S-Doped TiSe2 Nanoplates/Fe3O4 Nanoparticles Heterostructure

Article abstract of DOI:10.1002/smll.201702181

2D Sulfur-doped TiSe₂/Fe₃O₄ (named as S-TiSe₂/Fe₃O₄) heterostructures are synthesized successfully based on a facile oil phase process. The Fe₃O₄ nanoparticles, with an average size of 8 nm, grow uniformly on the surface of S-doped TiSe₂ (named as S-TiSe₂) nanoplates (300 nm in diameter and 15 nm in thickness). These heterostructures combine the advantages of both S-TiSe₂ with good electrical conductivity and Fe₃O₄ with high theoretical Li storage capacity. As demonstrated potential applications for energy storage, the S-TiSe₂/Fe₃O₄ heterostructures possess high reversible capacities (707.4 mAh g^?1 at 0.1 A g^?1 during the 100th cycle), excellent cycling stability (432.3 mAh g^?1 after 200 cycles at 5 A g^?1), and good rate capability (e.g., 301.7 mAh g^?1 at 20 A g^?1) in lithium-ion batteries. As for sodium-ion batteries, the S-TiSe₂/Fe₃O₄ heterostructures also maintain reversible capacities of 402.3 mAh g^?1 at 0.1 A g^?1 after 100 cycles, and a high rate capacity of 203.3 mAh g^?1 at 4 A g^?1.

Full text of DOI:10.1002/smll.201702181

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S-Doped TiSe2 Nanoplates/Fe3O4 Nanoparticles
Heterostructure
Jun Yang, Yufei Zhang, Yizhou Zhang, Jinjun Shao, Hongbo Geng, Yu Zhang,
Yun Zheng, Mani Ulaganathan, Zhengfei Dai, Bing Li, Yun Zong, Xiaochen Dong,*
Qingyu Yan,* and Wei Huang*
(SIBs) due to its safe and stable working
2D Sulfur-doped TiSe2/Fe3O4 (named as S-TiSe2/Fe3O4) heterostructures
are synthesized successfully based on a facile oil phase process. The Fe3O4
nanoparticles, with an average size of 8 nm, grow uniformly on the surface
of S-doped TiSe2 (named as S-TiSe2) nanoplates (300 nm in diameter and
15 nm in thickness). These heterostructures combine the advantages of both
S-TiSe2 with good electrical conductivity and Fe3O4 with high theoretical Li
storage capacity. As demonstrated potential applications for energy storage,
the S-TiSe2/Fe3O4 heterostructures possess high reversible capacities
(707.4 mAh g−1 at 0.1 A g−1 during the 100th cycle), excellent cycling stability
(432.3 mAh g−1 after 200 cycles at 5 A g−1), and good rate capability (e.g.,
301.7 mAh g−1 at 20 A g−1) in lithium-ion batteries. As for sodium-ion bat-
teries, the S-TiSe2/Fe3O4 heterostructures also maintain reversible capacities
of 402.3 mAh g−1 at 0.1 A g−1 after 100 cycles, and a high rate capacity of
203.3 mAh g−1 at 4 A g−1.
plateau potential (about 1.5–1.75 V vs
Li/Li+), good structural stability, long
lifespan, excellent rate capability as well
as the low processing cost and simplicity
of syntheses.[9,10] However, TiO2 possesses
a low theoretical capacity (335 mAh g−1),
which is less than that of commercial
graphite (372 mAh g−1).[11] Moreover, the
reported capacity of TiO2 is sometime far
below its theoretical value. For example,
Wang and co-workers reported a reversible
capacity of 237 mAh g−1 after 100 cycles at
a current rate of 1 C for the multishelled
TiO2 hollow microspheres;[12] and anatase
TiO2 nanosheets with the first charge
capacity of 169 mAh g−1 at a current rate
of 5 C can be achieved as reported by Lou
and co-workers.[13] This is possibly due
to the low ion diffusion coefficient and
In recent years, titanium-based compounds have attracted great
interests for various applications including thermoelectronics,[1]
charge density wave devices,[2] intelligent solar control,[3] photo-
thermal cancer therapy,[4] catalysis,[5,6] and energy storages.[7,8]
Among them, TiO2 has been explored as a promising electrode
material for lithium-ion batteries (LIBs) or sodium-ion batteries
intrinsic low electrical conductivity,[14] which limits its appli-
cation in LIBs and SIBs. Similar challenge is encountered for
TiS2 with a low theoretical capacity of 240 mAh g−1 due to the
insertion mechanism.[15,16] Nevertheless, it is reported that
TiS2 suffers from the irreversible changes at the surface and
distorted structures to form Li dendrites, resulting in rapid
capacity fading upon cycling, especially in the high potential
region.[17,18] On the other hand, modifications of the typical
anode materials, such as graphene,[19–21] transition-metal
dichalcogenides,[22] and MXenes[23] have been successfully iden-
tified, and their improved performances as an anode material
of Li/Na-ion batteries are expected to be explored.
Transition metal selenide, such as titanium selenide, shows
much lower optical bandgap and much higher electrical con-
ductivity compared to their sulfide counterparts or oxide.[24]
TiSe2 can be a good candidate in LIBs and SIBs because of the
relatively high electrical conductivity and theoretical capacity
(520 mAh g−1 based on conversion reaction). In addition,
sulfur doping in selenides has been demonstrated to effectively
increase the charge carrier concentration and electrical conduc-
tivity due to the bonding defects induced by sulfur doping.[25–27]
These are the remarkable advantages for sulfur doped TiSe2
as an active material for high-rate and high-capacity lithium
storage applications, which has not been explored. In addition,
further increasing the Li storage capacities of TiSe2-based elec-
trodes is desired as the theoretical capacity of TiSe2 is relatively
lower than many transition-metal oxide anodes.[28–31] Thus,
J. Yang, Dr. Y. F. Zhang, Dr. Y. Z. Zhang, Dr. J. Shao, Prof. X. Dong,
Prof. W. Huang
Key Laboratory of Flexible Electronics (KLOFE) and Institute
of Advanced Materials (IAM)
Jiangsu National Synergetic Innovation Center
for Advanced Materials (SICAM)
Nanjing Tech University (NanjingTech)
30 South Puzhu Road, Nanjing 211816, China
E-mail: iamxcdong@njtech.edu.cn; iamwhuang@njtech.edu.cn
Dr. H. Geng, Dr. Y. Zhang, Dr. Y. Zheng, Dr. M. Ulaganathan,
Dr. Z. Dai, Prof. Q. Yan
School of Materials Science and Engineering
Nanyang Technological University
Singapore 639798, Singapore
E-mail: alexyan@ntu.edu.sg
Dr. B. Li, Prof. Y. Zong
Institute of Materials Research and Engineering A*STAR
(Agency for Science, Technology and Research)
2 Fusionopolis Way Innovis #08-03
Singapore 138634, Singapore
DOI: 10.1002/smll.201702181
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a capping ligand, which is supposed to bind
strongly to Se.[33] The S-doping is introduced
by addition of the sulfur powder during the
synthesis of TiSe2 nanoplates. The XRD pat-
tern (Figure 1a) of S-TiSe2 reveals that the
diffraction peaks can also be indexed to hex-
agonal TiSe2 (JCPDS no. 71-4828). Compared
to the pure TiSe2, the XRD pattern of S-TiSe2
reveals the diffraction peaks with a slight dis-
placement to higher angles. This right shift
is probably due to the smaller ionic radius
of S atoms. In the Raman spectroscopy, both
nanoplate samples exhibit two peaks, which
are related to the composition-dependent
vibration modes (Figure S3a, Supporting
Figure 1. a) XRD pattern of the TiSe2 and S-TiSe2, b) XPS high-resolution spectrum of the
S 2p of S-TiSe2.
TiSe2 can be considered as a framework that incorporates a sec-
ondary phase with high specific capacities to form heterostruc-
tures. In this regard, Fe3O4 is a good choice to couple with TiSe2
due to its high theoretical capacity (e.g., 926 mAh g−1), natural
abundance, and environmental benignity.[32] Combination of
these two materials with a rationally designed heterostructure
and the complementary features may lead to improved Li/Na
storage properties.
Information). The observed peaks from pure TiSe2 at 185 and
354 cm−1 are attributed to the out-of-plane vibrational mode

A1g and in-plane vibrational mode E2g of Ti Se bond, respec-
tively.[34] The S-TiSe2 features the evident band broadening
and the two peaks shift to lower frequency range, implying a
possible structural disorder due to the S-doping.[35] The UV–vis–
NIR absorption of the TiSe2 and S-TiSe2 samples is shown in
the inset of Figure S3b (Supporting Information). From the
spectroscopy, the optical bandgaps of TiSe2 and S-TiSe2 can be
calculated to 0.38 and 0.31 eV, respectively (Figure S3b, Sup-
porting Information). The bandgap narrowing of the S-TiSe2
may lead to higher electrical conductivity.[36–38] The chemical
state of the S-TiSe2 sample is investigated by X-ray photo-
electron spectroscopy (XPS). The high-resolution Ti 2p XPS
spectrum (Figure S4a, Supporting Information) exhibits
the major Ti 2p3/2 peak at 457.8 eV and the Ti 2p1/2 peak at
464.1 eV, which reveals the predominant valence state of the Ti
is Ti4+.[39] As shown in Figure S4b (Supporting Information),
the 3d peaks of selenium at around 54.0 eV can be divided
into two well-defined peaks: Se 3d5/2 at 53.8 eV and Se 3d3/2
at 55.2 eV, confirming the existence of Se2− in the sample.[40]
Two XPS peaks of S 2p are observed at 160.8 and 166.7 eV,
respectively, in the S-TiSe2 (Figure 1b). They were assigned to
Herein, we report a two-step facile oil phase process to
prepare heterostructures with Fe3O4 nanoparticles anchored
on S-TiSe2 hexagonal nanoplates. The S-TiSe2 nanoplates
were obtained in oil phase with an average lateral size of
300 nm and a thickness of 15 nm. The heterostructure is
obtained by using the S-TiSe2 nanoplates as the hetero-
geneous nucleation supports to grow Fe3O4 nanoparticles
with an average size of ≈8 nm. The obtained heterostruc-
tured S-TiSe2 nanoplates/Fe3O4 nanoparticles (S-TiSe2/Fe3O4)
exhibit improved performance in Li/Na storage versus the
bare S-TiSe2 nanoplates. Serving as lithium-ion battery elec-
trodes, the S-TiSe2/Fe3O4 exhibits a large reversible capacities
of 707.4 mAh g−1 at 0.1 A g−1 during the 100th cycle, which is
higher than those of the S-TiSe2 nanoplates (168.8 mAh g−1)
and the pure Fe3O4 (487.7 mAh g−1). Besides, good rate capa-
bility (e.g., 301.7 mAh g−1 at 20 A g−1) and excellent cycling sta-
bility (432.3 mAh g−1 after 200 cycles at 5 A g−1) can be achieved
during the Li+ intercalation and deintercalation process. As for
Na storage, the S-TiSe2/Fe3O4 heterostructures also maintain
high reversible capacities of 402.3 mAh g−1 at 0.1 A g−1 during
the 100th cycle and a high rate capacity 203.3 mAh g−1 at 4 A g−1.
For comparison purpose, the pure TiSe2 nanoplates are first
synthesized by reacting TiCl4 and selenium powder in a hot
oleylamine/trioctylphosphine solution. The crystalline phase of
TiSe2 was first investigated by X-ray diffraction (XRD) and the
result is shown in Figure 1a. By comparing the XRD pattern
of the sample and standard data from the Joint Committee on
Powder Diffraction Standards (JCPDS), all the diffraction peaks
are in good agreement with the hexagonal type TiSe2 (JCPDS
no. 71-4828). The scanning electron microscope (SEM) images
of the as-prepared TiSe2 reveal that uniform nanoplates with an
average lateral size of a few µm can be obtained (Figure S1a,b,
Supporting Information). Without addition of the trioctylphos-
phine (TOP), flower-shaped TiSe2 nanoclusters are obtained
with obvious agglomeration of the nanocrystals (Figure S2, Sup-
porting Information). This indicates that the TOP can serve as

 
the Ti S and S Ti Se bonding due to the S dopants, respec-
tively.[41,42] From the XPS peaks, the atomic ratio of S/Se can be
calculated to be 0.19. These results demonstrate the successful
preparation of TiSe2 with S doping.
Figure 2a shows the SEM image of S-TiSe2, and it can be
clearly seen that uniform nanoplates can be obtained after
the addition of sulfur. The lateral size of these nanoplates was
300 nm, which is much smaller than that of pristine TiSe2. This
indicates S anions can act as a growth-control agent and lead to
a decrease of the nanostructure size as previously reported.[43,44]
The transmission electron microscope (TEM) image (Figure 2b)
the S-TiSe2 indicate that the product is hexagonal nanoplates
with the edge length of about 150 nm. The high-resolution
transmission electron microscope (HRTEM) image (Figure 2c)
of a single S-TiSe2 plate shows it is single crystalline. The
observed lattice fringes with interplanar distance of 0.17 nm
can be indexed to the (110) planes of TiSe2. Notably, the TEM
image of the side view (Figure 2d) of the nanoplates exhibits
that the thickness is about 15 nm. The scanning TEM image
and the corresponding energy dispersive X-ray spectroscopy
(EDX) elemental mapping images (Figure S5, Supporting
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Figure 2. a) SEM image and b) TEM images of S-TiSe2, c) HRTEM images of S-TiSe2, and d) sectional TEM image of S-TiSe2.
Information) reveal the uniform distribution of Ti, Se, S ele-
ments in the nanoplate. The different molar ratio of S/Se is
applied to adjust the doping content of the S and consequently
influence the nucleation and growth of S-TiSe2 nanocrystals
(Figure S6, Supporting Information). With the increased S con-
tent, the lateral dimension of individual nanoplate increases,
and finally nanosheets can be obtained with the molar ratio of
S/Se = 1:1.
S-TiSe2/Fe3O4 heterostructures were prepared by decom-
posing iron–oleate complex in oleyamine with the presence of
S-TiSe2 nanoplates. Figure 3a shows the XRD pattern of S-TiSe2/
Fe3O4 composites, and it can be seen that all the major diffrac-
tion peaks can also be assigned to hexagonal TiSe2 (JCPDS
no. 71-4828). Additionally, the prominent peak at 35.5° can be
indexed to (311) planes of cubic Fe3O4 (JCPDS no. 07-0322). The
XPS result was used to further analyze the samples. The XPS
spectrums of Ti 2p, S 2p, and Se 3d (Figure S7a–c, Supporting
Information) are almost the same as the above mentioned
S-TiSe2. Figure S7d (Supporting Information) shows the XPS
result of the Fe 2p regions, and the peak of Fe 2p3/2 at 710.8 eV
indicates the existence of both Fe2+ and Fe3+. The satellite peak
located at 718.8 eV is proven to be a characteristic peak of Fe3+ in
γ-Fe2O3, which suggests that the Fe3O4 is partly oxides.[45]
Figure 3b shows the SEM image of S-TiSe2/Fe3O4 hetero-
structures, where Fe3O4 nanoparticles anchored on hexagonal
nanoplates. The TEM images (Figure 3c) show that the size
of Fe3O4 nanoparticles is around 8 nm. The HRTEM image
(Figure 3d) reveals the interplanar distance of 0.30 nm in
the nanodomain corresponds to the (311) planes of Fe3O4, and
the interplanar distances of 0.30 nm can be assigned to (002)
planes of TiSe2. Further EDX analysis (Figure S8, Supporting
Information) shows the atomic ratio of Ti:Fe = 0.28:0.13. There-
after, the weight percentage of Fe3O4 in the composite material
is calculated to be 14.9%. For comparison, pure Fe3O4 nanopar-
ticles can be obtained by directly decomposing iron–oleate com-
plex in oleyamine. The XRD pattern (Figure S9a, Supporting
Information) of the pure Fe3O4 shows that all the diffrac-
tion peaks can also be assigned to the cubic Fe3O4 (JCPDS
no. 07-0322). The TEM image (Figure S9b, Supporting Infor-
mation) indicates the Fe3O4 is uniform nanoparticles with an
average diameter of ≈8 nm. Based on the N2 gas adsorption/
desorption measurements (Figure S10a, Supporting Infor-
mation), the Brunauer–Emmett–Teller (BET) surface area of
S-TiSe2 (51.1 m2 g−1) is much higher than that of the pure TiSe2
(8.8 m2 g−1), which may be related to the smaller size of S-TiSe2.
The BET surface area of S-TiSe2/Fe3O4 heterostructures is cal-
culated to be 45.9 m2 g−1, which is slightly lower than that of
the S-TiSe2 nanoplates (51.1 m2 g−1). This possibly due to the
growth of Fe3O4 nanoparticles on the S-TiSe2 nanoplates that
may block some pores. In addition, the pore size distribution of
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Figure 3. a) XRD pattern, b) SEM image and c) TEM image of S-TiSe2/Fe3O4, and d) HRTEM images of S-TiSe2/Fe3O4.
Li TiSe + 4 − x Li+ + 4 − x e− → Ti + 2Li Se
(2)
S-TiSe2/Fe3O4 heterostructures falls in the range of 18–40 nm
(Figure S10b, Supporting Information).
(
)
(
)
x
2
2
The LIBs performance of S-TiSe2/Fe3O4 heterostructures was
evaluated by assembling the sample into coin cells with lithium
foil as both the counter and reference electrodes. Figure 4a
shows the cyclic voltammetry (CV) curves of S-TiSe2/Fe3O4 het-
erostructures obtained between 0.005 to 3.0 V at a scan rate
of 0.2 mV s−1. The Li+ insertion and desertion mechanisms of
TiSe2 have not been identified in the previous studies, which
are considered to be similar to those of transition metal selenide
because of their similar crystal structure. The electrochemical
reaction of TiSe2 for Li-ion storage is proposed based on those
of transition metal selenide.[46–48] In the first cycle, three dom-
inating cathodic peaks are observed at 0.56, 1.35, and 1.54 V,
respectively. The reduction peak at 1.35 and 1.54 V can be attri-
buted to Li insertion into the interlayers of TiSe2, producing
LixTiSe2 (Equation (1)).[49] Additionally, the peak at 0.56 V is
assigned to formation of metal Ti and Li2Se (Equation (2)), and
is accompanied by the formation of a solid electrolyte inter-
phase (SEI) layer.[50,51] During the first anodic process, two
peaks located at 1.95 and 2.23 V correspond to the conversion
reaction of Li2Se and Ti to Ti ion (Equations (3) and (4)).[52] The
discharge and charge processes can be summarized below.
Discharge process:
Charge process:
Ti + 2Li Se → Li TiSe + 4 − x Li+ + 4 − x e−
(3)
(4)
(
)
(
)
2
x
2
Lix TiSe2 → TiSe2 + xLi+ + xe−
Upon subsequent cycling, the cathodic peaks shift slightly to
the higher voltage range than that in the first cycle, probably
because of the microstructure alteration after the first lithiation/
delithiation cycle. This is commonly observed in other conver-
sion reaction-based anode materials.[53,54] The CV curves of the
S-TiSe2/Fe3O4 electrode are almost overlapped during the sub-
sequent cycles, indicating a highly reversible electrochemical
process. For comparison, the CV curves of S-TiSe2 and S-TiSe2/
Fe3O4 between 0.005 and 3.0 V at a scan rate of 0.2 mV s−1 are
shown in Figure S11 (Supporting Information). It can be seen
obviously that the area of CV curves for S-TiSe2/Fe3O4 is much
larger than that of the S-TiSe2, indicating the significantly
improved capacity.
Figure 4b shows the charge–discharge voltage profiles of
S-TiSe2/Fe3O4 heterostructures at a current density of 0.1 A g−1.
In the first cycle, the S-TiSe2/Fe3O4 electrode delivers dis-
charge capacities of 1150.6 mAh g−1 and charge capacities of
TiSe2 + xLi+ + xe− → Lix TiSe2
(1)
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Figure 4. Electrochemical performance for LIBs at room temperature (25 °C). a) Cyclic voltammograms of S-TiSe2/Fe3O4 between 0.005 and 3.0 V at a
scan rate of 0.1 mV s−1. b) Charge–discharge voltage profiles of S-TiSe2/Fe3O4 at a current density of 0.1 A g−1. c) Cycling performance and Coulombic
efficiency of S-TiSe2 and S-TiSe2/Fe3O4 at 0.1 A g−1. d) Rate capability of S-TiSe2/Fe3O4 at various current rates, e) long-term cycling performance of
the S-TiSe2/Fe3O4 and pure Fe3O4 at a current density of 2.0 A g−1.
780.7 mAh g−1, with a Coulombic efficiency of 67.9%. All the
potential plateaus in the discharge–charge curves match the
redox peaks observed in the CV curves. During the subse-
quent cycle, The S-TiSe2/Fe3O4 shows discharge capacities of
789.3 mAh g−1 and a charge capacities of 763.2 mAh g−1, cor-
responding to a Coulombic efficiency of 96.7%. The cycling
performances of the S-TiSe2, pure Fe3O4, and S-TiSe2/Fe3O4
are shown in Figure 4c. The pure Fe3O4 and S-TiSe2 electrodes
give initial discharge capacities of 1251.3 and 411.2 mAh g−1
at 0.1 A g−1, respectively. But the capacities drop rapidly to
487.9 and 168.8 mAh g−1, respectively, after 100 cycles. By con-
trast, the S-TiSe2/Fe3O4 heterostructures shows higher revers-
ible capacities of 707.4 mAh g−1 at 0.1 A g−1 after 100 discharge/
charge cycles.
For comparison, the rate capability of S-TiSe2 and pure
TiSe2 is provided in Figure S12 (Supporting Information).
It can be observed that S-TiSe2 has better rate capability
than pure TiSe2 due to the significantly enhanced electrical
conductance with S-doping (Figure S13, Supporting Infor-
mation). Therefore, we choose S-TiSe2 as a framework that
incorporates Fe3O4 nanoparticles to form S-TiSe2/Fe3O4 het-
erostructures to further improve the capacity of S-TiSe2. The
rate capability of the as prepared S-TiSe2/Fe3O4 heterostruc-
tures is evaluated at various current densities from 0.1 to
20 A g−1. As demonstrated in Figure 4d, the reversible capaci-
ties of the composite are 739.8, 671.8, 623.6, 565.3, 495.4,
435.7, 360.7, and 301.7 mAh g−1 at current densities of 0.1,
0.2, 0.5, 1, 2, 5, 10, and 20 A g−1. After changing the current
density from 20 to 0.1 A g−1, the specific capacity returns to
724.4 mAh g−1. The S-TiSe2/Fe3O4 and Fe3O4 electrodes were
charged/discharged at a high current density of 2 A g−1 for
a long-term cycling test (Figure 4e). The S-TiSe2/Fe3O4 elec-
trode maintains a high specific capacities of 432.3 mAh g−1
after 200 cycles. The Fe3O4 electrode shows a specific capaci-
ties of 318.7 mAh g−1 after 200 cycles.
Electrochemical impedance spectroscopy (EIS) is carried
out to describe charge-transfer mechanisms of the S-TiSe2/
Fe3O4 heterostructures in LIBs. Figure S14 (Supporting Infor-
mation) shows the Nyquist plots of S-TiSe2 and S-TiSe2/
Fe3O4 after 1 and 100 cycles. Each Nyquist plot comprises a
depressed semicircle at medium frequency and a sloping line
in the low frequency region.[55] The semicircle is defined as the
charge-transfer impedance (Rct), and straight line represents
the Warburg impedance (Wo). Additionally, the high frequency
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region is related to the ohmic resistance (Re). It is obviously
seen that the Rct of S-TiSe2/Fe3O4 electrode (126 Ω) is much
smaller than that of S-TiSe2 (292 Ω), which manifests lower
charge-transfer resistance exists in the S-TiSe2/Fe3O4 electrode/
electrolyte interface. This is possibly due to the better infiltra-
tion of the electrolyte into Fe3O4, resulting in a faster Faradaic
reaction in the S-TiSe2/Fe3O4 heterostructures.[56,57] Moreover,
the heterostructures may also aid in reducing polarization
during the Li+ intercalation and deintercalation process.[58]
The Rct of S-TiSe2/Fe3O4 electrode increases a little bit after
100 cycles (171 Ω), revealing the excellent structural stability
of the electrode. Furthermore, the higher slope of the low-fre-
quency line of the S-TiSe2/Fe3O4 cell at low frequency means
the faster lithium-ion diffusion process can be achieved. For
the S-TiSe2/Fe3O4 sample after 50 cycles at 0.1 A g−1, the SEM
image (Figure S15, Supporting Information) indicates the nan-
oplate structure can be partially retained, suggesting its good
structural durability.
The sodium storage abilities of the S-TiSe2/Fe3O4 hetero-
structures are further evaluated. Figure 5a shows the
galvanostatic charge–discharge profiles of the S-TiSe2/Fe3O4
electrode at a current density of 0.1 A g−1 for the first three
cycles. Several voltage plateaus occur in the first discharge
curve, which are attributed to the intercalation reaction of
Na+ into S-TiSe2 and Fe3O4.[59] In the subsequent oxidation
curve, the plateaus are associated with the desodiation pro-
cess. The initial discharge and charge capacities of 984.6 and
497.5 mAh g−1 are delivered by the S-TiSe2/Fe3O4, resulting in
an initial Coulombic efficiency of 50.5%. The large irrevers-
ible capacity loss in the initial cycle can be mainly attributed
to the irreversible SEI formation, electrolyte decomposition,
and possibly electrochemical pulverization.[60] Except for the
initial cycle, the following charge/discharge curves are over-
lapped well. The rate capability of the S-TiSe2/Fe3O4 is shown
in Figure 5b. It is observed that high specific capacities of
430.0, 373.2, 332.7, 300.4, 260.3, 228.4, and 203.3 mAh g−1 can
be obtained at the current densities of 0.1, 0.2, 0.5, 1, 2, 3, and
4 A g−1, respectively. After the current density changing from
4 to 0.1 A g−1, the specific capacity returns to 410.0 mAh g−1.
In addition, the cycling performances of the S-TiSe2/Fe3O4 and
Fe3O4 are further tested at a current density of 0.1 A g−1 for
up to 100 cycles. As shown in Figure 5c, the specific capacity
of S-TiSe2/Fe3O4 during the 100th cycle is 402.3 mAh g−1,
which suggests good electrochemical stability for Na storage.
The reversible capacities of pure Fe3O4 electrode drop rapidly to
233.5 mAh g−1 after 100 cycles.
In summary, an oil phase synthesis of uniform S-TiSe2/Fe3O4
nanoplates is reported. The heterostructure with enhanced elec-
trical conductivity shows improved Li/Na storage properties.
Therefore, compared with S-TiSe2 nanoplates (168.8 mAh g−1 at
0.1 A g−1 during the 100th cycle), the as-prepared S-TiSe2/Fe3O4
nanoplates deliver larger reversible capacities of 707.4 mAh g−1
at 0.1 A g−1 during the 100th cycle, and also higher than that
of the pure Fe3O4 (487.7 mAh g−1). Besides, superior rate capa-
bility (e.g., 301.7 mAh g−1 at 20 A g−1) and excellent cycling sta-
bility (432.3 mAh g−1 after 200 cycles at 5 A g−1) can be achieved
during the Li+ intercalation and deintercalation process. For
the SIBs application, the S-TiSe2/Fe3O4 nanoplates also show
excellent Na storage properties with high reversible capaci-
ties (402.3 mAh g−1 at 0.1 A g−1 during the 100th cycle), and
excellent rate capabilities (203.3 mAh g−1 at 4 A g−1). This fasci-
nating electrochemical performance is attributed to the unique
Figure 5. Electrochemical performance for SIBs at room temperature (25 °C). a) Galvanostatic charge–discharge profiles of the S-TiSe2/Fe3O4 at a
current density of 0.1 A g−1. b) Rate performance of the S-TiSe2/Fe3O4. c) Cycling performance of the S-TiSe2/Fe3O4 and pure Fe3O4 at a current density
of 0.1 A g−1.
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electrodes. Celgard 2400 membrane was used as the separator, and
the electrolyte was 1.0 m LiPF6 in the 1:1 (volume) mixture of ethylene
carbonate/dimethyl carbonate. As for sodium-ion batteries, Na foil
was used as a counter electrode and a glass fiber was used as the
separator. The electrolyte was 1.0 m NaClO4 in propylene carbonate with
5% fluorinated ethylene carbonate. Cell was assembled in an Ar-filled
glovebox with concentrations of moisture and oxygen below 1.0 ppm.
All the cells of galvanostatic charge and discharge curves were tested in
NEWARE battery test system in the voltage range of 0.005–3.0 V. The CV
was used by Solartron analytical equipment (model 1470E) with a scan
rate of 0.2 mV s−1. EIS measurements were performed on an impedance
spectrum analyzer (Solatron, SI 1255B impedance/gain-phase analyzer;
software: ZView) at open-circuit potential with the frequency range from
10 kHz to 0.01 Hz at an AC perturbation of 5 mV. The electrode loading
for each battery is about 0.5 mg 0.1 mg cm−2.
heterostructure and the strong synergistic effects of the com-
bined contributions from the S-TiSe2 nanoplates and the Fe3O4
nanoparticles.
Experimental Section
Synthesis of S-Doped TiSe2 Nanoplates: In a typical synthesis, 500 mg
selenium powder (≈6 mmol) was first dissolved in 3 mL of TOP in a
three-neck flask. After vigorous stirring for 5 min, the solution become
clear, then 200 mg sulfur powder (≈6 mmol) and 15 mL of oleylamine
(OLA) were adding in the flask. The solution was heated to 120 °C and
staying at this temperature for 30 min with magnetic stirring and under
argon gas protection to eliminate the moisture. After the solution being
cooled down to room temperature, 3 mL of TiCl4 was injected into the
flask. It can be seen clearly that the color of solution become brown.
Subsequently, the solution was heated to 300 °C at 10 °C min−1, and
maintained at this temperature for 90 min. After being cooled down,
S-doped TiSe2 nanoplates product was collected by centrifugation at a
rotating speed of 9800 for 5 min, and washed by hexane for three times,
and then dried in a vacuum oven at 60 °C overnight. The pure TiSe2 can
be obtained using the same method without adding the sulfur powder.
Synthesis of Heterostructured S-Doped TiSe2 Nanoplates/Fe3O4
Nanoparticles Composites: The iron–oleate complex was synthesized
according to a reported procedure with minor modification. FeCl3·6H2O
(2 mmol) and Na–oleate (4 mmol) were dispersed in 10 mL ethanol,
10 mL deionized (DI) water, and 20 mL hexane. After stirring for
60 min, the upper organic layer containing the iron–oleate complex was
collected by separatory funnel, and washed three times with DI water to
remove the residual sodium salts, and then stored in a vacuum oven at
60 °C overnight to evaporate the solvent completely, resulting in iron–
oleate complex with waxy solid form. S-doped TiSe2 nanoplates/Fe3O4
nanoparticles composites were prepared through second oil phase
synthesis. First, 100 mg S-doped TiSe2 nanoplates product and 5 mg
iron–oleate complex were dissolved in 10 mL of OLA in a three neck
round-bottom flask, and degassed at 120 °C under argon gas protection
for 30 min to get rid of the moisture. After that, the temperature was
raised to 300 °C within 30 min and maintained for 30 min. The mixture
was cooled to room temperature naturally. The final product was washed
with hexane several times, followed by drying at 60 °C in a vacuum oven
overnight. Finally, the as-prepared samples were annealed under the
protection of argon at 400 °C for 2 h with a heat rate of 3 °C min−1.
The pure Fe3O4 can be obtained by the same method without adding the
S-doped TiSe2 nanoplates.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The work was supported by the NNSF of China (61525402 and
5161101159), QingLan Project, National Postdoctoral Program for
Innovative Talents (BX201600072), National Synergistic Innovation
Center for Advanced Materials (SICAM) Fellowship, Jiangsu Provincial
Founds for Graduate Student Innovation (KYZZ16_0243), Project
Singapore MOE AcRF Tier 1 under Grant Nos. RG2/13 and RG113/15,
and Singapore A*STAR Pharos program SERC 1527200022. The authors
would like to acknowledge the Facility for Analysis, Characterization,
Testing and Simulation (FACTS), Nanyang Technological University,
Singapore, for use of their electron microscopy (or XRD) facilities.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
Characterization: The morphology of all the samples was investigated
by field-emission scanning electron microscope (JEOL, Model
JSM-7600F) and TEM (JEOL, Model 2100F, operating at 200 kV),
including capturing HRTEM images and EDX mapping. Samples for TEM
analysis were prepared by drop casting the sample dispersed in hexane
onto carbon-coated copper grids, then followed by the evaporation of
solvent. The crystal structure characterization of the samples was taken
on a Shimadzu powder or thin film 6000 X-ray diffractometer with the
2θ range from 10° to 80° by using Cu Kα1 (λ = 0.15406 nm) radiation.
Surface area and porosimetry distribution system (ASAP 3020) were
used for the survey of nitrogen adsorption/desorption isotherms at
−196 °C, and all samples were degassed at 120 °C for 8 h under vacuum
before testing. The specific surface area was calculated by the Brunauer–
Emmett–Teller method. XPS was performed by AXIS-His (Kratos
Analytical). UV–vis–NIR absorption analysis was measured on a Cary
5000 mode with the wavelength range of 1000–3000 nm.
Electrochemical Measurements: The working electrode was fabricated
by mixing 80% wt of the samples with 10% wt of conductive carbon black
and 10% wt of polyvinylidene fluoride binder in N-methylpyrrolidone
solvent. After homogeneous dispersing, the slurry was coated on
a copper foil and dried in a vacuum oven at 60 °C overnight. The
lithium-ion batteries performance was carried out using two-electrode
coin cells with pure lithium foil as both the counter and the reference
heterostructures, lithium-ion batteries, S-doped TiSe2 nanoplates,
sodium-ion batteries
Received: June 26, 2017
Revised: August 2, 2017
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