Honeycomb-like Macro-Germanium as High-Capacity Anodes for Lithium-Ion Batteries with Good Cycling and Rate Performance

Article abstract of DOI:10.1021/acs.chemmater.5b01527

Macro-Ge powder has been synthesized with a novel hydrothermal reduction of commercial GeO₂ at 200°C in an autoclave. The obtained macro-Ge product demonstrates a honeycomb-like macroscopic network structure with a high tap density of 2.19 g cm^-3. As for the anode material of lithium ion batteries, the macro-Ge electrode exhibits 1350 mAh g^-1 at the current rate of 0.2 C and with 64% capacity retention over 3500 total cycles at 1 C. The macro-Ge contains a honeycomb porous structure, which allows for a high volumetric capacity (～3000 mAh cm^-3). Moreover, the symmetrical and asymmetric rate behaviors also provide its excellent electrochemical property. For example, the macro-Ge electrode can be rapidly charged to 1130 mAh g^-1 in 3 min (20 C) and 890 mAh g^-1 in 90 s (40 C) using the constant discharge mode of 1 C. Furthermore, the Ge electrode still maintains over 1020 mAh g^-1 at 1 C for 300 cycles at the high temperature (55°C) environment. When coupled with a commercial LiCoO₂ cathode, a 3.5 V lithium-ion battery with capacity retention of 91% (～364 Wh kg^-1) over 100 cycles is achieved. These outstanding properties may be attributed to the honeycomb structure, for which the porous architectures supply the high efficient ionic transport and buffers the volume change during the lithiation/delithiation processes. Moreover, with bulk frameworks it ensures the high tap density and further improves the energy density. It is supported that the macro-Ge acts as attractive anode materials for further application in rechargeable lithium ion batteries.

Full text of DOI:10.1021/acs.chemmater.5b01527

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Article
Honeycomb-like Macro-Germanium as High-Capacity Anodes for
Lithium-ion Batteries with Good Cycling and Rate Performance
Jianwen Liang, Xiaona Li, Zhiguo Hou, Tianwen Zhang, Yongchun Zhu, Xuedong Yan, and Yitai Qian
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01527 • Publication Date (Web): 12 May 2015
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Honeycomb-like Macro-Germanium as High-Capacity Anodes for
Lithium-ion Batteries with Good Cycling and Rate Performance
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Jianwen Liang, †,‡ Xiaona Li, †,‡ Zhiguo Hou, † Tianwen Zhang,† Yongchun Zhu,†,* Xuedong Yan§
and Yitai Qian† ,*
† Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry,
University of Science and Technology of China, Hefei, Anhui 230026, PR China;
E-mail: ychzhu@ustc.edu.cn; ytqian@ustc.edu.cn,
§ Ningbo Veken Battery Company Inc., West Bonded Zone, Ningbo, Zhejiang 315000, PR China.
KEYWORDS: Germanium; Lithium ion batteries; Anode; Full cell; Hydrothermal reduction;
ABSTRACT: Macro-Ge powder has been synthesized with a novel hydrothermal reduction of commercial GeO2 at 200 °C
in autoclave. The obtained macro-Ge product demonstrates a honeycomb-like macroscopic network structure with a high
tap density of 2.19 g cm-3. As for anode material of lithium ion batteries, the macro-Ge electrode exhibits 1350 mAh g-1 at
the current rate of 0.2 C, and with 64% capacity retention over 3500 total cycles at 1 C. The macro-Ge contains
honeycomb porous structure, which allows for a high volumetric capacity (~3000 mAh cm-3). Moreover, the symmetrical
and asymmetric rate behaviors also provide its excellent electrochemical property. For example, the macro-Ge electrode
can be rapidly charged to 1130 mAh g-1 in 3 min (20 C) and 890 mAh g-1 in 90 s (40 C) using the constant discharge mode
of 1 C. Furthermore, the Ge electrode still maintain over 1020 mAh g-1 at 1 C for 300 cycles at the high temperature (55 °C)
environment. When coupled with a commercial LiCoO2 cathode, a 3.5 V lithium-ion battery with capacity retention of
91% (~364 Wh kg-1) over 100 cycles is achieved. These outstanding properties may be attributed to the honeycomb
structure, which the porous architectures supplies the high efficient ionic transport and buffers the volume change during
the lithiation/delithiation processes. Moreover, with bulk frameworks ensures the high tap density and further improve
the energy density. It is supported that the macro-Ge as attractive anode materials for further application in rechargeable
lithium ion batteries.
1. INTRODUCTION
Ge2H6), chlorogermanes, or phenylgermanes at > 400 °C
with the presence of gold nanocrystals seeded9,
.
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Rechargeable lithium-ion batteries (LIBs) are key elec-
trical energy storage devices for next generation electrici-
ty storage and supply. To satisfy the increasing demand, it
is necessary to develop LIBs with higher energy density,
higher power density and more desirable stability.1,2 From
the viewpoint of energy density, group IVA elements are
most promising materials, especially Si and Ge, which can
deliver theoretical capacities of 4200 mAh g-1 (volumetric
capacity of 8344 Ah L−1) and 1600 mAh g-1 (volumetric
capacity of 7366 Ah L−1), respectively.3-5 Compared with
Si-based materials, Ge exhibits both a higher electronic
Nanostructure Ge materials were also prepared by the
typical thermal degradation of organogermane, GeI2 or
GeCl4 precursor in organic solvent11-14. Ge nanoparticles
were produced by oxidation of zintl alloy such as Na12Ge27
in a glove box or on a vacuum line15. With regard to the
using of GeO2 precursors in synthesizing of Ge, major
attention has been paid to the preparation in the thermal
reduction by H216, 17, Mg18 and Zn19 at the temperature
higher than 450 °C. Although many strategies have been
proposed to prepare the Ge materials, those procedures
generally need expensive catalyst such as gold nanocrys-
tals, high temperature, complicated procedures, or they
employ expensive Ge precursors such as GeH4, GeI2, and
diphenylgermane (C12H10Ge). Thus, for the desire of low-
ering the cost of Ge-based electronic devices, facile syn-
thesis of Ge by a new effective method is still a challenge
to overcome.
conductivity (104 times) and
a higher lithium ion
diffusivity (more than 400 times higher than silicon at
room temperature: 1.41×10−14 cm2 s−1 for Si and 6.51 ×10−12
cm2 s−1 for Ge),6,7 which can be expected to provide better
rate performance and cycling stability. In essence, Ge has
attracted less attention than Si recently because of its
expensive preparation process. However, the cost issue
can be surmountable by mass production and advance-
ments in the process technology as a result of the
abundance of Ge in the Earth’s crust8.
On the other hand, similar to silicon anode, the practi-
cal usage of Ge as anode material still suffers from huge
volume change (~ 300 %) during Ge lithiation-delithiation
process. These extreme volume changes can cause crack-
ing, pulverization, loss of electrode contact, and continu-
In terms of the synthesis in Ge, one of these methods
was chemical vapor deposition (CVD) of germane (like
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ous solid–electrolyte interface (SEI) films formation in the storage properties (with capacity retention of 91% (~364
Wh kg-1) over 100 cycles) is further demonstrated. Thus,
the novel hydrothermal reduction strategy was demon-
strated and proved to be an effective way to synthesize
the macro-Ge anodes. Such an approach greatly reduces
the preparation cost, not only avoiding expensive Ge
precursors, but also geting rid of the special device or
complicated processes. This advanced macro-Ge electrode
with high capacity and good stability provides a promis-
ing Ge anode for LIBs with high energy density and supe-
rior stability.
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electrode. Thus, many approaches have been explored to
improve the electrochemical performance of Ge anode,
such as the design of special morphology of Ge nanoparti-
cles20-22 and the preparation of Ge/carbon23-26 or Ge-
alloy27-29 nanocomposites. Nano-sized Ge can provide suf-
ficient space to tolerate the large volume change and the
employing of carbon or alloy metal can improve the con-
ductivity between Ge particles. However, preparation of
nano-sized Ge either requires expensive Ge precursors,
special device or involves complicated processes; again
the issues for cost and scaling up remain. Furthermore,
achieving a high tap density by using a micron framework
is particularly important for fabrication of high-energy
LIBs.30 Thus, Ge micro-particles are more promising as
electrode materials towards practical application because
they lead to higher volumetric energy density when com-
pared to nano-particle. Significantly, the macro-sized of
Ge is commercial available or easy to large scale prepara-
tion. Recently, Tobias Placke and co-workers showed that
macroporous-sized Ge particles can have excellent
charge–discharge cycling stability at certain condition.31
Their macroporous Ge material exhibits a capacity reten-
tion of 96.6% at 0.1 C for 200 cycles and 717 mAh g-1 reten-
tion when the rate up to 5 C.
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2. EXPERIMENTAL SECTION
Materials. GeO2, Mg (99%, 100-200 mesh powder) and
HCl (37%) were purchased from Sinopharm Chemical
Reagent Co., Ltd (China). Commercial LiCoO2 electrode
materials was come from Ningbo Veken Battery Company
(5th Gangxi Avenue,West Bonded Zone, Ningbo, China).
Hydrothermal synthesis of macro-Ge Particle. The
macro-Ge was prepared by reduction of commercial GeO2
with magnesium and water according to hydrothermal
reduction reaction under the stainless autoclave.
Typically, 0.8 g GeO2, 1.5 g deionized water and 2 g Mg
powder were mixed and added into a 20 mL stainless
autoclave before sealed. Subsequently, the autoclave was
maintained at 200 °C for 5 h and cooled to room
temperature. The as-prepared samples were immersed in
hydrochloric acid (1 mol L-1) for 1 hour to remove
impurities. The resultant powder was washed with
deionized water and ethanol and dried for overnight at
60 °C in vacuum oven to evaporate rest solvent to collect
Ge.
In our present work, a hydrothermal reduction process
to prepare silicon nanomaterials by using silica sol or
silica aerogel have been proposed.32 Such hydrothermal
reduction process can retain the original size of the
precursor with porous structure formation. Inspired by
the similarity between the silicon and germanium in
chemical properties and synthesized strategy11, 14, 18, 33-36, we
further develop the hydrothermal reduction process to
prepare the macro-Ge powder by using commercial bulk
GeO2 precursor at a low temperature (200 °C). Compared
with traditional synthesis route, the hydrothermal
reduction of GeO2 strategy has several advantages. Firstly,
the reaction temperature for the hydrothermal reduction
process is much lower than that for typical thermal reduc-
tion reactions; on the other hand, the fabrication process
is one-step synthetic reaction using environmentally be-
nign and cost-effective approach, which allows to produce
Ge powder on a large scale. The obtained product
demonstrates a honeycomb-like macroscopic network
structure with a high tap density of 2.19 g cm-3. As anode
material in LIBs, the macro-Ge delivers not only a high
reversible capacity of 1350 mAh g-1 (corresponding to
~3000 mAh cm-3) at the current rate of 0.2 C (1 C = 1600
mA g-1) and a superior stability (64% capacity retention
even over 3500 total cycles at 1 C). Moreover, the
performance of both galvanostatic and asymmetric rate
also exhibits its excellent electrochemical performance.
For instance, the macro-Ge electrodes can be rapidly
charged to 1130 mAh g-1 in 3 minutes (20 C) and 890 mAh
g-1 in 90 s (40 C) using the constant discharge mode of 1 C.
Even at a high temperature (55 °C) environment, the
macro-Ge electrode still maintain over 1020 mAh g-1 at 1 C
for 300 cycles. When coupled with a commercial LiCoO2
cathode, a 3.5 V full lithium-ion battery with excellent
Characterization. The morphology of the reaction
product were characterized by scanning electron micros-
copy (SEM, JEOL-JSM-6700F), transmission electron mi-
croscopy (TEM) images were digitally acquired using a
field emission Hitachi H7650 TEM operated at 100 kV
with macro-Ge disperse on 200 mesh lacey-carbon copper
TEM grids. X-ray diffractometer (XRD) was performed on
a Philips X’ Pert Super diffract meter with Cu Kα radiation
(λ=1.54178 Å). The Brunauer- Emmett-Teller (BET) surface
area and Barrett-Joyner-Halenda (BJH) pore distribution
plots were measured on a Micromeritics ASAP 2020
accelerated surface area and porosimetry system. The tap
density of the Ge powder was measured with a Powder
Autotap Density Meter (JT-1, Chengdu Jingxin Powder
Analyse Instrument Co., LTD). First, a stainless steel cyl-
inder is filled with the as-prepared Ge powder of known
weight (marked as m) and mounted onto the Autotap
instrument. The Autotap is then programmed to auto-
mate 1000 taps with the rate of one tap per second. Then,
the volume of the powder (marked as V) is recorded and
the tap density (marked as TD) is calculated based on the
formula of TD=m/V and expressed in g cm−3. The tap den-
sity of the Ge powder are averaged based on three repeat-
ed experiments.
Electrochemical Measurement of the macro-Ge.
The electrochemical properties of the macro-Ge elec-
trodes were measured with coin-type half cells (2016 R-
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type) which assemble under an argon-filled glove box
(H2O, O2 < 1 ppm). Working electrode was prepared by
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mixing the macro-Ge material, super P carbon black and
sodium carboxymethylcellulose (Na-CMC) binder in a
weight ratio of 60:20:20 in water solvent. The slurry was
pasted onto a Cu foil and then dried in a vacuum oven at
80 °C for 12 h. The active material density of each cell was
determined to be 0.8-1.4 mg cm-2. Metallic Li sheet was
used as counter electrode, and 1 M LiPF6 in a mixture of
ethylene carbonate/dimethylcarbonate (EC/DMC; 1:1 by
Volume) and 5 wt% fluoroethylene carbonate (FEC) was
used as the electrolyte (Zhuhai Smoothway Electronic
Materials Co., Ltd (china)). Galvanostatic measurements
were made using a LAND-CT2001A instrument at room
temperature that was cycled between 0.005 V and 1.50 V
versus Li+/Li at different current density from 0.16 to 80 A
g-1. The AC impedance spectra were recorded with a
CHI660e electrochemical station by applying an AC volt-
age of 5 mV in amplitude in the frequency range of 0.01
Hz to 100 kHz at room temperature. The obtained spectra
were analyzed using ZView software. Except for the high
temperature measurement, all electrochemical perfor-
mance data were measured at room temperature.
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Figure 1. XRD patterns of products obtained at different steps
during preparation. a) Commercial GeO2 percursor. b) The
reacted specimens after hydrothermal reduction at autoclave
in 200 °C for 5 h. c) the resulting Ge product.
The phase of the precursor and products obtained at
different steps in the synthesis process were characterized
by X-ray diffraction (XRD). The XRD pattern of the GeO2
precursor is shown in Figure 1a. All the peaks can be in-
dexed to the pure hexagonal GeO2 (JPCDS 85-1515). After
hydrothermal reduction at autoclave in 200 °C for 5 h,
XRD analysis (Figure 1b) confirmed the main component
of MgO, Mg(OH)2 and Ge in the reacted specimens. A
trace amount of Mg2GeO4 was also detected from the
XRD pattern, which might be due to the reaction between
GeO2 and Mg(OH)2. The composite reacted specimens
were then immersed in a 1 M HCl solution for several
hours to remove the impurity. The XRD pattern of the
resulting Ge product is shown in Figure 1c. No GeO2 phase
was detected during or after the process. The peaks at
27.5º, 45.4º, 53.8º, 66.2º and 73.0º can be completely in-
dexed to the standard cubic phase Ge with calculated lat-
tice constant of a = 5.64 Å, which is consistent with that
of crystalline Ge (JCPDS No.89-2768; Fd3m, a = 5.6568 Å).
Further structural information is provided by the Raman
spectra in Figure S2. The strong Raman peak located at
~300 cm-1 further confirms the fabrication of crystalline
Ge.39 The yield of this Ge material is above 40%.
Electrochemical Measurement for Ge-LCO full cell.
The electrochemical analysis was carried out at room
temperature with a CR2016 coin-type full-cell using LCO
electrode as cathode in potential range between 2.50 and
4.20 V. The LCO cathode electrode was prepared by mix-
ing the commercial LCO material, carbon black, and
poly(vinyl difluoride) (PVDF) binder in a weight ratio of
8:1:1. N-Methyl-2-pyrrolidone (NMP) was used as the sol-
vent to form homogeneous slurry. The resulting slurry
was coated onto the Al foil and dried at 120 °C for 12h.
After drying and pressing, the Al foil was cut into dish (12
mm in diameter) with LCO activity material loading of 7-
8 mg cm-2. Prior to their assembly in full-cells, the macro-
Ge electrodes were electrochemical pre-lithiation in half-
cell configuration (vs. metal Li) for one cycle. The coin
full cell was assembling in a glove box filled with argon
gas similar to that of the Ge anode with LCO electrode as
cathode and the pre-lithiation Ge electrode as anode
3. RESULTS AND DISCUSSION
3.1. Hydrothermal synthesis of the macro-Ge pow-
der
Figure 2 is the scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images of the as-
prepared Ge materials. From the SEM image (Figure 2a),
one can see that the as-prepared Ge is composed of hon-
eycomb-like morphology with macroporous structure.
The interconnected feature of Ge is clearly exhibited in
the high magnification SEM image (Figure 2b) and TEM
image (Figure 2c). Nitrogen adsorption (Brunauer–
Emmett–Teller, BET, Figure S3) measurements indicate
that the specific surface area of the macro-Ge is 3.3 m2 g-1.
Barrett–Joyner–Halenda (BJH, Figure 2d and Figure S4)
analyses of the nitrogen adsorption curves indicate that
the macro-Ge possesses nanopores (several to tens of na-
nometers in diameter) and macropores (hundreds of na-
nometers in diameter). The results above demonstrate
Commercially available macro-sized GeO2 is employed
as the Ge source. As shown in Figure S1, the initial GeO2
particle is macro-sized structure with a rough and angular
surface. Similar to our previous reports32, 37 in hydrother-
mal synthesis of Si, the hydrothermal reduction process of
GeO2 is promoted by the active H intermediates, which is
similar to the Clemmensen reaction in traditional organic
synthesis38. In Clemmensen reaction, active H intermedi-
ates generated from Zn and HCl to transform methylene
from carbonyl. Afterwards, followed by removal of the
impurity via an acid etching process, macro-Ge particles
are obtained.
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that honeycomb-like macroporous Ge particles consist of curves overlap well with each other and decrease slightly,
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several interconnected porous and aggregation structure.
The tap density of the macro-Ge powder was estimated to
be 2.19 g cm-3, which is higher than that of the commer-
cial graphite anode (1.3 g cm-3) and the previous reports40
of macro-sized Ge anode (1.8 g cm-3).
indicating stability lithiation/delithiation process. The
reversible Li charge/discharge capacity and the Cou-
lombic efficiency of the macro-Ge electrode versus cycle
number are plotted in Figure 3b. The macro-Ge electrode
exhibits a relatively stable reversible lithium capacity of
~1180 mAh g-1 at 1 C after activation and retained a quite
high capacity value of ~1040 mAh g-1 after 300 cycles, with
88% capacity retention. The Coulombic efficiency in-
creases to 99% after several cycles and stabilizes at ~100%
in later cycles. Significantly, even more than 3500 deep
cycles, the specific capacity still remain a high value of
~755 mAh g-1 (64% capacity retention, Figure 3c), which is
more than two times of the theoretical capacity of com-
mercial graphite anode (372 mAh g-1). The fact that the
capacity only degrades 1.02% per 100 cycles over 3500 to-
tal cycles indicates the superior stability of the macro-Ge
electrode. On the other hand, in virtue of the high pack-
ing density of the electrode materials, our macro-Ge with
a tap density of 2.19 g cm-3 is also expected to have a high
volumetric energy density (Figure S6 and 3c). Moreover，
there are some undulations in the specific capacity values
during the cycling test, which are not effect neither the
average value of the specific capacities, nor the ability of
cycling. The slight capacity undulations might result from
the temperature variation both inside of the cell and the
measurement environment, which is similar to our previ-
ous reports.17,32
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Figure 2. Morphology characteristic of the macro-Ge by using
the hydrothermal preparation. a) Typical SEM image, b)
enlarged image and c) TEM image of Macro/nano-porous Ge.
d) Barrett–Joyner–Halenda (BJH) distribution of the resulting
macro-Ge powder.
b)
a)
3.2. Electrochemical Performance of macro-Ge An-
odes in Lithium Batteries.
In order to evaluate the honeycomb-like macro-Ge as a
potential anode material for lithium ion batteries, CR2016
type coin-cells with lithium foil as a counter electrode
were prepared. Figure 3 shows the cyclic performance of
the macro-Ge electrodes at current density of 1 C after
first five cycles activated at 0.2 C (1 C = 1600 mA g-1). The
voltage profiles of the macro-Ge at first five cycles during
the electrode activation process are shown in Figure S5.
The first discharge curve displays a long flat plateau about
0.5 to 0.1 V, which is corresponds to the lithiation process
of crystalline Ge to form different LixGe alloys.41 Subse-
quently, the discharge and charge curves show the char-
acteristic of amorphous Ge anode. The electrode delivers
a capacity of 1512 mAh g-1 and 1263 mAh g-1 for the first
discharge and charge capacity, which corresponds to a
initial Coulombic efficiency of 84 %. Afterwards, the mac-
ro-Ge electrode affords superior storage performance with
reversible capacity about 1350 mAh g-1 at 0.2 C, which is
very close to the theoretical capacity of Ge (~1600 mAh g-
1). Moreover, the volumetric capacity was determined to
be ~3000 mAh cm-3, which is much higher than that of
commercial graphite anode (483 mAh cm-3). Such a high
initial Coulombic efficiency and reversible capacity value
indicates high activation of the macro-Ge for lithiation
and delithiation. Figure 3a shows the typical voltage-
capacity profiles for the macro-Ge electrodes after 1, 50,
100, 200, 500, 1000, 2000 and 3000 cycles at 1 C. With the
cycle number increasing, the discharge/charge voltage
c)
Figure 3. Electrochemical performances of macro-Ge anode. a)
Typical galvanastatic discharge-charge curves of the cell with
Ge anode in the potential region of 0.005–1.5 V versus Li+/Li
at a current density of 1C after 0.2 C activation. b) cycling
property and coulombic efficiency of the cell with macro-Ge
anode at the constant current density of 1C after 0.2 C
activation. ■ as discharge capacity (Qd), ● as charge capacity
▲
(Qc) and
property at 1 C over 3000 cycles.
as coulombic efficiency (Qc/Qd). c) cycling
Moreover, the rate capability for the macro-Ge anode is
evaluated using galvanostatic charge-discharge measure-
ments with increasing the current density from the low
current density of 0.1 C to a high current density of 50 C
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Additionally, in the real LIB application, the current
over the potential window of 0.005-1.50 V. Typical charge
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and discharge profiles are shown in Figure 4a. In good
agreement with the typical charge/discharge behavior of
Ge, the discharge curves have a dominant plateau at
about 0.3 V and the charge plateau at about 0.5 V. With
increasing current density, the discharge voltage plateau
only decreases slightly, indicating the small polarization
of < 0.2 V. Even at a high rate of 20 C (32 A g-1), the
macro-Ge anode still exhibits a significant discharge volt-
age plateau at ~0.2 V. The cycling performance at
different rates is shown in Figure 4b. A high reversible
capacity of ~1500 mAh g-1 is achieved at the charge and
discharge current density of 0.1 C. The macro-Ge mass
loading is about 0.8-1.4 mg cm-2 and the capacity is about
1.2-2.1 mAh cm-2. Although further work is needed to meet
the commercial requirement of LIBs (3 mAh cm-2), our
areal capacity is much higher than that of many other
previous reports.26,41 In addition, as the current density
increases from 0.1 to 0.2, 0.5, 1, 2, 5, 10, 20, 30, 40 C, the
discharge capacity decreases slightly from 1500 to 1330,
1220, 1150, 1070, 880, 650, 420, 300 and 260 mAh g-1,
respectively. Even at a higher current density of 50 C (80
A g-1), the electrode is able to maintain a capcaity of ~ 200
mAh g-1 (Figure 4b). When the current density returns to 1
C , the capacity can be recovered to 1119.7 mAh g-1, which
is nearly 94% of the initial discharge capacity at 1 C,
indicating that the sample has a good electrochemical
reversibility and structural integrity.
density of discharge process is not necessarily same as the
charge.43 For example, electric vehicles and high power
output machine (high current transmission), correspond-
ing to the fast charging process of anode materials, is
thirst of commercial applications. A more detail investiga-
tion of the rate capability for charge/discharge inde-
pendently becomes more significant. Thus, we also evalu-
ate the rate performance of macro-Ge during fast charge
or discharge processes independently. The cells are dis-
charged at a constant current density of 1 C and then
charged at various current densities ranging from 1 C to
40 C after activation process (Figure 5a and Figure S7). As
the fast charge rate density increases from 1 to 2, 5, 10, 20,
40 C, the specific capacity decreases slightly from 1190 to
1160, 1149, 1146, 1131, and 890 mA h g-1, respectively. Before
the fast charge rate current density of 20 C (32 A g-1), the
electrode is able to maintain a stable specific capcaity of ~
1100 mAh g-1. Although, there is a slightly decrease when
the current density over 20 C, the capacity is still much
higher than that of the symmetrical rate performance
with the same current density of charging (Figure 4b).
When the charge rate returns to 5 C , the capacity can be
largely recovered to 1080 mAh g-1. These results imply that
the macro-Ge electrodes can be rapidly charged to 95% in
3 minutes and 74% in 90 s using the constant current
mode of 1 C. More detail about the voltage-capacity
curves is shown in Figure S8 in supporting information.
The fast charge process still maintains the typical kinetics
feature when compared to galvanostatic charge-discharge
process, indicating a facile charge transported process.
Such excellent fast charge rate performance may poten-
tially bring significant impact on the development of high
power electric instrument such as electric vehicles and
power output machine.
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a)
On the other hand, the fast discharge rate performance
is also investigated by setting the charge current density
constant to 1 C and then increasing the discharge rates
from 1 to 40 C after the same activation process (Figure 5b,
Figure S9 and Figure S10). As the fast discharge rate
density increases from 1 to 40 C, the specific capacity
decreases dramatically beyond 2 C, and only ~150 mAh g-1
can be delivered under a current density of 40 C. Figure
5c reveals that such fast discharge performance can reach
comparable capacity at low current density. However,
storage capacity at high current rates is not comparable to
the fast charge performance. It seems that the fast charge
rate behavior of such macro-Ge is different from that of
fast discharge rate behavior.
b)
Figure 4. a) The typical galvanastatic discharge–charge
curves of macro-Ge anode at different current densities from
0.1 C to 20 C, and b) the rate performance of the macro-Ge
anode.
3.3. Asymmetric Rate Performance of the macro-Ge
anodes
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trode at the lithiation and de-lithiation states is similar
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and low as ~2 Ω, demonstrating that the macroporous Ge
electrode at both lithiation and de-lithiation states exhib-
its high Li+ transportation. In addition, during the de-
lithiation process, the R2 value just has a slight reduction
from 15.3 Ω (at 0.05 V, lithiation state) to 6.64 Ω (at 1.2 V,
de-lithiation state). This suggests a stable SEI film formed
on the macroporous electrode during cycling. However,
R3 values are significantly different between the lithiation
and de-lithiation state. The R3 value of the macroporous
Ge electrode at the lithiation state (4.26 Ω) is about 30
times smaller than that of the de-lithiation state (124.2 Ω),
indicating much faster electron and Li-ion diffusion on
the interface of the Ge electrode during the de-lithiation
process (fast charge).
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3.4. High temperature (55 °C) cycle performance
Based on the critical safety issue for many applications
such as automobile and electronic devices in LIBs at high
temperature, next, we process to examine the high tem-
perature electrochemical performance of the macro-Ge
electrodes. Figure 6a shows the typical voltage-capacity
profiles of the as-synthesized macro-Ge electrodes at a
current density of 1 C between 0.005-1.50 V under the
measurement temperature of 55 °C. The received voltage-
capacity profiles are similar to the cycling performance
obtained at room temperature. Moreover, after a cycles of
300 times at 1 C, the capacity still maintain over 1020 mAh
g-1 (Figure 6b). The fairly stable capacity shows the
thermal stability of macro-Ge as an potential anode
material at high working temperatures.
Figure 5. a) The fast charge rate performance. b) The fast
discharge rate performance. c) Reversible capacity of macro-
Ge electrode at different fast charge rate and fast discharge
rate current density. d) Nyquist plots obtained for the macro-
Ge electrode in the fresh, discharge (lithiation) and charge
(de-lithiation) state. e) Equivalent circuit used to model the
impedance spectra in (d).
Table 1. Equivalent-circuit parameters obtained from
fitting the experimental impedance spectra.
Statement
R1(Ω) R2(Ω) R3(Ω) CPE1(F)a) CPE2(F)a)
Discharge
1.74
15.3
4.26
2.76E-5
5.12E-3
(lithiation)
Charge (de- 2.24
lithiation)
6.64
124.2
3.68E-4
5.62E-3
a) CPE1 and CPE2: represent the two resistors with
constant phase elements.
In order to understand of such an asymmetric rate per-
formance, an AC impedance measurement was employed.
Electrochemical impedance spectroscopy (EIS) of the
macro-Ge electrode is tested in the lithiation and de-
lithiation state after 10 cycles at 0.5 C rate are presented in
a Nyquist plot (Figure 5d). The plots consist of two semi-
circles in the high frequency region which can be at-
tributed to charge transfer processes, and a sloped line in
the low frequency region related to the mass transfer of
lithium ion. The first small semicircle represents the con-
tribution of the charge transfer between the electrolyte
and the SEI layer (as indicated by the green arrow in the
insert picture of Figure 5d), and the second semicircle
represents the charge transfer resistance between SEI
layer and the Ge particles. The electrochemical system
can be modeled by an equivalent circuit as shown in Fig-
ure 5e. Table 1 lists the simulated kinetic parameter,
where R1 is the ohmic resistance for Li+ transport in the
electrolyte, R2 is the SEI layer resistance, R3 is the charge
transfer resistance between SEI layer and the Ge particles.
Two resistors with constant phase elements (CPE1 and
CPE2) in parallel, and the Warburg impedance (W) de-
scribing the solid state diffusion of Li+ in the macroporous
Ge electrode. The R1 value of the macroporous Ge elec-
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Chemistry of Materials
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b)
Figure 7. a-b) SEM images, c) TEM image and d) XRD pat-
tern of the macro-Ge electrode after 1000 charge/discharge
cycles at 1 C.
3.6. Full Cells Assembly and Testing
It is important to further check whether high capacity
of anode material can really increase the energy density of
the full-cell battery. Then, we established a single Ge-
LiCoO2 full cell configuration where macro-Ge was ap-
plied as an anode and commercial LiCoO2 (LCO) as a
cathode. Based on a stable capacity delivery of 1300 mAh
g-1 (Figure S5) and 150 mAh g-1 (Figure S11) for a macro-Ge
anode and LCO cathode respectively, a maximum energy
density of ~470 Wh kg-1 (based on the total weight of an-
ode and cathode materials) can be obtained for the 3.5 V
full cell system. Figure 8a shows typical discharge/charge
Figure 6. a) Galvanostatic charge-discharge profiles of the
macro-Ge electrode cycled at the current density of 1 C and b)
cycle performance. The experiment was carried out at 55 °C
in a CR2016 coin-type half-cell before 25 cycles at 25 °C.
curves of the Ge-LCO full cell at 0.5 C (1 C = 170 mA g-1
)
LCO
with capacity limited by the LCO cathode (0.2 C activated
process is shown in Figure S12). The Ge and LCO
electrode capacities were 1.4 and 1.2 mAh cm-2,
respectively, with the N/P (negative electrode
capacity/positive electrode capacity) ratio of 1.1. As ex-
pected from their respective characteristic voltages plat-
form, the Ge-LCO full-cell shows a discharge voltage of
around 3.5 V. The highest reversible capacity of the full-
cell at 0.5 C is ~130 mAh g-1 (based on the weight of cath-
ode active materials) and energy density of ~398 Wh kg-1
(based on the total weight of anode and cathode active
materials). Furthermore, the full-cell displays excellent
cycle stability, with capacity retention of 91% (~364 Wh
kg-1) even over 100 cycles (Figure 8b and Figure S13).
3.5. Characters Studies of the macro-Ge Electrode
after Electrochemical Cycling
Finally, to demonstrate the stability in repetitive
charge-discharge cycles, the cell was disassembled after
1000 cycles in the Ar-filled glove box. The Ge electrode
was rinsed in dimethyl carbonate (DMC) solvent and
dried overnight before SEM, TEM and XRD observation.
The macro-Ge basically retained the morphology with
several aggregation and pulverization (Figure 7a-c), which
is similar to the previous reports44, 45. The porous and in-
terconnected structure is still clearly observed and pre-
served. The smooth surface may be caused by the solid
electrolyte interface (SEI) film deposition. XRD result
(Figure 7d) shows that the Ge powder after cycling is de-
termined to be amorphous. We believe that the honey-
comb-like porous structure of the Ge particles can ac-
commodate the volume expansion and maintain the
structure integrity during cycling.
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*Yongchun Zhu, E-mail: ychzhu@ustc.edu.cn;
*Yitai Qian, E-mail: ytqian@ustc.edu.cn.
a)
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2
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Author Contributions
Yongchun Zhu and Yitai Qian supervised the whole project.
Jianwen Liang and Xiaona Li conceived the idea, experimen-
tally realized the idea and co-wrote the paper. Xuedon Yan
guided the assembly, test and analysis of lithium ion battery.
Jianwen Liang, Xiaona Li, Zhiguo Hou and Tianwen Zhang
carried out the examples synthesis, characterization and data
analysis. All authors have given approval to the final version
of the manuscript. ‡These authors contributed equally.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
b)
This work was supported by the 973 Project of China (No.
2011CB935901), the National Natural Science Fund of China
(No. 91022033, 21201158).
ASSOCIATED CONTENT
Supporting Information: Additional data associated with
this article are included: Raman spectra, Nitrogen adsorp-
tion-desorption isotherms and BJH Pore size distribution
curve of the the macro-Ge powder; The voltage profiles, vol-
ume capacity and long-term cycling under the fast charge
and discharge rate of the macro-Ge anode; The typical gal-
vanastatic discharge–charge curves of commercial LCO and
Ge-LCO full cell; The discharge energy density of Ge-LCO
full cell. This information is available free of charge via the
Internet at http://pubs.acs.org.
Figure 8. a) Galvanostatic charge-discharge profiles of the
Ge-LCO full cell at 0.5 C. b) Cycling property and coulombic
efficiency of the Ge-LCO full cell.
ABBREVIATIONS
LIBs, lithium-ion batteries; SEI, solid–electrolyte interface;
LCO, LiCoO2
4. CONCLUSION
A facile hydrothermal reduction process at 200 °C has
been proposed for synthesis of the macro-Ge powder.
When serving as anode materials, the macro-Ge electrode
exhibits high reversible specific (1350 mAh g-1 at 0.2 C)
and volume capacity (~3000 mAh cm-3), high rate
performance and stable cycle performance, thus
demonstrating promising anode materials for next high-
energy density and high power LIBs applications. Even at
the high temperature (55 °C) environment, the Ge
electrode still maintain over 1000 mAh g-1 at 1 C for 300
cycles with superior thermal stability. Moreover, the
asymmetric rate with fast charge rate performance and
fast discharge rate performance also have been
highlighted in this study. The macro-Ge electrodes can be
repaidly charge to 1130 mAh g-1 in 3 min (20 C) and 890
mAh g-1 in 90 s (40 C) using the constant discharge mode
of 1 C. A lithium-ion full cell using the Ge electrode as
anode and commercial LCO as cathode is assemble and
shows a high energy density as ~398 W h kg-1 at 1 C with
capacity retention of 91% even over 100 cycles, showing
that macro-Ge might be a viable alternative to graphite
for higher energy density.
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Chemistry of Materials
A novel hydrothermal reduction GeO2 process was developed to synthesize of Ge particles with honeycomb-like
macroporous network structure. These macro-Ge powders have a high tap density (2.19 g cm-3) and, as electrodes,
show superior stability, excellent rate capacity and high temperature performance. When further coupled with
commercial LiCoO2 cathode, a ~3.5 V lithium-ion battery with excellent energy storage properties have been pro-
vided.
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