nano-drilled multiwalled carbon nanotubes: characterizations and application for lib anode materials
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Nano-drilled multiwalled carbon nanotubes: characterizations and applicationfor LIB anode materials
Haryo S. Oktaviano,a Koichi Yamadab and Keiko Waki*ab
Received 17th July 2012, Accepted 8th October 2012
DOI: 10.1039/c2jm34684b
Nano-drilling of multiwalled carbon nanotubes (MWCNTs) has been conducted by taking advantage
of CoOx as the oxidation catalyst at a relatively low temperature of 250 �C. An increase of the ID/IGratio of Raman spectroscopy, together with TEM visualization, justifies the presence of hole defects on
the sidewalls, in comparison to the pristine and purified MWCNTs. Holes drilled on the basal plane of
MWCNTs could increase access into the inner core as observed by the N2 adsorption isotherm. Drilled
MWCNTs (DMWCNTs) show the largest quantity of functional groups of 5.4 � 10�3 mol g�1, which
suggests that our drilling method also introduces functional groups on the edges. We also tested the
electrochemical performance of our MWCNT samples as the anode material for lithium ion batteries.
After 20 cycles at a current density of 25 mA h g�1, the specific discharge capacities of pristine, purified
and DMWCNTs are 267, 421 and 625 mA h g�1, respectively. DMWCNTs exhibit the largest quantity
of Li extraction from the GIC and inner core as shown by the dQ/dV discharge profiles. This suggests
that increased access into the MWCNT inner core is proven to have positive effects on Li storage
properties.
1. Introduction
Owing to their high surface area, combined with good conduc-
tivity, carbon nanotubes (CNTs) are considered to be promising
materials for energy storage.1–6 The most prominent structural
characteristic of CNTs is their one dimensional, nanoscale
cylinder cavities enclosed by the graphitic sheet(s). The avail-
ability of the inner core is determined by the access into CNTs,
either through an opened end-cap or sidewall defects.7 In terms
of lithium ion batteries (LIBs), the Li storage sites for CNTs have
long been considered to be a graphitic intercalation compound
(GIC), inner tube and functional group related sites.8–11 A
comparison study of Li intercalation into open and closed single
wall carbon nanotubes (SWCNTs) has been conducted by Shi-
moda et al.12 The large reversible capacity observed on open
SWCNTs was attributed to an enhanced Li diffusion into the
interior of carbon nanotubes via the open ends and sidewall
defects. A similar consideration was also reported for short
multiwalled carbon nanotubes (MWCNTs) and vertically
aligned MWCNTs as Li anode materials.10,11 Both authors
related the enhancement of their Li storage properties, particu-
larly to the open access into the inner hole of tubes through
defects and edges formation. Such defects facilitate the Li
aDepartment of Energy Sciences, Tokyo Institute of Technology, 4529Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa 226-8502, Japan.E-mail: [email protected]; Fax: +81 459 24 5614; Tel: +81 45924 5614bCenter for Low Carbon Society Strategy, Japan Science and TechnologyAgency, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan
This journal is ª The Royal Society of Chemistry 2012
insertion and extraction, providing improved storage character-
istics. These results are in agreement with the previous MD
simulation carried out for nanotube-like structure in graphite,
revealing that the Li ion intrusion sites are to be open interstices
and/or void defects.13
Considering the important features of the CNT inner core for
various functional applications, many techniques have been
developed to open the holes on CNTs. Electron irradiation and
chemical oxidation treatments, whether by heating under a
certain gas atmosphere or treating in acids, have been intensively
studied to open the access into the inner surface of CNTs.14–20
However, such treatments have their own limitations. Although
the electron irradiation may produce well-controlled defects on a
specific location for surface nano-patterning application, this
technique will be difficult in preparing well-distributed open
holes on the bulk of CNTs. On the other hand, chemical
oxidation treatments may provide a plausible approach to make
well-distributed defects of CNTs, particularly due to the fact that
all samples have a uniform contact to the oxidation medium,
such as a gas atmosphere or liquid acid.17–20 Unfortunately, it is
difficult to form hole defects using such strong unlocalized
oxidation treatments, and the tube might deteriorate, which
would result in low electrical conductivity. It is important to note
that the gas oxidation treatment should be carried out at high
temperatures (above 500 �C), while acid-assisted oxidation
treatment is usually conducted by heating at a prolonged expo-
sure time. To overcome such drawbacks, our group has reported
the possibility of controlling the defect formation on the surface
of MWCNTs by solid-state reaction utilizing CoOx as an
J. Mater. Chem., 2012, 22, 25167–25173 | 25167
Fig. 1 Schematic procedure for drilling MWCNTs.
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oxidation catalyst at a relatively low temperature (250 �C),resulting in nano-drilled structures on the sidewalls of
MWCNTs.21
In this work, we further simplify the previous method21 by
excluding the pre-Ar treatment and directly proceeding to the
oxidation treatment in air. We attempt to characterize the nano-
drilled MWCNTs by TEM, Raman, BET and TDS to confirm
the openings of MWCNTs and to understand the defective
structures. We further conducted electrochemical studies on our
sample as the LIB anode material. From these results, we
correlate the presence of the defects with the enhancement of Li
storage capacities.
2. Experimental
Drilling multiwalled carbon nanotubes
Pristine multiwalled carbon nanotubes (MWCNTs) used for the
present study were received from Showa Denko KK, Japan
(VGCFX, diameter 15 nm, length approximately 3 mm). Prior to
the preparation of drilled structure MWCNTs (DMWCNTs),
pristine MWCNTs were purified by acid treatment as described
elsewhere.22 The purified MWCNTs were then impregnated with
10 wt% Co dispersed in ethanol. Subsequently, the as prepared
Co/purified MWCNTs underwent oxidation under air atmo-
sphere at 250 �C for 25 minutes. In order to obtain the
DMWCNTs, cobalt oxides were removed in a boiled concen-
trated nitric acid for 1 hour. Finally the sample was filtered,
washed and dried overnight.
Analysis
The morphologies of the MWCNTs were examined by FE-TEM
(JEOL, JEM-2010F) operated at 200 kV. Thermal analysis of
MWCNTswas conducted by TG-DTA (RIGAKU, Thermo-plus
EVO, TD-DTA/8120ST). The measurement was conducted by
heating in air (100 mL min�1) up to 800 �C at a rate 10 �Cmin�1.
Raman spectra were taken under ambient conditions using an
NRS-3000 Series (JASCO) with an Ar-ion laser beam at an exci-
tation wavelength of 514.5 nm. Nitrogen adsorption isotherms
were measured at 77 K on a BEL JAPAN, BELSORP-mini II
instrument. The samples were degassed at 120 �C under vacuum
for 3 h prior to the measurement. The specific surface area (SBET)
was calculated from the 0.05–0.3 P/P0 region of the adsorption
isotherm using the standard Brunauer–Emmett–Teller (BET)
method, while pore size distribution was derived by the Barrett–
Joyner–Halenda (BJH) method. The O-functionalities of the
MWCNTs were evaluated by thermal desorption spectroscopy
(TDS). TDS (ESCO, EMD-WA1000S) measurement was per-
formed by heating the sample up to 1100 �C at a rate of 30 �Cmin�1 under ultrahigh vacuum (base pressure: 2.0� 10�7 Pa) and
detecting the evolved gas using a quadrupole mass spectrometer
system (QMG422). The quantization of CO andCO2 gas evolved
from TDS measurement was conducted by calibrating the sensi-
tivity of the detector using a H-implanted Si substrate.
Electrochemical measurements
The lithium storage properties were evaluated using 2032 coin
cells; Li foil was used as the reference and counter electrodes. The
25168 | J. Mater. Chem., 2012, 22, 25167–25173
MWCNT samples were used as the working electrode. To
prepare the electrode material for the electrochemical test, a
mixture of 92% active material and 8% polyvinylidene fluoride
(PVDF) was dissolved in N-methyl-2-pyrrolidone (NMP) and
cast onto Cu foil (50 mm thickness). Typically 0.3 mg of active
material was coated on the Cu foil. After eliminating the solvent
in a vacuum oven at 120 �C, Cu foil was punched and hydrau-
lically pressed under a pressure of 40 MPa. All coin cells were
assembled in an Ar-filled glove box with less than 0.1 ppm
moisture. The electrodes were separated by a Celgard 2500
battery membrane soaked in a liquid electrolyte consisting of 1.0
MLiPF6, dissolved in a 3 : 7 (v/v) mixture of ethylene carbonate–
diethyl carbonate (EC/DEC; Tomiyama Chemicals). The gal-
vanostatic charge and discharge were controlled between 0.02
and 3.0 V at ambient temperature using a TOSCAT 3100 (TOYO
System Co. Ltd, Japan).
3. Results and discussions
The preparation of the multiwalled carbon nanotube (MWCNT)
drilled structures is outlined in Fig. 1. This method involves cobalt
deposition onto purified MWCNTs as the oxidation catalyst for
the solid-state reaction to formholes on carbonnanotube surfaces,
followed by cobalt removal in acid solution, resulting in drilling of
MWCNTs (DMWCNTs). The morphological structures on
MWCNT samples were examined by transmission electron
microscopy (TEM), as depicted in Fig. 2. Originally, the exterior
walls of the purifiedMWCNTs showed hole-free andwell-ordered
graphitic crystallites, suggesting that our purification method had
no effect on drilling the basal plane of MWCNTs.
After the oxidation of Co/purified MWCNTs, the TEM of
CoOx/DMWCNTs (Fig. 2b) shows how CoOx nanoparticles
(indicated by dark circular objects) drilled through the walls of
the nanotubes, resulting in defective structures with some
nanoparticles still residing in the holes. After Co removal by acid
digestion, the dark particles disappear and the opened hole
structures on the walls of MWCNTs remain (Fig. 2c). The
formation of hole defects indicates that carbon was partially
removed during the solid-state reaction. It can be expected that
This journal is ª The Royal Society of Chemistry 2012
Fig. 2 Transmission electron micrographs of MWCNT samples: (a) purified MWCNTs, (b) CoOx/DMWCNTs and (c) DMWCNTs.
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the size and depth of hole defects are highly related to the size of
Co catalysts and how the catalyst attached on the MWCNTs.
With the average size of about 4.9 nm for CoOx particles, we can
find many holes with similar dimensions. However, TEM visu-
alizations are only limited to some specific 2D projection loca-
tion. Controlling the size and preventing the agglomeration of
CoOx particles are the keys to control the defect structures.
Thermogravimetric analysis (TGA) was conducted to study
the oxidation profile of the cobalt assisted carbon oxidation for
defect formation (Fig. 3a). It is clear that Co/purified MWCNTs
completely oxidized at a much lower temperature than pristine
and purified MWCNTs. This demonstrates that the cobalt oxide
eased the oxidation of carbon by providing lattice oxygen in
promoting the C–O formation to produce CO gas as shown from
the reaction below.21,23
(i) 6CoO + O2 / 2Co3O4
(ii) Co3O4 + C / 3CoO + CO
Fig. 3 (a) TGA profiles and (b) Raman spectra obtained for MWCNT
samples.
This journal is ª The Royal Society of Chemistry 2012
The CoOx catalyst offers the possibility of forming holes on
the carbon surface at such a low oxidation temperature because
only carbon in contact with CoOx can be removed during the
solid-state reaction, leaving the other parts unaffected. TGA
curves also revealed that cobalt oxide particles were effectively
removed from DMWCNTs after acid washing, because of its
similar carbon oxidation profile to the purified MWCNTs. The
weight loss observed in the low temperatures range (<100 �C)could be attributed to the evaporation of the physisorbed water.
In the above mentioned temperature range, DMWCNTs have
faster weight loss than pristine and purified ones. This behavior
may be related to the presence of holes on the DMWCNTs,
which can absorb more water when in contact with the air.
Fig. 3b compares the Raman spectra of the three different
MWCNT samples. The Raman spectra show two strong peaks at
�1353 and�1581 cm�1, which are responsible for the disordered
sp2 carbon ‘‘D-band’’ and the graphite peak ‘‘G-band’’, respec-
tively.24 The D-band intensity relative to that of the G-band
(ID/IG) is often used as a measure of the nanotube quality. The
DMWCNTs showed the highest ID/IG ratio, followed by the
purified and pristine MWCNTs, respectively. This confirms that
DWMCNTs have the largest amount of graphitic edges and
disordered structures that come from the holes on the sidewalls.
The nitrogen adsorption isotherms of pristine, purified and
drilled MWCNTs are shown in Fig. 4. The isotherm of pristine
MWCNTs (Fig. 4a) is of type II in IUPAC classification and has
a sharp condensation step as the relative pressure approaches
unity,7,25,26 while the nitrogen isotherms of purified MWCNTs
and DMWCNTs are close to type IV with an obvious hysteresis
loop, indicating the main mesoporous characteristic of both
samples. Unlike the purified MWCNTs and DMWCNTs, pris-
tine MWCNTs demonstrate the largest saturated adsorption
amount at P/P0 ¼ 1, which corresponds to the multilayer filling
of large pores.26 Cheol-Min Yang et al. reported a similar
phenomenon on their pore structure study of purified and
unpurified SWCNTs.25 This suggests that pristine MWCNTs
may contain some kinds of metallic impurities, carbonaceous
agglomerates, and/or amorphous carbon forming incompact
structures, which are responsible for large quantity adsorption of
nitrogen gas at high relative pressure. We presume that our
pristine MWCNTs could be a promising gas adsorbent material,
however this is beyond the scope of this publication.
A clear understanding of porosity can be deduced through the
pore size distribution analysis, derived from the BJH method.
Fig. 5 reveals typical pore distribution among three different
MWCNT samples. It is shown that pristine MWCNTs have a
broad distribution of pore sizes, ranging in macropores and
J. Mater. Chem., 2012, 22, 25167–25173 | 25169
Fig. 4 N2 adsorption isotherms on MWCNT samples: (a) pristine
MWCNTs, (b) purified MWCNTs and (c) DMWCNTs.
Fig. 5 Pore size distribution of the MWCNT samples.
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mesopores. Surprisingly, after purification and drilling treat-
ment, pore sizes larger than 50 nm are significantly reduced. This
suggests that impurities and amorphous carbon, which are
responsible for large multilayer N2 filling structures at high
relative pressure (Fig. 4), were washed away during such
25170 | J. Mater. Chem., 2012, 22, 25167–25173
treatments. In purified MWCNTs and DMWCNTs, the absence
of macropore sites led to a shift of the pore size distribution to the
mesopores range. According to the TEM presented in Fig. 2, our
MWCNT has an outer diameter of about 15 nm and an inner
core diameter in the range of 3–8 nm. Since an inner core
diameter larger than 10 nm was not observed by TEM, the pore
size distribution observed in the range above 10 nm (Fig. 5) could
be related to the intertube mesopores, while pores with diameter
less than 10 nm were related to intratube mesopores. This is
supported by Inoue et al. who found two deviations of a high-
resolution as plot of the N2 adsorption isotherm of MWCNTs,
describing two different mesopore characteristics.27 The inter-
tube mesopores are formed between nanotube criss-crossing,
particularly due to the aggregation of individual MWCNTs. It is
noticeable that purified MWCNTs have a greater participation
of intertube mesopores than that of DMWCNTs. This could be
associated with the strong van der Waals forces of DMWCNTs,
consolidating the intimate contact between nanotubes allowing
for denser packing.28 On the other hand, the intratube mesopores
can be attributed to the inner core and defects on the sidewalls of
carbon nanotubes. A narrow pore size distribution was observed
with the peak at around 4 nm. As we expected, DMWCNTs have
shown the highest feature for such pore size, followed by purified
and pristine MWCNTs. It is evident that by drilling the sidewalls
of MWCNTs, we could create holes and access into the inner
core, in contrast to the pristine and purified ones. This has also
been confirmed by TEM observation (Fig. 2). The specific
surface area was calculated by the BET method (SBET), using
nitrogen adsorption data in the linear BET range (P/P0, 0.05–
0.3). SBET of pristine, purified and drilled MWCNTs are listed in
Table 1. SBET shows an increasing trend by applying purification
and drilling treatments as compared to the pristine MWCNTs.
Fig. 6 shows thermal desorption spectroscopy (TDS) of
DMWCNTs. TDS has been reported as one of the most
commonly used techniques to identify the surface functional
groups of CNTs, which decompose at different temperatures
releasing CO, CO2 and water.29 TDS offers accurate qualitative
and quantitative analysis of surface functionalities by heating the
samples up to a certain temperature and monitoring the evolved
gas by an online mass spectrometer. Generally, the groups of
phenols, carbonyls, quinones, and pyrone-type groups release in
the form of CO, while lactones and carboxylic acids release in the
form of CO2 (and H2O) under heating, with the exception that
carboxylic anhydrides release both CO and CO2. DMWCNTs
exhibit a broad distribution of O-functionalities on the surface,
but are mainly categorized as oxygen double bond functional
groups (carbonyl, quinone, anhydride, carboxyl, etc.). The
presence of a carboxyl hydrophilic group enhances the wetta-
bility nature, which is important for good dispersion in polar
solvents, allowing for the application of CNTs as advanced
energy materials, such as fuel cell electrocatalysts. It is widely
known that the oxidation of carbon not only results in graphitic
edges and disordered defects, but it also introduces functional
groups on the edges. The TDS quantization results demonstrate
that the O-functionalities and oxygen content are increased
noticeably after subsequent purification and drilling treatments,
when compared to pristine MWCNTs (Table 1). Owing to large
defects as observed by the highest value of ID/IG ratio,
DMWCNTs contained the largest amount of functional groups.
This journal is ª The Royal Society of Chemistry 2012
Table 1 The surface area and functional groups’ characteristics of MWCNT samples
Sample SBET (m2 g�1)P
CO (mol g�1)P
CO2 (mol g�1)
PO-functionalities
(mol g�1) O (wt%)Estimated Faradaiccapacitya (mA h g�1)
Pristine MWCNTs 270 1.3 � 10�4 6.9 � 10�6 1.4 � 10�4 0.2 4Purified MWCNTs 305.4 3.0 � 10�3 9.9 � 10�4 3.9 � 10�3 7.9 105DMWCNTs 313.8 4.3 � 10�3 1.1 � 10�3 5.4 � 10�3 10.4 144
a The estimated Faradaic capacity is calculated by the assumption that one functional group can react with one Li, as described by the reaction C]O +Li+ + e� 4 C–OLi.
Fig. 6 Thermal desorption profiles of DMWCNTs.
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To further study the hole effects on electrochemical perfor-
mance, we tested ourMWCNT samples as the anode material for
LIB. The electrochemical characterizations were conducted by
galvanostatic charge–discharge measurement of the as-prepared
coin cells. In this paper, we refer lithium extraction as the
discharge process and Li insertion as the charging process.
Fig. 7a presents the 20th cycle of charge–discharge curves for the
represented MWCNT samples at a current density of 25 mA g�1.
It shows that DMWCNTs have the largest charge–discharge
capacity. The specific discharge capacities observed for pristine,
purified and drilledMWCNTs at the 20th cycle were 267, 421 and
625 mA h g�1, respectively. Both purified MWCNTs and
DMWCNTs have a higher Li extraction capacity than the
theoretical capacity of graphite (372 mA h g�1, LiC6). This
suggests that purification and drilling treatment play an impor-
tant role in increasing the Li storage capacity. The cyclability for
both the specific discharge capacity and coulombic efficiency
(CE) is presented in Fig. 7b. The CE was derived from the ratio
of discharge to charge capacity on each cycle number. At the first
cycle, the CE of pristine, purified and drilled MWCNTs was 34,
38 and 40%, respectively. The rest of the capacity on the first
cycle is related to the irreversible processes, mainly contributed
from the reductive decomposition of the electrolyte and forma-
tion of Li organic compounds forming the so-called solid elec-
trolyte interface (SEI).2–4 Such a low initial CE characteristic has
been found in various nano-carbon materials because the large
surface area will result in a volume of SEI. Following subsequent
cycling, although the CE of all the samples increased and reached
a stable value, still some irreversible capacities remained. This
indicates that the SEI formed on the initial cycle may not
perfectly prevent the surface from further decomposition of
This journal is ª The Royal Society of Chemistry 2012
electrolyte. It is interesting to note that all MWCNT samples
reached a similar constant CE value of about 93%. This suggests
that the defects not only increase the specific reversible capacity,
but they also increase the electrochemical active surface area,
which is responsible for the increase of irreversible capacity.
All MWCNT samples exhibited a constant specific discharge
capacity over the cycling process (Fig. 7b). In contrast, graphene
materials have also been reported to have Li storage capacity
larger than graphite, however their extraction capacity fading
performances are poor even during a short cycling number.30–32
For instance, the oxidized graphene nanoribbons showed the
specific discharge capacity of 820 mA h g�1 at the first cycle,
which is then decreased to be about 550 mA h g�1 on the 15th
cycle.31 Due to these performance characteristics, CNTs are an
attractive Li anode material in comparison to the recent, widely
studied graphene materials. The stable and high conductivity of
CNTs comes from their rigid structure, which may be the reason
for the high cyclability.
So far many possible lithiation mechanisms in the carbon
materials have been proposed, such as Li intercalation in GIC
structure, Li hold in nanosize voids/cavities and Li bonded with
surface functionalities. Among them, GIC (graphitic intercala-
tion compound) is the most well known one, where the Li ions
are intercalated into the graphitic interlayers. Li insertion and
extraction occur at a very low potential (�0.1 V vs. Li+/Li) with
the maximum capacity of 372 mA h g�1 (LiC6). However, only
the GIC formation cannot explain the fact that many other
carbon materials have larger Li storage capacity than LiC6
value.8 To explain the large reversible capacity, the idea that Li
can be stored in small closed spaces in the carbon has been
proposed.33–36 One of the models has been constructed based on
the random stacking of small domain graphene layers, forming
small voids in between the carbon sheets (‘‘house-card’’
model).35,36 In spite of the different proposed void structures, all
explanations can be subscribed to the Li storage into a small
space in the carbon. Later, it has also been confirmed by low
temperature Li-NMR that lithium stored in the small space, like
cavities and voids, is in a metallic cluster form.37,38
Considering that carbon materials may also contain
functional groups on their edges and defects,8,9 another idea
based on the redox reaction of Li and functional groups has been
introduced. A reversible reduction and oxidation reaction
(C]O + Li+ + e� 4 C–OLi) between lithium ions and surface
functionalities, with the peak of 3.0 V vs. Li+/Li, has been
reported in the study of functional groups for high performance
batteries.39
In order to get better insight into the Li insertion and extrac-
tion process for our samples, the differential capacity (dQ/dV) of
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Fig. 7 The electrochemical characteristics of representedMWCNT samples as Li anode materials: (a) charge–discharge obtained from the 20th cycle of
the differentMWCNT samples at a current density of 25 mA h g�1, (b) cyclability performance of differentMWCNT electrode materials, (c) dQ/dV plot
of the 20th cycle charge (Li insertion) curves (inset figure is the closed-up dQ/dV plot of the 1st and 20th cycle charge curves, showing the SEI formation of the
DMWCNTs) and (d) dQ/dV plot of the 20th cycle discharge (Li extraction) curves for the represented MWCNT samples [black line: pristine MWCNTs;
red line: purified MWCNTs and blue line: DMWCNTs].
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the 20th cycle from the charge–discharge curves is plotted in
Fig. 7c and d. Fig. 7c shows that the insertion of Li mainly
occurred at a potential below 0.5 V vs. Li+/Li. For the first cycle,
a large and broad feature can be seen at 0.9 V, corresponding to
the SEI formation (inset, Fig. 7c). However, a small feature is still
left after continuing the cycles, which is responsible for the
irreversible capacities for each cycle, as we discussed before. For
the Li extraction, we found three features (Fig. 7d), which is in
agreement with the previous reports.8–12 The peak around 0.1 V is
associated with the extraction of Li ions from the GIC structure,
while the peak at about 1.2 V can be attributed to the Li
extraction out of the inner core of MWCNTs. A comparison
study of Li intercalation into open and closed SWCNTs has been
conducted by Shimoda et al.12 They found that Li extraction
from the inner core only occurs in open SWCNTs, as evidenced
by the presence of the peak at a similar potential on its dQ/dV of
the Li extraction process. Furthermore, the reversible capacity
increase with an increase of pores with sizes in the range of 3–8
nm is also shown in the BJH plot (Fig. 5). Thus, it seems
reasonable to consider that the peak at about 1.2 V is related to
the Li extraction from the inner core of MWCNTs. Nevertheless,
further investigation is required to clarify such Li species.
The last important feature on the Li extraction dQ/dV curve is
observed in the voltage range of 1.5–3.0 V. This pronounced
feature has been attributed to the extraction of Li ions bonded
with oxygen functionalities, due to some general facts that the
carbon materials are decorated by the functional groups.8–10
However, it was also reported that even after removing the
functional groups by vacuum annealing, such Li extraction in
the high potential range can still be observed.4 Nevertheless, the
origin of such a feature is not well discussed and is still unknown.
From the dQ/dV of Li insertion (Fig. 7c), it can be seen that there
is no obvious feature in the potential range of 1.5–3.0 V. This
indicates that a high potential is required to extract the Li
25172 | J. Mater. Chem., 2012, 22, 25167–25173
inserted at the potential below 0.5 V such that the feature cannot
be explained by the reversible redox reaction as reported
before.39 Additionally, it is clear for our MWCNT samples that
the amount of O-functionalities itself is too small to explain the
large storage capacity (Fig. 7a), as evidenced by the estimated Li
capacity derived from the TDS quantization of functional groups
(Table 1). We conclude that the corresponding extraction feature
is not related to the extraction of Li ions bonded with the
carbonyl groups as proposed in the literature.8–10
To explain a large Li storage capacity for conventionally
prepared carbon material by pyrolizing epoxy resins, the closed
volumes surrounded by a small domain size of graphene layers
was proposed to hold Li clusters.35–38 Due to the high tempera-
ture treatments, it is possible for the non-graphitizable carbon to
have closed cavities, surrounded with small but highly crystalline
graphene layers. The available space for storing Li metal clusters,
together with the less defective graphitic layers, may result in a
large reversible capacity with a relatively low potential of Li
insertion–extraction below 1.0 V. However, this is not the case
for our samples. For CNTs with long dimension rolled graphene
layers, it can be expected that a number of defects may form in
the plane due to the highly stressed structures.
Hence, we propose for CNTs that the Li might be trapped by
defective structures located in the plane of rolled graphene layers,
resulting in Li extraction at high potential. These in-plane defects
are probably located in the discontinuous graphene layers and not
accessible to the electrolyte, which may be formed in the prepa-
rationofCNTs and/or during the oxidation treatments, because of
the highly stressed structure of CNTs. Nevertheless, the origins of
the in-plane defects are not clarified. Further studies are required
to understand the defective structures with dangling bonds,
hydrogen terminated carbon (C–H), heptagon–pentagon rings
and other functional groups, to understand the mechanism and
improve the potential characteristics of Li insertion–extraction.
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4. Conclusions
In summary, we have prepared a drilled structure ofMWCNTsby
utilizing CoOx as the carbon oxidation catalyst. The Li storage
capacity, obtained from our drilled MWCNTs, is almost twice as
large as the theoretical LiC6 structure. This emphasizes that the
openings on the sidewalls increase the access into storage sites,
contributing to their large reversible capacity. While most of the
Li insertion occurs at a potential below 0.5 V, the large extraction
of Li at a high potential was explained by the idea of Li trapping
sites in graphene layers. In this study, not only a high reversible
capacity but also a better understanding of the Li storage mech-
anisms were achieved through the structural control of CNTs.
Acknowledgements
We acknowledge Prof. R. Kanno in the Department of Elec-
tronic Chemistry at Tokyo Institute of Technology (TIT) for
providing access for electrochemical measurements. We also
thank Dr K. Nakajima in the Department of Electronic Chem-
istry at TIT for Raman characterization. This work was con-
ducted in a collaborative research project between Tokyo
Institute of Technology and the Japan Science and Technology
Agency (JST). We greatly appreciate financial support by JST.
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