Electrochimica Acta 125 (2014) 659–665
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Yolk-shell ZnO-C microspheres with enhanced electrochemical
performance as anode material for lithium ion batteries
Qingshui Xie a , Xiaoqiang Zhang a , Xiaobiao Wu b , Huayi Wu b , Xiang Liu a , Guanghui Yue a ,
Yong Yang b , Dong-Liang Peng a,∗
a
Department of Materials Science and Engineering, College of Materials, Fujian Key Laboratory of Advanced Materials, Xiamen University,
Xiamen 361005, China
b
State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history:
Received 29 November 2013
Received in revised form 27 January 2014
Accepted 1 February 2014
Available online 15 February 2014
Keywords:
Yolk-shell
Zinc oxide
Carbon
Anode
Lithium ion batteries
a b s t r a c t
Three ZnO-C samples with distinct structures including yolk-shell microspheres, hollow microspheres
and solid microspheres are fabricated through a facile chemical solution reaction followed by calcination
in argon. When employed as the anode materials for lithium ion batteries, yolk-shell ZnO-C microspheres
exhibit the best electrochemical properties than the hollow and solid microspheres. After 150 cycles,
yolk-shell ZnO-C microspheres demonstrate a relative high capacity of 520 mA h g−1 at a current density
of 100 mA g−1 with a Coulombic efficiency of about 99.3%. The excellent cycling stability and good rate
capability of yolk-shell ZnO-C microspheres stem from the synergistic effect of the unique yolk-shell
structures and extra carbon support.
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
In recent years, hollow micro/nanostructures have been widely
investigated as anode materials for lithium ion batteries due
to their large surface area and internal void space which can
provide the large electrode/electrolyte contact area and effectively
accommodate the huge volume variation during the Li+ intercalation/deintercalation process, respectively [1–5]. More recently,
yolk-shell structures as one of the emerging hollow structures
have received increasingly attention in lithium ion batteries [6–8].
For example, Lou’s group reported the successful preparation
of yolk-shell V2 O5 microspheres through templating method,
which revealed a high reversible capacity of about 187 mA h
g−1 at 300 mA g−1 after 100 cycles [8]. Son et al. found that
yolk-shell Co3 O4 microspheres displayed significantly enhanced
electrochemical performance in comparison with the hollow Co3 O4
microspheres synthesized by the same route, which was ascribed
to the better structure stability of the appealing yolk-shell construction [9]. Yolk-shell Sn/C powders prepared by Zhang et al.
showed superior lithium storage capacity and excellent cycling stability [10]. Inspired by above studies, to fabricate various electrode
∗ Corresponding author. Tel.: +86 592 2180155; fax: +86 592 2183515.
E-mail address: [email protected] (D.-L. Peng).
http://dx.doi.org/10.1016/j.electacta.2014.02.003
0013-4686/© 2014 Elsevier Ltd. All rights reserved.
materials with yolk-shell construction would be an effective strategy to improve their electrochemical properties. However, it
is still a great challenge to develop a facile and cost-effective
route to large-scale synthesize various yolk-shell structures
till now.
Transition metal oxides have gained more and more interest
as electrode materials in lithium ion batteries in the last decades
because of their higher theoretical capacity and safety compared
with the conventional carbon materials [11–14]. Among them, ZnO
has some special advantages such as low cost, easy preparation,
morphologic diversity, environmental benignity and so on. Unfortunately, the poor electronic conductivity, large volume change
during lithium/delithium process and the resulting severe capacity
fading hinder its practical application. Hitherto, some efforts have
been made to improve its cycling performance including the
proper nanostructuring and decoration by metal, metal oxide and
carbon [15–20]. For example, Huang’s group directly fabricated
ZnO nanosheets on copper current-collecting substrates, which
delivered the higher capacity and better cycling stability than
commercial ZnO powders [19]. Although the reversible capacity of ZnO electrodes has been improved to some extent after
appropriate construction and modification, the cyclability of ZnO
anodes is still not satisfactory for developing advanced electrode
materials for next-generation lithium ion batteries. Therefore, to
further improve the cycling stability and reversible capacity of ZnO
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Q. Xie et al. / Electrochimica Acta 125 (2014) 659–665
electrode materials through elaborate construction and decoration
is still highly desirable.
In this article, three different ZnO-C structures including yolkshell microspheres, hollow microspheres and solid microspheres
were synthesized through a two step method. In the initial
synthesis process, zinc citrate yolk-shell microspheres, hollow
microspheres and solid microspheres were fabricated through a
simple chemical solution reaction. Then the carboxylic acid groups
in zinc citrate function as the in situ carbon source to prepare three
different ZnO-C samples in the subsequent carbonization treatment. To the best of our knowledge, this is the first report on the
successful synthesis of yolk-shell ZnO-C microspheres. The investigation of electrochemical properties suggests that all three samples
exhibit similarly excellent cycling stability and yolk-shell ZnO-C
microspheres possess the highest reversible capacity and best rate
capability than the hollow and solid counterparts.
2. Experimental
2.1. Synthesis
In a typical synthesis, amorphous zinc citrate yolk-shell microspheres, hollow microspheres and solid microspheres were firstly
prepared through a facile chemical solution route as described
in our previous literature [21]. Then the as-obtained zinc citrate
microspheres with different structures were carbonized at 500 ◦ C
for 2 h with a heating rate of 2 ◦ C/min in argon than air. After heat
treatment, the furnace was cooled to room temperature naturally
and then the black powders can be harvested.
2.2. Characterization
PANalytical X’ pert PRO x-ray diffractometer (Cu K␣ radiation
40 kV, 30 mA) was employed to characterize the crystal phase of
the products. Scanning electron microscopy (LEO-1530) and transmission electron microscopy (JEM-2100, 200 kV) were used to
investigate the morphology and microstructure of the products.
The thermogravimetric analysis (TGA) was carried out on a SDTQ600 thermal analyzer. The Brunauer-Emmett-Teller (BET) surface
area and pore size distribution were evaluated on a TriStar 3020
system.
2.3. Electrochemical measurements
Two-electrode cells were used to characterize the electrochemical performance of the as-produced products. The working
electrode was made up of 75 wt% active materials (ZnO-C microsheres with different structures), 15 wt% carbon black, and 10 wt%
poly (vinyl difluoride) (PVDF). Li metal and 1 M LiPF6 dissolved in
a mixed solution composed of equal volume of ethylene carbonate
(EC) and diethyl carbonate (DEC) were employed as the counterelectrode and electrolyte, respectively. All the cells were assembled
in an argon-filled glove box. The investigation of discharge-charge
cycling performance was conducted on a multichannel battery testing system (Newware, China). Cyclic voltammetry (CV) was carried
out using Autolab electrochemical workstation (NOVA 1.8) at a
scanning rate of 0.1 mV s−1 at room temperature.
3. Results and discussion
3.1. Morphology and structure characterization
Fig. 1 displays the morphology and microstructure of the
as-obtained yolk-shell ZnO-C microspheres. The panoramic SEM
image (Fig. 1a) implies that the sample is made up of dispersed
microspheres with an average diameter on the order of 1.6 ␮m.
From a broken microsphere as shown in the inset in Fig. 1a, the
inner core, outer shell and interstitial void space can be obviously
discerned, suggesting the yolk-shell structure of microsphere. From
the magnified SEM micrograph (Fig. 1b), it can be observed that
the shell is composed of numerous nanoparticles with an average
size of 20 nm. Energy dispersive spectroscopy (EDS) microanalysis confirms the presence of Zn, O, C elements (Fig. S1). The TEM
investigation was carried out to provide detailed structure of the
samples. As depicted in Fig. 1c, the yolk-shell structure is further
validated. The thickness of the shell and the diameter of the core are
about 280 and 400 nm, respectively. The lattice fringes with a spacing of 0.281 nm in HRTEM image (Fig. 1d) recorded from the shell
relate to the spacing of the (100) plane of ZnO. Nanocarbon in yolkshell ZnO-C microspheres is difficult to be directly distinguished
even under high-magnification TEM observation in view of the carbon stem from the in situ carbonization of carboxylic acid groups in
zinc citrate. Differently, for conventional carbon-coated materials,
nanocarbon-layer can be clearly seen under HRTEM observation
[18]. The corresponding SAED pattern (the inset in Fig. 1d) demonstrates the polycrystalline nature of the obtained yolk-shell ZnO-C
microspheres.
The morphology and microstructure of the products acquired
from the carbonization of zinc citrate hollow microspheres and
solid microspheres are displayed in Fig. 2. It is visible that ZnOC microspheres obtained from the carbonization of zinc citrate
hollow microspheres possess the hollow structure (the inset in
Fig. 2a and Fig. 2c). The average diameter of the hollow microspheres and thickness of the shell are about 1.6 ␮m and 250 nm,
respectively. The surface of the microsphere (Fig. 2b) is consisted
of many nanoparticles with the particle size ranging from 20 to
30 nm, which is similar with that of yolk-shell ZnO-C microspheres.
The SEM and TEM images revealed in Fig. 2d and 2f show the solid
structures of ZnO-C microspheres acquired from the carbonization of zinc citrate solid microspheres. The average diameter of
the solid microspheres is about 1.45 ␮m, which is slight smaller
than the yolk-shell and hollow microspheres. From the magnified
SEM image shown in Fig. 2e, the size of the nanoparticle building blocks for ZnO-C solid microspheres is approximate 45 nm.
Based on the above observation and analysis, it can be pronounced
that ZnO-C yolk-shell microspheres, hollow microspheres and solid
microspheres can be easily fabricated via the good morphology
inheritance of zinc citrate yolk-shell microspheres, hollow microspheres and solid microspheres by directly carbonizing various zinc
citrate microspheres in argon.
Shown in Fig. 3 are the XRD patterns of the as-obtained ZnO-C
yolk-shell microspheres, hollow microspheres and solid microspheres. All the diffraction peaks can be indexed to hexagonal ZnO
(JCPDS card no. 89-0510). The absence of the diffraction peaks
originated from carbon indicates the amorphous nature of such
derived carbon. Apart from ZnO diffraction peaks, no additional
peaks can be detected, suggesting the high purity of the products. In addition, the intensities of diffraction peaks for ZnO-C
yolk-shell microspheres and hollow microspheres are stronger
than ZnO-C solid counterparts, which may be ascribed to the
higher content of carbon in ZnO-C solid microspheres (TGA results
discussed below). The carbon contents in the products are determined based on the TGA measurement. As shown in Fig. 4, the
different amounts of weight loss below 350 ◦ C in all three samples are caused by the removal of physically adsorbed water. The
amorphous carbon decomposes between 350 and 500 ◦ C, which
is similar with the results of SnO2 -C hollow microspheres and
MoS2 -C nanotubes [22,23]. The carbon contents of ZnO-C yolk-shell
microspheres, hollow microspheres and solid microspheres are calculated to be 6.34%, 6.30% and 9.82%, respectively. The contents of
carbon for ZnO-C yolk-shell and hollow microspheres are almost
Q. Xie et al. / Electrochimica Acta 125 (2014) 659–665
661
Fig. 1. The low (a) and high (b) SEM micrographs of yolk-shell ZnO-C microspheres. The inset in Fig. 1a reveals a broken microsphere. The scale bar represents 200 nm. The
TEM (c) and HRTEM (d) images of yolk-shell ZnO-C hollow microspheres. The inset in Fig. 1d is the corresponding SAED pattern of yolk-shell ZnO-C microspheres.
Fig. 2. (a-c) The SEM and TEM micrographs of ZnO-C hollow microspheres. The scale bar in the inset represents 200 nm. (d-f) The SEM and TEM images of ZnO-C solid
microspheres.
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Q. Xie et al. / Electrochimica Acta 125 (2014) 659–665
Fig. 3. The XRD patterns of ZnO-C yolk-shell microspheres, hollow microspheres
and solid microspheres.
the same and ZnO-C solid microspheres possess the highest content
of carbon. N2 adsorption-desorption isotherms were performed to
gain insight into the textural features of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres. As shown in
Fig. 5, ZnO-C yolk-shell microspheres reveal the highest BrunauerEmmett-Teller (BET) surface area of 99.7 m2 g −1 than the hollow
microspheres (86.6 m2 g −1 ) and solid microspheres (53.2 m2 g −1 ).
The pore size distributions (the inset in Fig. 5) of above three
samples were determined based on the Barrett–Joyner–Halenda
method, which indicates the presence of numerous nanopores in
all three samples. Accordingly, the large surface area possess more
electrochemical active sites and the porosity can provide an effective channel for the diffusion of the electrolyte, which is beneficial
to improve the electrochemical properties of electrode materials
[24].
3.2. Electrochemical properties
The electrochemical properties of all three samples used as
anode materials for lithium ion batteries were investigated. Fig. 6a
reveals the cyclic voltammograms (CVs) of yolk-shell ZnO-C
microspheres at a scan rate of 0.1 mV s−1 in the voltage ranging
from 0.01 to 3 V. In the first lithium process, two cathodic peaks
located at 0.5 and 0.25 V can be found clearly. The relative weak
peak at 0.5 V originates from the reduction of ZnO into Zn and
Fig. 4. TG curves for ZnO-C yolk-shell microspheres, hollow microspheres and solid
microspheres.
Fig. 5. N2 adsorption-desorption isotherms of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres. The insets show the corresponding pore
size distributions.
the formation of amorphous Li2 O, while the strong peak nearby
0.25 V is caused by the generation of Li-Zn alloy together with
the decomposition of electrolyte and the resulting growth of
organic-like solid electrolyte interphase (SEI) layer [16,19,25,26].
In the subsequent delithium process, three weak oxidation peaks
centred at 0.27, 0.52 and 0.63 V can be carefully discerned which
can be attributed to a multistep dealloying of Li-Zn alloy [26,27].
The broad oxidation peak located at 1.35 V corresponds to the
formation of ZnO [28]. Recently, some researches demonstrated
that there is slight difference in the position of the broad oxidation
peak for various ZnO or ZnO-based electrodes [16,19,29,30]. For
example, Baibarac et al. reported that the broad oxidation peak of
ZnO nanoneedle electrodes located at 1.54 V during the first cycle
[29]. The broad oxidation peaks of ZnO porous nanosheets and
powders centered at 1.38 and 1.46 V, respectively [19]. ZnO/MnO2
sea urchans-like arrays and ZnO nanorod arrays fabricated by
Yuan exhibited the broad oxidation peaks nearby 1.33 and 1.36 V,
respectively [30]. From the above analysis, it is reasonable to
conclude that the position of the broad oxidation peak originated
from the regeneration of ZnO during charge process may relate to
the morphology and composition of ZnO or ZnO-based electrode
Q. Xie et al. / Electrochimica Acta 125 (2014) 659–665
663
Fig. 6. (a) Cyclic voltammograms of yolk-shell ZnO-C microspheres tested at 0.1 mV s−1 in 0.01-3 V. (b) Discharge-charge curves of yolk-shell ZnO-C microspheres at
100 mA g−1 . (c) Discharge capacities and Coulombic efficiencies vs cycle number of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres at 100 mA g−1 .
(d) Rate performance of ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres.
materials. In the second scan, there is only a peak nearby 0.6 V in
the cathodic curve. And the oxidation peaks in the anodic curve
slightly move to higher potential. From the second cycle onwards,
the CV curves almost superpose in shape, implying the good
reversibility of the electrochemical reactions. The CV results of
ZnO-C hollow microspheres and solid microspheres are similar
with that of ZnO-C yolk-shell microspheres (Fig. S2). According to
above results and previous studies, the electrochemical reactions
of yolk-shell ZnO-C microspheres are proposed as follows [19,31]:
ZnO + 2Li+ + 2e− ↔ Zn + Li2 O
(1)
Zn + xLi+ + xe− ↔ Lix Zn (x ≤ 1)
(2)
The galvanostatic discharge-charge curves for yolk-shell ZnOC electrode at 100 mA g−1 in the voltage range of 0.01-3.0 V are
depicted in Fig. 6b. Two plateaus in the first discharge curve can
be observed. The long plateau located at 0.5 V is attributed to the
reduction of ZnO into Zn as well as the formation of Li2 O, while
the relative weak plateau appeared at 0.25 V may be assigned to
the generation of Li-Zn alloy and the decomposition of electrolyte,
which is in good agreement with the above CV results [15,20,32].
In the second discharge curve, the long plateau is replaced by a
slope between 1.1 and 0.3 V, which is similar with other ZnO-based
electrodes reported previously [16,18,19]. However, no obvious
plateaus can be observed in the charge curves. After the first cycle,
the curves are similar in shape, implying the excellent reversibility
of the electrochemical reactions. The initial discharge and charge
capacities of yolk-shell ZnO-C microspheres are 1432 and 798 mA
h g−1 , respectively. The low Coulombic efficiency of about 55.7%
at first cycle is common for the most metal oxide electrode materials, which is mainly caused by the formation of SEI film [27,33].
The voltage-capacity profiles of ZnO-C hollow microspheres (Fig.
S3) and solid microspheres (Fig. S4) are analogous to that of the
above ZnO-C yolk-shell microspheres. The initial discharge/charge
capacities of ZnO-C hollow microspheres and solid microspheres
are 1397/682 and 1322/676 mA h g−1 with the corresponding
Coulombic efficiencies of 48.8% and 51.1%, respectively. Yolk-shell
ZnO-C microspheres reveal the highest discharge/charge capacities and Coulombic efficiency in the initial stage compared with
other two ZnO-C counterparts. Fig. 6c displays the cycling performance and the corresponding Coulombic efficiencies of all three
samples measured at 100 mA g−1 . In the first 25th cycles, a large
capacity fading can be found in all three samples, while in the following cycles all three samples show similarly excellent cycling
stability. After 150 cycles, ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres exhibit a reversibility
capacity of 520, 427 and 341 mA h g−1 with the Coulombic efficiencies of 99.3%, 99.7% and 99.7%, respectively. Yolk-shell ZnO-C
microspheres exhibit the highest discharge capacity. For comparison, the electrochemical properties of other ZnO nanostructures
or ZnO-based electrode materials are also provided in Table 1. For
instance, a reversible capacity of 392 mA h g−1 can be acquired at
120 mA g−1 for Au-ZnO flower-like nanostructures after 50 cycles
[16]. Ultrathin ZnO nanotubes fabricated by Park et al. demonstrate a reversible capacity of 386 mA h g−1 after 50 cycles at 0.5 C
[15]. Carbon/ZnO nanorod arrays deliver a discharge capacity of
330 mA h g−1 after 50 cycles [18]. It is obvious that yolk-shell ZnO-C
microspheres show a relative higher reversible capacity and better cycling stability in comparison with other ZnO or ZnO-based
electrode materials. The rate capabilities of all three samples were
also measured and the results are depicted in Fig. 6d. Compared to
ZnO-C hollow microspheres and solid microspheres, ZnO-C yolkshell microspheres display enhanced rate capability. The average
reversible capacities of ZnO-C yolk-shell microspheres range from
764, 465, 339 to 212 mA h g−1 as the current densities increase from
100, 200, 500 to 1000 mA g−1 . When the current density is back to
100 mA g−1 , a reversible capacity of 416 mA h g−1 can be acquired
again.
The enhanced electrochemical performance of yolk-shell ZnO-C
microspheres can be attributed to its special yolk-shell structures
and extra carbon support. First, yolk-shell ZnO-C microspheres
have the largest surface area than the hollow and solid counterparts, which can provide largest electrode/electrolyte contact area
and most active sites. In addition, the porosity of the samples can
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Q. Xie et al. / Electrochimica Acta 125 (2014) 659–665
Table 1
The electrochemical properties of various ZnO or ZnO-based electrode materials.
Materials
Morphology
Reversible capacity (mA h g−1 )
Cycles
Ref
ZnO
ZnO
ZnO
ZnO/C
ZnO/Au
ZnO/Se
ZnO/SnO2
ZnO/Fe2 O3
ZnO/NiO/C
ZnO/C
Ultrathin nanotubes
Dandelion-like nanorod arrays
Porous nanosheets
Nanorod arrays
Flower-like nanostructures
Nanocomposites
Nanocomposites
Flower-like films
Flower-like films
Yolk-shell/hollow/solid microspheres
386
310
400
330
392
<400
497
776
488
520/427/341
50
40
100
50
50
100
40
50
50
150
[15]
[26]
[19]
[18]
[16]
[34]
[35]
[36]
[20]
Our work
provide an effective channel for the diffusion of the electrolyte, benefiting to the improvement of the reversible capacity [24]. Second,
yolk-shell structures possess better structure stability than hollow
and solid structures during the repeated Li+ insertion/extraction
process. Compared to solid structures, the large void space in
yolk-shell structures can effectively accommodate the huge volume variation during discharge-charge process and then prevent
the electrode from pulverization to some extent, which is in favor
of the cycling stability [2,37]. Additionally, in recent years, studies have found that yolk-shell structures have more robust shells
and improved mechanical strength in comparison with the hollow structures due to the synergistic effect of the cores and shells,
which can provide better tolerance for structural degradation during lithium/delithium process [38–40]. For example, yolk-shell
CoMn2 O4 microspheres revealed better electrochemical properties
than hollow microspheres fabricated under the same conditions
[41]. Third, the building blocks of yolk-shell ZnO-C microspheres
are in nano-scale dimension, which can reduce the Li+ diffusion
length and then improve the kinetics of electrochemical reactions
[37]. Finally, the nanocarbon originated from the in situ carbonization of carboxylic acid groups in zinc citrate can not only improve
the conductivity of ZnO electrodes, but also provide extra support
to the structures, which plays a positive role in the high reversible
capacity and excellent cyclability of the electrode materials [42,43].
4. Conclusions
In summary, ZnO-C yolk-shell microspheres, hollow microspheres and solid microspheres were fabricated through a facile
route using zinc citrate microspheres with different structures as
precursors. The carboxylic acid groups in zinc citrate function as the
in situ carbon source in the carbonization process. When applied
as the anode materials for lithium ion batteries, all three samples exhibit similarly excellent cycling stability. Yolk-shell ZnO-C
microspheres possess the highest discharge/charge capacities and
Coulombic efficiency during the first lithium-delithium process
compared with other two ZnO-C counterparts. And after 150 cycles,
a high reversible capacity of 520 mA h g−1 can be acquired for yolkshell ZnO-C microspheres. The relative higher reversible capacity
and superior cyclability of yolk-shell ZnO-C microspheres are due
to its largest surface area, better structural stability and extra carbon support, which can provide more electrochemical active sites,
effectively alleviate the huge volume change during Li+ uptake and
removel process and enhance the conductivity of active materials.
This novel and facile method may be extended to prepare other
yolk-shell micro/nanostructures which may find applications in
lithium ion batteries.
Acknowledgements
The authors gratefully acknowledge financial support from the
National Basic Research Program of China (No. 2012CB933103),
the National Outstanding Youth Science Foundation of China
(Grant No. 50825101), the National Natural Science Foundation of
China (Grant Nos. 51171158 and 51371154) and the Fundamental Research Funds for the Central Universities of China (Grant no.
201312G003).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.electacta.
2014.02.003.
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