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Electrochemical properties of Mg-based hydrogen storagematerials modified with carbonaceous materials preparedby hydriding combustion synthesis and subsequentmechanical milling (HCS D MM)
Yunfeng Zhu, Wenfeng Zhang, Chen Yang, Liquan Li*
College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing, Jiangsu 210009, PR China
a r t i c l e i n f o
Article history:
Received 23 April 2010
Received in revised form
18 June 2010
Accepted 3 July 2010
Available online 1 August 2010
Keywords:
Mg-based hydrogen storage
materials
Hydriding combustion synthesis
Electrochemical properties
Mechanical milling
Carbonaceous materials
* Corresponding author. Tel.: þ86 25 8358725E-mail address: [email protected] (L. Li).
0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.07.031
a b s t r a c t
Mg2Ni-based hydride was prepared by hydriding combustion synthesis (HCS), and subse-
quently modified with various carbonaceous materials including graphite, multi-walled
carbon nanotubes (MWCNTs), carbon aerogels (CAs) and carbon nanofibers (CNFs) by
mechanical milling (MM) for 5 h. The structural properties of the modified hydrides were
characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). All of the
modified hydrides show amorphous or nanocrystalline-like phases. The hydride modified
with graphite exhibits the most homogenous distribution of particles and the smallest
particle size. The effects of the modifications on electrochemical properties of the hydride
were investigated by galvanostatic charge/discharge, linear polarization, Tafel polariza-
tion, electrochemical impedance spectroscopy and potentiostatic discharge measure-
ments. The results show that the maximum discharge capacity, the high rate
dischargeability (HRD), the exchange current density and the hydrogen diffusion ability of
the hydride modified with the carbonaceous materials are all increased. Especially, the
hydride modified with graphite possesses the highest discharge capacity of 531 mAh/g and
the best electrochemical kinetics property.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1. Introduction for the large-scale applications of Ni/MH batteries. Mg-based
Presently, nickelemetal hydride batteries (Ni/MH) have been
extensively applied in everyday life due to their superior
charge/discharge capability and pollution-free nature.
However, unsatisfactory theoretical specific capacities and
high cost of the commercial anode materials (i.e. LaNi5-based
and ZreTi-based hydrogen storage alloys) restrict their further
developments.
Consequently, the development of high specific capacity
and economical metal hydride as anode materials is crucial
5.
ssor T. Nejat Veziroglu. P
hydrogen storage materials have attracted considerable
interests among the most promising candidates in view of
their high theoretical specific capacity, light weight, abundant
resources, low cost, etc [1e4]. However, either poor cycling
stability or low practical discharge capacity hinders their
potential application. In order to overcome the above short-
comings,much research such asmechanical milling (alloying)
[5,6], element substitution [7e9], Mg-based composite prepa-
ration [10,11], surface modification [12,13] and novel prepa-
ration methods [14,15] has been conducted. Mechanical
ublished by Elsevier Ltd. All rights reserved.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 6 5 3e9 6 6 09654
milling/alloying has been widely used to synthesize Mg-based
alloys with amorphous or nanocrystalline microstructure,
which possesses higher discharge capacity than crystalline
alloys [5].
Hydriding combustion synthesis, put forward in 1997 by
Akiyama et al. [16], has been considered as a valuable method
to prepare Mg-based hydrogen storage alloys on account of its
short process time, low energy consumption and high purity
and activity of product [17e22]. By ways of HCS, Mg2Ni alloy
can be directly synthesized from the powder mixture of
magnesium and nickel at 850 K, which is below the melting
point of magnesium. As a consequence, the evaporation of
magnesium can be prevented effectively. Furthermore, the
preparation and hydrogenation of hydrogen storage alloy
proceed in a single process, which avoids the repeated
remelting and activating process as compared with the
conventional ingot melting. Based on the above advantages of
HCS and MM, we have prepared Mg2Ni-based hydrides with
high discharge capacity and activity by the process of
HCS þ MM (MM for only 5 h) previously [23,24].
It has been widely reported that carbonaceous materials,
such as graphite and multi-walled carbon nanotubes are
effective additives for improving the electrochemical proper-
ties of Mg-based hydrogen storage alloys [25e31]. Thus, the
aim of the present work is to investigate the influences of
various carbonaceous additives (including graphite,MWCNTs,
CAs and CNFs) on the structural and electrochemical proper-
ties of the Mg2Ni-based hydride prepared by the process of
HCS þ MM.
20 30 40 50 60 70
♦
♦
♦
♦
♦
♦
♦Mg2NiH0.3Mg2NiH4
Inte
nsity
(a.u
.)
2-Theta (degree)
(a)
(b)
(c)
(d)
(e)
(f)
HCS
HCS+MM
Graphite
MWCNTs
CAs
NiMgNi2
CNFs
Mg(OH)2
Fig. 1 e XRD patterns of the Mg2Ni-based hydride modified
with various carbonaceous additives: (a) HCS product;
(b) HCS D MM product; (c) HCS D MM-Graphite product;
(d) HCS D MM-MWCNTs product; (e) HCS D MM-CAs
product; (f) HCS D MM-CNFs product.
2. Experiment details
The HCS product was prepared from commercial Mg (99.9
mass% inpurity and<150 mminparticle size) andNi (99.7mass
% in purity and 2e3 mm in particle size) powders. The powders
were mixed in 2:1 of Mg/Ni molar ratio by an ultrasonic
homogenizer in acetone for 1 h. After completely dried in air,
the well-mixed powder was placed directly into the synthesis
reactor without compacting. Before heating, the reactor was
evacuated by a rotary pump, then argon at 0.1 MPa was
introduced and the reactorwas evacuated again. This cleaning
procedure was repeated twice to remove any oxygen in the
system. During HCS process, the mixed powder was heated
from room temperature to 850K at the rate of 7 K/min andheld
for 1 h under 1.9 MPa hydrogen pressure. In order to increase
the degree of hydrogenation of Mg2Ni, the samples were hold
at 623K for about 1 h during the cooling process. Subsequently,
the samples were cooled down to room temperature under
hydrogen atmosphere. After that, the HCS product was
mechanically milled with 3 wt% (vs. the HCS product)
commercial graphite, MWCNTs, CAs and CNFs for 5 h at
a speed of 400 rpm and 40:1 in ball-to-powder using a plane-
tary-type ball mill under argon atmosphere, respectively.
All the testing electrodes were prepared as follows: 0.1 g
HCS þ MM product was mixed with 0.4 g carbonyl nickel
powder, and then cold-pressed into a pellet of 10mmdiameter
and about 1 mm thickness under a pressure of 12 MPa. The
electrochemical measurements were performed in 6 M KOH
aqueous solutionusinga three-compartment cell comprisedof
a metal hydride testing electrode, a sintered NiOOH/Ni(OH)2counter electrode and a Hg/HgO reference electrode. The
charge/discharge cycles were carried out with a LAND Battery
Test instrument. As the initial anode are in their hydride state,
all the testing electrodeswere first discharged at 30mA/g up to
�0.6 V (vs. Hg/HgO), and then charged at 300mA/g for 2 h after
resting for 10 min at 30 � 1 �C. The discharge capacities of
electrodes were evaluated by the mass of active substances.
The discharge capacities at different discharge current densi-
ties (100, 200, 400mA/g) weremeasured to investigate the high
rate dischargeability (HRD). Linear polarization, Tafel polari-
zation, electrochemical impedance spectroscopy (EIS) and
potentiostatic discharge of the electrodes were performed at
room temperature on a CHI660C electrochemical workstation
at 50%, 100%, 50% depth of discharge (DOD) and 100% depth of
charge (DOC), respectively. Linear polarization and Tafel
polarization were measured at scanning rates of 0.1 mV/s and
1mV/s from�5 toþ5mVand�300 to 1500mV (vs. open circuit
potential), respectively. The EIS spectra of the electrodes were
obtained in the frequency range of 100 kHze5mHzwith an AC
amplitude of 5mVunder open circuit conditions. The obtained
EIS spectra were analyzed by ZPLOT electrochemical imped-
ance software. As for potentiostatic discharge, electrodeswere
discharge at þ600 mV potential steps for 3600 s.
The crystal structures of the samples were characterized
by X-ray diffraction (XRD) on an ARL X’TRA diffractometer
with Cu Ka radiation. The morphologies of the samples were
analyzed by a JSM-6360LV scanning electron microscopy
(SEM).
3. Results and discussion
3.1. Structural and morphological characteristics
Fig. 1 shows the XRD patterns of the bare (unmodified) and
modified Mg2Ni-based hydrides. It can be seen from Fig. 1(a)
that the HCS product consists of the main phase Mg2NiH4 and
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 6 5 3e9 6 6 0 9655
very small amount of Mg2NiH0.3, MgNi2 and Ni. As the raw
materials Mg and Ni were mixed in molar ratio of 2:1, the
appearance of small peaks of Ni and MgNi2 means the evap-
oration of Mg still takes place, while the appearance of
Mg2NiH0.3 peak indicates incomplete hydrogenation of Mg2Ni
during the HCS process. The phase content of Mg2NiH0.3 is less
than that of the sample in our previous work [24], indicating
that the hydrogenation degree of Mg2Ni is increased in the
current work. Therefore, the electrochemical properties may
be different as compared to the previous work. After
mechanical milling (Fig. 1(b)), the sharp peaks are broadened
and the peak intensities are also decreased, indicating an
increase in intrinsic stress and a decrease in crystallite size.
After modification with carbonaceous materials, the
Mg2NiH4 peaks around 23� even disappear, suggesting that
the carbonaceous additives can facilitate amorphization or
nanocrystallization of the HCS þ MM product effectively.
Meanwhile, the Mg2NiH0.3 peak around 20� appears after
various carbonaceous modifications, indicating part of
Mg2NiH4 was dehydrogenated during the MM process prob-
ably due to the catalytic effect of carbonaceous additives.
The peaks of Ni and MgNi2 become more distinct after
mechanical milling, which can be attributed to the less
brittle nature of Ni and MgNi2 than the Mg2NiH4 hydride,
whose peaks are broadened easily during the MM process. As
seen in Fig. 1 (c)e(f), it can be found that graphite and
MWCNTs additions have more effect on peak broadening of
Ni around 45� than CAs and CNFs additions. Cui et al. [6]
reported the smaller nickel particles might be inlaid to
surface of large magnesium alloy particles by cold welding
through MM. Hence, it can be expected that the nickel
particles with smaller crystallite size were inlaid to the
surface of graphite and MWCNTsmodified hydrides. Besides,
Mg(OH)2 peaks were detected in the modified hydrides due to
exposure of the samples to air, which is also found previ-
ously [20].
The SEM images of the carbonaceous materials are shown
in Fig. 2aed. It can be seen that the microscale graphite flakes
and nanoscale MWCNTs possess homogenous distribution,
while agglomerates are detected in microscale CAs and
nanoscale CNFs. As shown in Fig. 2 e and f, the HCS product is
severely pulverized during intensive ball milling. However,
the particle size of the HCS þ MM product is irregular and
serious agglomerates took place during the unmodified MM
process. After carbonaceous modifications, the particles of
the hydrides become smaller and more homogenous
(Fig. 2gej). It has been reported that lots of carbon accumu-
lates on the surface, and hence lessens the agglomeration
and adhesion of magnesium hydride particles [32]. In addi-
tion, the hydride modified with graphite exhibits the most
uniform distribution and the smallest particle size. No
distinct macro-aggregated graphite flakes are detected,
implying the structure has been broken down adequately
during the MM process. Owing to the low magnifications
employed, no MWCNTs and CNFs traces can be detected in
the hydrides. In general, we believe that the graphite exhibits
the best process control effect. However, considering its
largest initial particle size, this effect still needs to be further
studied.
3.2. Electrochemical properties
Fig. 3 shows the discharge curves of the Mg2Ni-based hydrides
modified with various carbonaceous additives. The discharge
capacities are listed in Table 1. For the modified hydrides, the
discharge potential is shifted to the negative direction and
show clear discharge potential plateaus. This may be caused
by the decrease of particle size of the hydride, since smaller
particles having large surface areas result in a smaller current
density at particle surface [33], which helps to reduce the
electrochemical polarization. For galvanostatic method, the
discharge capacity of the hydride is determined by two
factors. One is the quantity of the active substances that can
release hydrogen (i.e. the nanocrystalline or amorphous phase
in the hydride), and the other is the electrochemical kinetics
of the hydride. As stated above, the modified hydrides have
more amorphous or nanocrystalline phases and crystal
defects, such as dislocations, grain boundaries, etc. The larger
number of interfaces and grain boundaries provide more
efficient active sites for charge-transfer reaction. Besides,
hydrogen may reach the surface more rapidly for a smaller
particle. Therefore, the addition of carbonaceous materials
significantly improves both the amount of active substance
and the electrochemical kinetics, and hence increases the
discharge capacity. It is well-known that these carbonaceous
materials are important hydrogen storage materials.
However, in present case, this small amount of uncharged
carbonaceous materials can hardly make a contribution to
discharge capacity. Thus, the high discharge capacity must
come from the amorphous or nanocrystalline Mg2Ni-based
hydride phase. Modifications of the hydride with graphite and
MWCNTs result in more remarkable increments of the initial
discharge capacity than those with CAs and CNFs. This may
be due to the differences of the structural and morphological
characteristics mentioned above.
Fig. 4 shows the evolution of the discharge capacity of the
original and modified Mg2Ni-based hydrides with cycle
numbers. The capacity retention rate R8 (C8/Cmax) is also listed
in Table 1. The rapid degradation in discharge capacity of
Mg-based alloys is attributed to the well-known reason of the
oxidation and corrosion of Mg in alkaline solution [34]. As
shown in Fig. 4 and Table 1, themodifications of carbonaceous
additives cannot improve the cycling stability under present
conditions, which does not agree with the results reported by
Iwakura et al. [26] and Guo et al. [28]. There may be
a competitive mechanism on the cycling stability. On one
hand, the defective structures and small particle size, i.e. large
specific surface area, account for serious oxidation of the
modified hydrides, corresponding to a decrease of anti-
corrosion capability, which is proved by the Tafel polarization
result as follows. On the other hand, the carbonaceous
materials can obstruct the formation of Mg(OH)2 during
cycling, which is found by the above authors. In present work,
it should be noted that the content of carbonaceous additives
(3 wt%) are much smaller than those in their works (10 wt%).
We also prepared the samples with increased amount of
graphite additive, and the results show that the cycling
stability was improved. Therefore, the protection effect of
carbonaceous additives in the current work is weak. As
Fig. 2 e Morphologies of the carbonaceous materials and the hydrides observed by SEM: (a) Graphite; (b) MWCNTs; (c) CAs;
(d) CNFs; (e) HCS product; (f) HCS D MM product; (g) HCS D MM-Graphite product; (h) HCS D MM-MWCNTs product;
(i) HCS D MM-CAs product; (j) HCS D MM-CNFs product.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 6 5 3e9 6 6 09656
0 100 200 300 400 5000.6
0.7
0.8
0.9
54321
1 Bare 2 CNFs 3 CAs 4 MWCNTs 5 Graphite
-Pot
entia
l (V
vs. H
g/H
gO)
Discharge capacity (mAh/g)
Fig. 3 e Discharge curves for the Mg2Ni-based hydride
modified with various carbonaceous additives (Discharge
rate: 30 mA/g; the first cycle).
0 1 2 3 4 5 6 7 8 90
100
200
300
400
500
Dis
char
ge c
apac
ity (m
Ah/g
)
Cycle number
Bare Graphite MWCNTs CAs CNFs
Fig. 4 e Discharge capacities as a function of cycle number
for the Mg2Ni-based hydride modified with various
carbonaceous additives.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 6 5 3e9 6 6 0 9657
a result, the cycling stability of the modified hydride
decreases. The Tafel polarization curves of the hydrides
modified with various carbonaceous materials are shown in
Fig. 5 and the corrosion potential Ecorr are also listed in Table 1.
The result shows that the Ecorr shifts toward the negative
direction, indicating the anti-corrosion capability is impaired
after the modifications.
The high rate dischargeability of the Mg2Ni-based hydride
modified with various carbonaceous additives is shown in
Fig. 6. The HRD is determined as follows:
HRD ¼ Cd
Cd þ C60� 100% (1)
where Cd is the discharge capacity with cut-off potential of
�0.6 V (vs. Hg/HgO) at the discharge current density Id (100,
200, 400 mA/g), C60 is the residual discharge capacity with the
same cut-off potential at the discharge current density
I (60 mA/g) after the alloy electrode has been fully discharged
at the discharge current density Id. It is obvious that the HRD is
improved notably by the carbonaceous modifications, sug-
gesting favorable effect of the modification on electro-
chemical kinetics. Generally speaking, the HRD is determined
by the charge-transfer on the alloy surface and the hydrogen
diffusion inside the alloy [35]. For further understanding the
relationship between and the electrochemical kinetics prop-
erties of the electrodes and the modifications, linear polari-
zation, EIS and potentiostatic discharge measurements were
performed.
Table 1 e The electrochemical properties of the Mg2Ni-based h
Modifications Cmax (mAh/g) R8 (%) Ecorr (
Bare 339 13.9 �0.86
Graphite 531 5.4 �0.87
MWCNTs 510 4.1 �0.87
CAs 441 4.4 �0.94
CNFs 416 4.3 �0.91
The exchange current density I0, which is the rate of the
charge-transfer reaction, can be obtained according to the
following formula [36]:
I0 ¼ RTIFh
(2)
where R is the gas constant, T is the absolute temperature (K),
I is the applied current density (mA/g), F is the Faraday
constant and h is the total overpotential (mV). The linear
polarization curves for the modified hydrides are shown in
Fig. 7 and the calculated I0 values are also listed in Table 1. It
can be seen that the value of I0 is increased significantly after
the modifications. This result certifies that the rate of charge-
transfer reaction on the alloy surface is greatly improved,
hence reducing the overpotential during the discharge
process and leading to an increase of discharge capacity of the
electrode. It has been reported that the surface Ni/Mg ratio of
MgNi alloy increases after graphite modification [25].
Furthermore, as stated above, there might have some
unreacted nanocrystalline nickel particles inlaid on the
hydride surface. These nickel particles are beneficial to
enhance the charge-transfer rate on the hydride surface due
to the good electrocatalytic activity of nickel in alkaline elec-
trolyte. The hydride modified with graphite possesses the
largest I0 due to its structural characteristic, for small particles
can enhance the reactivity of the electrode desirably [37].
The EIS spectra and corresponding equivalent circuit of the
modified hydrides are presented in Fig. 8. The spectra are
composed of two semicircles and a straight line. Based on the
ydride modified with various carbonaceous additives.
V) I0 (mA/g) Rct (mU) D/a2 (�10�5 s�1)
6 36.7 493.1 1.3
2 66.7 212.2 2.2
5 60.0 342.4 1.9
2 42.3 410.9 1.8
8 52.0 391.0 1.6
-6 -4 -2 0 2 4 6
-10
-5
0
5
10 Bare Graphite MWCNTs CAs CNFs
Cur
rent
den
sity
(mA/
g)
Overpotential (mV)
Fig. 7 e Linear polarization curves of the Mg2Ni-based
hydride modified with various carbonaceous additives
(scan rate: 0.1 mV/s).
0.6
-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9-6
-5
-4
-3
-2
-1
0
-0.94 -0.92 -0.90 -0.88 -0.86-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
CAs
CNFs
MWCNTs
Graphite
Bare
log i (
A/g)
Potential (V, vs Hg/HgO)
Fig. 5 e Tafel polarization curves of the Mg2Ni-based
hydride modified with various carbonaceous additives
(scan rate: 1 mV/s).
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 6 5 3e9 6 6 09658
EIS model of hydrogen storage alloy electrode [38], the high
frequency semicircle, the lower frequency semicircle and the
low frequency straight line are related to the contact resis-
tance (Rc) between the current collector and the alloy pellet,
the charge-transfer reaction resistance (Rct) and the Warburg
impedance (W ), respectively. The values of Rct estimated
according to the equivalent circuit are also listed in Table 1. It
is obvious that the charge-transfer resistance is decreased
after the modifications, which is consistent with the results
obtained by linear polarization. Besides, judging from the
radius of semicircle in the high frequency, we find the modi-
fications also decrease the contact resistance Rc, which is
helpful to improve the discharge capacity as well.
To investigate the hydrogen diffusion behavior in the
hydrides, potentiostatic discharge measurements of the
hydrides were performed and the result is shown in Fig. 9. At
the initial stage of discharging (<500 s), the current declines
sharply. After a time response of about 2000 s, the current
decreases slowly in a linear fashion. In the linear region, the
hydrogen diffusion in the bulk of hydride controls the
100 200 300 400
20
40
60
80
100
HR
D (
%)
Discharge current density (mA/g)
Bare Graphite MWCNTs CAs CNFs
Fig. 6 e High rate dischargeability of the Mg2Ni-based
hydride modified with various carbonaceous additives.
electrode process and the hydrogen diffusion ability D/a2 can
be calculated according to the following equation [39]:
log i ¼ log
�� 6FD
da2ðC0 � CsÞ
�� p2
2:303Da2t (3)
where i (A/g) is the current density, F is Faraday constant,
D (cm2/s) is the hydrogen diffusion coefficient, d (g/cm3) is the
density of the hydrogen storage alloy, a (cm) is the alloy
particle radius, C0 (mol/cm3) is the initial hydrogen concen-
tration in the bulk of the alloy, Cs (mol/cm3) is the hydrogen
concentration on the surface of the alloy particles, and t (s) is
the discharge time. Since the average size of the hydride
particles are not similar for the different samples, it is
reasonable to useD/a2 to evaluate the discharge kinetics of the
electrode contributed by hydrogen diffusion. The values of
D/a2 for the samples were determined from the slopes of
0.8 1.2 1.6 2.0 2.40.0
0.2
0.4
-Zimag ( Ω
)
Zreal (Ω)
Bare Graphite MWCNTs CAs CNFs
CPE2Rs
Rc
CPE1
Rct W
o
Fig. 8 e Electrochemical impedance spectra and
corresponding equivalent circuit of the Mg2Ni-based
hydride modified with various carbonaceous additives.
Fig. 9 e Potentiostatic discharge curves of the Mg2Ni-based
hydride modified with various carbonaceous additives.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 6 5 3e9 6 6 0 9659
linear current responses and listed in Table 1. It is found that
the value of D/a2 is increased after themodifications. Graphite
addition leads to the largest D/a2 value, indicating that the
hydride modified with graphite possesses the best hydrogen
diffusion ability.
In summary, both exchange current density I0 and D/a2
value are enhanced by the carbonaceous modifications. As
HRD is determined by both of the hydrogen diffusion in the
alloy and charge-transfer on the alloy surface, it is reasonable
that the electrochemical kinetics property is improved after
the modifications. For the hydride modified with graphite, it
has the largest I0 and D/a2 value, and hence possesses the best
high rate dischargeability.
4. Conclusions
The structural and electrochemical properties of the Mg2Ni-
based hydride modified with various carbonaceous additives
were investigated systematically. The XRD and SEM results
reveal that additions of carbonaceous materials can signifi-
cantly facilitate amorphization or nanocrystallization and the
decrease of particle size of the hydride. Electrochemical
measurements indicate that the carbonaceous modifications
improve both the discharge capacity and electrochemical
kinetics. More specifically, the maximum discharge capacity,
the high rate dischargeability, the exchange current density I0and the D/a2 value are all increased after the modifications,
while the electrochemical reaction resistance Rct decreases.
Furthermore, the hydride modified with graphite exhibits the
highest discharge capacity (531 mAh/g), the largest D/a2 value
(2.2 � 10�5 s�1), I0 (66.7 mA/g) and the smallest Rct (212.2 mU).
Acknowledgements
This work is supported by National Natural Science Founda-
tion of China (Nos. 50871052, 50601014), the National Hi-Tech
Research and Development Program of China (863 Program)
(No. 2007AA05Z110), the PhD Programs Foundation ofMinistry
of Education of China (No. 20093221110008), Natural Science
Foundation of Jiangsu Province (No. BK2009361) and Key
Laboratory of Inorganic and Related New Composite Materials
of Jiangsu Province.
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