Highly reversible Na-ion reaction in nanostructured Sb2Te3-C composites as Na-ion battery anodes

Article abstract of DOI:10.1149/2.1161709jes

Sb₂Te₃ and its amorphous carbon-modified nanocomposite (Sb₂Te₃-C) are synthesized by simple solid-state synthetic methods, and their potential as anode materials for rechargeable Na-ion batteries (NIBs) is evaluated. Ex situ X-ray diffraction and high-resolution transmission electron microscopy clearly demonstrate the sodiation/desodiation reaction mechanism of the Sb₂Te₃ and Sb₂Te₃-C nanocomposite electrodes. In both electrodes, Sb₂Te₃ is converted into Na₃Sb and Na₂Te during Na insertion. However, Na₃Sb and Na₂Te in only the Sb₂Te₃-C nanocomposite electrode recombine into the original Sb₂Te₃ phase after full Na extraction. As a consequence of its interesting conversion/recombination reaction during sodiation/desodiation reaction, the Sb₂Te₃-C nanocomposite electrode exhibits a long cycle life with highly reversible gravimetric and volumetric capacity (373 mAh g^-1 and 765 mAh cm^-3 over 50 cycles) and fast rate capability (1 C: 391 mAh g^-1 and 802 mAh cm^-3; 2 C: 377 mAh g^-1 and 773 mAh cm^-3).

Full text of DOI:10.1149/2.1161709jes

A2056
Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
0013-4651/2017/164(9)/A2056/9/$37.00 © The Electrochemical Society
Highly Reversible Na-Ion Reaction in Nanostructured Sb2Te3-C
Composites as Na-Ion Battery Anodes
Ki-Hun Nam,a Jeong-Hee Choi,b,z and Cheol-Min Parka,z
aSchool of Materials Science and Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk 39177,
Korea
bBattery Research Center, Korea Electrotechnology Research Institute, Changwon, Gyeongnam 51543, Korea
Sb2Te3 and its amorphous carbon-modified nanocomposite (Sb2Te3–C) are synthesized by simple solid-state synthetic methods, and
their potential as anode materials for rechargeable Na-ion batteries (NIBs) is evaluated. Ex situ X-ray diffraction and high-resolution
transmission electron microscopy clearly demonstrate the sodiation/desodiation reaction mechanism of the Sb2Te3 and Sb2Te3–C
nanocomposite electrodes. In both electrodes, Sb2Te3 is converted into Na3Sb and Na2Te during Na insertion. However, Na3Sb
and Na2Te in only the Sb2Te3–C nanocomposite electrode recombine into the original Sb2Te3 phase after full Na extraction. As a
consequence of its interesting conversion/recombination reaction during sodiation/desodiation reaction, the Sb2Te3–C nanocomposite
electrode exhibits a long cycle life with highly reversible gravimetric and volumetric capacity (373 mAh g−1 and 765 mAh cm−3
over 50 cycles) and fast rate capability (1 C: 391 mAh g−1 and 802 mAh cm−3; 2 C: 377 mAh g−1 and 773 mAh cm−3).
© 2017 The Electrochemical Society. [DOI: 10.1149/2.1161709jes] All rights reserved.
Manuscript submitted April 11, 2017; revised manuscript received June 2, 2017. Published July 15, 2017.
cm−3) owing to its high density (6.24 g cm−3). Additionally, the elec-
tronic conductivity of Te (2 × 10−4 MS m−1) is considerably higher
than those of S (5 × 10−22 MS m−1) and Se (1 × 10−10 MS m−1).39
Despite the many advantages of Te, these materials have not been
widely researched as materials for high-capacity NIB electrodes.40,41
In this study, in order to overcome the limitations of the Sb and
Te electrodes, their compound Sb2Te3 and its nanostructured com-
posite (Sb2Te3–C) was synthesized by a simple high energy ball
milling (HEBM) process and investigated for use as anode materi-
als in NIBs. Furthermore, the electrochemical sodiation/desodiation
reaction mechanisms in Sb2Te3 and the Sb2Te3–C nanocomposite
were completely elucidated using ex situ X-ray diffraction (XRD) and
ex situ high-resolution transmission electron microscopy (TEM) based
on the results of the differential capacity plots (DCPs).
Although Li-ion batteries (LIBs) are the most common recharge-
able battery systems used in electric cars and mobile electronic devices
owing to their large energy density and high output voltage, the limited
reserves of Li considerably influence the price of LIBs.1–3 Therefore,
as an alternative to LIBs, the rechargeable Na-ion battery (NIB) is at-
tracting attention as a next-generation secondary battery system, due
to the abundance of Na in the earth and similarity to Li in terms of
chemical properties.4–8
The cathode materials used in the NIB system are the same as those
in LIBs, such as layered oxides and polyanions, while identifying the
anode materials for NIBs remains a challenge.4–8 The commercial
graphite anode used for LIBs is not applicable for NIBs because Na
ions have a larger radius than Li ions, and cannot intercalate into
the graphene layers.9–12 Therefore, although various carbon-based
materials such as hard carbon, expanded graphite, and hollow-type
carbon materials have been investigated as anodes for NIBs, they
generally exhibit a limited reversible capacity of ∼250 mAh g−1 and
poor rate capability.13–19
Experimental
Materials preparation.—Sb2Te3 was synthesized by the follow-
ing solid-state synthesis route: stoichiometric amounts of Sb (Sigma-
Aldrich, 99.9%) and Te (Sigma-Aldrich, 99.5%) powders were placed
in a hardened-steel vial (volume: 80 cm3) along with hardened-steel
balls (diameter: 3/8 in. and 3/16 in.) to achieve a ball-to-powder ratio
of 20:1 by weight. This vial was assembled in an Ar-filled glove box
and was tightly sealed to prevent oxidation of the samples; HEBM
(Spex-8000) was conducted under an argon atmosphere for 12 h. For
preparation of the Sb2Te3–C nanocomposite, the HEBM process was
carried out for an additional 6 h using mixtures of the synthesized
Sb2Te3 and amorphous carbon black (Super P, Timcal). On the basis
of the electrochemical performance of the nanostructured Sb2Te3–C
composite electrodes, the optimal amounts of Sb2Te3 and C were
revealed to be 60% and 40% by weight, respectively.
Recently, materials based on Na-alloy-forming elements such as
P, Sb, and Sn, have been proposed as possible candidates for NIB an-
odes, because they can react reversibly with large amounts of Na.20–28
However, despite their high capacities, these Na-alloy-based elements
show large volume changes during sodiation/desodiation reactions,
resulting in poor cycling behavior. To overcome this problem, nano-
structured or nanocomposite materials have been suggested. These
materials promote stable cycling behavior through the accommoda-
tion of the strain generated during cycling and enhance Na storage
kinetics because of their larger surface areas, shorter diffusion lengths,
and faster diffusion rates.29–32
Among Na-alloy-forming materials, Sb-based ones have been in-
vestigated as promising anode materials for rechargeable NIBs, due
to their high theoretical capacity of 660 mAh g−1 and the ability to
form various Sb–M binary compounds.33–38 Although Sb-based an-
odes have been observed to exhibit high capacities, they also suffer
from poor cycling behavior as a consequence of the large volume
variation accompanying the formation/release of Na3Sb. The group
16 elements S, Se, and Te can be used toward rechargeable NIB elec-
trodes, because these elements can form the Na-alloy phases Na2S,
Na2Se, and Na2Te, respectively. Therefore, several S- and Se-based
materials have also been investigated as high-capacity rechargeable
NIB electrodes. Although Te has a small theoretical gravimetric ca-
pacity (420 mAh g−1) and LD50 (5000 mg/kg, LD50 is the individual
dose required to kill 50% of a population of test animals), compared to
S (1672 mAh g−1, LD50: 14500 mg/kg) and Se (679 mAh g−1, LD50:
6700 mg/kg), it has a high theoretical volumetric capacity (2621 mAh
Materials characterization.—The Sb2Te3 and Sb2Te3–C com-
posite samples were characterized by XRD (DMAX2500-PC,
Rigaku, 2θ range 10–80◦), high-resolution TEM (HRTEM, JEM-
ARM200F/JEOL, operating at 200 kV), and energy dispersive X-ray
spectroscopy (EDS, attached to the high-resolution TEM). In addition,
to observe the structural and phase changes occurring in the Sb2Te3
and Sb2Te3–C composite electrodes during Na insertion/extraction,
ex situ XRD and ex situ HRTEM analyses were carried out. To avoid
exposure of the electrodes to air, they were laminated using polyimide
tape (Kapton) in an Ar-filled glove box.
Electrochemical measurements.—For electrochemical evalua-
tion, the electrodes were prepared by coating slurries on Cu foil sub-
strates. The slurries consisted of the active material powder (80 wt%),
conducting carbon black agent (Denka, 10 wt%), and polyvinylidene
zE-mail: dodgers@keri.re.kr; cmpark@kumoh.ac.kr
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Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
fluoride (PVDF, 10 wt%) binder, dissolved in N-methyl-2-pyrrolidone
A2057
(NMP) solvent. Coated slurries of each mixture were dried in a vac-
uum oven at 120◦C for 3 h. Each electrode was then pressed and
punched. In an Ar-filled glove box, coin-type electrochemical cells
were assembled using Na foil as the counter and reference electrodes,
glass microfiber filter separator (GF/D, Whatman), and electrolyte (1
M NaClO4 in ethylene carbonate/propylene carbonate solvent (1/1 by
volume) with 5% fluoroethylene carbonate. With the exception of the
rate capability tests, all coin cells were cycled at room temperature
with a Maccor automated test system from 0–3.0 V vs. Na+/Na, at a
current density of 50 mA g−1. Cyclic voltammetry (CV) profiles were
obtained by measuring the current-voltage (I-V) response at a scan-
ning rate of 0.10 mV s−1 (potential window: 0.0–2.5 V vs. Na+/Na)
using a ZIVE-MP2A electrochemical workstation (ZIVELAB). To
investigate the electrochemical reaction resistance of the electrodes,
electrochemical impedance spectroscopy (EIS) was conducted using
an impedance analyzer (ZIVE MP2A, WonATech, amplitude: 5 mV,
frequency range 100 kHz–10 mHz). The gravimetric capacity was
calculated based on the weight of the active materials, and the volu-
metric capacity was calculated by multiplying the tap density of the
active materials (Sb2Te3: 3.68 g cm−3, Sb2Te3–C composite: 2.05 g
cm−3, and hard carbon: 0.86 g cm−3), measured using a tap density
tester (BT-301, Bettersize).
Results and Discussion
Figure 1a shows the XRD pattern of the Sb2Te3 sample synthe-
sized by the HEBM route. All the peaks corresponded to the crys-
talline Sb2Te3 phase (JCPDS No. 71-0393) and no impurity phases
were detected. The layered crystal structure of rhombohedral Sb2Te3
¯
(S.G. R3m, a = 4.264 Å, c = 30.458 Å) is shown in Figure 1b, where
each layer has a triangular lattice with a single Sb/Te atom, and one
quintuple layer unit consists of five layers. These interesting quintuple
layers in Sb2Te3 enable facile Na-ion diffusion and storage within its
structure.
The voltage profiles and electrochemical data of Sb, Te, and the
synthesized Sb2Te3 electrodes are shown in Figures 2a–2c and 2d, re-
spectively. The Sb electrode showed a high first sodiation/desodiation
capacity of 792/555 mAh g−1 within the voltage range 0–3.0 V,
at a current density of 50 mA g−1 (Figure 2a). The first sodiation
capacity of Sb electrode exceeded its theoretical capacity, which
demonstrates that severe interface reactions between electrode and
electrolyte occurred. However, the desodiation capacity dramatically
decreased to 54 mAh g−1 after 5 cycles, showing very poor capac-
ity retention of 9.7%. This likely originated from the huge volume
change caused by the formation of the Na3Sb phase. The Te elec-
trode showed a first sodiation/desodiation capacity of 208/98 mAh
g−1 in the voltage range 0–3.0 V, at a current density of 50 mA g−1
(Figure 2b). Considering the theoretical gravimetric capacity of Te
(Na2Te: 420 mAh g−1), it showed very poor reversibility, which could
originate from the poor electrical conductivity of Te. However, the
Sb2Te3 electrode exhibited a high sodiation/desodiation capacity of
550/410 mAh g−1 with an initial coulombic efficiency of 74.5% (Fig-
ure 2c). From the tap density of synthesized Sb2Te3 (3.68 g cm−3),
the Sb2Te3 electrode was revealed to have a high volumetric sodia-
tion/desodiation capacity of 2024/1509 mAh cm−3. The theoretical
gravimetric capacity of Sb2Te3 is 513 mAh g−1, which demonstrates
that it had fully reacted with Na, taking into account the capacity
corresponding to the solid electrolyte interface (SEI) formation re-
action. Even though the capacity retention of the Sb2Te3 electrode
after 5 cycles is 40.2%, which is better than those of the Sb and
Te electrodes, the electrochemical performance is still unsatisfactory
(Figure 2d). The relatively poor capacity retention was originated
form the huge volume change caused by the formations of the sodi-
ated phases (Na2Te and Na3Sb) and the dissociation of Na2Te phase
in the organic electrolyte.
Figure 1. Synthesis and crystal structure of Sb2Te3. (a) XRD patterns of the
synthesized and standard (JCPDS # 71-0393) Sb2Te3. (b) Crystal structure of
the layered rhombohedral Sb2Te3 (green atoms: Sb, blue atoms: Te).
of the first and second cycles for the Sb2Te3 electrode (Figures 3a and
S1) showed two large and sharp peaks during the sodiation and des-
odiation reactions. On the basis of the DCPs, ex situ XRD analyses
were carried out at selected potentials and the results are shown in
Figure 3b. When the potential was decreased from the open circuit
potential (t0 in Figure 3b) to 0.9 V (t1 in Figure 3b), the Sb2Te3 phase
disappeared and only the Na2Te phase was seen, demonstrating that
Sb2Te3 transformed into Na2Te and amorphous Sb. In the fully sodi-
ated state at 0 V (t2 in Figure 3b), the main peaks of Na2Te and Na3Sb
phases, which are overlapped, increased. For further elucidation of
the Na2Te and Na3Sb phases in the fully sodiated state at 0 V, ex
situ HRTEM analysis was performed, confirming the small nanocrys-
talline Na2Te and Na3Sb phases (Figure 3c). These results show that
Sb2Te3 transformed into the Na2Te and Na3Sb phases after the full so-
diation reaction. In reverse, when the potential was increased to 1.5 V
(t3 in Figure 3b), only the Na2Te phase remained, which confirms that
Na was extracted from the Na3Sb. Additionally, in the fully desodi-
ated state at 3.0 V (t4 in Figure 3b), the XRD peaks were amorphized.
However, ex situ HRTEM results show that only the Sb and Te phases
remained (Figure 3d), and thus these phases do not recombine after
the desodiation reaction. Therefore, on the basis of the above ex situ
XRD and HRTEM results, the electrochemical sodiation/desodiation
The electrochemical sodiation/desodiation reaction mechanism of
the Sb2Te3 electrode is demonstrated in Figure 3. The DCPs and CVs
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Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
Figure 2. Electrochemical performances of the Sb, Te, and Sb2Te3 electrodes for NIBs. Voltage profiles for cycles 1–5 at a constant current density of 50 mA g−1
for the (a) Sb electrode, (b) Te electrode, and (c) Sb2Te3 electrode. (d) Comparison of the initial coulombic efficiencies and capacity retentions after five cycles for
the Sb, Te, and Sb2Te3 electrodes.
reaction mechanism of the Sb2Te3 electrode is suggested as
follows:
firmed by comparison of their EIS results (Figure 4b). The Nyquist
plots of the Sb2Te3 and Sb2Te3–C nanocomposite electrodes were ob-
tained at open circuit potential between 100 kHz and 10 mHz. Using a
relationship between Zꢀ and the square root of frequency (ω−1/2) in the
low frequency range, the Na-ion diffusion coefficients (D) of Sb2Te3
and Sb2Te3–C nanocomposite were determined as 5.864 × 10−15 and
4.921 × 10−14, respectively. The HRTEM image with correspond-
ing diffraction patterns (Figure 4c) and the scanning TEM image
with EDS mapping images (Figure 4d) reveal that the nanostructured
Sb2Te3–C composite was composed of uniformly dispersed small
Sb2Te3 nanocrystallites (size ∼10 nm) within the amorphous carbon
matrix.
To evaluate and compare the electrochemical performances of the
synthesized Sb2Te3–C nanocomposite as an anode material for NIBs,
the voltage profiles of Sb–C, Te–C, and Sb2Te3–C nanocomposite
electrodes are displayed in Figures 5a–5c. The voltage profiles of the
Sb–C nanocomposite electrode shows a first sodiation/desodiation
capacity of 601/447 mAh g−1 with an initial coulombic efficiency of
74.4% (Figure 5a), while the voltage profiles of the Te–C nanocom-
posite electrode (Figure 5b) show a first sodiation/desodiation ca-
pacity of 532/315 mAh g−1 with an initial coulombic efficiency of
59.2%. Although the Sb–C and Te–C electrodes showed high ini-
tial sodiation/desodiation capacities, the desodiation capacities af-
ter 10 cycles decreased to 381 and 268 mAh g−1, corresponding to
During sodiation:
Sb2Te3 → Na2Te + Sb → Na2Te + Na3Sb
During desodiation:
Na2Te + Na3Sb → Na2Te + Sb → Sb + Te
[1]
[2]
Figure 3e shows the crystallographic representation for the
conversion/non-recombination reaction of the Sb2Te3 electrode
(Eqs. 1 and 2) during the sodiation/desodiation process. Rhombo-
hedral Sb2Te3 was converted into hexagonal Na3Sb and cubic Na2Te,
and then they transformed into rhombohedral Sb and hexagonal Te
after the full desodiation reaction.
As mentioned in the introduction, various carbon-decorated nanos-
tructured composites have been investigated as alternative solutions
for Na-alloy-based anodes. The carbon in the composites hinders ag-
glomeration of the nanosized metal crystallites during cycling, yield-
ing enhanced electrical conductivity.29–32 Therefore, we synthesized
nanostructured Sb2Te3–C composite by an additional HEBM route us-
ing the Sb2Te3 and amorphous carbon (Super P) powders. The XRD
pattern of the nanostructured Sb2Te3–C composite (Figure 4a) con-
firms a single pure phase. This material exhibits dramatically reduced
charge transfer resistance when compared to Sb2Te3 which was con-
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Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
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Figure 3. Electrochemical characterization and reaction mechanism of the Sb2Te3 electrode during sodiation/desodiation. (a) DCP for the first and second cycles
of Sb2Te3 electrode. (b) Ex situ XRD results of the Sb2Te3 electrode during the first cycle. (c) Ex situ HRTEM results of the fully sodiated state at 0 V with
corresponding diffraction and FT patterns. (d) Ex situ HRTEM results of the fully desodiated state at 3.0 V with corresponding diffraction and FT patterns. (e)
Crystallographic phase change reaction mechanism of Sb2Te3 during the sodiation/desodiation process.
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Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
Figure 4. Synthesis and morphological characterization of the Sb2Te3–C nanocomposite. (a) XRD patterns of the synthesized and standard (JCPDS # 71-0393)
Sb2Te3–C nanocomposite. (b) Comparison of EIS measurement results for Sb2Te3 and the Sb2Te3–C nanocomposite. (c) Bright-field and HRTEM images
combined with corresponding diffraction patterns. (d) Scanning TEM images and the corresponding EDS mapping images of Sb, Te, and C.
capacity retentions of 85.2% and 85.1% of the first desodiation ca-
pacity, respectively. On the other hand, the Sb2Te3–C nanocomposite
electrode showed a highly reversible reaction showing a high first
sodiation/desodiation capacity of 643/410 mAh g−1 with a corre-
sponding initial coulombic efficiency of 63.8% (Figure 5c). Consid-
ering that the contribution of the HEBM-treated amorphous carbon
(40 wt%) toward the sodiation/desodiation capacity in the Sb2Te3–C
nanocomposite was 194/84 mAh g−1 (Figure 5d) for the first cy-
cle, the Sb2Te3–C nanocomposite electrode displayed a highly re-
versible reaction with Na. Additionally, the reversible desodiation
capacity of the Sb2Te3–C nanocomposite was 401 mAh g−1 after
10 cycles, indicating an extremely high capacity retention of 97.8%
of the first desodiation capacity. Additionally, an electrode com-
prising of simple mixture containing same composition (48 wt%
of Sb2Te3, 32 wt% of Super P, 10 wt% of Denka, and 10 wt%
PVDF binder) of the Sb2Te3–C nanocomposite electrode was manu-
factured and electrochemically tested (Figure S2). However, it showed
very poor electrochemical performances, which demonstrates that the
superior electrochemical performances of Sb2Te3–C nanocomposite
were attributed to the effect of nanostructured composite prepared by
HEBM.
The electrochemical sodiation/desodiation reaction mechanism of
the Sb2Te3–C nanocomposite electrode is also demonstrated in Fig-
ure 6. The DCPs and CVs of the first and second cycles for the
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Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
A2061
Figure 5. Electrochemical performances of Sb–C, Te–C, Sb2Te3–C nanocomposites, and HEBM-treated amorphous carbon electrodes for NIBs. Voltage profiles
for cycles 1–10 at a constant current density of 50 mA g−1 for (a) Sb–C nanocomposite, (b) Te–C nanocomposite, (c) Sb2Te3–C nanocomposite, and (d)
HEBM-treated amorphous carbon electrodes.
Sb2Te3–C nanocomposite electrode showed two large and sharp peaks
during both the sodiation and desodiation reactions (Figure 6a and S3),
which is similar to those of the Sb2Te3 electrode (Figure 3a). On the
basis of the DCPs, ex situ XRD and HRTEM analyses were carried
out at selected potentials and the results are shown in Figures 6b–6d.
During the sodiation reaction in the Sb2Te3–C nanocomposite elec-
trode (t1 and t2 in Figure 6b), the ex situ XRD analysis results were
similar to that of the Sb2Te3 electrode, confirming that Sb2Te3 in the
composite was converted into the Na3Sb and Na2Te. Additionally, the
ex situ HRTEM image at the fully sodiated state of 0 V also shows
the nanocrystalline Na3Sb and Na2Te phases, which indicates a con-
version reaction. In reverse, at the desodiated state of 1.5 V (t3 in
Figure 6b), the XRD peaks showed Na3Sb and amorphized Sb. At the
fully desodiated state of 3.0 V (t4 in Figure 6b), the XRD peaks were
amorphized. Therefore, to confirm the amorphized phases, additional
ex situ HRTEM analysis was carried out at this potential (Figure 6d).
The ex situ HRTEM image with the corresponding diffraction and
Fourier transformed (FT) patterns revealed homogeneously dispersed
small (size 5–8 nm) Sb2Te3 nanocrystallites within the conducting
amorphous carbon matrix, showing that the Na2Te and Na3Sb phases
transformed back into the original Sb2Te3 phase in the conducting
amorphous carbon matrix after the full desodiation process. Based
on ex situ XRD and ex situ HRTEM results, the sodiation/desodiation
reaction mechanism of the Sb2Te3–C nanocomposite electrode is sum-
marized as follows:
During sodiation:
Sb2Te3 (in Sb2Te3-C nanocomposite) →
Na2Te + Sb → Na2Te + Na3Sb
[3]
During desodiation:
Na2Te + Na3Sb → Na2Te + Sb → Sb2Te3
(in Sb2Te3-C nanocomposite)
[4]
The
crystallographic
representation
of
the
conver-
sion/recombination reaction of the Sb2Te3–C nanocomposite
electrode (Eqs. 3 and 4) are shown in Figure 6e. Rhombohedral
Sb2Te3 was converted into hexagonal Na3Sb and cubic Na2Te, which
then recombined into the original rhombohedral Sb2Te3 within
the nanocomposite, after the complete desodiation reaction. These
interesting conversion/recombination reactions in the compound
electrodes provide advantages in terms of the electrochemical behav-
ior, because the size of the compound crystallites in the composite
decreases gradually to 2–3 nm after several electrochemical cycles,
and then remains the same upon repeated conversion/recombination
reactions. In addition, several anode materials, such as ZnSe, GeS2,
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Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
Figure 6. Electrochemical characterization and reaction mechanism of the Sb2Te3–C nanocomposite electrode during sodiation/desodiation. (a) DCP for the first
and second cycles of the Sb2Te3–C nanocomposite electrode. (b) Ex situ XRD results of the Sb2Te3–C nanocomposite electrode during the first cycle. (c) Ex situ
HRTEM results of the fully sodiated state at 0 V with corresponding diffraction and FT patterns. (d) Ex situ HRTEM results of the fully desodiated state at 3.0
V with corresponding diffraction and FT patterns. (e) Crystallographic phase change reaction mechanism of the nanostructured Sb2Te3–C composite during the
sodiation/desodiation reaction.
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Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
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Figure 7. Electrochemical performances of the Sb2Te3–C nanocomposite electrode. (a) Gravimetric capacity vs. cycling number (cycling rate: 50 mA g−1) for
Sb2Te3, Sb2Te3–C nanocomposite, and commercial hard carbon electrodes. (b) Volumetric capacity vs. cycling number (cycling rate: 50 mA g−1) for Sb2Te3–C
nanocomposite and hard-carbon electrodes. (c) Voltage profiles at different C-rates. (d) Comparison of volumetric capacities at various C-rates for the Sb2Te3–C
nanocomposite and hard carbon electrodes.
SiP2, SnSb, and nanosized-metal oxides for LIBs also showed
similar conversion/recombination reactions after discharge/charge
reactions.42–47
Additionally, the rate capability of the Sb2Te3–C nanocomposite
electrode as a function of the C rate (1 C is defined as the full use
of the restricted desodiated capacity of 400 mAh g−1 in 1 hour) was
tested and compared with that of the commercial hard carbon electrode
(Figures 7c and 7d). The Sb2Te3–C nanocomposite electrode showed
high reversible gravimetric and volumetric capacities at very fast C
rates (1 C: 391 mAh g−1 and 802 mAh cm−3; 2C: 377 mAh g−1 and 773
mAh cm−3), which is much faster than that of commercial hard carbon
electrodes (Figure 7d). This high rate capability was attributed to the
short Na-ion diffusion paths and enhanced electrical conductivity by
the synthesis of small Sb2Te3 nanocrystallites (size ∼10 nm) within
the conducting amorphous carbon matrix.
The cycling performances of Sb2Te3, Sb2Te3–C nanocomposite,
and commercial hard carbon electrodes were compared at a cycling
rate of 50 mA g−1 (Figures 7a and 7b). Although the cycling per-
formance of the Sb2Te3 electrode was poor due to the large vol-
ume changes that accompany the formation/release of the Na3Sb and
Na2Te phases, the Sb2Te3–C nanocomposite electrode showed ex-
cellent cycling performance with a high reversible gravimetric (373
mAh g−1) and volumetric (765 mAh cm−3) capacity over 50 cy-
cles (Figures 7a and 7b). Notably, the volumetric capacity of the
Sb2Te3–C nanocomposite electrode was four-times higher than that
of the commercial hard carbon electrode. The excellent cycling per-
formance was attributed to the uniformly distributed approximately
10 nm-sized Sb2Te3 nanocrystallites in the buffering amorphous car-
bon matrix and their repeatable conversion (formation of Na3Sb and
Na2Te)/recombination (formation of Sb2Te3 within the carbon matrix)
reactions.
Conclusions
In summary, Sb2Te3 and Sb2Te3–C nanocomposites were syn-
thesized and electrochemical tests carried out for their use as
high performance NIB anodes. The Na insertion/extraction mech-
anisms of the Sb2Te3 and Sb2Te3-C nanocomposite electrodes
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A2064
Journal of The Electrochemical Society, 164 (9) A2056-A2064 (2017)
were demonstrated using various ex situ analytical tools. The
Sb2Te3 electrode displayed conversion/non-recombination reactions
during the sodiation/desodiation process, whereas Sb2Te3 in the
nanocomposite showed conversion/recombination reactions dur-
ing sodiation/desodiation reaction. The interesting conver-
sion/recombination mechanism of the Sb2Te3–C nanocomposite elec-
trode yielded good electrochemical performances with long cycle life,
accompanied by highly reversible gravimetric and volumetric capac-
ity (373 mAh g−1 and 765 mAh cm−3 over 50 cycles) and fast rate
capability (1C: 391 mAh g−1 and 802 mAh cm−3; 2C: 377 mAh g−1
and 773 mAh cm−3). Based on these results, Sb2Te3 and its nanocom-
posite should be considered as possible alternative electrode materials
for NIBs.
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This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea Government (MSIP)
(NRF-2014R1A2A1A11053057). This work was also supported by
the MSIP (Ministry of Science, ICT and Future Planning), Korea,
under the ITRC (Information Technology Research Center) support
program (IITP-2017-2014-0-00639) supervised by the IITP (Institute
for Information & Communications Technology Promotion).
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