indicates
composition
of
the
prepared
nanoparticles
analyzed
from
X-ray
oxide nanoparticles. Moreover, Figure 2 indicates composition of the prepared nanoparticles analyzed
diffractometry
(XRD)
spectra.
These
compositions
of
the
prepared
nanoparticles
were
used
to
from X-ray diffractometry (XRD) spectra. These compositions of the prepared nanoparticles were used
evaluate
the
ratio
of
metal
that
reacted
with
Li.
Evaluated
reaction
ratio
will
be
discussed
in
the
to evaluate the ratio of metal that reacted with Li. Evaluated reaction ratio will be discussed in the
following section.
section.
following
LiMn2O4
Intensity [a.u.]
Li-Mn
Li-Cr
Li-Co
Mn3O4
LiCrO2
Cr2O3
LiCoO2
Li0.21Co0.79O
Co3O4
Li0.4Ni1.6O2
Li-Ni
Li2CO3
10 20 30 40 50 60 70 80 90
2 [Degree]
Figure
(XRD)
spectra
of as-prepared
nanoparticles
by induction
thermalthermal
plasma
Figure 1.1. X-ray
X-raydiffractometry
diffractometry
(XRD)
spectra
of as-prepared
nanoparticles
by induction
in
different
systems,
Li–Mn,
Li–Co,
Li–Cr,
and
Li–Ni.
plasma in different systems, Li–Mn, Li–Co, Li–Cr, and Li–Ni.
Nanomaterials
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2016,
6,
60
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2016, 6,
6, 60
60
333of
of
of999
Mole Fraction
Fraction [%]
[%]
Mole
100
80
60
LiMn2O4
2 4
LiCrO2
2
LiCoO2
2
40
Li
Co
O
Li0.21
Co0.79
O
0.21
0.79
Li
Ni
Li0.4
Ni1.6O
O22
0.4 1.6
20
0
Mn
Mn33O
O44
Cr
Cr22O
O33
Co
Co33O
O44
Li
Li22CO
CO33
Mn
Cr
Co
Ni
Figure
2. Composition
of prepared nanoparticles
nanoparticles by induction
induction thermal plasma
plasma in different
different systems,
Figure
Figure 2.
2. Composition
Composition of
of prepared
prepared nanoparticles by
by induction thermal
thermal plasma in
in different systems,
systems,
Li–Mn,
Li–Co,
Li–Cr,
and
Li–Ni.
Li–Mn,
Li–Mn, Li–Co,
Li–Co, Li–Cr,
Li–Cr, and
and Li–Ni.
Li–Ni.
Morphologies
Morphologies and particle sizes of the
the prepared
prepared nanoparticles
nanoparticles were
were observed
observed via
via transmission
transmission
electron
microscopy
shows
the
representative
TEM
images
and
the
(TEM).
Figure
3
electron microscopy
shows the representative TEM images and the particle
particle size
size
distributions
of
the
nanoparticles
in
different
systems.
Many
spherical
particles
with
60
distributions of the nanoparticles in different systems. Many spherical particles with 60 nm
nm in
in mean
mean
diameter
thethe
Li–Ni
system.
In the
of Li–Mn,
Li–Co,Li–Co,
and Li–Cr,
of the
particles
diameter are
areobserved
observedinin
Li–Ni
system.
In cases
the cases
of Li–Mn,
and most
Li–Cr,
most
of the
with
50–80with
nm in
meannm
diameters
have
polyhedral
shapes
including
quadrangular,
and
particles
50–80
in mean
diameters
have
polyhedral
shapes
includingpentagonal,
quadrangular,
hexagonal
In particular,
many
particles many
with aparticles
hexagonal
can be shape
found can
as shown
in
pentagonal,shapes.
and hexagonal
shapes.
In particular,
withshape
a hexagonal
be found
Figure
3a.inHence,
LiMn2 O4 isLiMn
considered
to have a to
hexagonal
shape because
as shown
Figurethe
3a. spinel-structured
Hence, the spinel-structured
22O44 is considered
have a hexagonal
shape
LiMn
is a major
product
in the Li–Mn
according
to XRD results.
because
22O44 is a
major product
in thesystem,
Li–Mn system,
according
to XRD results.
2 O4LiMn
Figure 3.3.
(a)
Representative
transmission
electron
microscopy
(TEM)
image
and
size
3.(a)
(a)Representative
Representative
transmission
electron
microscopy
(TEM)and
image
and
particle
size
Figure
transmission
electron
microscopy
(TEM) image
particle
sizeparticle
distribution
distribution
prepared
induction
thermal
plasma
in
(b)
in
distribution
of
prepared nanoparticles
nanoparticles
bythermal
induction
thermal
plasmasystem;
in Li–Mn
Li–Mn
system;
(b) that
thatsystem;
in Li–
Li–
of
prepared of
nanoparticles
by inductionby
plasma
in Li–Mn
(b)system;
that in Li–Cr
Cr
system;
(c)
that
in
Li–Co
system;
(d)
that
in
Li–Ni
system.
Cr
system;
(c)
that
in
Li–Co
system;
(d)
that
in
Li–Ni
system.
(c) that in Li–Co system; (d) that in Li–Ni system.
Scanning electron microscopy (SEM) observation was carried out to clarify morphology of
Scanning electron microscopy (SEM) observation was carried out to clarify morphology of
LiMn2O4 more specifically than TEM observation. Figure 4 shows the representative SEM image of
LiMn22 O44 more specifically than TEM observation. Figure 4 shows the representative SEM image
the prepared nanoparticles in the Li–Mn system. Many particles have truncated an octahedral shape
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as the
shown
in Figure
4. Previousinresearch
on morphology
of ferrite
nanoparticles
synthesized
by the
of
prepared
nanoparticles
the Li–Mn
system. Many
particles
have truncated
an octahedral
induction
thermal
plasma
reported
that
the
spinel-structured
nanoparticles
had
a
truncated
shape as shown in Figure 4. Previous research on morphology of ferrite nanoparticles synthesized
octahedral
shapethermal
[24]. The
spinel-structured
synthesized
in thermalhad
plasma
have a
by
the induction
plasma
reported thatnanoparticles
the spinel-structured
nanoparticles
a truncated
truncated
octahedral
shape,
although
the
stable
structure
of
the
spinel-structured
particles
are
octahedral shape [24]. The spinel-structured nanoparticles synthesized in thermal plasma have
a
generally
considered
to
be
of
an
octahedral
shape.
The
reason
for
this
truncation
is
currently
under
truncated octahedral shape, although the stable structure of the spinel-structured particles are generally
investigation.
considered
to be of an octahedral shape. The reason for this truncation is currently under investigation.
Figure 4. Representative
Representativescanning
scanningelectron
electronmicroscopy
microscopy
(SEM)
image
of prepared
nanoparticles
in
(SEM)
image
of prepared
nanoparticles
in Li–
Li–Mn
system.
Mn system.
2.2. Nanoparticle Formation Mechanism
Homogeneous nucleation
nucleationtemperatures
temperatures
of metals
considered
in the study
present
study
were
Homogeneous
of metals
considered
in the present
were
estimated
estimated
based on theory
nucleation
theory non-dimensional
considering non-dimensional
surface
tension
[25]. The
based
on nucleation
considering
surface tension
[25]. The
homogeneous
homogeneous
rate J canasbe expressed as
nucleation
ratenucleation
J can be expressed
“
JJ 
βijijnns22sSS
12
12


43 3 
Θ
4Θ
exp
exp  Θ
´

2 2 

27
plnSq
22π

27
ln
S




c
˜
¸
(1)
(1)
where
where SS is
is the
the saturation
saturation ratio
ratio and
and nnss is
is the
the equilibrium
equilibrium saturation
saturation monomer
monomer concentration
concentration at
at
temperature
T.
β
is
the
collision
frequency
function.
The
dimensionless
surface
tension
is given
temperature T. β is the collision frequency function. The dimensionless surface tension
is given
by
by
following
equation:
thethe
following
equation:
σs
Θ“ 1
(2)
kTs

 1
(2)
where σ is the surface tension and s1 is the monomer
kT surface area. The surface tension and the
saturation ratio have a dominant influence on determining the nucleation rate. Stable nuclei are
where σ is
the surface tension
andnucleation
s1 is the monomer
surface
surface
and the
´3 ¨ s´The
1 . Hence,
observed
experimentally
when the
rate is over
1.0 cmarea.
thetension
corresponding
saturation
ratio
have
a
dominant
influence
on
determining
the
nucleation
rate.
Stable
nuclei
are
value of the saturation ratio is defined as the critical saturation ratio. The detailed procedure to estimate
−3·s−1. Hence, the corresponding value
observed
experimentally
when
the
nucleation
rate
is
over
1.0
cm
the nucleation rate and corresponding nucleation temperature can be found in previous works [10,25].
of the
saturation
ratio
is defined
as the critical
saturation
ratio. The
detailed
procedure
to estimate
The
relationship
between
the calculated
nucleation
temperature
and
the boiling
and melting
points
the
nucleation
rate
and
corresponding
nucleation
temperature
can
be
found
in
previous
works
is summarized in Figure 5. Because of the unknown properties of metal oxides, only melting[10,25].
point
The
relationship
between
the
calculated
nucleation
temperature
and
the
boiling
and
melting
oxides are plotted for metal oxides. These temperatures indicate that the melting points of the oxides
points
is summarized
in Figure
5. Because of
ofpure
the unknown
properties
of metalTherefore,
oxides, only
melting
are
higher
than the nucleation
temperatures
metals in each
Li–Me system.
nucleation
point
oxides
are
plotted
for
metal
oxides.
These
temperatures
indicate
that
the
melting
points
of the
of metal oxides is considered to occur at first. Li oxide and metal vapors co-condense onto the nuclei
oxides
arethe
higher
than the
nucleation temperatures of pure metals in each Li–Me system. Therefore,
just
after
nucleation
starts.
nucleation of metal oxides is considered to occur at first. Li oxide and metal vapors co-condense onto
the nuclei just after the nucleation starts.
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NiO
Ni
Co3O4
CrO2
Ni
Co
Co3O4
Co
CrO2
Mn3O4
Mn2O3
MnO2
Li
0
0
Li
Melting point
Nucleation temperature
Boiling point
Melting point
CoO
Cr2O3
Cr
MnO
Boiling point
Cr2O3
Cr
MnO
Mn3O4
Mn2O3 Mn
MnO2
Nucleation temperature
CoO
NiO
Li2O
Mn
500
1000
500
1000
Li2O
1500
2000
Temperature [K]
1500
2000
2500
3000
3500
2500
3000
3500
Figure 5. Relationship between nucleation temperature and melting and boiling points of considered
Temperature [K]
metals 5.
and
metal oxides.
Figure
Relationship
between nucleation temperature and melting and boiling points of considered
Figure
Relationship
between nucleation temperature and melting and boiling points of considered
metals 5.
and
metal oxides.
The above
mechanism
metals
and metal
oxides. can be proposed as a common mechanism for all considered Li–Me
Ratio of
Metal
with Lithium
Ratio
of Reacted
Metal Reacted
with Lithium
to Total
to Metal
Total [-]
Metal [-]
systems
the relationship
between the
temperature
andconsidered
the melting
andsystems
boiling
The because
above mechanism
can be proposed
as a nucleation
common mechanism
for all
Li–Me
The above
mechanism
can bethe
proposed
as a However,
common mechanism
forresults
all considered
Li–Me
temperature
of
their
oxides
shows
same
trend.
experimental
show
a
different
because the relationship between the nucleation temperature and the melting and boiling temperature
systems
because
the relationship
between
the nucleation
temperature
andLi–Mn,
the melting
and boiling
ratio
of
metal
reactive
with
Li
to
total
metal.
In
the
cases
of
Li–Cr
and
high
ratios
the
of their oxides shows the same trend. However, experimental results show a different ratio of of
metal
temperature
of
their
oxides
shows
the
same
trend.
However,
experimental
results
show
a
different
reactive metals
obtained,
low ratio
was obtained
in Li–Ni
meltingmetals
points
reactive
with Liwere
to total
metal. while
In thea cases
of Li–Cr
and Li–Mn,
high system.
ratios ofThen,
the reactive
ratio
of oxide
metalin
reactive
with
Li towere
totalfocused
metal. In
the cases
of Li–Crrate
andofLi–Mn,
high particles
ratios of and
the
of
each
different
system
because
the
reaction
metal
oxide
were obtained, while a low ratio was obtained in Li–Ni system. Then, melting points of each oxide in
reactive
metals
were
obtained,
while
a
low
ratio
was
obtained
in
Li–Ni
system.
Then,
melting
points
the condensed
wouldbecause
drastically
decreaserate
after
complete
the
growing
different
systemlithium
were focused
the reaction
of metal
oxidesolidification
particles andofthe
condensed
of
each oxide in different system were focused because the reaction rate of metal oxide particles and
particles.
lithium would drastically decrease after complete solidification of the growing particles.
the condensed
lithium
would drastically
after
complete
solidification
the growing
Figure 66 shows
shows
the relationship
betweendecrease
the lowest
lowest
melting
temperature
of metal
metalofoxides
oxides
for each
each
Figure
the
relationship between
the
melting
temperature
of
for
particles.
Li–Me system
Li–Me
system and
and the
the reaction
reaction ratios.
ratios. The
Theresults
resultssuggest
suggestaalower
lowermelting
meltingpoint
pointofofoxide
oxideleads
leadstotoa
Figure
6 shows
theThis
relationship
between the
lowest
melting
temperature
of metalthe
oxides
for each
higher
reaction
ratio.
can
be
explained
by
the
different
reaction
time
during
nanoparticle
a higher reaction ratio. This can be explained by the different reaction time during the nanoparticle
Li–Me
system
and the reaction
ratios. The results
suggest a lower melting
point
of nanoparticles.
oxide leads to A
a
formation
process.
ofof
Li–Me
oxide
formation
process. Figure
Figure77summarizes
summarizesthe
theformation
formationmechanism
mechanism
Li–Me
oxide
nanoparticles.
higher
reaction
ratio.
This
can
be
explained
by
the
different
reaction
time
during
the
nanoparticle
lower
melting
point
of oxide
leads
to ato
longer
residence
timetime
of growing
particles
in a liquid-like
state,
A
lower
melting
point
of oxide
leads
a longer
residence
of growing
particles
in a liquid-like
formation
process.
Figure
7 summarizes
the
formation
mechanism
of
Li–Me oxide ananoparticles.
A
resulting
in
the
longer
reaction
time
with
condensed
lithium
oxide.
Consequently,
higher
reaction
state, resulting in the longer reaction time with condensed lithium oxide. Consequently, a higher
lower
melting
point
of
oxide
leads
to
a
longer
residence
time
of
growing
particles
in
a
liquid-like
state,
ratio canratio
be obtained
in the Li–Me
system,
in thatas
of in
Li–Mn
and
Li–Cr.
OnLi–Cr.
the other
hand,
a shorter
reaction
can be obtained
in the
Li–Meas
system,
that of
Li–Mn
and
On the
other
hand,
resulting
in
theof
longer
reactionparticles
time with
condensed
lithium
oxide.
Consequently,
a higher
reaction
residence
time
the
growing
in
a
liquid-like
state
leads
to
a
shorter
reaction
time,
resulting
a shorter residence time of the growing particles in a liquid-like state leads to a shorter reaction time,
ratio
can be reaction
obtained in theThese
Li–Meresults
system,
as in that
ofthe
Li–Mn
and Li–Cr. of
On the other
hand,
aashorter
in a lower
suggest
melting
hasoxide
strong
resulting
in a lower ratio.
reaction ratio.
These
resultsthat
suggest
that thepoint
melting metal
point oxide
of metal
has
residence
time
of
the
growing
particles
in
a
liquid-like
state
leads
to
a
shorter
reaction
time,
resulting
influence
on
the
reaction
with
lithium
oxide.
a strong influence on the reaction with lithium oxide.
in a lower reaction ratio. These results suggest that the melting point of metal oxide has a strong
influence on the reaction with lithium oxide.
0.6
0.5
0.6
Cr
0.4
0.5
Cr
0.3
0.4
0.2
0.3
Mn
Co
Mn
Co
0.1
0.2
0.10500
Ni
1500
2000 Ni 2500
Temperature [K]
0
500
1000
1500
2000
2500
Figure
Relationship between
points
[K]oxides
Figure 6.
6. Relationship
between lowest
lowest melting
meltingTemperature
points of
of metal
metal
oxides and
and the
the reaction
reaction ratios
ratios for
for each
each
Li–Me
system.
Li–Me system.
Figure 6. Relationship between lowest melting points of metal oxides and the reaction ratios for each
Li–Me system.
1000
Nanomaterials 2016, 6, 60
Nanomaterials 2016, 6, 60
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Figure 7.
7. Conceptual
Conceptual diagram
diagram of
of formation
formation mechanism
mechanism of
of Li–Me
Li–Me oxide
oxide nanoparticles
nanoparticles in
in induction
induction
Figure
thermal
plasma.
thermal plasma.
The above mechanism can explain the obtained results well, although this is still only a
The above mechanism can explain the obtained results well, although this is still only a hypothesis.
hypothesis. Further experimental and theoretical investigation will be required to understand the
Further experimental and theoretical investigation will be required to understand the formation
formation mechanism of complicated oxide nanoparticles in thermal plasmas.
mechanism of complicated oxide nanoparticles in thermal plasmas.
3. Experimental
3.
ExperimentalSection
Section
3.1. Experimental
Experimental Setup
Setup and
and Conditions
Conditions
3.1.
A schematic
schematic illustration
illustration of
of the
the experimental
experimental setup
setup for
for Li–Me
Li–Me oxide
oxide nanoparticle
nanoparticle fabrication
fabrication is
is
A
presented in
in Figure
Figure 8a.
8a. This
This equipment
equipment is
is composed
composed of
of the
the plasma
the reaction
chamber where
where
presented
plasma torch,
torch, the
reaction chamber
the nanoparticles
8b8b
shows
thethe
enlarged
illustration
of the
the
nanoparticles are
are synthesized,
synthesized,and
andthe
thefilter
filterunit.
unit.Figure
Figure
shows
enlarged
illustration
of
plasma
torch.
HighHigh
temperature
plasma
of more
K was generated
in the plasma
by
the
plasma
torch.
temperature
plasma
of than
more10,000
than 10,000
K was generated
in thetorch
plasma
induction
heating at
4 MHz.
input
power
controlled
at 20 kW.
was Ar
used
forused
the for
carrier
torch
by induction
heating
at The
4 MHz.
The
inputwas
power
was controlled
at Ar
20 kW.
was
the
gas
of
the
raw
powder
at
3
L/min
and
inner
gas
at
5
L/min.
A
mixture
of
argon
and
oxygen
were
used
carrier gas of the raw powder at 3 L/min and inner gas at 5 L/min. A mixture of argon and oxygen
for the
plasma
forming
gasforming
at 60 L/min.
Mixture
of LiMixture
2CO3 with
diameter and metal or metal
were
used
for the
plasma
gas at
60 L/min.
of3.5
Li2μm
COin
3 with 3.5 µm in diameter and
oxide
with
3–10
μm
in
mean
diameter
were
injected
from
the
powder
by Arfeeder
carrierbygas.
metal or metal oxide with 3–10 µm in mean diameter were injected from feeder
the powder
Ar
Different
including
Mn, Cr, Mn,
Co, Cr,
and
compared
to to
investigate
carrier
gas.metals
Different
metals including
Co,Ni
andwere
Ni were
compared
investigatethe
the formation
formation
mechanism of
of Li–Me
Li–Me (Mn,
(Mn, Cr,
Cr, Co,
Co, and
andNi)
Ni)nanoparticles.
nanoparticles. The
The composition
composition ratio
ratio of
ofLi
Li22CO
CO33 to
to metal
metal
mechanism
was
0.5.
The
powder
feed
rate
was
fixed
at
400
mg/min.
These
experimental
conditions
are
listed
in
was 0.5. The powder feed rate was fixed at 400 mg/min. These experimental conditions are listed in
Table 1.
1.
Table
Prepared nanoparticles were collected from the filter and the inner wall of the reaction chamber.
Unevaporated raw materials were not confirmed according to SEM observation of the collected
particles. This fact implies that the fed raw materials were completely evaporated in the high
temperature region of the thermal plasma and converted into nanoparticles during the quenching
process. Therefore, the mole fractions of the Li–Me oxides indicated in Figure 2 correspond to the
yields of the Li–Me oxide in this nanoparticle fabrication process by the induction thermal plasma.
Figure 8. Schematic illustration of induction thermal plasma system for nanoparticles fabrication (a)
and enlarged illustration of plasma torch (b).
for the plasma forming gas at 60 L/min. Mixture of Li2CO3 with 3.5 μm in diameter and metal or metal
oxide with 3–10 μm in mean diameter were injected from the powder feeder by Ar carrier gas.
Different metals including Mn, Cr, Co, and Ni were compared to investigate the formation
mechanism of Li–Me (Mn, Cr, Co, and Ni) nanoparticles. The composition ratio of Li2CO3 to metal
was 0.5. The powder feed rate was fixed at 400 mg/min. These experimental conditions are listed in
Nanomaterials 2016, 6, 60
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Table 1.
Figure 8. Schematic illustration of induction thermal plasma system for nanoparticles fabrication (a)
and enlarged illustration
illustration of
of plasma
plasma torch
torch (b).
(b).
Table 1. Typical experimental conditions.
Plasma Conditions
Input power
Frequency
Pressure
Sheath gas
Inner gas
Carrier gas
Discharge time
Feed rate
20 kW
4 MHz
101.3 kPa
Ar: 57.5 L/min
O2 : 2.5 L/min
Ar: 5 L/min
Ar: 3 L/min
5 min
400 mg/min
Raw Materials
System
Raw powders
Li/Me ratio
Li–Mn
Li2 CO3 , MnO2
Li–Cr
Li2 CO3 , Cr
Li–Co
Li2 CO3 , Co
Li–Ni
Li2 CO3 , Ni
0.5
3.2. Characterization of Prepared Nanoparticles
The phase identification of the prepared nanoparticles was determined by X-ray diffractometry
(XRD, Multiflex, Rigaku Co., Tokyo, Japan), operated with Cu Kα source (λ = 0.1541 nm). The diffraction
data was collected using a continuous scan mode with a speed of 2 degree/min in the region of
10–90 degrees with a step size of 0.04 degrees. The accelerating voltage and applied current was 40 kV
and 50 mA, respectively. The quantitative analysis of the composition of the prepared nanoparticles was
conducted based on the whole-powder-pattern-decomposition (WPPD) method with the assumption
that no amorphous particles were included in the prepared nanoparticles.
The particle morphology and size distribution of the prepared nanoparticles were observed
by TEM (JEM-2100HCKM, JEOL Ltd., Tokyo, Japan), operated at an accelerating voltage of 200 kV.
The TEM specimens were prepared by dispersing the as-prepared nanoparticles in ethanol and placing
a few drops of the dispersion on a carbon-grid. Furthermore, the 3D particle morphology was observed
by field emission (FE)-SEM (SII TES+ Zeiss ULTRA55, Carl Zeiss, Oberkochen, Germany).
4. Conclusions
Lithium metal oxide nanoparticles were synthesized in induction thermal plasma and formation
mechanism was investigated. Obtained remarks are as follows:
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(a) Lithium metal oxide nanoparticles were synthesized in different Li–Me (Mn, Cr, Co, and Ni)
systems. In the case of Li–Mn, Li–Cr, and Li–Co, lithium-metal oxide nanoparticles were
successfully synthesized, while Li–Ni oxides were not synthesized in the Li–Ni system.
(b) The spinel-structured LiMn2 O4 with a truncated octahedral shape was synthesized in Li–Mn
system, although the stable shape of the spinel structure was an octahedral shape.
(c) The relationship between nucleation temperature and boiling and melting points of the considered
metals and their oxides suggests the following formation mechanism: Metal oxide starts to
nucleate at first. Then, vapors of metal and lithium oxide co-condense on the metal nuclei with an
oxidation reaction.
(d) Melting point of metal oxides is an important factor in determining the final product of the Li–Me
composite. A lower melting point of metal oxide leads to a longer reaction time, resulting in
higher yields of the Li–Me composite.
(e) Nanomaterial fabrication with induction thermal plasma enables the production of high-purity
nanoparticles of Li–Me oxide at high productivity.
Acknowledgments: The authors thank the Ultramicroscopy Research Center, Kyushu University for TEM and
FE-SEM observation. The authors also thank the Center of Advanced Instrumental Analysis, Kyushu University
for XRD analyses.
Author Contributions: Manabu Tanaka, Hirotaka Sone and Takayuki Watanabe coordinated the study. Manabu
Tanaka, Takuya Kageyama, Shuhei Yoshida and Daisuke Okamoto conducted the experiments. Takuya Kageyama,
Hirotaka Sone and Shuhei Yoshida conducted analyses of the produced nanoparticles. Takuya Kageyama
conducted the data analyses. Manabu Tanaka and Takayuki Watanabe prepared the manuscript. All authors read
and approved the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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