Preparation and Electrochemical Performance of a Honeycomb

Int. J. Electrochem. Sci., 12 (2017) 9619 – 9625, doi: 10.20964/2017.10.08
International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org
Short Communication
Preparation and Electrochemical Performance of a Honeycomblike Porous Anode Material
Xin-xiu Li1, Jiao Hou2, Xing-wei Wang2, Xiong-fei Liu1, and Chun-ping Hou2,*
1
China University of Mining and Technology Yinchuan College, Yinchuan Ningxia 750021, P.R.
China
2
Ningxia BOLT Technologies Co., Ltd., Yinchuan Ningxia, 750002, P.R. China
*
E-mail: hcp400@163.com
Received: 22 June 2017 / Accepted: 2 August 2017 / Published: 12 September 2017
Ground coffee is used to prepare a honeycomb-like porous anode material for Li-ion batteries. The
microstructure and morphology of the as-prepared samples are characterized by X-ray diffraction and
scanning electron microscopy. The material has a honeycomb-like porous morphology, a smaller
specific surface area than graphitized coke, and a relatively low degree of graphitization.
Electrochemical tests show as-prepared anode material to have good low-temperature performance,
large rate capability, and good cycling performance. At -30 °C, the material delivers a discharge
capacity of 52.1 mAh g-1, which is more than three times that of graphitized needle coke artificial
graphite anode material that is commercially available. Thus, it is suitable for high-power or energy
storage batteries.
Keywords: Electrochemical performance; Coffee; Honeycomb-like; Li-ion batteries; Anode material
1. INTRODUCTION
Lithium ion batteries (LIBs) have many advantages, and have been used in many types of
electronics [1-4]. Recently, much effort has been devoted to expanding their practical use in highpower and long-cycle applications, such as electric vehicles and dispersed energy storage, which
require LIBs to have an excellent life span, and be reliable and safe [5, 6]. Graphite is the most widely
used anode material due to its flat charge/discharge profiles, giving it a theoretical specific capacity of
372 mAh g-1 [7].
However, at low temperatures, graphite anodes suffer from severe limitations in rate capability
and cycling, and produce an abrupt decrease in the Li intercalation capacity at temperatures below -20
°C. At -30 °C, graphite completely loses its intercalation ability [8-10]. At low temperatures, graphite
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anodes suffer from high polarization that occurs during Li loading, a reduced Li+ ion solid-state
diffusivity, as well as to an increase in the ohmic resistance of the active material. Materials including
mesophase carbon micro beads [11], hard carbon [12, 13], soft carbon [14], and metallic oxides [15,
16] for high-performance anodes have been the focus of research, and have overcome the limitations
of graphite anodes. In addition, many porous structure designs have been used to improve greatly the
electrochemical performance of the electrode materials owing to their many pores and large specific
surface area, which keep active anode materials in full contact with the electrolyte and facilitate Li +
intercalation/deintercalation [17-22]. Tian et al [23] synthesized porous carbon anode material with
coffee grounds by an easy carbonization and activation approach, which obtained a specific surface
area of 352 m2 g-1.
In this study, a honeycomb-like porous (HP) anode material was synthesized from ground
coffee by a facile method and showed much better electrochemical performances compared with a
graphitized needle coke artificial graphite (CAG) anode material that is commercially available.
2. EXPERIMENTAL
Ground coffee was carbonized at 350 °C for 4 h in a pure N2 atmosphere to obtain the
precursor. This was ground to 200 mesh and coated uniformly with 8wt % high-temperature petroleum
asphalt. The coated precursor was carbonized at 900 °C for 7 h in a pure N 2 atmosphere to obtain the
target sample. A CAG sample with a diameter of approximately 16 μm and a carbon content of 99.98
wt% was supplied by Ningxia BOLT Technologies Co., Ltd. and used for comparison.
The structure of the synthesized materials was characterized by an X-ray diffraction (XRD;
XRD-7000S, Shimadzu, Japan) using Cu Kα radiation (λ=0.15423 nm). The surface morphologies of
the samples were observed by scanning electron microscopy (SEM; JSM-6700F, JEOL, Japan). The
specific surface area was characterized with a surface area analyser (NOVA 4000e, Quantachrome,
USA).
The electrochemical properties of the samples were characterised by first assembling CR2025
coin cells. The composite electrodes were prepared by mixing HP or the comparison sample (CAG)
with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 92:3:5 in Nmethylpyrrolidone (NMP) solvent to form a homogeneous slurry. Then, the mixtures were coated on a
copper foil and punched to disks. After drying under ambient conditions, the disks were further dried
in a vacuum oven at 120 °C for 12 h. Finally, the cells were assembled in an Ar-filled glove box
(LABSTAR 1250/750, MBRAUN) using lithium foil as the counter and reference electrode, a
polypropylene micro-porous film (Cellgard2400) as the separator, and 1 M LiPF6 in ethylene
carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1, v/v/v)
(GuangzhouTinci) as the electrolyte. A LAND batteries testing system (LAND CT2001A, Wuhan
Jinnuo, China) was used to perform the galvanostatic charge/discharge tests in the potential range of
0.03-2.0 V (vs.Li+/Li) at a rate of 0.1 C, 0.5 C and 1C (where 1 C = 372 mA g-1).
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3. RESULTS AND DISCUSSION
Figure 1. XRD patterns of HP and CAG.
The XRD patterns of HP and CAG are shown in Fig. 1. In contrast to the perfect graphite
crystal structure of CAG, the characteristic features of the as-prepared HP sample were typical of
poorly organized carbon with a very low degree of graphitization. The XRD diffractogram for the asprepared samples contained clear diffraction peaks at 2θ of 23°-25° and 42°-44°, which corresponded
to the (002) and (101) plane signals of hard carbon, respectively [12, 24]. Compared with diffraction
peaks at 26.4° and 44.5° of CAG, the shift to a lower 2θ indicated that HP had a larger interlamellar
spacing that allowed Li+ to diffuse more easily.
Figure 2. SEM images of the HP samples carbonized at (a) 350 and (b) 900 °C.
Fig. 2 presents SEM images of the HP samples carbonized at 350 and 900 °C. The honeycomblike porous structure appeared after carbonization at 900 °C, and no obvious porous structure was
produced at 350 °C. As the temperature and reaction time increases, the gases are released from the
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material, creating many micro- and nano-pores. Although the as-prepared sample had a honeycomblike porous structure, it also had a small specific surface area of 2.512 m2 g−1, which is different from
previous studies [17, 24-26]. The specific surface area of anode materials for LIBs is too high to be
used in practical application.
Figure 3. Electrochemical performance of HP and CAG. (a) Charge and discharge curves at a rate of
0.1C at room temperature, (b) rate capabilities at room temperature, (c) cycle performances at a
rate of 0.5C at room temperature, and (d) charge and discharge curves at -30 °C.
The charge-discharge curves of HP and CAG at a rate of 0.1C at room temperature are shown
in Fig. 3 (a). HP exhibited a lower discharge capacity than CAG (296.3 vs. 354.4 mAh g-1). However,
its charge/discharge plateaus were much higher than those of CAG, and these special characteristics
would suppress Li plating during electrochemical Li+ intercalation and de-intercalation, improving the
safety of an LIB.
Fig. 3 (b) shows the rate capabilities of HP and CAG at rates of 0.1, 0.5, and 1.0 C at room
temperature. Although HP delivered a lower discharge capacity of 296.3 mAh g−1 at a rate of 0.1 C, its
discharge ability increased as the discharge rate increased. For discharge at a rate of 1.0 C, HP
delivered a much higher discharge capacity of 129.1 mAh g-1, which was more than twice that of
CAG. It also exhibited better cycling performance, with a capacity retention of 85.45% at a rate of 0.5
C at room temperature after 60 cycles, whereas CAG only had a capacity retention of 40.4% (as shown
in Fig. 3 (c)).
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Shown in Fig. 3 (d), good low-temperature performance of an anode material is necessary for
electric vehicles and dispersed energy storage. At -30 °C, HP delivered a discharge capacity of 52.1
mAh g-1, which was more than three times that of CAG, and exhibited excellent low-temperature
performance. The discharge capacities of CAG with similar anode materials that were described in
literatures at low temperature are also listed in Table 1. The above-mentioned discharge capacity is far
beyond the results in previous literatures about untreated graphite anode. Nobili et al. [9] reported that
graphite without Sn coating delivered almost no capacity at -30 °C. Markevich et al. [10] found that at
-30 °C only 3.6% (13 mAh g-1) of Li was extracted from the fully charged graphite anode. Yaqub et al.
[27] revealed that pitch coated graphite electrode delivered a discharge capacity of 1.40 mAh g-1 with
high loading at -32 °C which caused by abrupt decrease of Li-ion diffusion. The DLi+ of graphite
electrode with low loading downgraded from 1.38×10-8 cm2 s-1 at room temperature to 4.79×10-13
cm2 s-1 at -32 °C. The improvement in electrochemical performance, including rate capability, cycle
stability, and low-temperature characteristics, were attributed to the larger interlamellar spacing and
honeycomb-like porous structure of HP. The larger the interlamellar spacing, the more easily Li +
diffuses and intercalates/de-intercalates in the HP anode material. Moreover, the honeycomb-like
porous microstructure has various advantages, such as short solid-state diffusion lengths and more
diffusion passageways for Li+, full contact with the electrolyte for the active material, reasonable
electrical conductivity of the porous carbon due to a well-interconnected wall structure, and a large
number of active sites for charge-transfer reactions, which are important factors in improving the
electrochemical performance of HP anode material [17, 25, 28].
Table 1. The discharge capacities of CAG with similar anode materials that were described in
literatures at low temperature.
Discharge capacity
(mAh g-1)
Temperature
(°C)
Ref.
NG
1
-30
[9]
Sn-graphite
94
-30
[9]
NG
13
-30
[10]
C-graphite
1.4
-32
[27]
CAG
15
-30
This work
HP
52.1
-30
This work
Sample
4. CONCLUSIONS
An HP anode material was prepared by a facile method from ground coffee. The sample had a
honeycomb-like porous morphology, a smaller specific surface area of 2.512 m2 g−1 compared with
CAG, a relatively low degree of graphitization. The HP anode material showed much better
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electrochemical performance than CAG, especially at low temperatures. Thus, HP is a potential anode
material for powerful LIBs, and is suitable for high-power applications or energy storage.
ACKNOWLEDGEMENTS
This work was supported by the Science and Technology Innovation Leader Program of Ningxia Hui
Autonomous Region, the Key Research Project of Ningxia Hui Autonomous Region (2016), and the
Key Research Project of Beifang Univesity of Nationality (grant 2015KJ30).
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