Electrochimica Acta 55 (2010) 4915–4920
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Highly crystalline macroporous ␤-MnO2 : Hydrothermal synthesis and
application in lithium battery
Xingkang Huang a,b,∗ , Dongping Lv b , Qingshun Zhang a , Haitao Chang a , Jianlong Gan a , Yong Yang b,∗∗
a
b
Fujian Nanping Nanfu Battery Company, Limited, Nanping 353000, PR China
State Key Lab for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China
a r t i c l e
i n f o
Article history:
Received 14 November 2009
Received in revised form 26 March 2010
Accepted 27 March 2010
Available online 2 April 2010
Keywords:
Manganese oxide
Macroporous material
Hydrothermal
Electrochemical performance
Lithium battery
a b s t r a c t
A highly crystalline macroporous ␤-MnO2 was hydrothermally synthesized using stoichiometric reaction
between KMnO4 and MnCl2 . The as-prepared material has a pore size of ca. 400 nm and a shell thickness of
300–500 nm. The formation of the macroporous morphology is related to self-assembling from nanowires
of ␣-MnO2 , and could be obtained at high reactant concentrations (e.g., 0.8 M KMnO4 ) but not at low ones
(e.g., below 0.04 M KMnO4 ). Compared to conventional bulk ␤-MnO2 processing very low capacity, our
macroporous material exhibits good electrochemical activity, e.g., obtaining an initial discharge capacity
of 251 mAh g−1 and sustaining as ca. 165 mAh g−1 at 10 mA g−1 . The electrochemical activity of the asprepared ␤-MnO2 is related to its macroporous morphology and small shell thickness; the former leads to
that electrolyte can flood pore of the material and its inner surface is available for lithium ion diffusion,
while the latter helps to release the stress from phase transformation during the initial discharging.
The X-ray diffraction characterizations of the macroporous ␤-MnO2 electrodes suggest that, upon initial
discharging, such a ␤-MnO2 will be irreversibly transformed to an orthorhombic Lix MnO2 and then cycled
within the new developed phase in the subsequent lithium insertion/extraction processes.
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Manganese dioxide is known as various polymorphs, including ␣, ␤, ␦, ␥, ␭, and ␧, forms; among them, ␤-MnO2 is the most
stable in thermodynamics, and is very easy to be prepared, e.g.,
by thermal decomposition of Mn(NO3 )2 [1], pyrolysis of MnOOH
[2,3], or hydrothermal reaction [4,5]. ␤-MnO2 has a rutile-type
tetragonal symmetry (P42 /mnm) with a distorted hexagonal-closepacked oxygen array, where edge-sharing MnO6 octahedra stacks
and forms 1 × 1 (2.3 Å × 2.3 Å) tunnels [6]. Such a narrow tunnel
makes it difficult for lithium ions to diffuse into bulk upon electrochemically intercalating. As a result, conventional ␤-MnO2 with
high crystallinity usually exhibits very low electrochemical activity [7–9], e.g., inserting 0.2 Li+ per Mn [10]. On the contrary, those
relatively poorly crystalline ␤-MnO2 , like nanocomposite [11] and
mesoporous materials [8,9], delivered high capacities. Thackeray
et al. [7] acid-treated a LiMn2 O4 to obtain a ␥-MnO2 which was
heated at 300 ◦ C to form a final product of ␤-MnO2 ; the obtained
␤-MnO2 delivered an initial discharge capacity of 210 mAh g−1 . Jiao
∗ Corresponding author at: State Key Lab for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Siming South Road 422, Xiamen,
Fujian 361005, PR China. Tel.: +86 592 2185753/2186337; fax: +86 592 2185753.
∗∗ Corresponding author. Tel.: +86 592 2185753; fax: +86 592 2185753.
E-mail addresses: [email protected] (X. Huang), [email protected] (Y. Yang).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.03.090
and Bruce [9] prepared a mesoporous ␤-MnO2 , displaying a capacity of 283 mAh g−1 till a cutoff voltage of 2.0 V at a current density
of 15 mA g−1 . Tang et al. [11] synthesized a ␤-MnO2 /C nanocomposite by pyrolysing mixtures of Mn(NO3 )2 and acetylene black
at 160–320 ◦ C; the product, obtained at 300 ◦ C, inserted 1.15 Li+
per Mn at a cutoff voltage of 1.0 V. All the above-mentioned electrochemically active ␤-MnO2 , are not well-crystallized ␤-MnO2 as
suggested by the broad and weak peaks on their XRD patterns. Here,
we report a highly crystalline ␤-MnO2 with macroporous morphology, which, however, shows good electrochemical performance.
We aim, in this study, to investigate the formation mechanism
as well as the origin of the electrochemical activity of the highly
crystalline macroporous ␤-MnO2 .
2. Experimental
The highly crystalline macroporous ␤-MnO2 was hydrothermally synthesized by the method similar to that reported
previously [5]. In brief, 0.8 M KMnO4 was mixed by 1.2 M MnCl2 in
a 100 mL Teflon-lined stainless steel autoclave for 30 min (60 mL
solution in total). The autoclave was sealed, heated for 24 h
at 180 ◦ C, and then cooled naturally to room temperature. The
obtained product was filtered, washed, and then dried at 120 ◦ C
for 24 h.
X-ray diffraction (XRD) measurements were performed on
a PANalytical X’Pert diffractometer with Cu K␣ radiation. The
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X. Huang et al. / Electrochimica Acta 55 (2010) 4915–4920
Fig. 1. XRD pattern of the as-prepared ␤-MnO2 .
Fig. 2. SEM images of the macroporous ␤-MnO2 .
morphology of the as-prepared MnO2 was observed with scanning
electron microscopy (SEM) on LEO 1530. The tap density of the
as-prepared material was measured according to standard test
method for determination of tap density of metallic powers and
compounds (ASTM B527–06), where 20 g materials were loaded to
a 25 mL graduated cylinder and then tapped till a constant volume.
The electrodes were fabricated by pressing a mixture of
80 wt% active material, 10 wt% acetylene black, and 10 wt%
poly(vinylidenefluoride) to aluminum foil (current collector). Coin
cells were assembled with the prepared cathode, lithium anode,
Celgard 2400 polypropylene separator, and 1 M LiClO4 in propylene
carbonate:1,2-dimethoxyethane (PC:DME; 1:1, by volume) electrolyte as described in our previous work [12]. The cell tests were
carried out at 30 ◦ C, unless otherwise stated.
Ex situ XRD experiments of cathodes at various states of
discharge/charge (SOD/SOC) were carried out according to our previous report [13]. Coin cells were charged or discharged to various
cutoff voltages at a current rate of 10 mA g−1 and then unassembled in an argon-filled glove box and rinsed with PC. The cathodes
were characterized by XRD after drying under vacuum at room
temperature.
3. Results and discussion
Fig. 1 shows the XRD pattern of the as-prepared sample, where
all the peaks can be indexed to ␤-MnO2 with a space group of
P42 /mnm; the sharp and narrow peaks indicate that the ␤-MnO2 is
highly crystalline. By contrast, those reported ␤-MnO2 with electrochemical activity process weak and broad XRD peaks [9,11].
SEM images of the ␤-MnO2 were shown in Fig. 2, indicating its
macroporous structure; the pore size is ca. 400 nm and the shell
thickness is distributed in 300–500 nm. The well-defined shape
of the macroporous ␤-MnO2 is in good agreement with its high
crystallinity as indicated by the XRD characterization (Fig. 1). The
formation of such a macroporous structure is believed to be related
with concentration of reactants. As shown in Fig. 3, we did not
observe the macroporous morphology when 0.01–0.04 M KMnO4
reacted stoichiometricly with MnSO4 , although all the products
obtained over this concentration rage are ␤-MnO2 as reported previously [5]. At very low concentration (such as 0.01 M KMnO4 ), we
obtained solid rods (Fig. 3a), while observed well-defined cuboids
with split along the axis direction (Fig. 3b) at mediate concentration
(such as 0.04 M KMnO4 ). Further increase reactant concentration
(such as 0.8 M KMnO4 ) resulted in the macroporous MnO2 .
The formation mechanism is complicate and difficult to reveal;
however, according to our present observation, it is possibly associated with self-assembly of ␣-MnO2 . When we limited the reaction
time within 1 h, we obtained an ␣-MnO2 with 20 nm in diameter
as indicated by its XRD pattern and SEM image (Fig. 4). The
nanowire shape of the ␣-MnO2 is consistent with those reported
in literature [5,14,15]. Considering hydrothermal growth of MnO2
is usually concerned with a dissolution- recrystallization process
[5,16,17], such ␣-MnO2 is believed to be a mediate product for our
macroporous ␤-MnO2 . Note also that when we transformed an
␣-MnO2 to ␤-MnO2 by adjusting the pH value of the hydrothermal
system to 0.5, we observed the macroporous structure similar to
the as-prepared sample, but with some nanowires inside (Fig. 5).
This phenomenon supports the self-assembly mechanism for
Fig. 3. SEM images of the ␤-MnO2 obtained in the cases of (a) 0.01 and (b) 0.04 M KMnO4 employed.
X. Huang et al. / Electrochimica Acta 55 (2010) 4915–4920
Fig. 4. XRD pattern and SEM image of the ␣-MnO2 obtained after 1-h hydrothermal
treatment.
hydrothermal formation of macroporous ␤-MnO2 . In other words,
during hydrothermal growth of ␤-MnO2 , the precursor produced
at room temperature formed nanowires of ␣-MnO2 which subsequently self-assemble to a big size of ␤-MnO2 . The self-assembly
mechanism for formation of ␤-MnO2 is consistent with the recent
report of Zhang et al. [18]. In addition, Zhang et al. [19] recently
reported a hollow ␤-MnO2 by KMnO4 , CuCl2 , and HCl, where they
proposed that the formation of the porous morphology is related
to the evolved Cl2 gas as a soft template, where the details of
Fig. 5. SEM image of a ␤-MnO2 from hydrothermal transformation of an ␣-MnO2
in HCl solution (at pH 0.5).
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the template mechanism lacks of discussion. Our results seem
to support such a proposal since we obtained the macroporous
␤-MnO2 at high reactant concentrations. The reaction between
KMnO4 and MnCl2 results in HCl as one of the product. High reactant concentrations result in high HCl concentration, which will
etch MnO2 and produce Cl2 gas. The evolved Cl2 bobbles would be
attached to the gaps between nanowires of MnO2 , which, to some
extent, blocks crystal growth of the inner surface of MnO2 particle.
In other words, the outer surface of MnO2 particle has more chance
to grow than the inner one due to the presence of Cl2 bobbles
attached, which finally results in the macroporous morphology.
In recent years, three-dimension nanoarchitectures attract a lot
of interest for energy storage and conversion [20]. We therefore
expect that the macroporous ␤-MnO2 could show good electrochemical performance since electrolyte can enter pores of the
macroporous ␤-MnO2 which consequently benefits lithium ion
diffusing into shell of ␤-MnO2 . In fact, unlike those conventional well-crystallized ␤-MnO2 showing very low electrochemical
activity, our macroporous ␤-MnO2 , in spite of high crystallinity,
delivered a high initial capacity of 251 mAh g−1 at a current density
of 10 mA g−1 (Fig. 6a). After initially decayed to 177 mAh g−1 , the
discharge capacity then sustained to be ca. 165 mAh g−1 (Fig. 6c).
By contrast, when a commercial ␤-MnO2 discharged at the same
condition, it delivered only 29 mAh g−1 (Fig. 6b), which is in agreement with those previously reported results that conventional bulk
␤-MnO2 processes low electrochemical activity [7–9]. The electrochemical activity of the macroporous ␤-MnO2 is related to its
macroporous morphology which leads to that electrolyte can flood
pore of the material and its inner surface is therefore available
for lithium ion diffusion. In addition, small size of shell thickness
(300–500 nm) of the macroporous ␤-MnO2 is also contributed to
obtaining electrochemical activity.
Note that a transformation to spinel from rutile was observed
upon electrochemically cycling the as-prepared ␤-MnO2 as indicated by the development of 4 V discharge plateau (Fig. 6a). This
agrees well with the result of Tang et al. [11]. Such a transformation
is related to the distorted hexagonal-close-packed oxygen lattice in
rutile to the cubic-close-packed lattice in spinel, accompanied by
one half of Mn migrating to neighboring octahedral sites [7].
Fig. 7a exhibits the charge/discharge curves of the macroporous
␤-MnO2 performed at 200 mAh g−1 , obtaining an initial capacity of
130 mAh g−1 and a reversible capacity of ca. 75 mAh g−1 (Fig. 7b).
This result suggests such a macroporous ␤-MnO2 processing a
high polarization at high current density. However, comparing
to the conventional ␤-MnO2 with low electrochemical capacity,
our macroporous ␤-MnO2 has much high electrochemical activity.
Therefore, we believe that such a macroporous ␤-MnO2 can be used
as an active material for a battery requiring high capacity but low
loading current density.
Those mesoporous ␤-MnO2 show better rate capability [8,9],
e.g., delivering ca. 210 mAh g−1 (at 300 mA g−1 ), 74% of capacity
obtained at 15 mA g−1 [9]. However, as we know, the tap densities of mesoporous materials are commonly much lower than bulk
ones although there are few values of tap densities of mesoporous
materials available in the reported literature; for example, GordonSmith and Spong [21] presented the tap densities of mesoporous
Ni(OH)2 as 0.8–0.98 cm3 g−1 , less than a half of the typical vale of
conventional Ni(OH)2 (ca. 2.1 cm3 g−1 ). By contrast, our macroporous ␤-MnO2 processes a tap density of 1.7 cm3 g−1 , close to the
value of the battery-level MnO2 , and higher than those of mesoporous materials, which benefits the practical application of the
macroporous ␤-MnO2 .
To further understanding the origin of electrochemical activity
of the as-prepared macroporous ␤-MnO2 with high crystallinity,
we would like to investigate its structure changes along with discharging and charging processes. There are several studies on the
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X. Huang et al. / Electrochimica Acta 55 (2010) 4915–4920
Fig. 7. (a) Discharge and charge profiles and (b) cyclic performance of the macroporous ␤-MnO2 at a current density of 200 mA g−1 (2.0–4.2 V).
Fig. 6. Discharge and charge profiles of (a) the macroporous ␤-MnO2 and (b) a commercial ␤-MnO2 and (c) their cyclic performances at a current density of 10 mA g−1
(2.0–4.2 V).
structure variation during electrochemical cycling. Tang et al. [11]
synthesized ␤-MnO2 /C nanocomposites by pyrolysing mixtures of
Mn(NO3 )2 and acetylene black at 160–320 ◦ C, and investigated the
structure changes of two samples upon discharging and charging,
where they believe that lithium insertion process is an one-phase
cathodic reaction. Jiao and Bruce [9] prepared a mesoporous ␤MnO2 , and proposed that charge/discharge of the mesoporous
material only caused lattice variation without structural conver-
sion. Compared to these materials with electrochemical activity
based on observation of XRD patterns, our macroporous ␤-MnO2
shows much higher crystallinity; this means that it will be helpful
to observe more accurate information on structure changes upon
charging and discharging. Electrodes, from the macroporous ␤MnO2 , at various state of discharge or charge were characterized by
XRD as shown in Fig. 8. Upon discharging to 2.0 V, similar to Tang et
al. [11], we observed the 1 1 0 peak (at ca. 29◦ ) on the XRD pattern
decreased its intensity gradually, accompanied by the development
of the peaks at ca. 18◦ and 24◦ . However, we noticed the interesting variation of two peaks at 23–25◦ , different with the previous
reports; a XRD peak at ca. 23.8◦ appeared after 2 h discharge, and
its intensity increased at 8 h and then decreased at 16 h; opposite to
the variation of the peak at 23.8◦ , the peak at 24.5◦ developed at 8 h
and then increased its intensity during the whole subsequent discharge process, which was accompanied by the development of the
peak at 18.3◦ . These results suggest that the macroporous ␤-MnO2
preserved its rutile framework at the initial stage (e.g., before 2 h)
with lattice constant a changed from 0.440 to 0.454 nm (one-phase
reaction); further lithium insertion resulted in structural transformation as indicated by vanishment of the peaks at 23.8◦ and 28.7◦
and development of the peaks at 18◦ and 24.5◦ (two-phase reaction). The formed phase is likely to be an orthorhombic Lix MnO2
[11,22] and would not be regained to the original rutile structure of
␤-MnO2 on charging as indicated by the XRD peaks at 18◦ and 37◦ ;
X. Huang et al. / Electrochimica Acta 55 (2010) 4915–4920
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Fig. 8. XRD patterns of the ␤-MnO2 electrode at (a) SOD-OCP, (b) SOD-2h, (c) SOD-8h, (d) SOD-16h and (e) SOD-2V, (f) SOC-6h, (g) SOC-12h, (h) SOC-4.2V for the first
charge-discharge process, (i) SOD-8h and (j) SOD-2.0V for the second discharge process; enlarged patterns between 17–27◦ and 35–44◦ are shown on right. SOD and SOC
mean the state of discharge and charge, respectively.
the two peaks, developed during the initial discharge process, did
not disappear in the initial charge process. Note also that the peak
at ca. 37◦ shifted towards higher 2 angles upon the initial charging
but regained during the subsequent discharge, suggesting that after
the initial discharge, the electrode did not electrochemically cycle
in the rutile framework but in the new developed Lix MnO2 phase.
Such a Lix MnO2 phase needs a part of lithium ions remained to
stabilize its structure, which is the reason causing the irreversible
capacity for the initial electrochemical process.
From above results, we can conclude that phase transformation
of the macroporous ␤-MnO2 is necessary for the initial discharging
process. Furthermore, it becomes easier to understand why those
␤-MnO2 with poor crystallization and/or small particle size deliver
high capacities while highly crystalline bulk ␤-MnO2 do not; i.e.,
the former case helps release of stress of phase transformation.
The necessary occurrence of phase transformation of the highly
crystalline macroporous ␤-MnO2 for the initial discharge process
is also noticed to help to understand its low- and high- temperature performance. Fig. 9 shows the electrochemical performance of
the macroporous ␤-MnO2 at various temperatures. The discharge
capacities decreased significantly with temperature, e.g., obtaining
initial capacities of 251, 121, 69, and 21 mAh g−1 for the temper-
Fig. 9. The initial discharge/charge curves of the macroporous ␤-MnO2 at various
temperatures (10 mA g−1 , 2.0–4.2 V).
atures of 30, 0, −10, and −30 ◦ C, respectively, which indicates the
poor low-temperature performance of the macroporous ␤-MnO2 .
On the contrary, at 60 ◦ C the initial discharge capacity increased
to ca. 269 mAh g−1 . The marked effect of temperature on discharge capacity of the macroporous ␤-MnO2 is associated with the
structure variation during the electrochemical insertion of lithium
ions. At low temperature, it turns more difficult for phase transformation of the highly crystalline ␤-MnO2 , resulting in its poor
low-temperature performance. On the contrary, higher temperature helps it transform the rutile framework into the orthorhombic
Lix MnO2 phase, which would be converted further to spinel phase
upon electrochemical cycling. In fact, an easier transformation into
spinel phase was observed at 60 ◦ C than at 30 ◦ C as indicated by the
development of 4 V discharge plateau (not shown).
4. Conclusion
A macroporous ␤-MnO2 with high crystallinity was hydrothermally synthesized with KMnO4 and MnCl2 as starting materials.
Its pore size is ca. 400 nm and the shell thickness is distributed
in 300–500 nm. The formation of the macroporous morphology is
related to self-assembling from nanowires of ␣-MnO2 , and could
be obtained at high reactant concentrations (e.g., 0.8 M KMnO4 ) but
not at low ones (e.g., below 0.04 M KMnO4 ). In addition, the resulted
HCl with high concentration would etch MnO2 and produce Cl2
gas which is believed to assist the formation of the porous structure. Compared to those conventional well-crystallized ␤-MnO2
with very low electrochemical activity, the as-prepared macroporous material exhibits high electrochemical capacity, e.g., of
251 mAh g−1 at a current density of 10 mA g−1 . The electrochemical
activity of the macroporous ␤-MnO2 is related to its macroporous
morphology which leads to that electrolyte can flood pore of the
material and its inner surface is available for lithium ion diffusion. In
addition, its small particle size (shell thickness) benefits the release
of stress due to phase transformation upon initial discharging, and
therefore improves the electrochemical activity of ␤-MnO2 .
The high crystallinity of the as-prepared macroporous ␤-MnO2
is contributed to investigating the structural variation upon discharging/charging. The present results suggest that in the first
discharge process the macroporous ␤-MnO2 proceeded electrochemical insertion of lithium by one-phase and two-phase
reactions at the initial stage (e.g., before 2 h at 10 mA g−1 ) and at the
subsequent stage, respectively; at the latter stage, the rutile struc-
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X. Huang et al. / Electrochimica Acta 55 (2010) 4915–4920
ture of ␤-MnO2 converted irreversibly to orthorhombic Lix MnO2 .
The electrode was then electrochemically cycled within the frameworks of the new developed phase, and a fraction of lithium ions
is required in structure to stabilize the new developed phase. As a
result, the ␤-MnO2 initially decayed from 251 to 177 mAh g−1 and
then sustained as ca. 165 mAh g−1 at a current density of 10 mA g−1 .
Acknowledgements
Y.Y. thanks the financial support from the National
Basic Research Program of China (973 Program) (grant no.
2007CB209702), and the National Natural Science Foundation of
China Grants (nos. 20473060, 29925310, and 20021002).
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