Nano Energy (2013) 2, 498–504
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
RAPID COMMUNICATION
Amorphous Zn2GeO4 nanoparticles as anodes
with high reversible capacity and long cycling
life for Li-ion batteries
Ran Yia, Jinkui Fenga, Dongping Lva, Mikhail L. Gordina,
Shuru Chena, Daiwon Choib, Donghai Wanga,n
a
Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park,
PA 16802, USA
b
Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, USA
Received 13 November 2012; received in revised form 10 December 2012; accepted 12 December 2012
Available online 27 December 2012
KEYWORDS
Abstract
Amorphous;
Nanoparticles;
Zinc germanate;
Anode;
Lithium-ion batteries
Amorphous and crystalline Zn2GeO4 nanoparticles were prepared and characterized as anode
materials for Li-ion batteries. A higher reversible specific capacity of 1250 mAh/g after 500
cycles and excellent rate capability were obtained for amorphous Zn2GeO4 nanoparticles,
compared to that of crystalline Zn2GeO4 nanoparticles. Small particle size, amorphous phase
and incorporation of zinc and oxygen contribute synergetically to the improved performance by
effectively mitigating the huge volume variations during lithiation and delithiation process.
& 2012 Elsevier Ltd. All rights reserved.
Introduction
Lithium-ion batteries (LIBs) have been intensively pursued as
the most promising electrochemical power sources for transportation applications such aselectric vehicles and plug-in
hybrid electric vehicles, and for small-scale stationary energy
storage [1–3]. As a key part of LIBs, anode materials with high
energy density and long cycle lifetime are indispensable for
these applications. High energy anode materials can be
obtained by employing group IV elements such as silicon and
n
Corresponding author. Tel.: +1 8148631287; fax: +1 8148634848.
E-mail address: [email protected] (D. Wang).
2211-2855/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.nanoen.2012.12.001
germanium, owing to their abilities to uptake 4.4 lithium atoms
per atom [4]. Ge has a high theoretical capacity of 1600 mAh/g
and higher lithium ion diffusivity and electronic conductivity
than those of Si, i.e., 400 and 104 times greater at room
temperature, respectively [5]. This makes Ge a promising
anode material for both high-power and high-energy applications. However, the severe pulverization triggered by large
volume change of Ge during lithation and delithation leads to
poor cycling performance [5,6]. Tremendous efforts have been
made to tackle this problem. For example, decreasing the
dimensions of Ge-based anode materials to the nanoscale
(nanotubes [7], nanowires [8], nanoparticles [5], etc.) can
mitigate physical strains and thereby provide a large capacity
with minimal fading. Amorphization of the anode materials has
Nanoparticles as anodes with high reversible capacity and long cycling life
also been proven to strengthen the cycling stability by reducing
anisotropic expansion/extraction and eliminating stresses deriving from phase transition [9,10]. Recently, an amorphous GeOx
nanostructure has been demonstrated to exhibit excellent
capacity retention of 96.7% after 600 cycles [11]. Another
effective approach to improve cycling performance is to make
composites with carbon and/or other components [12–15].
However, the overall energy density of the composites is
decreased due to the incorporation of the inactive and/or
low capacity components. Also, multi-step synthesis is usually
required, which makes preparing these materials long and
costly.
Cost is an important factor for practical applications of
high-energy anode materials. Despiteits advantages, Ge has
received less attention than the more widely investigated
Si, mainly because of its high cost. The development of
novel Ge-containing materials with low Ge content but high
energy density and long cycling life is thus attractive but
challenging. Crystalline Zn2GeO4 nanorods have been
reported as a promising low-cost Ge-containing anode
material with good cyclability and rate performance [16].
The mass percentage of Ge in Zn2GeO4 is only 27%, much
lower than that in other Ge-containing anode materials,
making Zn2GeO4 a more affordable choice. Herein, we
report for the first time that amorphous Zn2GeO4 shows
significantly improved electrochemical performance as an
anode material for Li-ion battery. The processing-structureperformance relationships of Zn2GeO4 have also been
invesigated.
Material and methods
Synthesis of Zn2GeO4 powders
Zn2GeO4 powders were prepared by an ion-exchange route
at room temperature with various reaction times based on a
modified procedure reported by Zhang et al. [17]. Firstly,
0.0025 mol Na2GeO3 powders were prepared by solid state
reaction between stoichiometric mixture of Na2CO3 and
GeO2 at 900 1C for 10 h and then dispersed in 50 mL
deionized water to form a transparent colloidal suspension
of Na2GeO3 hydrates. The Zn2GeO4 were prepared by ions
exchange in aqueous solution. In a typical process,
0.0025 mol Zn(CH3COO)2 2H2O was dissolved in 50 mL
deionized water to form a clear solution. The obtained
Na2GeO3 hydrates colloidal suspension was added dropwise
to the Zn(CH3COO)2 solution and white precipitation was
formed immediately. The mixture was stirred for 1–52 h at
room temperature. The white precipitation was separated
by centrifugation and then dried under vacuum at room
temperature for 12 h.
499
Brunauer–Emmett–Teller (BET) specific surface area of the
samples was determined by an ASAP 2010 using the standard
N2 adsorption and desorption isotherm measurements at 77 K.
A Fourier transform infrared (FT-IR) spectrometer (IFS 66/S,
Bruker) was used to characterize the Zn2GeO4 sample.
Electrochemical measurements
The electrochemical experiments were performed using 2016
coin cells, which were assembled in an argon-filled dry glovebox (MBraun, Inc.) with the Zn2GeO4 electrode as the working
electrode and the Li metal as the counter electrode. The
working electrodes were prepared by casting the slurry
consisting of 80 wt% of active material, 10 wt% of conductive
Super P carbon black, and 10 wt% of lithium polyacrylate
(Li-PAA) binder. 1 mol L 1 LiPF6 in a mixture of ethylene
carbonate, diethyl carbonate and dimethyl carbonate (EC:
DEC: DMC, 2:1:2 by vol%) and 10 wt% fluoroethylene carbonate
(FEC) was used as the electrolyte (Novolyte Technologies,
Independence, OH). The electrochemical performance was
evaluated by galvanostatic charge/discharge cycling on an
Arbin battery tester BT-2000 at room temperature under
different current densities in the voltage range between
3.00 and 0 V versus Li + /Li.
Results and discussion
Characterization of Zn2GeO4 powders
The phase and crystallinity of Zn2GeO4 samples were examined
by X-ray diffraction (XRD). Amorphous structure was formed
when the ion-exchange time was limited to 1 h (Figure 1a).
The composition of the sample was confirmed by EDS, showing
similar mass percentage of Zn, Ge and O to the theoretical
values in Zn2GeO4 (Figure S1in SI). In addition, the chemical
bondings of ZnO4 and GeO4 in Zn2GeO4 were verified by FT-IR
spectra (Figure S2 in SI). Prolonging reaction time to 20 h
(Figure 1b) produces sharp diffraction peaks, which indicates
the transformation from amorphous to crystalline phase.
All sharp peaks are indexed to the hexagonal phase Zn2GeO4
Characterizations
The obtained samples were characterized on a Rigaku Dmax2000 X-ray powder diffractometer (XRD) with Cu Ka radiation
(l =1.5418 Å). The operation voltage and current were kept at
40 kV and 30 mA, respectively. The size and morphology of the
as-synthesized products were determined by a JEOL-1200
transmission electron microscope (TEM) and an FEI Nova
NanoSEM 630 scanning electron microscope (SEM). The
Figure 1 XRD patterns of Zn2GeO4 prepared with a reaction
time of (a) 1 h, (b) 20 h and (c) 52 h.
500
R. Yi et al.
Figure 2 SEM images of(a) 1 h, (c) 20 h and (d) 52 h Zn2GeO4 samples and (b) TEM image of 1 h Zn2GeO4 sample.
and are consistent with data in JCPDS no. 11-0687. However,
the presence of broad bumps around 201, 271 and 421 and a
broad peak at 341 (marked by asterisks in Figure 1b) suggest
that the transformation was not complete and the product is a
mixture of amorphous and crystalline phases. When the reaction time was further prolonged to 52 h (Figure 1c), the
crystallinity of Zn2GeO4 was improved, as indicated by sharper
existing peaks and sharp new peaks.
The morphologies of products were characterized by SEM
and TEM. As shown in Figure 2a, homogeneous floccule-like
morphology is observed for the 1 h sample. A typical TEM
image (Figure 2b) reveals its nanoscale structure, in which
the product composed of particles with diameters about
8 nm is clearly exhibited. When the reaction time was
increased to 20 h, the product shows mixed morphologies
(Figure 2c). Besides nanoparticles (inset of Figure S3a in SI)
similar to those in the 1 h sample shown in Figure 2b, large
irregular particles with the size greater than 100 nm are
also observed, indicating transformation of nano-sized
particles into large ones due to Ostwald ripening. This
mixed nature agrees well with the presence of both bumps
and sharp reflection peaks in its XRD pattern (Figure 1b).
The amorphous and crystalline structures of the 1 h and 20 h
samples were also confirmed by their selected area electron
diffraction patterns (Figure S4 in SI). Figure 2d is an SEM
image of Zn2GeO4 obtained with a longer reaction time of
52 h. Only rod-like particles with a diameter of 100 nm and
length of 500 nm are observed. The morphology of the 52 h
sample can also be seen in the TEM image (Figure S3b in SI).
Electrochemical studies of Zn2GeO4 powders
The electrochemical performance of all three Zn2GeO4
samples was tested by a galvanostatic discharge–charge
technique at a current density of 400 mA/g over a voltage
window from 0 to 3.0 V vs Li + /Li. The voltage profiles of the
1 h sample at different cycles are shown in Figure 3a. The
initial discharge and charge capacities are 1619 and
560 mAh/g, respectively, giving the first cycle columbic
efficiency(CE) of 34%. It has been reported that a high
specific surface area could lead to a low first cycle CE
because more solid electrolyte interphase (SEI) layer is
irreversibly formed [18,19]. The presence of large particles
in the 20 h sample leads to a lower specific surface area
(53.4 m2/g, Brunauer–Emmett–Teller surface area) than that
of the 1 h sample (147.6 m2/g), and therefore a higher first
cycle CE of 44% (Figure S5 in SI). With an even lower specific
Nanoparticles as anodes with high reversible capacity and long cycling life
Figure 4
Figure 3 (a) Voltage profiles of the 1 h Zn2GeO4 sample at
various discharge and charge cycles with a current density of
400 mA/g, and (b) cycling performance of Zn2GeO4 prepared
under different conditions at 400 mA/g.
surface area of 12.4 m2/g, a higher first cycle CE of 51% was
achieved (Figure S5 in SI) by the 52 h sample. In spite of the
large irreversible capacity loss in the first cycle of the 1 h
sample, which was also observed in other Ge-based anode
materials [18] the CE increases above 99% in 7 cycles and
thereafter remains at that level. The first cycle CE could be
improved by a facile pre-lithiation method, as previously
reported [20].
It is well known that amorphization and decreased particle
size can improve the cycling stability of high capacity anode
materials that suffer drastic volume change during lithiation/
delithiation [21,22]. Excellent cyclability is thus expected for
the amorphous Zn2GeO4 nanoparticles. The discharge (lithiation) capacities of all three samples increase during the first 10
cycles, similar to the previous reports on CaSnO3 [23,24]. The
capacity of both the 1 h and 20 h samples continuously
increases for 380 cycles and 200 cycles, respectively
(Figure 3b). The phenomena of the capacity increase along
cycling beyond initial cycles (4100 cycles) have been observed
in a variety of transition metal oxide based anodes for lithiumion batteries [25–27]. For example of a-Fe2O3 anode, the
capacity was found to be increasing from the 100th to 600th
501
Rate performance of the 1 h Zn2GeO4 sample.
cycle [28]. The exact underlying mechanism is not clear yet. In
the Zn2GeO4 case, the capacity increasing is possibly related to
the slow activation process of Zn2GeO4 upon lithiation/delithiation reflected by the gradual change of the charge/discharge
voltage profiles during cycling (Figure 3a). In contrast, the wellcrystallized Zn2GeO4 product (52 h sample) quickly shows
decreased cycling stability (Figure 3b), as its large and crystalline particles adversely affect its ability to tolerate the
dramatic volume change during cycling, which is consistent
with our previous report on well-crystallized Zn2GeO4 nanoparticles [16]. The capacity of the 20 h sample peaks at 1090 mAh/
g after 200 cycles and then starts to decrease, which may also
be caused by large crystalline particles with low tolerance to
volume change, like those in the 52 h sample. As for the
amorphous nanoparticles (1 h sample), contrary to the frequently observed capacity fading over cycling of other Gebased anode materials [5,8], its capacity stabilizes at
1250 mAh/g from the 380th to the 500th cycle (Figure 3b)
despiteful depth of discharge. The excellent cycling stability is
also evidenced by the almost overlapped voltage profiles of the
400th and 500th cycles as shown in Figure 3a.
To further investigate the electrochemical performance
of the 1 h sample, rate capability was examined. As shown
in Figure 4, a good capacity retention is obtained at each
current rate ranging from 800 mA/g to 6.4 A/g. Even cycling
at high current densities of 3.2 A/g and 6.4 A/g, capacities
of 610 and 470 mAh/g can be obtained, respectively,
exhibiting significantly improved rate performance
compared with the previous report on well-crystallized
Zn2GeO4 [16]. In addition, the CE remains above 99% even
at very high rates. Figure S6 in SI plots the corresponding
discharge–charge voltage profiles at different rates,
which remain similar in shape as the current rate is
increased.
Usually the second metal in alloy anodes and oxygen in alloymetal oxide anodes are introduced to form a buffer matrix to
alleviate the damage caused by volume change of the active
materials. Unfortunately, in most cases, either the second
metal is electrochemically inert with respect to lithium [29]
or Li2O formed during the first lithiation process cannot be
reversibly charged/discharged in the following cycles [23,24],
leading to decreased reversible capacities of the whole anodes.
However, in our case of the amorphous Zn2GeO4 nanoparticles,
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R. Yi et al.
Figure 5 SEM images of the 52 h Zn2GeO4 electrode(a and c)after 200 cycles and the 1 h Zn2GeO4 electrode (b and d) after 500 cycles.
not only can zinc form zinc–lithium alloy reversibly as demonstrated by previous study on zinc-containing anode materials
[30] but also the reactions between oxide and lithium (ZnO+2Li2Zn+Li2O and GeO2 +4Li2Ge+2Li2O) are at least partially
reversible [11,31,32]. Otherwise, a high reversible capacity of
1250 mAh/g, close to the theoretical capacity of 1443 mAh/g
assuming fully reversible formation/decomposition of Li2O and
reversible lithiation/delithiation in zinc, could not be achieved.
Therefore, the following reaction mechanism between Zn2GeO4
and Li during lithiation and delithiation in the amorphous
Zn2GeO4 nanoparticles is proposed:
Zn2GeO4 +8Li + + 8e - 2Zn +Ge +4Li2O (Ref. 16)
Zn +Li + +e 2LiZn (Ref. [30,31])
Ge +4.4Li + +4.4e 2Li4.4Ge (Ref. [4])
Zn +Li2O2ZnO+2Li + +2e (Ref. [31])
Ge +2Li2O2GeO2 +4Li + + 4e (Ref. [11])
To better understand cyclability difference between
amorphous and crystalline Zn2GeO4, post-cycling TEM and
SEM analyses have been studied. Irregular particles other
than original rods were found in TEM of the 52 h sample
(Figure S7a in SI), indicating that the large crystalline rods
underwent pulverization upon cycling. In contrast, the 1 h
sample still shows nanoparticles with diameter of 10 nm
(Figure S7b in SI) that is similar to the morphology of the 1 h
sample before lithiation/delithiation. We also investigated
the morphology of electrodes prepared by amorphous (1 h)
and crystalline (52 h) Zn2GeO4. It is found that active
materials were peeled off (Figure 5a) from the current
collector and large cracks can be observed (Figure 5c) in the
52 h electrode, suggesting severe damage of the electrode
after extended cycling. In comparison, the structure of 1 h
electrode maintains well as only small cracks (Figure 5d) are
present without any active material peeling off (Figure 5b).
The sharp difference on the Zn2GeO4 electrodes from TEM
and SEM observation serves as strong evidence that small,
amorphous particles shows better resistance to volume
change and pulverization than the large, crystalline
particles.
The superior electrochemical performance of amorphous
Zn2GeO4 nanoparticles (1 h sample) in terms of high reversible capacities, excellent capacity retention and improved
rate capability can be attributed to the synergy of nanoparticles, amorphous phase and incorporation of zinc and
oxygen. First, the nano-sized particles play the most
important role in the improved performance. Nano-sized
particles could efficiently tolerate volume changes of the
Ge-based high capacity anodes and alleviate the problem of
pulverization. On the other hand, anode materials with
nano-sized particles show higher specific capacity and
enhanced rate performance compared to their bulk counterparts due to the shorter ionic and electronic transport
paths, a better contact between the electrode material and
the electrolyte, and larger specific surface area favorable
for high reactivity [3,33–35]. In addition, amorphous structure would not suffer from stresses accompanying phase
transition of crystalline materials upon lithiation, which
Nanoparticles as anodes with high reversible capacity and long cycling life
mitigates the pulverization of the material. Finally, the
presence of zinc and oxygen also plays an important role in
enhancing the capacity retention by providing a buffer
matrix. As discussed above, Zn is electrochemically active
to Li and both Zn and Ge are able to reversibly react with
Li2O. Therefore, aside from helping to accommodate the
volume expansion, the incorporation of Zn and especially O
(considering its molar fraction) contributes to the reversible
capacity of Zn2GeO4.
Conclusions
In summary, amorphous and crystalline Zn2GeO4nanostructures
have been synthesized via a facile ion-exchange route by
simply tuning the reaction time. The Zn2GeO4 composed of
amorphous nanoparticles exhibits much higher reversible
capacity, better capacity retention and enhanced rate capability compared with crystalline counterparts. This sample
has a reversible capacity of 1250 mAh/g after 500 cycles at
400 mA/g with full depth of discharge and capacities of 610
and 470 mAh/g can be obtained at high current densities of
3.2 A/g and 6.4 A/g, respectively. The improved electrochemical performance of amorphous Zn2GeO4 is ascribed to the
synergy of its nano-sized particles, amorphous nature and the
incorporation of Zn and O. The present work provides an
effective strategy for the development of new highperformance anode materials for Li-ion batteries.
Acknowledgements
The authors would like to acknowledge financial support
from the U.S.Department of Energy’s (DOE’s) Office of
Electricity Delivery & EnergyReliability (OE) (under Contract
no. 57558). We are also grateful foruseful discussions with
Dr. Imre Gyuk of the DOE-OE Grid Storage Program.
Appendix A.
Supporting information
Supplementary data associated with this article can be
found in the online version at http://dx.doi.org/10.1016/
j.nanoen.2012.12.001.
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Ran Yi received his B.S. (2007) and M.S.
(2010) in Materials Science from Central
South University, China. Now he is a Ph.D.
candidate at Department of Mechanical
Engineering,
the Pennsylvania
State
University working in Dr. Donghai Wang’s
group. His research is focused on novel
nanomaterialsas high performance anode
for lithium-ion batteries.
504
R. Yi et al.
Dr. Jinkui Feng is currently an Associate
Professor at Shandong University (SDU) in
China. Prior to joining SDU, Dr. Feng worked
on the development of novel materials for
energy storage as a postdocat Department
of Mechanical Engineering at the Pennsylvania State University. He also conducted
research on solid state batteries as a
research fellow at National University of
Singapore between 2008 and 2011. Dr. Feng
received his B.S. (2003) in physical chemistry and Ph.D. (2008) in
electrochemistry at Wuhan University, China. His current research
interests are focused on advanced materials for sustainable energy.
Dr. Daiwon Choi joined Pacific Northwest
National Laboratory in 2007 and has been
conducting researches in hydrogen storage,
biomaterials, electrochemical sensor, water
purification and energy storage devices.
With more than 15 years of experience in
materials science, his research interest lies
in material design for various applications.
His current specific area of interest is in Liion batteries for electrical vehicle and
stationary energy storage for smart grid. Dr. Choi is an author of
more than 45 research papers, 3 book chapters, and an inventor of
three US patents.
Dr. Dongping Lv received Ph.D. degree in
electrochemistry from Xiamen University in
2011, and since then worked as a postdoc
fellow at the Pennsylvania State University.
His research interest is focused on physics
and chemistry of materials in energy
storage, and electrochemistry and in situ
techniques (XRD and XAFS) for batteries.
Dr. Donghai Wang is currently Assistant
Professor at Department of Mechanical and
Nuclear Engineering at the Pennsylvania
State University. Before joining Penn State
in 2009, he was a postdoc and subsequently
became a staff scientist at Pacific Northwest National Laboratories. He received a
B.S. and Ph.D. degree in Chemical Engineering from Tsinghua University and Tulane
University in 1997 and 2006, respectively.
Dr. Donghai Wang’s research interests have been related to design
and synthesis of nanostructured materials for a variety of applications. His recent research is focused on material development for
energy conversion and storage technologies such as fuel cells,
batteries and solar cells/fuels.
Mikhail L. Gordin is currently a Ph.D.
candidate working with Dr. Donghai Wang’s
group at the Pennsylvania State University.
He received his B.S.E. degree in Mechanical
Engineering from Duke University in 2009.
His research interests include development
and characterization of new battery materials and in-situ analysis of battery.
Shuru Chen is currently a Ph.D. candidate
working with Professor Donghai Wang at
Department of Mechanical and Nuclear
Engineering at the Pennsylvania State University. He obtained his B.S. degree in 2007
and M.S. degree in 2010 in chemistry from
Xiamen University in China. His research
involves development of nanostructured
materials for rechargeable Li-ion and Li-S
batteries.