Electrochimica Acta 285 (2018) 16e22
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Improving the electrochemical performance of LithiumeSulfur
batteries using an Nb-Doped TiO2 additive layer for the chemisorption
of lithium polysulfides
Wan-Ting Tsou, Cheng-Yu Wu, Hao Yang, Jenq-Gong Duh*
Department of Materials Science and Engineering, National Tsing-Hua University, 101, Kuang Fu Road, Sec. 2, Hsin-Chu, 300, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 January 2018
Received in revised form
26 July 2018
Accepted 29 July 2018
Available online 2 August 2018
This study presents a method to suppress the migration of lithium polysulfides in lithiumesulfur batteries by introducing a dual-layer electrode structure. Herein, unlike conventional methods of mixing the
polar additives with sulfur/carbon composites, melted sulfur mixed with mesocarbon microbeads are
used as the electrode and covered with an additive layer of Nb-doped TiO2/graphite composite via twostep blade coating. By doping TiO2 with Nb, electrical and lithium ion conductivity of TiO2 can be
increased, thereby enhancing the redox reaction kinetics. Most importantly, chemisorption of lithium
polysulfides to NbeTiO2 can effectively mitigate the shuttle effect, resulting in higher capacity and longer
cycle life. The electrode with the NbeTiO2 additive layer results in a 1st and 100th cycle specific capacity
of 1883 mAh g1 and 894 mAh g1, respectively, at 0.1 C (1 C ¼ 1675 mAh), indicating enhanced electrochemical performance as compared with that of bare lithium-sulfur batteries. X-ray photoelectron
spectroscopy (XPS) study was conducted to investigate the interaction between polysulfides and Nb
eTiO2. The results indicate that the NbeTiO2elayered electrode efficiently traps polysulfides on the
cathode and improves the rate capability, cycle performance, and specific capacity.
© 2018 Published by Elsevier Ltd.
Keywords:
Lithiumesulfur battery
Shuttle effect
Metaleoxide additives
Dual layer
Chemisorption
1. Introduction
Recently, given the profound environmental issues associated
with fossil-fuel-based transportation, replacing traditional cars
with electric vehicles or hybrid electric vehicles has become a
global effort. The rapidly increasing demand for high-power sources and energy storage systems has turned lithiumesulfur (LieS)
batteries into a potentially promising energy storage alternative
because the active material, sulfur, is low-cost and eco-friendly. In
addition, LieS batteries deliver a high gravimetric capacity of
1675 mAh g1 and a high theoretical specific energy of 2600 Wh
kg1 [1]. Although LieS batteries have many advantages, several
critical problems must be solved prior to wider commercial applications. The primary drawbacks include the insulating nature of
sulfur (5 1030 S cm1 at 25 C) [2], large volume expansion
(~80%) during the formation of Li2S, and dissolution of lithium
polysulfides (Li2Sx, 4 x 8) through the reduction of S8 or
oxidation of short-chain polysulfides in liquid electrolyte, which
* Corresponding author.
E-mail address:
[email protected] (J.-G. Duh).
https://doi.org/10.1016/j.electacta.2018.07.214
0013-4686/© 2018 Published by Elsevier Ltd.
ultimately leads to the shuttle effect [3], causing low utilization of
sulfur, poor Coulombic efficiency, and rapid LieS battery capacity
fading during the chargeedischarge process [4,5]. To overcome
these obstacles, mesoporous/microporous carbons [6,7], graphene
[8], carbon nanotubes [9], and carbon fiber [10] have been used to
encapsulate elemental sulfur. Carbonesulfur composite cathodes
enhance the electrical conductivity and the physically confined
LiPSs (lithium polysulfides) in the carbon pores or layers. However,
polar LiPSs still diffuse out of non-polar carbon after long cycling
owing to the lack of chemical interaction between LiPSs and carbon.
Hence, incorporating polar metal oxide or sulfides in new cathode
designs is a rapidly developing research area [11]. These polar
inorganic additives can be classified by the electrical conductivity
of conductors (e.g., Ti4O7 [12] and Mxene [13]), semiconductors
(e.g., TiO2 [14,15], Nb2O5 [16], and FeS2 [17]), and insulators (e.g.,
SiO2 [18], Mg0.6Ni0.4O [19], and Al2O3 [20]). These additives provide
active sites to absorb LiPSs and enhance surface electrochemical
kinetics, effectively reducing the shuttle effect and prolonging the
cycle life [21].
Previous studies have reported that Nb-doped TiO2 has many
electrochemical applications [22]. NbeTiO2 is selected as the
W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22
absorbent to anchor LiPSs owing to relatively high electrical and
better lithium ion conductivity as compared to other semiconductors [23,24]. Additionally, NbeTiO2 participates in the
electrochemical reaction, thereby providing additional capacity for
the LieS battery. Here, instead of mixing metal oxide additives with
sulfur and carbon, a dual-layer cathode is developed with an
NbeTiO2 coating on an MCMBesulfur layer via the doctor-blading
technique. The NbeTiO2 layer acts as a protective layer that physically and chemically prevents LiPSs from dissolving directly into
the electrolyte. Moreover, NbeTiO2 layer renders a pathway for
lithium ions to diffuse into the cathode. NbeTiO2 was synthesized
to fabricate dual-layer LieS cathodes, and the electrochemical behaviors were studied in detail. Furthermore, lithium ion diffusivity
was analyzed using AC impedance, and chemisorption was
analyzed via X-ray photoelectron spectroscopy (XPS).
17
2.5. Electrochemical measurements
2. Experimental
Electrochemical studies were conducted in 2230-type coin cells
in an Ar-filled glove box (O2 < 0.1 ppm; H2O < 0.1 ppm) using
lithium foil as the counter electrode. The sulfur fraction in MCMB
was determined to be 46.8 wt% via thermogravimetric analysis
(TGA). The sulfur content in the MCMB/S/NTO15 was calculated
using an average pristine electrode weight of 1.2e1.3 mg. The
electrolyte was a solution of lithium bis(trifluoromethanesulfonyl)
imide (1 M) in 1:1 v/v 1,2-dimethoxyethane (DME) and 1,3dioxolane (DOL) containing LiNO3 (1 wt%). The cells were charged
and discharged using an Arbin battery tester, and cycled between
1.6 and 2.6 V (vs. Li/Liþ) at 25 C ± 0.1 C. Electrochemical impedance spectroscopy (EIS) measurements were carried out over the
frequency range of 100 to 0.01 kHz at 2.15 V. The ac impedance and
CV measurements were obtained using an Ametek 263A electrochemical workstation.
2.1. Preparation of materials
3. Results and discussion
The MCMBesulfur composites were prepared by mixing and
melting sulfur (95% purity, SHOWA KAKO) and MCMB at 155 C for
5 h in a sealed glass bottle. NbeTiO2 was synthesized according to
procedures reported elsewhere [25], Nb0.15Ti0.85O2 (NTO15) was
selected for this study.
3.1. Cathode characterization
2.2. Preparation of cathodes
The pristine cathodes abbreviated as MCMB/S were prepared by
mixing 80 wt% MCMBesulfur composite powder, 10 wt% Super P,
and 10 wt% polyethylene oxide/polyvinylpyrrolidone (PEO/PVP) in
DI water to form a slurry that was cast on aluminum foil via doctor
blading. On the other hand, the slurry of 80 wt% NTO15s, 10 wt%
graphite, and 10 wt% PEO/PVP was cast on the MCMB/S cathodes to
construct the dual-layer structure. The dual-layer cathodes were
abbreviated as MCMB/S/NTO15. All electrodes were dried for
24 h at 40 C in a vacuum oven.
2.3. Preparation of XPS samples
To investigate the interaction between NTO15s and LiPSs, Li2S8
was selected as the representative. A 20 mM Li2S8 solution was
prepared by dissolving dry sulfur powder and Li2S in a molar ratio
of 7:1 in anhydrous tetrahydrofuran (THF) at 55 C in a glove box.
After stirring for 48 h, the Li2S8 solution with a redebrown color
was obtained. The solvent was dried out in a vacuum oven, and the
resulting solid was heated at 130 C in a glove box to remove any
residual solvent to produce Li2S8 powder. 50 mg NTO15s were
added to 5 mL Li2S8 solution and stirred for 48 h to obtain the
NTO15/Li2S8 solution. Then, the precipitated product was dried
under vacuum to obtain NTO15/Li2S8 powder for XPS analysis.
2.4. Characterization
X-ray diffraction (XRD) patterns were measured on a Bruker D2phaser using Cu-Ka radiation (l ¼ 1.5418 Å) at 30 kV. Sample
morphology was analyzed via field-emission scanning electron
microscopy (FE-SEM, JEOL JSM-7600F) equipped with energydispersive spectroscopy (EDX). High-resolution XPS (HR-XPS) performed with a ULVAC PHI Quantera SXM spectrometer with a
monochromatic Al-Ka X-ray source was used to investigate the
interaction between NTO15s and LiPSs. The XPS spectra were fitted
using GaussianeLorentzian functions and a Shirley-type background; C 1s peak at 285.0 eV was used to calibrate the spectra.
Fig. 1a shows the morphology of NTO15s synthesized via the
hydrothermal method, which are hollow spherical nanoparticles
with a mesoporous surface. Fig. 1b presents the XRD pattern of
NTO15s. As Compared to the spectrum of anatase TiO2, the XRD
pattern of NTO15s shifts to lower angles, suggesting that TiO2 is
doped with Nb atoms, resulting in lattice expansion (Nb5þ ¼ 0.64 Å,
Ti4þ ¼ 0.61 Å), as reported in literature [25,27]. Based on the XRD
pattern of NTO15s, a small trace of rutile TiO2 is found in NTO15s.
Fig. 2a presents the structure of the cell with the MCMB/S/
NTO15, comprising an Al foil, an MCMBesulfur layer, and an NTO15
layer. The SEM images in Fig. 2b and c shows that the MCMB/S/
NTO15 is covered by NTO15s over the entire surface of the
MCMBesulfur layer. Furthermore, EDS mapping results presented
in Fig. 2d clearly show that some NTO15s interfuse with the sulfurrich layer. The same binder and solvent are used in each layer,
suggesting that NTO15s possibly penetrate the MCMBesulfur layer
during the casting and drying process. Therefore, NTO15s can form
a thin protective layer and additives for MCMB/S-layer. Furthermore, the binder could rebind these two layers together to provide
better contact.
3.2. Interaction between NTO15s and the lithium polysulfide
The reaction between NTO15s and LiPS was examined by XPS.
L2S8 was prepared by combining sulfur powder with Li2S and
chosen as the LiPS representative. Fig. 3a and b presents the Ti 2p
spectra of NTO15s before and after stirring with L2S8. A small shift
(0.2 eV) to a lower binding energy in the Ti 2p spectrum of
NTO15s/L2S8, and a lower Ti3þ oxidation state are observed. The
same phenomenon is observed in the Nb 3d spectrum (Fig. 3d),
which shifts to a lower binding energy as compared to the NTO15s
spectrum (Fig. 3c). The peak of Nb4þ is evaluated at 206.5 (3d5/2)
and 209.3 eV (3d3/2) in the NTO15s/L2S8 spectrum. As shown in
Fig. 3e, the S 2p spectrum of Li2S8 exhibits two 2p3/2 peaks at 161.1
and 163.0 eV referred to terminal (ST1) and bridge sulfur (S0B). A 3:1
ratio between these two 2p3/2 peaks is consistent with the Li2S8
composition. In the S 2P spectrum (Fig. 3f), post stirring with
NTO15s, ST1 and S0B move to 161.8 and 163.4 eV, respectively,
indicating a shift (þ0.7 eV; þ0.4 eV) to a higher binding energy. The
XPS study represents oxygen atoms binding with Nb or Ti, which
may oxidize sulfur atoms in Li2S8 and form SeO bonds. The
replacement of lower electronegativity sulfur results in the shifting
peaks [26]. Based on Tao et al.'s DFT calculations [27], LiPSs tend to
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W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22
Fig. 1. The characterization of NTO15s: (a) SEM image of NTO15s; (b) the XRD pattern of NTO15s.
Fig. 2. Scheme of the dual-layer LiS battery and the morphology of MCMB/S and MCMB/S/NTO15: (a) schematic for the LieS battery with the MCMB/S/NTO15; SEM top views of
the (b) MCMB/S and (c) MCMB/S/NTO15; (d) EDS mapping results of the MCMB/S/NTO15.
W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22
19
Fig. 3. Demonstration of the interaction between the LiPS and NTO15s: XPS Ti 2p spectrum of (a) NTO15s and (b) NTO15s/Li2S8; Nb 3d spectrum of (c) NTO15s and (d) NTO15s/Li2S8;
S 2P spectrum of (e) Li2S8 and (f) NTO15s/Li2S8 (black line: experimental value, red line: sum of fitted value, and lines in other colors: fitted individual components). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 1
The XPS binding energies (eV) of NTO15s, NTO15s/Li2S8, and Li2S8 in Ti 2p, Nb 3d, and S 2p spectrum.
Spectrum
Ti
Sample
Peak 1
Peak 2
NTO15s
4þ
4þ
NTO15s/Li2S8
Nb
NTO15s
NTO15s/Li2S8
S
Li2S8
NTO15s/Li2S8
Ti 2p3/2
459.4
Ti3þ2p3/2
458.2
Nb5þ3d5/2
207.6
Nb4þ3d5/2
206.5
S1T 2p3/2
161.1
S1T 2p3/2
161.8
Ti 2p1/2
465.1
Ti4þ2p3/2
459.2
Nb5þ3d3/2
210.4
Nb5þ3d5/2
207.4
S1T 2p1/2
162.3
S1T 2p1/2
162.6
react with the bridging oxygen atoms instead of Ti. The XPS results
of this study are in accordance with Tao's observation, wherein no
TieS or NbeS binding formation is observed. Table 1 presents the
corresponding details.
3.3. Electrochemical behavior of MCMB/S/NTO15 cathodes
To evaluate the electrochemical performances of the MCMB/S/
NTO15, all batteries were measured between 1.6 and 2.6 V (vs. Li/
Liþ), then compared to the performances of the MCMB/S under the
same conditions. As shown in Fig. 4, the cells were activated by a
multichannel testing system under 0.05 C (1C ¼ 1675 mAh g1) for
3 cycles before the CV test, conducted at a sweep rate of
Peak 3
Peak 4
Sulfite
Sulfate
e
e
e
e
3þ
4þ
Ti 2p1/2
463.8
e
Ti 2p1/2
464.9
e
e
e
e
e
Nb4þ3d3/2
209.3
S0B 2p3/2
163.0
S0B 2p3/2
163.4
Nb5þ3d3/2
210.1
S0B 2p1/2
164.1
S0B 2p1/2
164.4
e
e
166.4
167.7
166.5
167.3
(2p3/2)
(2p1/2)
(2p3/2)
(2p1/2)
e
168.2 (2p3/2)
169.3 (2p1/2)
0.075 mV s1. For the cell with the MCMB/S/NTO15, two cathodic
peaks are observed at 2.35 and 2.02 V, corresponding to the
reduction of elemental sulfur to LiPSs (Li2Sx, 4 x 8) and longchain LiPSs to insoluble discharge products (Li2S2 and Li2S). Two
overlapped anodic peaks at 2.41 and 2.36 V are attributed to the
oxidation of LiPSs [28,29]. The peaks at 1.96 and 1.75 V belong to the
delithiation and lithiation of NTO15s, respectively [27]. For the cell
with the MCMB/S, a lower current intensity, a broader anodic peak
at 2.43 V, and a larger peak separation between the first redox
couple (DV ¼ 0.1 V) indicate higher polarization.
Fig. 5a presents the EIS results. The EIS studies show that the
resistance of the cell with the MCMB/S/NTO15 is significantly lower
than that of the cell with MCMB/S owing to the better electron-
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W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22
the MCMB/S/NTO15. The MCMB/S/NTO15 exhibits three longer
plateaus (2.6e2.3 V, 2.3e2.1 V, and 2.1e1.9 V), which are primarily
caused by the reactions as expressed through Eqs. (3)e(5) [33].
Fig. 4. CV curves of LiS batteries with the MCMB/S and MCMB/S/NTO15.
conductivity network [30]. The lithium ion diffusion coefficient (D)
can be calculated by the slope ðsÞ of the fitting lines of the Zre vs.
u0.5 diagram (Fig. 5b) at low frequency, using Eqs. (1) and (2) [31]:
Zre ¼ Rs þ Rct þ su0:5
D¼
R2 T 2
2A2 n4 F 4 C 2 s2
(1)
(2)
where u is frequency, R is the gas constant (8.314 J mol1 K1), T is
room temperature (298 K), A is the electrode area (1.37 cm2), n is
the number of electrons transferred in the reaction (n ¼ 2), F is
Faraday constant (96485 C mol1), and C is the molar concentration
of Li ions (1 M of LiTFSI in the electrolyte; C ¼ 0.001 mol cm3). The
lithium
ion
diffusion
in
the
MCMB/S/NTO15
(D ¼ 1.11 109 cm2 s1) is an order of magnitude faster than that
in the MCMB/S (D ¼ 4.83 1010 cm2 s1).
In Fig. 6a, the discharging curves depict two plateaus. The upper
plateau indicates that S8 is reduced to long-chain LiPSs, and the
lower plateau indicates that long-chain LiPSs are further reduced to
Li2S2 and Li2S [32]. It is observed that the additional capacity
contributed by NTO15s results in a sluggish slope at about 1.7 V in
S8 þ 2Liþ þ 2e / Li2S8
(3)
2Li2S6 þ 2Liþ þ 2e / 3Li2S4
(4)
Li2S4 þ 2Liþ þ 2e / 2Li2S2
(5)
More soluble Li2S8, Li2S6, and Li2S4 products are immobilized on
the MCMB/S/NTO15 than those on the MCMB/S and continue
participating in the charging and discharging process. To further
investigate the electrochemical performance improvement associated with the NTO15s additive layer, the lower-voltage capacity
(Qlow), referring to short-chain polysulfides is used to divide the
higher-voltage capacity (Qhigh), as a parameter [34,35]. The parameters can eliminate the error caused by the weight of cathodes.
The higher Qlow/Qhigh ratio indicates that LiPSs can be effectively
trapped on the cathode, contributing to the capacity. In Table 2, the
MCMB/S/NTO15 displays a higher Qlow/Qhigh ratio in the 30th and
100th cycle than that of the MCMB/S, suggesting the potential
suppression of the shuttle effect. The relatively large difference
between Qlow/Qhigh in the 30th and 100th cycle implies the rapid
dissolution of active materials into the electrolyte and/or conversion to insulated Li2S, which is in accordance with the XPS study,
wherein LiPSs bind with NTO15s.
Fig. 6b shows the rate capabilities of the cells, as measured by
increasing the charge/discharge current density from 0.1 to 2 C. The
MCMB/S delivers discharge capacities of 966, 664, 520 and
408 mAh g1 (vs. the active material mass of the electrode) at 0.1,
0.2, 1, and 2 C, respectively. By contrast, the MCMB/S/NTO15 achieves considerably higher discharge capacities of 1717, 1069, 815,
and 761 mAh g1 at 0.1, 0.2, 1, and 2 C, respectively. However, high
irreversibility of capacity is observed during the initial cycles. This
phenomenon is also observed during long term cycling under the
current rate of 0.1 C (Fig. 6c), wherein the lithiation of NTO15s
causes the decay [25]. The MCMB/S/NTO15 demonstrates an initial
capacity of 1883 mAh g1, which is maintained over 894 mAh
g1after 100 cycles. However, the MCMB/S exhibits a lower capacity
of 793 mAh g1 during the 1st cycle and a final capacity of 246 mAh
g1. Moreover, retention of the MCMB/S/NTO15 represented by
comparing the capacity at the 100th (Q100th) and 20th cycles (Q20th)
Fig. 5. (a) Nyquist plots of the cells with MCMB/S and MCMB/S/NTO15 at 2.15 V. (b) The fitting lines of Zre vs. u0.5 at low frequency region.
W.-T. Tsou et al. / Electrochimica Acta 285 (2018) 16e22
21
Fig. 6. Electrochemical performances of LieS batteries with MCMB/S and MCMB/S/NTO15: (a) charging and discharging profiles with the current density of 0.1 C at 30th cycle and
100th cycle; (b) rate performances from 0.1 to 2 C; (c) long term cycling capability at 1 C for 100 cycles.
Table 2
The parameter of capacity ratio of Qlow/Qhigh in the 30th and 100th cycle for the
MCMB/S and MCMB/S/NTO15.
Q 30th
high
Q 30th
low
Q 30th
low
Q 100th
high
Q 100th
low
Q 30th
high
MCMB/S
MCMB/S/NTO15
122.39
277.88
229.98
612.70
1.88
2.20
Q 100th
low
Q 100th
high
81.00
228.73
137.19
514.14
1.69
2.24
Additionally, the shifts and different oxidation states from the XPS
results indicate chemical interaction between NTO15s and LiPSs.
Moreover, the polysulfide shuttle effect can be effectively mitigated
by the chemisorption of LiPSs on the NTO15 additive layer. Therefore, by fabricating NbeTiO2 additiveelayer cathodes, the cyclability, c-rate performance, and specific capacity of LieS batteries
can be enhanced.
Acknowledgments
is significantly improved (78% vs. 53%). The improvement in
discharge capacity, cycle life, and rate capability is attributed to the
protective layer of NTO15s, which is an excellent absorbent to LiPSs.
The shuttle reaction can be suppressed owing to the chemical
attraction between LiPSs and NTO15s.
The authors would like to acknowledge the financial support
from Nice Success International Ltd, Hong Kong and the Ministry of
Science and Technology, Taiwan, under the Contract No. MOST 1062811-E-007-012.
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method and cast on the MCMBesulfur layer using the doctorblading method to construct an NTO15s additive layer. The
MCMBesulfur layer was physically confined to prevent direct
contact between the electrolyte and the sulfur-rich region.
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