Jeong et al.
Synthesis of Amorphous Carbon Materials for Lithium Secondary Batteries
carbons. Their electrochemical properties were evaluated
by using coin-type cells and compared with those of
graphite and undoped amorphous carbons.
under vacuum for 12 h and assembled into cells. The elec-
trolyte was 1.0 M LiPF6 in ethylene carbonate (EC)/ethyl
methyl carbonate (EMC) (3:7, v/v, Panax Etec) with
2 wt% vinylene carbonate (VC) as an additive. Coin-type
cells (CR2016) were assembled with a lithium metal
foil as a counter electrode in an argon-filled glove box.
Charge/discharge tests of the cells were conducted in the
voltage range between 0.005 and 1.5 V versus Li/Li+
2. EXPERIMENTAL DETAILS
2.1. Preparation and Characterization of
Phosphorus-Doped Amorphous Carbons
The PO(OC3H7ꢀ3 was used as a doping agent of car-
bon, which was in situ prepared by reacting POCl3 and
isopropyl alcohol (35 mL) for 1 h at room temperature.
PO(OC3H7ꢀ3 was incorporated into petroleum cokes (GS
Caltex, Korea) by simply dispersing petroleum cokes in
the isopropyl alcohol solution containing PO(OC3H7ꢀ3.
The mixing ratios of POCl3 and petroleum cokes were
10:90 and 20:80. The total weight of POCl3 and petroleum
cokes was 25 g. The resulting mixture was dried by
agitation at room temperature and then carbonized at
ꢀ
at 25 C. A constant current–constant voltage (CC–CV)
mode was used for the charge tests, whereas a constant
current (CC) mode was used for the discharge test. The
CC–CV tests were carried out in two steps: (1) charge to
5 mV under a constant current; and (2) subsequent charge
at 5 mV until the current reaches the cut-off value of
0.02 C (1 C = 372 mA h/g). During the first two cycles,
the charge (lithium insertion) and discharge (lithium de
insertion) were carried out at rates of 0.1 C and 0.2 C,
respectively. Subsequently, the C rate was fixed at 1.0 C
for the cycle performance tests. In this work, the third
cycle is defined as the first cycle of the cycle tests. For rate
capability tests, the charge rates were sequentially changed
(0.2 C → 0ꢄ5 C → 1ꢄ0 C → 2ꢄ0 C → 5ꢄ0 C) after the first
two cycles, whereas the discharge rate was fixed at 0.2 C.
The charge rate tests were examined under a CC mode
with cut-off voltages of 5 mV.
ꢀ
850 C under argon for 1 h to make phosphorus-doped
amorphous carbons. The amount of phosphorus element
in the carbons was analyzed by an inductively coupled
plasma-atomic emission spectroscopy (ICP-AES, Horiba
Ultima 2). The two phosphorus-doped amorphous carbons,
denoted as PD-AC-L and PD-AC-H, showed phosphorus
contents of 0.78% and 1.2%, respectively. Undoped amor-
phous carbons (UD-AC) were prepared by carbonizing
pure petroleum cokes under the same conditions to eluci-
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date the effect of phosphorus doping into the amorphous
3. RESULTS AND DISCUSSION
IP: 50.24.109.28 On: Fri, 23 Oct 2015 03:35:46
carbons. The characterization of the prepared carbons was
Elemental analysis of the undoped amorphous carbons
(UD-AC) and the two phosphorus-doped amorphous car-
bons (PD-AC-L and PD-AC-H) showed that the H/C
atomic ratios were 0.08 in the three carbons (Table I). It
was revealed from ICP-AES that PD-AC-L and PD-AC-H
contained 0.78% and 1.2% phosphorus within each amor-
phous carbon, respectively (Table I).
This indicates that ca. 40% phosphorus at the PD-AC-L
and ca. 30% phosphorus at the PD-AC-H remained after
carbonization when considering the initial amount of phos-
phorus within POCl3 used in the early mixing step. The
particle size of the carbons was not affected by phosphorus
doping, which was very similar to one another (Table I).
Figure 1 shows XRD patterns for the undoped amor-
phous carbon and the two phosphorus-doped amorphous
carbons. The (002) peaks with similar full width at half
maximum (FWHM) due to the stacking of carbon layers
were observed at approximately 2ꢂ = 26ꢀ in all the three
Copyright: American Scientific Publishers
carried out through scanning electron microscopy (SEM),
X-ray diffraction (XRD), and elemental analysis. SEM
images were collected with a field emission scanning elec-
tron microscope (JEOL JSM-7000F). XRD patterns were
taken in the 2ꢂ range of 10∼40 with a Rigaku Ru200B
diffractometer using Cu Kꢃ radiation. The nitrogen sorp-
tion tests were carried out with a Micromeritics TriStar to
get pore size distribution curves, which was determined by
Barrett–Joyner–Halenda (BJH) analysis from the adsorp-
tion branch. Elemental analysis was performed by a Perkin
Elmer 2400 Series II. The particle size of the prepared car-
bons was measured by a particle size analyzer (Microtrac
S3500).
2.2. Electrochemical Evaluation of Phosphorus-Doped
Amorphous Carbons
Electrodes were composed of 90 wt% amorphous car-
bons or graphite (mesocarbon microbeads, MCMB) as
an active material, 2 wt% carbon black (Super-P) as
a conducing agent and 8 wt% polyvinylidene difluoride
(PVDF) as a binder. To make a composite slurry, active
material and carbon black were dispersed in the binder
solution, which was prepared by dissolving PVDF in
N-methyl-2-pyrrolidone (NMP) solvent. The slurry was
coated on copper foil with a doctor blade and the car-
Table I. Results of composition and particle size analysis for the amor-
phous carbons.
Ratio of
POCl3 to coke
H/C atomic
ratio
Weight percent
of phosphorous (%) (ꢅm)
D50
Samples
UD-AC
PD-AC-L
PD-AC-H
0:100
10:90
20:80
0.08
0.08
0.08
0
0.78
1.2
5.5
5.6
5.6
ꢀ
ꢀ
bon electrodes were dried in a convection oven at 120 C
for 1 h. After that, they were pressed, dried at 100 C
J. Nanosci. Nanotechnol. 14, 7788–7792, 2014
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