Cobalt-Doped FeS2Nanospheres with Complete Solid Solubility as a High-Performance Anode Material for Sodium-Ion Batteries

Article abstract of DOI:10.1002/anie.201607469

Considering that the high capacity, long-term cycle life, and high-rate capability of anode materials for sodium-ion batteries (SIBs) is a bottleneck currently, a series of Co-doped FeS₂solid solutions with different Co contents were prepared by a facile solvothermal method, and for the first time their Na-storage properties were investigated. The optimized Co_0.5Fe_0.5S₂(Fe0.5) has discharge capacities of 0.220 Ah g^?1after 5000 cycles at 2 A g^?1and 0.172 Ah g^?1even at 20 A g^?1with compatible ether-based electrolyte in a voltage window of 0.8–2.9 V. The Fe0.5 sample transforms to layered Na_xCo_0.5Fe_0.5S₂by initial activation, and the layered structure is maintained during following cycles. The redox reactions of Na_xCo_0.5Fe_0.5S₂are dominated by pseudocapacitive behavior, leading to fast Na⁺insertion/extraction and durable cycle life. A Na₃V₂(PO₄)₃/Fe0.5 full cell was assembled, delivering an initial capacity of 0.340 Ah g^?1.

Full text of DOI:10.1002/anie.201607469

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201607469
German Edition: DOI: 10.1002/ange.201607469
Sodium-Ion Batteries
Cobalt-Doped FeS2 Nanospheres with Complete Solid Solubility as
a High-Performance Anode Material for Sodium-Ion Batteries
Kai Zhang, Mihui Park, Limin Zhou, Gi-Hyeok Lee, Jeongyim Shin, Zhe Hu, Shu-Lei Chou,
Jun Chen,* and Yong-Mook Kang*
Abstract: Considering that the high capacity, long-term cycle
life, and high-rate capability of anode materials for sodium-ion
batteries (SIBs) is a bottleneck currently, a series of Co-doped
FeS2 solid solutions with different Co contents were prepared
by a facile solvothermal method, and for the first time their Na-
storage properties were investigated. The optimized Co0.5Fe0.5S2
(Fe0.5) has discharge capacities of 0.220 AhgÀ1 after 5000
cycles at 2 AgÀ1 and 0.172 AhgÀ1 even at 20 AgÀ1 with
compatible ether-based electrolyte in a voltage window of
0.8–2.9 V. The Fe0.5 sample transforms to layered
NaxCo0.5Fe0.5S2 by initial activation, and the layered structure
is maintained during following cycles. The redox reactions of
NaxCo0.5Fe0.5S2 are dominated by pseudocapacitive behavior,
leading to fast Na+ insertion/extraction and durable cycle life.
A Na3V2(PO4)3/Fe0.5 full cell was assembled, delivering an
initial capacity of 0.340 AhgÀ1.
retain a high capacity after over 1000 cycles or at a high
current rate of above 10 AgÀ1. Thus, it is urgent and necessary
to propose novel solutions to develop high-capacity TMDSs
with stable cyclic retention and high rate capability.
To address these problems, controlling discharge cut-off
voltages and utilizing an ether-based electrolyte (1m
NaSO3CF3 in diglyme (DGM)) have been attempted for
TMDSs without the carbon modification.[5,9] Raising the
discharge cut-off voltage prohibits conversion reactions and
makes intercalation reactions take place during charge–
discharge cycles. The ether-based electrolytes present
a higher chemical stability with intermediate products of
TMDSs during charge/discharge than the carbonate-based
electrolytes. Nevertheless, a single TMDS is difficult to meet
all the requirements as a promising electrode material. Thus,
in term of intrinsic feature of substance, it would be
interesting to know if binary TMDSs could integrate their
own advantages.
FeS2, as an easily accessible natural mineral, shows good
cycling and rate performance when cycling between 0.8–
2.9 V.[9] However, its capacity needs be further enhanced. Fuꢀs
group first reported Na-storage property of CoS2 with an
ether-based electrolyte.[6] The CoS2 delivered a higher
capacity in a voltage range of 1.0–2.9 V than that of FeS2,
but its rate performance is worse than that of FeS2. Because
CoS2 has similar pyrite structure to FeS2, Co2+ ions can be
doped into FeS2 to form a complete solid solution,[10] inspiring
us to prepare Co-doped FeS2 to combine the high rate
capability of FeS2 and high capacity of CoS2.
S
odium-ion batteries (SIBs) are highly attractive as a prom-
ising battery system in a post Li-ion battery (LIB) era because
sodium precursors are cheap and abundant.[1] To date,
learning from LIBs, high-capacity anode materials for SIBs
include simple substances, alloys, and transition-metal chal-
cogenides.[2–4] Among them, transition-metal disulfides
(TMDSs) have received remarkable interests owing to their
robust structure, low cost, simple preparation, and high
theoretical capacities.[3–7] To further improve their electro-
chemical properties, many reports have focused on introduc-
ing conductive carbon materials to enhance electronic con-
ductivity and accommodate volume changes during charge–
discharge process.[8] However, the carbon contents are
typically more than 40 wt%, which reduces a practical
capacity of the electrode. Furthermore, it is still difficult to
Herein, a series of Co-doped FeS2 with different Co
contents have been synthesized by a facile one-step solvo-
thermal route and for the first time used as anode materials
for SIBs. Co0.5Fe0.5S2 sample exhibits the best electrochemical
performance among the doping series, FeS2, and CoS2. It
delivers a discharge capacity of 0.220 AhgÀ1 after 5000 cycles
at 2 AgÀ1. Even at 20 AgÀ1, discharge capacity is still
0.172 AhgÀ1. Moreover, Na3V2(PO4)3/Co0.5Fe0.5S2 full cell
[*] Dr. K. Zhang, M. Park, G.-H. Lee, J. Shin, Prof. Y.-M. Kang
Department of Energy and Materials Engineering
Dongguk University-Seoul
Seoul 04620 (Republic of Korea)
E-mail: dake1234@dongguk.edu
shows an initial capacity of 0.340 AhgÀ1
.
The Co-doped FeS2 was synthesized by a simple solvo-
thermal route (see the Experimental Section in the Support-
ing Information). According to the concentration ratio of
FeSO4·7H2O to CoSO4·7H2O (x: (1Àx) (x = 1.0, 0.9, 0.7, 0.5,
0.4, and 0.0)), the corresponding samples are referred to as
FeS2, Fe0.9, Fe0.7, Fe0.5, Fe0.4, and Fe0.0, respectively.
Figure 1a and the Supporting Information, Figure S1 show X-
ray diffraction (XRD) patterns of the as-prepared six samples.
When x is more than or equal to 0.5, all diffraction peaks can
be indexed to the standard JCPDS No. 71-53, indicating that
L. Zhou, Prof. J. Chen
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of
Education), Collaborative Innovation Center of Chemical Science and
Engineering, College of Chemistry, Nankai University
Tianjin 300071 (China)
E-mail: chenabc@nankai.edu.cn
Dr. Z. Hu, Dr. S.-L. Chou
Institute for Superconducting and Electronic Materials
University of Wollongong
Wollongong, New South Wales 2522 (Australia)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607469.
¯
they have a cubic pyrite phase with the Pa3 space group. CoS2
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mapping images of the
Fe0.5 sample clearly indi-
cate that Co shows a dis-
tinct contrast at the edge of
the particles while Fe signal
blurs at the edges (Fig-
ure 1g–j). In the bulk, S,
Fe, and Co are distributed
uniformly. EDS line scan
data (Supporting Informa-
tion, Figure S5) confirms
that Co concentration is
much higher than Fe con-
tent at the edge of the
particles. The formation
process of the Co-doped
FeS2 is shown in the Sup-
porting Information, Fig-
ure S6. Because Co2+ and
S2À ions tend to form hex-
agonal CoS rather than
CoS2 when raw materials
do not include Fe salts
under the given synthesis
conditions, Co-doped FeS2
Figure 1. a) XRD patterns of the as-prepared FeS2, Fe0.9, Fe0.7, Fe0.5, and Fe0.4 samples and b) the
corresponding partial magnification patterns in a 2q range of 31–358. c),e) SEM and d),f) TEM images of the
as-prepared FeS2 (c,d) and Fe0.5 (e,f) samples. g)–j) EDS color mapping images and k) high-resolution TEM
(HRTEM) of the Fe0.5 sample.
and any other impurities are not observed in the four doping
samples. The partial magnification of the (200) peaks are
displayed in Figure 1b. After the Co doping, the (200) peaks
of the Fe0.9, Fe0.7, and Fe0.5 powders shift slightly to lower
angles. When x = 0.4, CoS phase is detected at 30.68, 35.28,
and 54.68, suggesting that the Fe0.4 sample is impure. Besides,
the results of energy dispersive spectrometer (EDS) and
inductively coupled plasma-atomic emission spectroscopy
(ICP-AES) demonstrate that the compositions of the Fe0.9,
Fe0.7, and Fe0.5 powders are Co0.1Fe0.9S2, Co0.3Fe0.7S2, and
Co0.5Fe0.5S2, respectively (Supporting Information, Figure S2
and Table S1). All peaks of the as-prepared Fe0.0 sample are
matched to the standard JCPDS No. 65-8977 (Supporting
Information, Figure S1). It illustrates that the product is CoS
when raw materials do not include Fe salts in the given
synthetic condition. The composition of the Fe0.9, Fe0.7,
Fe0.5, Fe0.4, and Fe0.0 samples is summarized in the
Supporting Information, Table S2. The first three samples
are pyrite phase, the Fe0.4 powder includes both pyrite phase
and impure CoS, and the Fe0.0 sample is CoS.
Figure 1c–f and the Supporting Information, Figure S3
display scanning electron microscopy (SEM) and transmis-
sion electron microscopy (TEM) images of the as-prepared
FeS2 and Fe0.5 powders. All samples show spherical particle
shapes, but FeS2 and the Co-doped FeS2 samples have notably
different sizes. The average diameter of FeS2 particles is about
500 nm, while the Fe0.9, Fe0.7, and Fe0.5 particles exhibit
similar average diameters of about 100 nm. Furthermore, the
particle surface is slightly damaged after the Co doping. The
selected area electron diffraction (SAED) patterns of the four
samples in the Supporting Information, Figure S4 show that
lattice spacing of the (200) plane slightly increases after the
Co doping, which is in accordance with the XRD patterns in
Figure 1b. Energy-dispersive X-ray spectroscopy (EDS)
samples originate from the ionic exchange between the initial
FeS2 nuclei and Co2+ ions. As a result, the diameter of the Co-
doped FeS2 is smaller than that of FeS2, and the nanospheres
has slightly damaged edge and high Co content on the edge
(Supporting Information, page 8). The HRTEM image of the
Fe0.5 sample (Figure 1k) reveals that lattices in the surface
and the center of the particles have the same spacing,
indicating that the Fe0.5 sample is a complete solid solution
despite the concentration gradient.
Figure 2 presents electrochemical properties of the as-
prepared FeS2, Fe0.9, Fe0.7, Fe0.5, and Fe0.4 powders. Before
the test, the liquid electrolyte and operating voltage range
were optimized first. Only charging/discharging between 0.8
and 2.9 V with ether-based electrolytes shows good cycle life.
Upgrading the discharge terminal voltage avoids the con-
version reactions to enable structural stability (Supporting
Information, Figures S7, S8). Ether-based electrolytes show
not only chemical stability with polysulfides but also solvent-
co-intercalation behaviors (Supporting Information, Fig-
ure S9–S12). Figure 2a compares their cycling performance.
As Co content increases, the first discharge capacities
increase. However, after 400 cycles, the Fe0.5 sample shows
the highest capacity, and the Fe0.4 sample displays the lowest
capacity. The cycling performance of the Fe0.0 (CoS) sample
with a discharge capacity of only 0.065 AhgÀ1 after 150 cycles
is depicted in the Supporting Information, Figure S13. Since
the Fe0.4 sample consists of the Co-doped FeS2 and impure
CoS, the capacity decreases more sharply than those of the
other samples. Figure 2b shows typical charge–discharge
voltage curves of the five samples. FeS2 and all of the Co-
doped FeS2 solid solution displayed three discharge voltage
plateaus at about 1.7, 1.5, and 1.1 Vas well as one charge slope
combined with multi-step reactions. The average voltage for
charge–discharge process is around 1.5 V, which is similar to
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Figure 2. a) Cycling performance and b) charge–discharge curves in the 400th cycle of the as-prepared FeS2 and four doping samples. c) Rate performance
of the as-prepared FeS2, Fe0.9, Fe0.7, and Fe0.5 samples. d) Typical CV curves of the as-prepared FeS2, Fe0.9, Fe0.7, Fe0.5, and CoS2 samples at a scan rate
of 1.5 mVsÀ1. e) Log(i) vs. log(v) plots at each redox peak and f) bar chart showing the percent of pseudocapacitive contribution of the Fe0.5 sample.
that of widely reported Li4Ti5O12 anodes for LIBs. A
moderately high voltage of the anode material can be
beneficial to the safety of batteries. Furthermore, Co-doped
FeS2 samples deliver higher capacities than that of Li4Ti5O12,
showing more competitive merit. The charge–discharge
voltage curves of the Fe0.4 sample are obviously different
from the other curves probably owing to the presence of CoS.
Rate capabilities of the FeS2, Fe0.9, Fe0.7, and Fe0.5
samples were also characterized (Figure 2c). At low current
densities from 0.05 to 1 AgÀ1, the capacities increase as the Co
contents increase. At 10 AgÀ1, the capacities of both Fe0.9
and Fe0.7 samples are lower than that of the FeS2 although
they possess higher surface area (Supporting Information,
Figure S14) and smaller particle size than those of the FeS2.
Remarkably, the Fe0.5 sample displays the highest capacity
among the four samples even at 20 AgÀ1. However, pure CoS2
only delivers a higher capacity than that of the FeS2 at a very
low rate (0.05 AgÀ1) (Supporting Information, Figures S15,
S16). The results indicate that Co-doping increases the
capacity but weakens the rate performance. When the ratio
of Fe to Co is more than 1:1, the capacity increase does not
make up for rate-capability loss. Thus, the Fe0.5 delivers the
best performance among all as-prepared samples. Charge–
discharge voltage curves of the Fe0.5 sample at different rates
are presented in the Supporting Information, Figure S17.
Although the differences between charge and discharge
voltage plateaus slight increase, the distinct plateaus are
observed even at a very high current density of 10 AgÀ1. The
Fe0.5 sample shows discharge capacities of 0.261, 0.192, and
0.173 AhgÀ1 at 1, 10, and 20 AgÀ1, respectively. It can be
concluded that the Co-doped FeS2 samples not only enhance
capacities of FeS2 but also improve the rate performance of
CoS2.
and CoS2 samples. Because the Co-doped FeS2 and pure FeS2
and CoS2 samples display similar redox peaks (R1, R2, O2,
O1À1, and O1À2), they should have similar intercalation
reactions.[9] To further investigate the rate capability of each
reaction, the CV tests at different scan rates were carried out.
As shown in the Supporting Information, Figure S18, all peak
currents (i) increase as the scan rates (v) increase. Figure 2e
and the Supporting Information, Figure S19 exhibit log(i) vs.
log(v) plots of the five samples at each redox peak, and the
slope of the fitted line is denoted by b. A high b-value
positively affects rate capability.[4,11] The calculated b-values
are listed in the Supporting Information, Table S3. For the
Co-doped FeS2 samples, the b-values of the O2 and O3 peaks
are about 0.8, which are higher than that of O2 peak of CoS2
but lower than that of O2 peak of FeS2. As a result, the Co-
doped FeS2 samples have higher rate capabilities than CoS2,
but the Fe0.9 and Fe0.7 samples deliver lower capacities at
10 AgÀ1 than that of FeS2. Figure 2 f summarizes contribu-
tions of the pseudocapacitive behaviors at various scan rates.
The contributions are 75.8%, 78.2%, 79.8%, 83.3%, 87.8%,
and 90.5% at the scan rates of 0.1, 0.2, 0.3, 0.5, 0.8, and
1.0 mVsÀ1, respectively. The results reveal that the pseudo-
capacitive charge-storage amount occupies a relatively high
portion of the whole capacity.
Reaction mechanism of the Fe0.5 sample was also
investigated. Figure 3a shows in situ XRD analysis of the
first cycle. During the discharge process, the characteristic
peak of pyrite phase at 32.88 gradually shifts to about 318,
indicating the formation of a monoclinic NaFe0.5Co0.5S2.[9]
During the charge process, the peak near 318 gradually
changes to about 298, which is attributed to the transforma-
tion
from
monoclinic
NaFe0.5Co0.5S2
to
layered
NaxFe0.5Co0.5S2.[9] After about 50 cycles, the active materials
totally turn into layered NaxFe0.5Co0.5S2, but it leads to initial
capacity declination as shown in Figure 2a. The discharge and
charge products in the 50th cycle were also analyzed by ex situ
To study the effect of the Co-doping on reaction kinetics,
a cyclic voltammetry (CV) test was performed. Figure 2d
shows CV curves of the as-prepared FeS2, Fe0.9, Fe0.7, Fe0.5,
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tion, Figure S22, the full
cell with excessive Na3V2-
(PO4)3 delivers an initial
capacity of 0.340 AhgÀ1 at
0.2 AgÀ1 based on the mass
of Fe0.5 with an average
output voltage of 1.6 V.
The capacity still reaches
0.296 AhgÀ1 after 100
cycles, corresponding to
a
capacity retention of
90.2% calculated from the
2nd cycle. The results sug-
gest that the Co-doped
FeS2 can be used for both
rechargeable Na and Na-
ion batteries. The electro-
chemical performance of
the Fe0.5 is better than
those of pure FeS2 and
CoS2 and comparable to
Figure 3. a) In situ XRD analysis of the Fe0.5 electrode during the first cycle and the corresponding image
plots of diffraction patterns in a 2q range of 28–338. b),c) Ex situ SAED patterns, d) XPS of S 2p peak, and
e),f) XANES for Fe (e) and Co (f) of the as-prepared Fe0.5 sample, fully discharged (0.8 V) and charged (2.9 V)
after 50 cycles.
methods. The characteristic d-spaces of 0.316 and 0.311 nm
are detected at fully discharged 0.8 V and charged 2.9 V,
respectively (Figure 3b,c). It indicates that the layered
structure is remained, but interlayer spacing changes during
the following cycles.[9] Figure 3d shows ex situ XPS spectra of
S 2p peak of the charge–discharge products. The binding
energies of S 2p peaks of the discharge and charge products
are 161.6 and 162.5 eV, respectively, indicating electrochem-
ical oxidation of S during the charge process. X-ray absorp-
tion near-edge spectra (XANES) reveals that cationic valen-
ces remain during the charge-discharge process (Figure 3e,f).
However, the Fe valence is as similar as that in Fe2O3, and the
Co valence is close to + 2. The results indicate that only
intercalation reactions occur when cycling between 0.8 and
2.9 V, which enables good cycle stability.
Figure 4 exhibits composition and cycling performance of
the Na/Fe0.5 half-cell and the Na3V2(PO4)3/Fe0.5 full cell. For
Figure 4. Illustration of the composition and cycling performance of
the half-cell, the discharge capacity is 0.220 AhgÀ1 after 5000
cycles at 2 AgÀ1, corresponding to an average capacity
retention of about 99.999% per cycle. Furthermore, coulom-
bic efficiencies of the Fe0.5 powder in the 3rd, 1000th, and
5000th cycles are 99.78%, 99.84%, and 99.91%, respectively.
The superior Coulombic efficiency is owing to upgradation of
discharge terminal voltage and utilization of ether-based
electrolyte. Increasing the terminal voltage to 0.8 V avoid the
conversion reactions to improve the structural stability, and
the ether-based electrolytes have higher chemical stability for
polysulfides compared with carbonate-based electrolytes.[9,12]
Furthermore, the nanospheres of the Fe0.5 sample are well
maintained and homogeneously surrounded by carbon addi-
tives after 1000 cycles (Supporting Information, Figure S20),
and Fe, Co, and S elements are also dispersed uniformly in
bulk of the nanospheres (Supporting Information, Fig-
ure S21). The superior electrochemical performance of the
Fe0.5 half-cell encouraged us to assemble Na3V2(PO4)3/Fe0.5
full cell. As shown in Figure 4b and the Supporting Informa-
a),b) Na/Fe0.5 half-cell and c),d) Na3V2(PO4)3/Fe0.5 full cell.
that of FeSe2.[4] Furthermore, S resource has lower toxicity
and cost compared with Se resource, which is favorable for
large-scale application.
In summary, a series of the Co-doped FeS2 nanospheres
have been synthesized by a facile solvothermal method. The
optimized Co0.5Fe0.5S2 (Fe0.5) sample delivers a discharge
capacity of 0.220 AhgÀ1 after 5000 cycles at 2 AgÀ1 with an
ether-based electrolyte and in a voltage range of 0.8–2.9 V.
Even at 20 AgÀ1, its discharge capacity is still 0.172 AhgÀ1. It
not only increases the capacity of FeS2 but also promotes the
rate performance of CoS2. Its redox reactions are mainly
controlled by pseudocapacitive behaviors, leading to fast
kinetics and durable cyclic life. Furthermore, the Na3V2-
(PO4)3/Fe0.5 full cell delivers an initial capacity of
0.340 AhgÀ1 with an average output voltage of 1.6 V. The
facile preparation and superior performance of Co0.5Fe0.5S2
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Acknowledgements
This work was supported by the energy efficiency and
resources of the Korea Institute of Energy Technology
Evaluation and Planning (KETEP) grant funded by the
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Received: August 2, 2016
Published online: && &&, &&&&
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Communications
Sodium-Ion Batteries
K. Zhang, M. Park, L. Zhou, G.-H. Lee,
J. Shin, Z. Hu, S.-L. Chou, J. Chen,*
Y.-M. Kang*
&&&& — &&&&
Cobalt-Doped FeS2 Nanospheres with
Complete Solid Solubility as a High-
Performance Anode Material for Sodium-
Ion Batteries
A series of Co-doped FeS2 nanospheres
were synthesized by a facile one-step
solvothermal method and used as anode
materials for sodium-ion batteries. The
Co-doped FeS2 sample delivers excellent
performance, not only increasing the
capacity of FeS2 but also promoting the
rate performance of CoS2. Na red, S yel-
low, Fe/Co green.
6
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!