ISSN 1998-0124 CN 11-5974/O4
2019, 12(10): 2609–2613 https://doi.org/10.1007/s12274-019-2495-4
Research Art
Iron sulfides with dopamine-derived carbon coating as superior performance anodes for sodium-ion batteries
Aihua Jin1,2,§, Seung-Ho Yu3,§, Jae-Hyuk Park1,2, Seok Mun Kang1,2, Mi-Ju Kim1,2, Tae-Yeol Jeon4, Junyoung Mun5 (), and Yung-Eun Sung1,2 ()
1 Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea
2 School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea
3 Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea
4 Beamline Department, Pohang Accelerator Laboratory (PAL), Pohang 37673, Republic of Korea
5 Department of Energy and Chemical Engineering, Incheon National University, 12-1, Songdo-dong, Yeonsu-gu, Incheon 22012, Republic of Korea
§ Aihua Jin and Seung-Ho Yu contributed equally to this work.
© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Received: 1 May 2019 / Revised: 5 July 2019 / Accepted: 27 July 2019
ABSTRACT
High energy ball-milled iron sulfides with thin carbon layer coating (BM-FeS/C composites) were prepared by the simple and economical process. Ball-milled process, followed by carbon coating, reduced the particle size and increased the electrical conductivity. When employed as sodium-ion battery anodes, BM-FeS/C composites showed extremely high electrochemical performance with reversible specific capacity of 589.8 mAh·g−1 after 100 cycles at a current density of 100 mA·g−1. They also exhibited superior rate capabilities of 375.9 mAh·g−1 even at 3.2 A·g−1 and 423.6 mAh·g−1 at 1.5 A·g−1. X-ray absorption near edge structure analysis confirmed the electrochemical pathway for conversion reaction of BM-FeS/C composites.
KEYWORDS
sodium-ion batteries, iron sulfides, high energy ball-milling, dopamine, X-ray absorption near edge structure
1 Introduction
Due to the restriction of Li resources and high-cost of lithium-ion batteries (LIBs), enormous efforts have been focused on developing new battery systems to replace LIBs. Sodium-ion batteries (SIBs) have emerged as a promising alternative because sodium resources are widely distributed and seemly inexhaustible. However, electrochemical performance still needs to be further improved for practical application [1–4]. In particular, poor cycle performance and low reversible capacity of anode are considered to be the main issues. Graphite, the anode material for commercial LIBs, delivers very low reversible capacity in SIB compared to LIB application [5, 6]. Therefore, it is essential to explore the new anode materials which can possess higher volumetric and gravimetric specific capacities.
Conversion reaction-based materials usually performed higher specific capacities than carbonaceous materials. For this reason, metal sulfides have undergone remarkable development in SIBs [7–10]. In particular, iron sulfides have attracted great attention due to their high capacity as well as abundance, low-cost, and environmental benignity [11, 12]. However, there are two problems to be overcome.
First, the volume expansion during sodiation is huge, and this causes severe pulverization during cycling, ultimately leading to cycle fading.
One strategy, to solve this problem, is to prepare the nanoscale active materials to accommodate the volume change effectively, which also can help the transport of electrons and Na ions by shortening the length of transport. For example, Cho et al. prepared porous FeS nanofibers by electrospinning methods, and Li et al. synthesized Fe3S4 (greigite) nanoparticles, and their nanomaterials showed much
improved electrochemical performance [11, 14]. Second, iron sulfides have low electrical conductivity, leading to poor rate capability.
Incorporating the iron sulfide with the carbonaceous materials, such as reduced graphene oxide, porous carbon, and carbon cloth, is one of the effective ways to enhance the conductivity [15–17].
Nonetheless, the synthetic methods to prepare nanomaterials incorporated with carbonaceous materials are too complex, time- consuming and unsuitable for commercial application. Ball-milling is a favorable way to reduce the particle size, mechanical mixing, and synthetic materials without further treatment, which can easily utilize and reduce the cost of production [18–21].
In this study, we report a simple method to prepare carbon-coated iron sulfide (BM-FeS/C) and their application as anode material for SIBs. The FeS particles were reduced by high energy ball-milling, followed by carbon coating. BM-FeS/C composites showed extremely high reversible capacity of 589.8 mAh·g−1 after 100 cycles at a constant current density of 100 mA·g−1, while FeS exhibited only 295.0 mAh·g−1. They also exhibited superior rate capabilities of 375.9 mAh·g−1 even at 3.2 A·g−1 and 423.6 mAh·g−1 at 1.5 A·g−1 during 100 cycles. X-ray absorption near edge structure analysis was performed in order to further understand the reaction mechanism of BM-FeS/C composites for SIB anodes.
2 Experimental
2.1 Preparation of materials
Iron(II) sulfide (Sigma-Aldrich) was ground to BM-FeS by the simple Address correspondence to Junyoung Mun, [email protected] ; Yung-Eun Sung, [email protected]
ball-milling method. It was conducted using PULVERISETTE7 (Fritsch) with a speed of 300 rpm for 10 h. The prepared BM-FeS/C was coated by carbon by dopamine hydrochloride (Sigma-Aldrich) in tris-HCl buffer solution (Sigma-Aldrich). After stirring for 2 h, the precipitates were annealed at 550 °C for 4 h under Ar atmosphere.
2.2 Characterizations
X-ray diffraction patterns were obtained using Bruker D-5005 with Cu Kα radiation. Field-emission scanning electron microscopy (FE-SEM) analysis was conducted with a SUPRA 55VP (Carl Zeiss).
Transmission electron microscopy (TEM) images were obtained by JEOL JEM-2100F (Jeol). TA SDT Q600 was used as the Thermo- Gravimetric Analysis (TGA). X-ray photoelectron spectrometer was operated by Sigma Probe instrument (ThermoFisher Scientific) with Al Kα. X-ray absorption near edge structure (XANES) experiments were employed at the beamline 8C of Pohang Light Source II (PLS II), Republic of Korea.
2.3 Electrochemical characterizations
The electrodes were made by coating the slurry on Cu current collector.
The slurry was made by mixing active material (70 wt.%), super P (15 wt.%) and poly (acrylic acid) (15 wt.%) in n-methyl-2- pyrrolidinone solvent. Then the electrodes were dried in a vacuum oven. The loading mass of active materials on electrode is over 1.2 mg·cm−2. The electrochemical cells (2032-type coin type) were assembled in Ar-filled glove box. Sodium metal was used as counter and reference electrodes. A glass fiber was used as separator, and 1 M NaClO4 in ethylene carbonate (EC)/propylene carbonate (PC) (1:1 by volume) with 5 wt.% fluoroethylene carbonate (FEC) was used as electrolyte. All coin cells were tested with a WBCS 3000 cycler in a voltage range of 0.01–3 V (vs. Na+/Na) at 25 °C.
3 Results and discussion
The schematic illustration of the synthetic process of BM-FeS/C is shown in Fig. 1(a). First, the ball-milled FeS (denoted as BM-FeS) was prepared by grinding commercial FeS for 10 h by high energy ball-milling machine. After then, the collected BM-FeS was carbon- coated via dopamine coating for 2 h, followed by annealing at 550 °C under Ar atmosphere. Dopamine can spontaneously deposit on virtually any surface by self-polymerization, and the coating thickness can be easily tuned by controlling the time [22, 23]. The morphologies of FeS and BM-FeS/C composites were characterized by SEM and TEM (Figs. 1(b), 1(c), and Fig. S2 in the Electronic Supplementary Material (ESM)). The morphology of FeS has irregular shapes, and the size of FeS particles becomes small after ball-milling, as can be seen in Fig. 1(b) and Fig. S2(a) in the ESM. BM-FeS and BM-FeS/C composites have similar particle size indicating that the polydopamine coating rarely affects the particle size (Fig. S1 in the ESM). As observed in Figs. 1(e) and 1(g), the carbon shell perfectly surrounds the ball-milled FeS particles. In addition, the thickness of the carbon shell is about 10 nm. The high-resolution TEM (HR-TEM) image of BM-FeS/C composites is shown in Fig. 1(f). The lattice fringe is 0.26 nm and that value is well matched with d-spacing of FeS (112) planes. The EDS elemental mapping of BM-FeS/C composites (Fig. 1(g)) demonstrates that the FeS particle was well coated by the uniform carbon shells with the thickness of about 10 nm [16].
Figure S3(a) in the ESM presents X-ray diffraction (XRD) patterns of FeS, BM-FeS, and BM-FeS/C composites. The peaks corresponding to the hexagonal FeS (JCPDS No. 65-3356) were observed in all three samples, indicating that the crystal structure does not change during ball-milling and polydopamine coating [24, 25]. To further analyze the chemical states of FeS, BM-FeS, and BM-FeS/C composites, X-ray photoelectron spectroscopy (XPS) was conducted (Fig. S3(b)–
S3(f) in the ESM). In the survey spectra (Fig. S3(b) in the ESM),
Figure 1 (a) Schematic illustration of the preparation of BM-FeS/C composites.
SEM images of (b) FeS and (c) BM-FeS/C composites. ((d), (e)) TEM images of BM-FeS/C composites. (f) HR-TEM image of BM-FeS/C composites and (g) EDS mapping of BM-FeS/C composites.
only peaks related to Fe, S, C, and O elements were observed. In the Fe 2p spectra, the Fe 2p1/2 and Fe2p3/2 peaks are located at 724.1 and 711.0 eV, respectively, which are attributed to FeS, and no binding energy shift was observed during the annealing process. In the S 2p spectrum, the binding energies at 161.5 and 162.6 eV were attributed to the bond between S and Fe [26, 27]. In order to confirm the carbon contents in BM-FeS/C composites, TGA was performed (Fig. S4 in the ESM). The weight gained at about 300 °C is attributed to the conversion reaction of FeS to FeSO4. The decomposition of FeSO4 to Fe2O3 and carbon thermal reduction take place after the conversion reaction. The final weight loss is 16.6%, detemining that the ratios of carbon and FeS are 8.3% and 91.7%, repectively [11, 15].
FeS and BM-FeS/C composites were evaluated as anode materials for SIBs. The cyclic voltammograms (CVs) of FeS and BM-FeS/C composites at a scan rate of 0.1 mV·s−1 in a voltage range of 0.01–3.0 V (vs. Na+/Na) were shown in Figs. 2(a) and 2(c), respectively (see Fig. S6 for CVs of BM-FeS/C at different scan rates from 0.05 to 1.5 V·s−1).
In Fig. 2(a), a large cathodic peak at 0.45 V can be observed in the first cycle, which is attributed to the electrolyte decomposition, formation of the SEI layer and the conversion reaction of FeS. This first huge cathodic peak is observed at much higher potential (about 0.65 V) in BM-FeS/C composites (Fig. 2(c)), indicating that BM-FeS/C composites have smaller overpotential than FeS. BM-FeS also has a large cathodic peak at similar potential (~ 0.65 V) (Fig. S5 in the ESM), demonstrating that the reduced particle size can improve the kinetics. After the first cycle, the reduction peak is separated into two peaks at 0.88 and 0.32 V, and their redox peaks were formed at 1.72 and 1.43 V, respectively. These two redox pairs are corresponding to the reversible phase transitions of FeS. These reduction peaks also can be seen in BM-FeS/C composites (Fig. 2(c)) [11, 16]. The subsequent CV curves are almost overlapped after the first reduction reaction, especially for BM-FeS/C composites. The galvanostatical charge/discharge profiles are shown in Figs. 2(b) and 2(d). During the first cycle, the plateau was observed at higher potential in BM-FeS/C composites than that in FeS, which well agreed to the CV curve results. The high reversibility of BM-FeS/C composites, confirmed both in CVs and voltage profiles, is due to the reduced size of FeS particles and facile electron transport through thin carbon shell.
Figure 2 (a) Cyclic voltammograms and (b) voltage profiles of FeS. (c) Cyclic voltammograms and (d) voltage profiles of BM-FeS/C. (e) Cycle performance of FeS and BM-FeS/C composites at a current density of 100 mA·g−1.
Figure 2(e) shows the cycle performance of FeS and BM-FeS/C composites at a constant current density of 100 mA·g−1. The initial
discharge and charge capacities of BM-FeS/C composites are 632.5 and 494.16 mAh·g−1, respectively. The irreversible capacities were mainly from the electrolyte decomposition and formation of the SEI layer. For FeS, the reversible capacity is 458.3 mAh·g−1 after the first desodiation. The initial Coulombic efficiency for BM-FeS/C composites is 78.1% which is high for metal sulfides [28, 29].
The discharge capacity of BM-FeS/C composites are maintained at 490 mAh·g−1 for initial 20 cycles, and then it is gradually increased as shown in Fig. 3(e). This phenomena often can be seen in other transition metal chalcogenides [30, 31]. At the same time, FeS rapidly decreased to 295.0 mAh·g−1, when BM-FeS/C composites exhibit the highest discharge capacity of 589.8 mAh·g−1 even after 100 cycles.
The rate performance of FeS and BM-FeS/C composites were evaluated by changing the current density stepwise from 50 to 3,200 mA·g−1, and back to 50 mA·g−1 in Fig. 3(a). BM-FeS/C composites showed the final discharge capacities of 492.6, 474.9, 459.74, 444.0, 426.0, 399.5, 375.9, and 517.1 mAh·g−1 at current densities of 50, 100, 200, 400, 800, 1,600, 3,200 and 50 mA·g−1, respectively. After the current densities increased to 1,600 and 3,200 mA·g−1, the capacity gap between two becomes the largest. When the current density back to 50 mA·g−1, the capacity of BM-FeS/C composites still recovered to the high value even after high C-rate cycling. The voltage profiles of BM-FeS/C composites and FeS for the second cycle of each current density are shown in Figs. 3(b) and 3(c), respectively. The BM-FeS/C composites capacities decrease slowly with the increasing current densities (Fig. 3(b)), whereas the reversible capacities of FeS decrease much rapidly (Fig. 3(c)). Figure 3(d) presents the cycle performance of BM-FeS/C composites at various current densities (800, 1,000, and 1,500 mA·g−1). At all current densities, BM-FeS/C composites show excellent cycle stability. In particular, the reversible capacity was 423.6 mA·g−1 and the Coulombic efficiency was over 98%
Figure 3 (a) Rate properties of FeS and BM-FeS/C composites at different current densities. Voltage profiles of (b) FeS and (c) BM-FeS/C composites at various current densities. (d) Cycle performance of FeS and BM-FeS/C composites at current densities of 800, 1,000 and 1,500 mA·g−1. (e) Nyquist plots of FeS and BM-FeS/C composites before and after the first cycle (desodiated state, 3.0 V). (f) Comparison of relative capacity at various current densities for metal sulfides anode materials.
at 1,500 mA·g−1. It is noteworthy that the kinetics is much improved by the simple ball-milling method and polydopamine coating.
In Fig. 3(e), the electrochemical impedance spectroscopy (EIS) measurements of FeS and BM-FeS/C composites were performed before test and after 1 cycle. The semicircle of BM-FeS/C composites is much smaller than that of FeS, indicating that the BM-FeS/C composites have much improved charge transfer resistance. The comparison of relative capacity with other iron and metal sulfides anode materials at various current densities were displayed in Fig. 3(f).
The BM-FeS/C composites have superior rate properties when compared with other samples. All the results can clearly demonstrate the structural advantages of BM-FeS/C composites, which enables BM-FeS/C composites to have excellent performance with improved conductivity [12, 13, 16, 17, 32–38].
XANES was performed to elucidate the reaction mechanism of BM-FeS/C composites during charge/discharge (Fig. 4 and Fig. S7 in the ESM). During sodiation, the pre-edge peak shifted to lower energy with increasing peak intensity. The fully sodiated sample reached to the Fe metal, indicating the conversion electrochemical reactions.
This can be seen clearer in the first derivative plots (Fig. S7(a) in the ESM). During desodiation, the pre-edge peak of BM-FeS/C composites gradually returned to the initial state (Fig. 4(b)). At the same time, the increasing intensity of the white line also demonstrates the reversible oxidation of metallic Fe to FeS when the voltage up to 3V [39].
The excellent cycle performance and superior rate capabilities of BM-FeS/C composites can be attributed to the reduced size by simple high energy ball-milling method. Additionally, the thin carbon shell, from poly dopamine coating, can increase the electrical conductivity.
4 Conclusions
In summary, we prepared ball-milled FeS with thin carbon layer coating through the simple and economical process. As an anode material, the electrochemical properties of FeS were much improved by polydopamine coating, when compared with only commercial FeS and ball-milled FeS. The ball-milled FeS with thin carbon layer coating exhibits high reversible capacity of 589.8 mAh·g−1 after 100 cycles at a constant current density of 100 mA·g−1. Furthermore, the high capacity of 375.9 mAh·g−1 is delivered even at a very high current density of 3.2 A·g−1. These excellent electrochemical properties are due to the structural advantages. The reduced particle size of FeS can effectively accommodate the volume expansion. The carbon shell from poly dopamine can increase the electrical conductivity and play
Figure 4 XANES spectra of BM-FeS/C composites at Fe K edge during the first (a) sodiation and (b) desodiation.
a key role, especially for the high rate capability. In addition, the electrochemical reactions of BM-FeS/C composites are studied by X-ray absorption near edge structure analysis.
Acknowledgements
This work is based upon research conducted at the Institute for Basic Science (IBS) in Republic of Korea and Y. E. S. acknowledges the financial support by IBS-R006-A2.
Electronic Supplementary Material: Supplementary material (supporting figures showing SEM image, XRD patterns, XPS spectra, TGA curve, cyclic voltammograms, and voltage profiles) is available in the online version of this article at https://doi.org/10.1007/
s12274-019-2495-4.
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