Triple-shelled Mn2O3 hollow nanocubes: Force-induced synthesis and excellent performance as the anode in lithium-ion batteries

Article abstract of DOI:10.1039/c4ta02666g

In this paper, we report a novel structure of Mn₂O₃, the triple-shelled Mn₂O₃ hollow nanocube, as the anode material for high-energy lithium-ion batteries, synthesized through a programmed annealing treatment with cubic MnCO₃ as precursor. This hierarchical structure is developed through the interaction between the contraction force from the decomposition of MnCO₃ and the adhesion force from the formation of Mn₂O₃. The structure has been confirmed by characterization with XRD, FESEM, TEM, and HRTEM. The charge-discharge tests demonstrate that the resulting Mn₂O₃ exhibits excellent cycling stability and rate capability when evaluated as an anode material for lithium-ion batteries. It delivers a reversible capacity of 606 mA h g ^-1 at a current rate of 500 mA g^-1 with a capacity retention of 88% and a remaining capacity of 350 mA h g^-1 at 2000 mA g^-1. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ta02666g

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PAPER
Triple-shelled Mn2O3 hollow nanocubes: force-
induced synthesis and excellent performance as the
anode in lithium-ion batteries†
Cite this: J. Mater. Chem. A, 2014, 2,
14189
*
*
H. B. Lin, H. B. Rong, W. Z. Huang, Y. H. Liao, L. D. Xing, M. Q. Xu, X. P. Li and W. S. Li
In this paper, we report a novel structure of Mn2O3, the triple-shelled Mn2O3 hollow nanocube, as the anode
material for high-energy lithium-ion batteries, synthesized through a programmed annealing treatment
with cubic MnCO3 as precursor. This hierarchical structure is developed through the interaction between
the contraction force from the decomposition of MnCO3 and the adhesion force from the formation of
Mn2O3. The structure has been confirmed by characterization with XRD, FESEM, TEM, and HRTEM. The
charge–discharge tests demonstrate that the resulting Mn2O3 exhibits excellent cycling stability and rate
capability when evaluated as an anode material for lithium-ion batteries. It delivers a reversible capacity
of 606 mA h gÀ1 at a current rate of 500 mA gÀ1 with a capacity retention of 88% and a remaining
Received 28th May 2014
Accepted 11th July 2014
DOI: 10.1039/c4ta02666g
www.rsc.org/MaterialsA
capacity of 350 mA h gÀ1 at 2000 mA gÀ1
.
has been devoted to developing new strategies for rational
synthesis of complex hollow structures, such as multi-shelled
hollow structures and yolk–shelled structures, as these complex
structures are expected to improve the properties of lithium-ion
batteries.21–24 A general approach for the fabrication of multi-
shelled hollow structures involves the employment of sacricial
templates, such as monodispersed polymer,25 silica,13 or
carbon.26 However, methods employing sacricial templates
usually require multiple steps and post-treatment, which might
not be suitable for cost-effective and environmentally-friendly
large-scale manufacture. It is still a great challenge to develop a
template-free method that enables facile synthesis of multi-
shelled hollow structures.
In this paper, we report a novel structure of Mn2O3, the
triple-shelled Mn2O3 hollow nanocube, synthesized through a
programmed annealing treatment with cubic MnCO3 as
precursor. The crystal structure and morphology of the result-
ing product were characterized by XRD, FESEM, TEM, and
HRTEM, and its performance as anode material in lithium-ion
batteries, in terms of cyclic stability and capability, was evalu-
ated with charge–discharge tests.
Introduction
With the increasing demand for large-scale electrochemical
energy storage, rechargeable lithium-ion batteries with a high-
energy density, high rate capability and long-term stability are
urgently required.1,2 Transition metal oxides have been widely
investigated as anodes in lithium-ion batteries because of their
higher theoretical specic capacities compared with the
commercial graphite anode currently available.3–7 Among the
various transition metal oxide anodes, Mn2O3 with a theoretical
capacity of 1018 mA h gÀ1 is the most attractive owing to its
abundance and environmentally friendly characteristics.8–10
However, its major volume expansion during the charge–
discharge process leads to a signicant capacity loss and poor
cycling stability, which greatly hinders its practical application
in lithium-ion batteries.11,12
The synthesis of
a hollow micro-/nanostructure with
controlled size, shape and internal structure is a promising
approach to improve the performance of transition metal oxide
anodes.13–16 The nano-size units reduce the path of lithium-ion
diffusion and increase the number of reaction sites for lithium
intercalation/deintercalation, and the interior hollow spaces
buffer the volume changes during charge–discharge and thus
enhance the rate capability and cyclic stability.17–20 Much effort
Experimental section
Material synthesis
The cubic MnCO3 was synthesized by a precipitation reaction in
a microemulsion.27–29 Typically, 4 g of cetyltrimethyl ammo-
nium bromide (CTAB) was dissolved in a mixture of 100 mL
cyclohexane, 5 mL n-pentanol and 5 mL 0.8 M (NH4)HCO3
aqueous solution. The mixture was stirred for 20 min until it
became transparent, then 5 mL 0.4 M MnSO4 was dripped into
the continuously stirred solution to produce a milky white
School of Chemistry and Environment, Key Laboratory of Electrochemical Technology
on Energy Storage and Power Generation of Guangdong Higher Education Institutes,
Engineering Research Center of Materials and Technology for Electrochemical
Energy Storage (Ministry of Education), South China Normal University, Guangzhou,
China. E-mail: liwsh@scnu.edu.cn; Fax: +86-20-39310256; Tel: +86-20-39310256
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta02666g
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solution. Finally, the white MnCO3 was ltered, washed several As illustrated in Fig. 1, the fabrication process mainly consists of
times with ethanol and distilled water, and dried in a vacuum at two annealing treatment steps. The MnCO3 nanocubes are rst
90 ꢀC. The triple-shelled Mn2O3 hollow nanocubes were sintered at 300 ꢀC for 1 h with a ramping rate of 1 ꢀC minÀ1, and
synthesized by a two-step annealing process, using the cubic partially converted into manganese oxide, leading to the forma-
MnCO3 as template. It was rst sintered at 300 ꢀC for 1 h with a tion of the rst shell, which is mainly based on the heteroge-
À1
ꢀ
ꢀ
ramping rate of 1 ꢀ C min and then at 600 C for 1 h with a neous contraction caused by non-equilibrium heat
ramping rate of 2 C minÀ1 in air.
treatment.15,16,32 During the calcination, a large temperature
gradient (DT) is present along the radial direction, which leads to
the formation of a manganese oxide shell on the surface of the
MnCO3 core.32,33 There are two forces in opposite directions
acting on the interface between the manganese oxide shell and
the MnCO3 core. One is the contraction force (Fc) as a result of the
large weight loss during the calcinations, which promotes the
inward shrinkage of the MnCO3 core. The other is the adhesion
force (Fa) from the relatively rigid shell, which prevents its inward
contraction. With DT1 at the rst heat treatment, Fc exceeds Fa so
that the inner core contracts inward and detaches from the outer
shell. It can be observed that, aer the rst heat treatment, the
particle maintains its cubic shape but its surface becomes coarser
due to the release of CO2 (Fig. S2A and B†). The TEM image of the
product conrms the core–shell structure (Fig. S2C†). The XRD
pattern provides evidence of the MnCO3 core from the partially
decomposed MnCO3 nanocube (Fig. S2D†). Compared with the
highly crystalline MnCO3 (Fig. 2), the intensity of the XRD peaks
decreased considerably upon partial decomposition of MnCO3.
This process was accompanied by a color change of the material
from milky-white to brown (Fig. S3†). Similar to the rst heat
treatment, the product obtained was then annealed at 600 ꢀC for
1 h with a ramping rate of 2 ꢀC minÀ1. The same process as the
rst heat treatment occurs in the precursor cube, leading to the
formation of the second shell. When the heating is prolonged at
high temperature, Fc decreases rapidly and Fa surpasses Fc,
leading to the reversed direction for movement of materials. As a
result, the inner core shrinks outward, leaving a hollow cavity in
the center.
Materials characterization
The morphology of the materials was observed by eld emission
scanning electron microscopy (FESEM, ZEISS Ultra 55), trans-
mission electron microscopy (TEM, JEOL JEM-2100HR) and high-
resolution TEM (HRTEM). The crystal structures of the samples
were analyzed by X-ray diffraction (XRD, BRUKER D8 ADVANCE,
Germany) with Cu Ka radiation. The surface area was determined
by the Brunauer–Emmett–Teller method (BET, Micromeritics
ASAP 2020 M) at liquid nitrogen temperature (77 K).
Electrochemical measurements
The electrode was prepared by mixing 70 wt% active material
with 20 wt% acetylene black and 10 wt% polyvinylidene
diuoride (PVDF) binder, coating the mixture on an aluminum
sheet, and then cutting the sheet into 1 cm  1 cm samples. The
typical mass of loaded active material was 5–7 mg. The CR2025
coin cell was assembled in an Ar-lled MBraun glove box using
the prepared electrode as cathode, lithium lm as anode, Cel-
gard 2400 as separator, and 1 M LiPF6 in ethylene carbonate
(EC) and dimethyl carbonate (DMC) (EC/DMC 1 : 2 by volume)
as electrolyte. The charge–discharge tests were performed on a
Land cell test system (Land CT2001A, China). Cells were cycled
between 3 V and 0.01 V (vs. Li/Li+) at 25 ꢀC. Cyclic voltammetry
ꢀ
was performed on a Solartron-1470E CellTest at 25 C between
3 V and 0.01 V at a sweep rate of 0.1 mV sÀ1
.
The as-synthesized samples were characterized by XRD to
determine their crystallographic structure. As shown in Fig. 2, the
Results and discussion
Triple-shelled Mn2O3 hollow nanocubes are synthesized through resulting MnCO3 exhibits a hexagonal phase (PDF#44-1472). All
a thermally driven contraction process during the decomposition of the diffraction peaks of the Mn2O3 can be indexed to a cubic
of MnCO3, a process which was developed in the fabrication of phase (PDF#73-1826) by comparing them with the standard
double-shelled CoMn2O4.15 Uniform MnCO3 nanocubes prepared pattern. No residues or contaminants have been detected in the
by the microemulsion method are applied as the precursor.27,28 two XRD patterns, indicating the high purity of the product.
The formation of Mn2O3 can be demonstrated by the thermog-
ravimetric analysis curve for MnCO3 nanocubes (Fig. S1†).
Thermal decomposition in air leads to the conversion of MnCO3
into manganese oxide, MnO2, Mn2O3 or Mn3O4, depending upon
the sintering temperature. The rst steep weight loss (>300 ꢀC) is
associated with the decomposition of MnCO3 to form MnO2
phase, while the second steep weight loss (>490 ꢀC) is the result of
transformation of MnO2 to Mn2O3. Usually, to obtain the pure
phase of Mn2O3, as guided by the thermogravimetric analysis
curve, thermal decomposition should be conducted at
a
temperature above 490 ꢀC for a certain time. However, if the
thermal decomposition process is simply stopped halfway,
MnCO3 can only be partially decomposed, which could lead to
the development of a manganese oxide layer on the surface of the
Fig. 1 Schematic illustration of the process of formation for triple-
particles, whereas the inner cores should remain as MnCO3.30,31 shelled Mn2O3 hollow nanocubes.
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Fig. 2 XRD patterns for the MnCO3 nanocube and the triple-shelled
Mn2O3 hollow nanocube.
The morphology and detailed structure of the samples were
observed by eld-emission scan electron microscopy (FESEM)
and transmission electron microscopy (TEM). As can be seen
from Fig. 3A and B, the MnCO3 obtained adopted a homoge-
nous cubic shape with edge length of around 200 nm. When
MnSO4 and (NH4)2CO3 solutions were mixed, MnCO3 nucleated
gradually in the microemulsion droplets. The selective inter-
action between the crystallographic planes of MnCO3 and
CTAB, which acts as a morphology controller,27–29 resulted in the
cubic shape and microemulsion denes the nanoparticle size.
Aer the programmed heat treatment, the Mn2O3 product
maintained the cubic shape, as shown in Fig. 3C. From the inset
in Fig. 3C, it can be clearly observed that the Mn2O3 cube adopts
a porous architecture composed of nanosized subunits. The
porous structure is attributed to gas evolution from the MnCO3
bulk during the thermal decomposition procedure to form
manganese oxide. From the broken nanocubes with exfoliation
of the shell as shown in Fig. 3D and E, the triple-shelled
structure with the shell marked by the number and the hollow
interior can be identied. Further FESEM images (Fig. S4†)
indicate that the inner shells also retain the porous cubic shape.
TEM images further conrmed the triple-shelled structure of
the synthesized Mn2O3 nanocube. As shown in Fig. 3F and G,
clear gaps can be distinguished between the shells, and the
inner core can be conrmed to adopt the hollow cubic structure.
Fig. 3H presents the high-resolution TEM (HRTEM) image of
Fig. 3 FESEM images for the MnCO3 nanocube (A and B), and FESEM
(C–E), TEM (F and G) and HRTEM (H) images for the triple-shelled
Mn2O3 hollow nanocube.
the Mn2O3 nanocube. The lattice fringes with interplanar lithium plate as reference and counter electrodes. The product
spacing of about 0.54 nm can be indexed to the family of the obtained was rst evaluated by cyclic voltammetry in the voltage
(111) plane. The inset is the corresponding fast Fourier-trans- range 0.01–3.0 V at a scan rate of 0.1 mV sÀ1, as shown in
formed (FFT) pattern, indicating that the Mn2O3 nanocube Fig. 5A. During the rst cathodic process, three main peaks were
adopts a highly crystalline structure. The BET result illustrated present centered at 1.25 V, 0.75 V and 0.2 V. The peaks located at
in Fig. 4 indicates that the pore sizes of triple-shelled Mn2O3 1.25 V and 0.2 V are attributed to the reduction of Mn3+ to Mn2+
hollow nanocubes are mainly less than 30 nm, contributing to a and Mn2+ to Mn0, respectively,9,10 whereas the peak at 0.75 V is
surface area of 21.18 m2 gÀ1
.
ascribed to the irreversible decomposition of the solvent in the
The electrochemical properties of the triple-shelled Mn2O3 electrolyte to form the solid–electrolyte interface (SEI).16
hollow nanocube were investigated in coin cells by both cyclic The minor peak located at around 0.9 V may be attributed to the
voltammetry and galvanostatic charge–discharge test using a lithium-ion insertion into the conductive agent acetylene
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Fig.
4 N2 adsorption/desorption isotherm for the triple-shelled
Mn2O3 hollow nanocube and the corresponding pore size distribution.
black.8,34 There are two oxidation peaks at 1.25 V and 2.35 V in
the following anodic process, which are associated with the
oxidation of Mn0 to Mn2+ and Mn2+ to Mn3+, respectively.
Compared with the rst cycle, two major peaks are observed in
the following cycles, which correspond to the reduction/oxida-
tion of Mn2+. Note that the third curve and the h one overlap,
indicating the good reversibility of the electrochemical reaction.
Fig. 5B and C illustrate the cycling performance of the triple-
shelled Mn2O3 hollow nanocube at 100 mA gÀ1 current density
between 3.0 V and 0.01 V and the corresponding discharge–
charge proles. The initial discharge and charge capacities are
1200 mA h gÀ1 and 845 mA h gÀ1, respectively. The irreversible
capacity loss of the rst cycle can be attributed to the formation
of the SEI lm and the irreversible change between Mn3+ and
Mn2+, which correspond to the capacity in the voltage range
between 1.5 V and 0.5 V. In the rst discharge curve, two
plateaus at around 1.3 V and 0.25 V could be attributed to the
reduction of Mn3+ to Mn2+ and Mn2+ to Mn0 and the plateaus at
about 1.3 V and 2.4 V could be associated with the oxidation of
Mn0 to Mn2+ and Mn2+ to Mn3+, respectively. Aer the rst cycle,
there are two main plateaus located at around 0.5 V and 1.5 V for
the discharge and charge proles, respectively, which are
related to the Mn2+/Mn0 redox reaction. This result is consistent
with observations from cyclic voltammetry.
It is well known that the major volume expansion of transi-
tion metal oxide anodes during the charge–discharge process
leads to serious capacity loss and poor cycling stability. The
cyclic stability of the resulting triple-shelled Mn2O3 hollow
nanocube was evaluated with 100 charge–discharge cycles at
current densities of 200 mA gÀ1 and 500 mA gÀ1 in the voltage
range of 0.01–3.0 V. Fig. 6 presents the discharge capacity as a
function of cycle number. All of the cells were rst charged and
discharged at a current density of 100 mA gÀ1 for two cycles. The
rst discharge capacity is around 718 mA h gÀ1 for the current
Fig. 5 (A) Cyclic voltammograms for the triple-shelled Mn2O3 nano-
cube electrode at a scan rate of 0.1 mV sÀ1 in the voltage range of
0.01–3.0 V. (B) Cycling performance in the voltage range of 0.01–3.0 V
at a current density of 100 mA gÀ1 and (C) the corresponding
discharge–charge profiles.
density of 200 mA gÀ1 and 606 mA h gÀ1 for 500 mA gÀ1
.
Interestingly, the discharge capacities gradually decrease and
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then start to increase aer certain cycles. This phenomenon can ranging from 100 mA gÀ1 to 2000 mA gÀ1. Fig. 8A and B show
be attributed to the presence of a possible activation process in the rate capability of the triple-shelled Mn2O3 hollow nanocube
the electrode, which is commonly observed for transition metal and the corresponding discharge–charge proles. The triple-
oxides.4 Note that the discharge capacities stay above the shelled Mn2O3 hollow nanocube delivers a capacity of 350 mA h
specic capacity of graphite (372 mA h gÀ1), although they gÀ1 at 2000 mA gÀ1. To the best of our knowledge, this is one of
change slightly. The discharge capacity at 500 mA gÀ1 remains the best rate performances for Mn2O3 anodes that have been
533 mA h gÀ1 aer 100 cycles with a capacity retention of 88%, reported in the literature.8,9,35 This excellent rate capability
indicative of the excellent cyclic stability of the triple-shelled should be related to the nano-size units of Mn2O3. It should be
Mn2O3 hollow nanocube. This excellent cyclic stability should noted that the discharge capacity recovers to 700 mA h gÀ1 when
be related to the structural stability of the triple-shelled Mn2O3 the current rate is returned back to 200 mA gÀ1. These results
hollow nanocube. Fig. 7 presents the FESEM images of the indicate that the resulting triple-shelled Mn2O3 hollow nano-
electrode aer cycling. Compared with the image for the pris- cube simultaneously exhibits excellent cyclic stability and rate
tine nanoparticles (Fig. 3D), the porous cubic structure is capability.
maintained aer cycling although the cube size increases
The excellent performance of the resulting triple-shelled
slightly due to the SEI layer formed, conrming the structural Mn2O3 hollow nanocube can be attributed to its special struc-
stability of the triple-shelled Mn2O3 hollow nanocube. Appar- ture. The hollow space buffers the mechanical strain induced by
ently, the hierarchical structure of Mn2O3 provides space to the volume change during cycling, while the nano-size units
buffer the mechanical strain induced by the volume change reduce the path of lithium-ion diffusion and increase the
resulting from lithium insertion/extraction, maintaining the number of reaction sites for lithium insertion/extraction,
structural stability of Mn2O3.
To evaluate the rate capability, the resulting triple-shelled
Mn2O3 hollow nanocube was cycled at various current densities
Fig. 6 Cyclic stability of the triple-shelled Mn2O3 nanocube electrode
at current densities of 200 and 500 mA gÀ1
.
Fig. 7 FESEM images of the triple-shelled Mn2O3 nanocube electrode Fig. 8 (A) Specific capacity as a function of cycle number at different
after cycling. (B) is a higher magnification FESEM image.
current densities and (B) the corresponding discharge–charge profiles.
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leading to the excellent cyclic stability and rate capability of the 10 X. Fang, X. Lu, X. Guo, Y. Mao, Y.-S. Hu, J. Wang, Z. Wang,
resulting triple-shelled Mn2O3 hollow nanocube.
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Conclusions
In summary, we have developed a novel structure of Mn2O3, the
triple-shelled Mn2O3 hollow nanocube with nanosize subunits,
as the anode material for high-energy lithium-ion batteries. The
nanocube was synthesized through a programmed annealing
treatment with cubic MnCO3 as precursor. The resulting hier-
archical structure is formed via the interaction between the
contraction force from the decomposition of MnCO3 and the
adhesion force from the formation of Mn2O3. When evaluated
as an anode material for lithium-ion batteries, the triple-shelled
Mn2O3 hollow nanocube exhibits excellent cyclic stability and
rate capability. The hollow space buffers the mechanical strain
induced by the volume change during cycling, while the nano-
size units reduce the path of lithium-ion diffusion and increase
the number of reaction sites for lithium insertion/extraction,
leading to the excellent cyclic stability and rate capability of the
resulting triple-shelled Mn2O3 hollow nanocube.
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Acknowledgements
This work is nancially supported from the joint project of the
National Natural Science Foundation of China and the Natural
Science Foundation of Guangdong Province (Grant no.
U1134002), the National Natural Science Foundation (Grant no.
21273084), the Natural Science Fund of Guangdong Province
(Grant no. 10351063101000001), the key project of Science and
Technology in Guangdong Province (Grant no. 2012A010702003),
and the scientic research project of the Department of Educa-
tion of Guangdong Province (Grant no. 2013CXZDA013).
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