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Porous ternary complex metal oxide nanoparticles converted from core/shell nanoparticles
Jaewon Lee2, Huazhang Zhu3, Gautam Ganapati Yadav2,†, James Caruthers2, and Yue Wu1,3 ()
1 School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 200235, China 2 School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA 3 Department of Chemical and Biological Engineering, Iowa State University, Sweeney Hall, Ames, IA 50011, USA † Present address: Energy Institute, City College of New York, New York, NY 10031, USA
carbonate, ratio = 1 :1:1 (MTI Corporation)) as the
electrolyte.
The crystal structures of CoO nanoparticles, CoO/
Mn3O4 nanoparticles, and Co2MnO4 nanoparticles
were characterized via X-ray diffraction (XRD) on a
Bruker D8 Focus X-ray diffractometer with a Cu Kα
source. The morphology of these different types of
nanoparticles was analyzed through transmission
electron microscopy (TEM, FEI Tecnai-20) and high-
resolution TEM (HRTEM, Titan 80-300 kV Environ-
mental Electron Microscope). The surface compositions
of CoO nanoparticles, CoO/Mn3O4 nanoparticles,
and Co2MnO4 nanoparticles were studied by X-ray
photoelectron spectroscopy (XPS), using an AXIS
ULTRA DLD system under ultra-high vacuum (<10−9
torr/mbar). Elemental mapping studies of CoO/Mn3O4
nanoparticle were performed via energy dispersive
spectroscopy (EDX) on a scanning transmission electron
microscope (STEM, FEI Tecnai G2 F30 Super Twin
microscope). Raman spectroscopy of Co2MnO4 nano-
particles and Co3O4 nanoparticles was performed
with a HORIBA Jobin Yvon LabRam HR800 with an
excitation wavelength of 632.18 nm. Finally, the pre-
pared lithium half-cells were galvanostatically cycled
between 3.0 and 0.01 V by using a BAS8-MA analyzer
(MTI Corporation). The charge–discharge curve was
measured with a current density of 0.1 g/A between
0.01 and 3.00 V (vs. Li/Li+). The rate capability was
measured at various current densities between 0.01
and 3.00 V (vs. Li/Li+). After 35 cycles, the current
density switched back to 0.1 g/A. Cyclic voltammetry
(CV) analysis of the Co2MnO4 nanoparticles was also
performed in a lithium half-cell using a Maccor
testing station (model 4304). The scan rate of the CV
test was 0.05 mV/s between 0.01 and 3 V (vs. Li/Li+).
The electrodes after certain electrochemical cycles are
dissembled and immersed in acetonitrile completely
for 10 min to remove the residual electrolyte. The
used electrodes were analyzed by scanning electron
microscopy (SEM, Hitachi S-4800 Field Emission
Microscope) with energy dispersive X-ray spectroscopy.
3 Results and discussion
The XRD studies confirmed the compositions of
the CoO nanoparticles (Fig. 2(a), red curve) and the
CoO/Mn3O4 core/shell nanoparticles (Fig. 2(a), blue
curve), which were consistent with the standard XRD
database. The low-magnification TEM studies (Figs. 2(b)
and 2(c)) of the materials prepared in this way showed
that the CoO nanoparticles (Fig. 2(b)) and the CoO/
Mn3O4 core/shell nanoparticles (Fig. 2(c)) were uniform
in size. The analysis of samples showed that the CoO
nanoparticles had diameters of 47 ± 10 nm. After the
growth of Mn3O4, spherical Mn3O4 nanoparticles with
diameters of about 5–10 nm formed on the surface of
the CoO nanoparticles. The Mn3O4 nanoparticles did
not completely coat the surface, and many empty
spaces between the Mn3O4 nanoparticles were visible
in the core/shell nanoparticle structures.
The HRTEM studies (Figs. 2(d) and 2(e)) clearly
demonstrated the crystallinity of the CoO nanoparticles
(Fig. 2(d)) and the Mn3O4 shell (Fig. 2(e)). The reciprocal
lattice peaks obtained from the two-dimensional fast
Fourier transform (2D FFT) of the lattice-resolved
images (insets, Figs. 2(d) and 2(e)) could be indexed
to the (422) and (220) reflections of the face-centered
cubic CoO structure (inset, Fig. 2(d)) and the (112)
and (211) reflections of Mn3O4 shells with extra (111)
and (220) reflections from the CoO cores (inset, Fig. 2(e)).
In addition, the XPS (Fig. 2(f)) studies showed the
transition from the CoO nanoparticles with only Co
and O peaks (Fig. 2(f), red curve) to the CoO/Mn3O4
core/shell nanoparticles with extra peaks from Mn
(Fig. 2(f), blue curve). Lastly, the EDX elemental
mapping studies on the CoO/Mn3O4 nanoparticles
further confirmed the core/shell structures (Figs. 2(g)
and 2(h), with the cores composed of CoO shown in
Fig. 2(g) and the shells composed of Mn3O4 shown in
Fig. 2(h)).
Figure 3(a) shows the XRD pattern of the converted
Co2MnO4 nanoparticles after annealing. All the peaks
could be indexed to pure spinel phase Co2MnO4.
Further TEM analysis (Figs. 3(b) and 3(c)) showed that
there was no significant change in morphology after
annealing. Visible voids in the Co2MnO4 nanoparticles
could have resulted from the initial empty spaces
between the Mn3O4 nanoparticles coated on the CoO
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999 Nano Res. 2016, 9(4): 996–1004
cores as well as the relatively density increases when
Mn3O4 changed to Co2MnO4. The large surface area
and tolerance to volume expansion associated with
the voids could lead to improved electrochemical and
mechanical properties, which will be discussed later.
Because of the similarity in crystal structure between
Co3O4 and Co2MnO4, the Co spectrum of the Co2MnO4
nanoparticles was further measured by high-resolution
XPS and compared with that of the Co3O4 nano-
particles to investigate the binding energy change
(Fig. 3(d)). The two peaks in the Co2MnO4 obviously
shifted compared to the two peaks of Co3O4. This
resulted from the lower electronegativity of Mn (1.55)
compared to that of Co (1.88); thus, electrons were
distorted toward Co, leading to a decrease in the
measured binding energy for the Co core electrons
in Co2MnO4. Lastly, the Raman spectroscopy analysis
(Fig. 3(e)) performed on both Co3O4 nanoparticles
and Co2MnO4 nanoparticles showed that the Co3O4
nanoparticles had two weak peaks at 520 and 608 cm−1,
which disappeared in the Co2MnO4 spectrum. In
addition, the strong peak at 680 cm−1 in Co3O4 nano-
particles shifted to 650 cm−1, further confirming the
successful conversion to Co2MnO4. The yield of the
Co2MnO4 nanoparticles calculated from the starting
precursors was approximately 50%, showing a truly
scalable process.
The rational control to grow porous ternary complex
metal oxide nanoparticles gave us the unique capability
to further investigate their potential applications. In
Figure 2 (a) XRD pattern of CoO nanoparticles and CoO/Mn3O4 core/shell nanoparticles. (b) TEM overview image of CoO nanoparticles.(c) TEM overview image of CoO/Mn3O4 core/shell nanoparticles. (d) HRTEM image of CoO nanoparticle and inset showing the FFTimage. (e) HRTEM image of CoO/Mn3O4 core/shell nanoparticle and upper inset showing the FFT image of CoO core region (orangeregion), and lower inset showing the FFT image of Mn3O4 shell region (red region). (f) XPS evolution pattern of CoO nanoparticles andCoO/Mn3O4 core/shell nanoparticles. The Mn 2p core level spectra were observed at 652 eV (2p1/2) and 640 ev (2p3/2). (g) and (h) EDX images of CoO/Mn3O4 core/shell nanparticles. The green color of (g) indicates the spatial distribution of Co and the orange color of (h) indicatesthe spatial distribution of Mn.
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1000 Nano Res. 2016, 9(4): 996–1004
the case of Co2MnO4 nanoparticles, we investigated
their electrochemical properties for possible appli-
cations in LIBs. Figure 4(a) shows the galvanostatic
discharge/charge voltage profiles for a Co2MnO4
nanocomposite electrode. In the first discharge profile,
a broad potential plateau at around 0.8–0.2 V, which
is lower than the potential plateau of Co3O4 (Fig. S7
in the Electronic Supplementary Material (ESM)),
corresponds to the conversion reaction of Co2MnO4.
The Co2MnO4 nanocomposite electrode showed a
discharge capacity of 1,518 mA·h/g (Fig. 4(a)), which is
much higher than the theoretical capacity of Co2MnO4
(905 mA·h/g). The discharge capacity decreased after
the first cycle and reached 942 mA·h/g at the 10th
cycle, which is similar to the theoretical capacity. After
that, the capacity continuously increased until the
200th cycle (1,385 mA·h/g) and stabilized between the
200th and 300th cycles. The capacity of Co2MnO4
nanocomposite was much higher than that of Co3O4
nanoparticles (Fig. S7 in the ESM). We believe that
the additional capacity could arise from the surface
absorption of Li+ and its conversion to Li2O and LiOH,
which has been observed in other metal oxide systems
[18]. The cyclic voltammetry (Fig. 4(b)) measurement
on Co2MnO4 shows two peaks at ~0.8 and ~0.32 V in
the cathodic process. The sharp peak at 0.32 V can be
assigned to the reductions of Co3+ to Co2+ and Co2+
and Mn2+ to metallic Co and Mn. The broad peak at
0.8 V can be attributed to Li2O formation and decom-
position of the organic electrolyte to form a SEI at the
electrode/electrolyte interphase. Two broad oxidation
peaks could be observed at ~1.28 and ~1.98 V in the
anodic scan, corresponding to the oxidation of Mn
to Mn2+ and Co to Co2+. After the second cycle, the
reduction peak gradually moved to 0.4 and 0.9 V, which
was different than the irreversible electrochemical
reaction during the first discharge cycle.
The rate capability was evaluated using multiple-step
charging–discharging at different current densities
ranging from 0.1 to 6.4 A/g (Fig. 4(c)). The discharge
Figure 3 (a) XRD pattern of Co2MnO4 nanoparticles. (b) TEM overview image of Co2MnO4 nanoparticles. (c) HRTEM images of individual Co2MnO4 nanoparticle and inset showing the FFT image. (d) XPS spectra of the Co2MnO4 nanoparticles and the Co3O4
nanoparticles. (e) Raman spectra of the Co2MnO4 nanoparticles and the Co3O4 nanoparticles.
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1001 Nano Res. 2016, 9(4): 996–1004
capacity decreased from 800 mA·h/g at 0.1 A/g in the
2nd cycle to 790, 700, 500, 300, 200, and 100 mA·h/g at
current densities of 0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 A/g
in the following cycles, respectively. After charging/
discharging at 6.4 A/g, the capacity of the Co2MnO4
nanocomposite electrode could fully recover and
even start to increase to achieve a capacity higher
than that during the 2nd cycle. However, the capacity
of the Co3O4 nanoparticle electrode did not recover
completely (Fig. S8 in the ESM).
A more interesting observation came from the
long-term lifecycle test. As shown in Fig. 4(d), when
charging/discharging at a rate of 0.1 A/g, a reversible
capacity of 942 mA·h/g was obtained at the 2nd cycle.
After the 2nd cycle, the capacity of Co2MnO4 continued
increasing until it reached 1,400 mA·h/g, which is
more than 3.5 times higher than the theoretical capacity
of graphite (372 mA·h/g) [19]. Even after 300 cycles,
our Co2MnO4 nanocomposite electrode not only showed
very stable behavior but also still possessed a capacity
Figure 4 (a) Charging/discharging curves of the Co2MnO4/Li half-cell cycled at 0.1 A/g. (b) Cyclic voltammetry curves of Co2MnO4/Li half-cell cycled. (c) Retention of discharge capacity of the Co2MnO4/Li half-cell at different charge rates. (d) Discharge capacity of the Co2MnO4/Li half-cell cycled at 0.1 A/g. (e) SEM overview image of the electrode surface after 328 cycles. (f) and (g) areSEM EDX images of (e). The red color of (f) indicates the spatial distribution of Co, and the turquoise color of (g) indicates the spatialdistribution of Mn.
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1002 Nano Res. 2016, 9(4): 996–1004
over 1,200 mA·h/g, which is still more than 3 times
higher than the theoretical capacity of graphite. In
addition, the Coulombic efficiency reached 99.11% and
gradually stabilized after 5 cycles. It was maintained
over 300 cycles. Thus, the Co2MnO4 nanocomposite
electrode was superior to graphite as well as Co3O4 in
terms of life cycle (Fig. S9 in the ESM).
Previously, many metal oxide materials such as
Co3O4, Mn3O4, Fe3O4, and NiO have been investigated
as anode materials for LIBs because of their remarkably
high capacity [20–23]. For instance, Co3O4 has twice
the theoretical capacity of graphite [20]. Among
metal oxide materials, complex oxide materials, which
consist of two transition metal oxides such as ZnFe2O4,
NiCo2O4, ZnCo2O4, ZnMn2O4, and MnCo2O4, are
beginning to be studied as anode materials for LIBs
because they are able to create a synergy effect through
complementarity in the Li+ charge–discharge process
[7, 24–31]. For example, manganese has a higher
capacity than cobalt per unit mass because cobalt is
heavier [12]. Moreover, it has been reported that cobalt
has a higher oxidation potential than manganese, thus
leading to a reduced output voltage when it is applied
as an anode for LIBs [7]. However, cobalt-based oxide
materials have excellent conductivity, which is a great
advantage for high current rate or high power density
applications [7, 32–34]. Recently, complex metal oxide
nanomaterials have shown a great advantage in terms
of a high surface area and a short diffusion path length
for Li ions compared to their bulk materials [35–37].
It has been reported that there is a different reaction
step between the surface and bulk [18]. Although
metal oxide materials should be changed to metal,
such as with Li2O and LiH in the bulk reaction, LiOH
can be produced at the surface. Further, LiOH can
react with additional Li in order to form LiO2 and
generate additional capacity [18]. In addition, there are
minor contributions that lead to additional capacity,
such as reversible solid-electrolyte interphase (SEI)
formation and Li adsorption on the surface of active
materials [37, 38]. Therefore, nanomaterials can pro-
vide a large additional capacity because of their high
surface area. Moreover, their short diffusion path
length can lead to an increase in the power density
and a decrease in charging time [36]. However, most
of these nanomaterials show a limited life cycle due
to their habitual aggregation and unstable SEI [12, 19].
The advantages of nanomaterial could quickly
disappear because of aggregation during the cycling
process [39]. In addition, the process of breakdown
and reformation induced by lithiation/delithiation
reactions on the metal oxide surface [14–16] can
negatively affect the Coulombic efficiency because
the SEI could impede charge transport [35, 40–42].
As a result, the Coulombic efficiency might decrease
because accumulated SEI may block charge transport
[35]. In our Co2MnO4 nanoparticles, the void spaces
formed during the conversion from core/shell nano-
particles can provide a buffer zone during volume
expansion/contraction [14, 35, 43]. It can prevent the
deformation of the SEI layer such that the cell has
good Coulombic efficiency. Moreover, the large surface
area in the porous structure also leads to additional
capacity through the interfacial reversible reaction.
Therefore, our Co2MnO4 nanoparticles show a stable
long life cycle with a large energy storage capacity.
To further explore our Co2MnO4 nanoparticle-based
composite LIB electrode, we conducted a series of
experiments to study when and how the breakdown
could occur. After continuously charging/discharging at
1,300 mA·h/g for 328 cycles, we observed a degradation/
breakdown of the nanocomposite electrode (Fig. 4(e)).
Elemental mapping performed on the working
electrode and the electrode after the breakdown
showed a completely new breakdown mechanism;
unlike other material-based electrode systems where
Cui, Y. Stable Li-ion battery anodes by in-situ polymerization
of conducting hydrogel to conformally coat silicon nano-
particles. Nat. Commun. 2013, 4, 1943.
Nano Res.
Table of contents
Porous ternary complex metal oxide nanoparticles are synthesized by noble thermal annealing of core/shell nanoparticles with well- controlled sizes and properties. It is found that their porous morphology and large surface area can lead to high-performance lithium-ion battery electrode materials.
Nano Res.
Electronic Supplementary Material
Porous ternary complex metal oxide nanoparticles converted from core/shell nanoparticles
Jaewon Lee2, Huazhang Zhu3, Gautam Ganapati Yadav2,†, James Caruthers2, and Yue Wu1,3 ()
1 School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 200235, China 2 School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA 3 Department of Chemical and Biological Engineering, Iowa State University, Sweeney Hall, Ames, IA 50011, USA † Present address: Energy Institute, City College of New York, New York, NY 10031, USA
Supporting information to DOI 10.1007/s12274-016-0987-z
Figure S1 TEM (a) and HADDF STEM (high-angle annular dark field scanning transmission electron microscopy) overview images of CoO/Mn3O4 nanoparticles. Both images are taken by Titan 80-300 kV Environmental Electron Microscope.
Figure S2 HRTEM (a) and HADDF STEM images of CoO/Mn3O4 nanoparticles. Both images are taken by Titan 80-300 kV Environmental Electron Microscope.
Figure S3 High-resolution HADDF STEM images of CoO/Mn3O4 nanoparticles with EDX line scanning. This image is taken by Titan 80-300 kV Environmental Electron Microscope.
Figure S4 HRTEM images of Co2MnO4 nanoparticle. This image is taken by Titan 80-300 kV Environmental Electron Microscope.
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Nano Res.
Figure S5 FT-IR (Fourier transform infrared) spectra of annealed Co2MnO4 (blue line), oleylamine (red line), and CCl4 (black line). Oleylamine is dissolved in CCl4 solution. FT-IR spectra of Co2MnO4, oleylamine and CCl4 are measured by Nicolet 6700, Thermo Scientific. It is demonstrated that oleylamine attached on surface of CoO/Mn3O4 is removed completely via annealing process with 4% hydrogen forming gas environment.
Figure S6 XRD pattern of Co3O4 nanoparticles. CoO nanoparticles are changed to Co3O4 nanoparticles through annealing with 4% hydrogen forming gas environment.
Figure S7 Charging/discharging curves of the Co3O4/Li half-cell cycled at 0.1 A/g.
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Nano Res.
Figure S8 Retention of discharge capacity of the Co3O4/Li half-cell at different charge rates.
Figure S9 Discharge capacity and Coulombic efficiency of the Co3O4/Li half-cell cycled at 0.1 A/g.