Spherical NiO-C composite for anode material of lithium ion batteries

Article abstract of DOI:10.1016/j.electacta.2006.11.034

Spherical NiO-C composite was prepared by dispersing spherical NiO in glucose solution and subsequent carbonization under hydrothermal conditions at 180 °C. The microstructure and morphology of the NiO-C and NiO powders were characterized by means of X-ra

Full text of DOI:10.1016/j.electacta.2006.11.034

Electrochimica Acta 52 (2007) 4177–4181
Spherical NiO-C composite for anode
material of lithium ion batteries
X.H. Huang, J.P. Tu∗, C.Q. Zhang, X.T. Chen, Y.F. Yuan, H.M. Wu
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
Received 24 August 2006; received in revised form 7 November 2006; accepted 26 November 2006
Available online 9 January 2007
Abstract
Spherical NiO-C composite was prepared by dispersing spherical NiO in glucose solution and subsequent carbonization under hydrothermal
conditions at 180 ◦C. The microstructure and morphology of the NiO-C and NiO powders were characterized by means of X-ray diffraction (XRD)
and scanning electron microscopy (SEM). The electrochemical properties of the electrodes were measured by galvanostatic charge–discharge tests,
cyclic voltammetric analysis (CV), and electrochemical impedance spectroscopy (EIS). SEM images showed that the amorphous carbon not only
coated on the surface but also filled the inner pores of the NiO spheres. Electrochemical tests showed that the NiO-C composite exhibited higher
initial coulombic efficiency (66.6%) than NiO (56.4%), and better cycling performances. The improvement of these properties is attributed to the
carbon, as it can reduce the specific surface area of porous sphere, and enhance the conductivity of porous NiO.
© 2007 Published by Elsevier Ltd.
Keywords: NiO; NiO-C composite; Anode material; Lithium ion battery
1. Introduction
CuO, and Cu2O [2,3]. The electrochemical properties of metal
oxide electrodes are related to various factors, such as the spe-
cific surface areas, the large change in volume and the serious
aggregation of active particles during charge–discharge. Spher-
ical particle, as it is known, with the smallest specific surface
area which results in less of SEI film during the discharge, can
increase the initial coulombic efficiency. It was reported that the
cycling performance of Sn [14,15], SnO2 [16,17], Si [18,19],
TiO2 [20] had been significantly enhanced by forming compos-
ite with carbon. The carbon can act as a barrier to suppress the
aggregation of active particles and thus increase their structure
stability during cycling [14,16,20], and also act as a buffering
matrix to relax the expansion that occurred within the electrode
upon lithiation/delithiation process [18,19]. Furthermore, the
carbon has a high electronic conductivity and it can improve
the conductance of the active materials [17]. Therefore, a syn-
thesis of composite with carbon is an effective way to improve
the electrochemical performance of electrodes. In this present
work, spherical NiO-C composite was prepared by dispersing
spherical NiO in glucose solution and subsequent carbonization
under hydrothermal conditions at 180 ◦C. The electrochemical
properties of the NiO-C composite were investigated by gal-
vanostatic discharge–charge test, cyclic voltammetric analysis
(CV), and electrochemical impedance spectroscopy (EIS).
Transition-metal oxides such as CoO, Co3O4, NiO, Cu2O,
CuO, FeO, Fe2O3, and MnO have attracted much attention.
These oxides offer a promising carbon alternative to negative-
electrode materials in lithium ion batteries [1–13], which is
attributed to their high specific capacity and good cycling
performance. A mechanism different from the classical Li
insertion/deinsertion in carbonaceous compounds or Li-alloying
processes in alloy electrode is proposed, which can be written
as: MxOy + 2y Li ↔ y Li2O + xM [2]. Solid electrolyte interface
(SEI) will also be formed during the discharge, but it will par-
tially decompose during the subsequent charge process, which
is attributed to the catalytic activity of metallic nanoparticles
[11]. The partially reversible formation/decomposition of SEI
film will lead to an extra capacity.
NiO has a theoretic capacity of 718 mAh g−1 when it is used
as anode material for lithium ion batteries. However, the ini-
tial coulombic efficiency and cycling performance of NiO are
worse than those of other transition metal oxides such as CoO,
∗
Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856.
E-mail address: tujp@cmsce.zju.edu.cn (J.P. Tu).
0013-4686/$ – see front matter © 2007 Published by Elsevier Ltd.
doi:10.1016/j.electacta.2006.11.034

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Fig. 1. SEM images of the spherical Ni(OH)2 precursor: (a) the morphology of a whole sphere and (b) the magnified image of the surface of the sphere.
2. Experimental
3. Results and discussion
Commercial spherical ␤-Ni(OH)2 (about 10 ␮m in diameter)
was used as the precursor. Spherical NiO was obtained by cal-
cining Ni(OH)2 under flowing oxygen in a quartz tube furnace
at 350 ◦C for 30 min. The NiO-C composite was prepared as fol-
lows. Glucose (3.17 g) was dissolved in deionized water (80 mL)
until a clear solution was observed. Then the as-prepared NiO
(2.88 g) was dispersed in the resulting solution. It was reported
thatglucosewouldbecarbonizedunderhydrothermalconditions
at 180 ◦C [15,21]. In this work, the mixture was placed in a flask
under magnetic stirring for 2 days, and then removed to a 120 mL
teflon-sealed autoclave filled with Ar maintained at 180 ◦C for
3 h. The products were centrifuged and washed with deion-
ized water and ethanol for three times, respectively, and were
dried in vacuum at 160 ◦C for 24 h. The structure and morphol-
ogy of these products were characterized by X-ray diffraction
(XRD, Rigaku D/max-rA; Cu K␣ radiation) and scanning elec-
tron microscopy (SEM, FEI SIRION, equipped with EDX).
The working electrodes were prepared by a slurry proce-
dure. The slurry consisted of 75 wt.% active materials, 15 wt.%
acetylene black and 10 wt.% polyvinylidene fluoride (PVDF)
dissolved in N-methyl pyrrolidinone (NMP), and was coated on
a copper foam (1.3 cm in diameter) acted as current collector.
The foam was pressed under a pressure of 20 MPa after drying at
95 ◦C for 12 h in vacuum. Test cells were assembled in an argon-
filled glove box using Li foil as counter electrode, polypropylene
(PP) film (Celgard 2300) as separator. The electrolyte was 1 M
LiPF6 in a 50:50 (w/w) mixture of ethylene carbonate (EC) and
diethyl carbonate (DEC).
3.1. Characterization of materials
Themorphologyof ␤-Ni(OH)2 precursorisshownin Fig. 1. It
is clearly observed in the magnified image (Fig. 1b) that the sur-
face of the ␤-Ni(OH)2 microsphere is composed of spindle-like
particles. XRD pattern of the precursor is shown in Fig. 2a and
the diffraction peaks are corresponded with ␤-Ni(OH)2. After
The galvanostatic charge–discharge tests were conducted
on a PCPT-138-32D battery program-control test system with
the cut-off voltages of 0.02 and 3.0 V (versus Li+/Li). Cyclic
voltammetric measurements of the electrodes were performed
on a CHI604B Electrochemical Workstation with a scan rate
of 0.1 mV s−1 between 0 and 3 V (versus Li+/Li). The elec-
trochemical impedance spectroscopy of the electrode was also
carried out on a CHI604B Electrochemical Workstation. Before
the measurement, the electrodes were cycled for 3 cycles, then
discharged to 2.0 V and kept until the open-circuit voltage stabi-
lized. The frequency of EIS ranged from 0.01 Hz to 100 kHz. A
smallac signal of 5 mV in amplitudewasusedasthe perturbation
of the system throughout the tests.
Fig. 2. XRD patterns of (a) the Ni(OH)2 precursor and (b) the as-prepared
spherical NiO and NiO-C composite.

X.H. Huang et al. / Electrochimica Acta 52 (2007) 4177–4181
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Fig. 3. SEM images of the as-prepared spherical (a) NiO and (b) NiO-C composite (insets: higher magnifications). The cross-sectional morphology of (c) a NiO
sphere and (d) a NiO-C sphere. (e) EDX spectrum of the identical sample in (d).
the heat treatment at 350 ◦C for 30 min, the green Ni(OH)2 pow-
der turned to black. The XRD pattern of the as-calcined sample is
shown in Fig. 2b. All the peaks can be assigned to cubic NiO and
noimpuritiescanbeobserved, indicatingacompletedecomposi-
tion of the Ni(OH)2 precursor. A SEM image of the as-prepared
NiO is given in Fig. 3a. The morphology of the microspheres was
preserved after the decomposition of the Ni(OH)2 precursor, but
less compact spheres can be observed. The inset figure in Fig. 3a
and the cross-sectional morphology of the NiO microspheres
(Fig. 3c) indicate that these spheres have a porous structure.
The XRD pattern of the NiO-C composite is shown in
Fig. 2b. Only peaks of cubic NiO can be observed, indicating
that the carbon in the composite is amorphous. The surface
of the NiO-C sphere is coated by a carbon layer (Fig. 3(b and
d)), and no pores can be observed in the cross-section image
of NiO-C spheres (Fig. 3d). All the original micropores in
NiO spheres are filled with carbon. The content of carbon is
7.35 wt%, calculated according to the EDX result (Fig. 3e).
Since the NiO microspheres are porous, the glucose solution
can permeate into the inner pores of the NiO spheres. After
carbonization in hydrothermal conditions, the carbon filled the
pores. It can be also observed in Fig. 3d that there is a carbon
layer coated on the surface of the spheres.
3.2. Electrochemical analysis
Fig. 4 shows the first galvanostatic discharge–charge curves
for NiO and NiO-C electrodes measured between 0.02 and
3.0 V versus Li+/Li at a rate of 0.1–4C (1C = 718 mA g−1).
Lower discharge capacity (849 mAh g−1 at 0.5C) was observed
for NiO-C at low rates (0.1, 0.5 and 1C) as compared to NiO
(1025 mAh g−1 at 0.5C). This would be due to its smaller
surface area, which means less SEI. However, the capacity
of NiO-C was remarkably higher at high rates (2 and 4C),

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Fig. 5. Cyclic voltammograms of NiO and NiO-C electrodes measured between
0 and 3 V at the scan rate of 0.1 mV s−1
.
results were obtained for core/shell TiO2-C composite prepared
by emulsion polymerization [20].
Fig. 6 shows the capacity retention properties of NiO-C and
NiO electrodes at 0.5 C. Much better cycling performance was
obtained for the NiO-C electrode. The specific capacity after 40
cycles for the NiO-C electrode is 430 mAh g−1, much higher
than that of the NiO electrode (200 mAh g−1). There are sev-
eral reasons for the poor cycling performance of NiO spheres.
The SEI film is a poor conductive gel-like polymer, which con-
tained LiF, Li2CO3, and lithium alkyl carbonate (ROCO2Li)
[11], so more SEI will lead to poorer conductivity. The conduc-
tive carbon filled in the inner pores and coated on the shell of
the spheres, it was able to keep the NiO particles in the spheres
electrically connected and thus facilitate the charge transfer
in the interface and in the inner of the spheres. Fig. 7 shows
the electrochemical impedance spectrum (EIS) of the NiO and
NiO-C electrodes. The high-frequency semicircle is attributed
to SEI film and/or contact resistance, the semicircle in medium-
frequency region is assigned to the charge-transfer impedance
on electrode/electrolyte interface, and the inclined line at an
Fig. 4. Galvanostaticlithium-insertion/extraction curvesof (a) NiOand (b)NiO-
C at a rate of 0.1–4C (1C = 718 mA g−1).
suggesting that the carbon facilitated the charge transfer at high
charging–discharging rates. It can be seen that the potential
hysteresis increased upon increasing the charging–discharging
rate, but it was suppressed by the incorporation of carbon. It
is calculated that the initial coulombic efficiency of NiO-C
composite is higher than that of NiO at each charging–discharge
rate. The initial coulombic efficiency of NiO-C composite is
66.6% at a rate of 0.5 C as compared to that for NiO (56.4%).
The cyclic voltammograms (CVs) of the NiO-C and NiO
electrodes measured between 0 and 3 V (versus Li+/Li) at a
scanning rate of 0.1 mV s−1 are shown in Fig. 5. The curves
for both the electrodes show similar reduction and oxidation
peaks. For the NiO, the reduction peak located at 0.89 V corre-
sponds to the decomposition of NiO into Ni, and the formation
of amorphous Li2O and the SEI. For the NiO-C composite, this
peak shifts to 0.94 V. The two oxidation peaks located at about
1.51 and 2.35 V can be attributed to the decomposition of the
SEI and Li2O, respectively [11,12]. For the NiO-C composite,
these peaks shift to 1.47 and 2.32 V, respectively. The separa-
tion between the reduction and oxidation peaks (ꢀU) of the
NiO-C composite decreases in comparison with that of the NiO,
demonstrative of weaker polarization and better reversibility.
This is because the high electronic conductive carbon in the NiO
spheres is beneficial for the diffusion of lithium ions. Similar
Fig. 6. Cyclic performances of the NiO and NiO-C electrodes cycled between
0.02 and 3.0 V at a current density of 359 mA g−1 (0.5C).

X.H. Huang et al. / Electrochimica Acta 52 (2007) 4177–4181
4181
improvements can be attributed to the conductive carbon and its
combination with the pores. The carbon can reduce the specific
surface area of the original porous sphere, keep the particles in
the spheres electrically connected and thus improve the electro-
chemical performance of NiO.
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4. Conclusions
Spherical NiO-C composite was synthesized successfully
by dispersing porous NiO microspheres in glucose solution
and subsequent carbonization under hydrothermal conditions
at 180 ◦C. The carbon coated on the surface, and also filled in
the inner pores of the NiO sphere. The NiO-C composite exhib-
ited higher initial coulombic efficiency (66.6%) than the NiO
(56.4%) at a charging–discharging rate of 0.5 C. Better cycling
performance was also obtained for the NiO-C electrode. The
specific capacity after 40 cycles for the NiO-C composite is
430 mAh g−1, higher than that of NiO (200 mAh g−1). These
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