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MoS2 composite intertwined on rGO nanosheets as a high-performance anode material for sodium-ion battery
Journal Pre-proof Facile synthesis of FeS2/MoS2 composite intertwined on rGO nanosheets as a highperformance anode material for sodium-ion battery Subramanian Yuvaraj, Ganesh Kumar Veerasubramani, Myung-Soo Park, Pandiarajan Thangavel, Dong-Won Kim PII:
S0925-8388(19)34468-8
DOI:
https://doi.org/10.1016/j.jallcom.2019.153222
Reference:
JALCOM 153222
To appear in:
Journal of Alloys and Compounds
Received Date: 9 August 2019 Revised Date:
14 November 2019
Accepted Date: 29 November 2019
Please cite this article as: S. Yuvaraj, G.K. Veerasubramani, M.-S. Park, P. Thangavel, D.-W. Kim, Facile synthesis of FeS2/MoS2 composite intertwined on rGO nanosheets as a high-performance anode material for sodium-ion battery, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.153222. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Facile synthesis of FeS2/MoS2 composite intertwined on rGO nanosheets as a high-performance anode material for sodium-ion battery
Subramanian Yuvaraja, Ganesh Kumar Veerasubramania, Myung-Soo Parka, Pandiarajan Thangavelb, Dong-Won Kima,*
a
Department of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea
b
Department of Chemistry, Ulsan National Institute of Science and Technology, Ulsan 44919,
Republic of Korea
*Corresponding author Tel.: +82 2 2220 2337; Fax: +82 2 2298 4101 E-mail address: [email protected] (D.-W. Kim)
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Abstract In this work, we demonstrate the FeS2/MoS2 composite embedded in rGO nanosheets (FeS2/MoS2-rGO composite) as an anode material for sodium-ion battery. The FeS2/MoS2 composite material was synthesized by one step hydrothermal method, and the prepared particles were wrapped by reduced graphene oxide (rGO) nanosheets, which facilitated the electronic transport between the particles, and suppressed the volume expansion of active materials as well as the polysulfide dissolution into the electrolyte during cycling. The FeS2/MoS2-rGO composite electrode delivered a high initial discharge capacity of 468.0 mAh g-1 and exhibited good cycling stability at a current density of 100 mA g-1. Due to the pseudocapacitive behavior of FeS2/MoS2rGO electrode, a high reversible capacity of 346.5 mAh g-1 was achieved at 3000 mA g-1, which was much higher than those of FeS2, MoS2 and FeS2/MoS2 composite electrodes. The sodiumion full cell assembled with FeS2/MoS2-rGO composite anode and Na3V2(PO4)2F3 cathode exhibited a high reversible capacity with good cycling stability, which demonstrates that the prepared FeS2/MoS2-rGO composite can be used as a promising anode material for sodium-ion batteries.
Keywords: FeS2/MoS2-rGO composite, Reduced graphene oxide, Anode material, Sodium-ion battery, Cycling performance
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1. Introduction The discovery of energy storage devices with high energy density is one of the most important research topics in the recent years. In the last three decades, lithium-ion batteries have been considered as the superior electrochemical storage device due to their high energy density and long cycle life [1-8]. However, lithium sources are limited in the earth crust, which lay down to find the alternative energy storage devices. In this respect, sodium-ion batteries have been attracted great interest as the cost-effective energy storage system due to largely abundant sodium sources. However, the ionic radius of sodium is slightly larger than the lithium, which may affect the cycling performance of electrode materials [9-12]. A lot of studies have been dedicated to discover the novel efficient electrode materials for sodium-ion batteries. In case of cathode materials, the sodium layered materials (NaMO2, M= Mn, Co, Ni and Fe), Nasicon-type sodium vanadium phosphates and fluorophosphates have been investigated for Na-ion battery application [13-17]. As the anode materials, hard carbon can be a good candidate owing to its low potential and high reversibility [18]. However, its low capacity and poor rate capability are hurdles to its practical implementation. Recently, pyrite FeS2 material has been considered as a suitable anode candidate due to its highly abundance in earth surface [19,20]. During the electrochemical reaction, it adopts four electron transfer reaction (FeS2 + 4Na ↔ Fe + 2Na2S), which results in high specific capacity (894 mAh g-1). However, it has some demerits such as poor electronic conductivity, large volume expansion during sodiation/de-sodiation process and polysulfide dissolution into the electrolyte, leading to large capacity fading and low rate capability [20-22]. Many research groups have paid attention to enhance the electrochemical performance of FeS2 materials. Walter et al. mitigated the volume expansion of FeS2 material by reducing the particle size [20]. It exhibited a high capacity of 410 mAh g-1 after 600 cycles, 3
however, the capacity loss was observed in nano-sized FeS2 particles due to polysulfide dissolution. Liu et al. reported the FeS2@C yolk-shell nanoboxes, which delivered a high capacity of 511 mAh g-1 with moderate capacity retention and good rate capability, since yolkshell structure can accommodate the volume strain and enhance the electronic conductivity [23]. Wang et al. synthesized the FeS2 and rGO composite, which exhibited the excellent electrochemical performance [24]. In addition, the studies on electrolyte optimization have been reported to increase the cycling performance of FeS2. Zhu et al. reported the ether-based electrolyte in Na/FeS2 system, which demonstrated the superior performance than carbonatebased electrolyte due to the suppression of polysulfide dissolution [25]. However, they reduced the cut-off voltage to avoid the conversion reaction, which decreased the reversible capacity of the FeS2 material. Binary metal chalcogenides are also attractive anode materials for Na-ion batteries due to their excellent sodium storage capability and long term cyclability [26-28]. Zhao et al. reported the Lychee-like FeS2@FeSe2 core-shell microspheres, which showed the excellent electrochemical performance due to the synergistic effects between FeS2 and FeSe2 [26]. Geng et al. also investigated the Co9S8/MoS2 yolk-shell spheres that showed the remarkable enhancement in cycling stability and rate capability than its individual counterparts [29]. To concern the above issues and advantages of binary chalcogenides, we investigate the FeS2/MoS2 composite embedded in rGO nanosheets (FeS2/MoS2-rGO composite) as an anode material for Na-ion batteries. Here, MoS2 is an electrochemically active material for Na-ion storage and can also effectively adsorb the polysulfide species during electrochemical reaction [30-32]. rGO nanosheets in the composite can facilitate fast electron transport in the electrode and accommodate the volume strain due to its high mechanical strength. In this respect, it is believed that the FeS2/MoS2-rGO composite can be a suitable anode material for sodium-based
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energy storage applications.
In this work, we synthesized the FeS2/MoS2-rGO composite
through single step hydrothermal method and investigated its electrochemical performance. The FeS2/MoS2-rGO electrode exhibited a high initial discharge capacity of 468.0 mAh g-1 with a capacity retention of 92.4% after 150 cycles and delivered the high capacity of 346.5 mAh g-1 even at current density of 3000 mA g-1. The sodium-ion full cell assembled with FeS2/MoS2-rGO anode and Na3V2(PO4)2F3 cathode exhibited high reversible capacity with good cycling stability. Such a superior cycling performance can be attributed to the synergistic effect of FeS2, MoS2 and rGO, and our results demonstrate that the FeS2/MoS2-rGO composite can be a promising anode material for sodium-ion batteries.
2. Experimental section 2.1. Synthesis of FeS2/MoS2 and FeS2/MoS2-rGO composite The FeS2/MoS2 composite was synthesized using hydrothermal method. In this process, 2 mmol of FeCl2, 2 mmol (NH4)6Mo7O24·4H2O (ammonium molybdate tetra hydrate) and 12 mmol of CH3CSNH2 (thioacetamide) were dissolved in 50 mL of distilled water under magnetic stirring. The solution was transferred to the Teflon lined autoclave and it was kept at 180 oC for 24 h. The obtained black colored FeS2/MoS2 powder was washed with distilled water/ethanol and dried at 80 oC for 12 h. The sample was finally heated at 400 oC for 2 h under Ar atmosphere. The FeS2/MoS2-rGO composite was prepared using same way with addition of 120 mg of graphene oxide. For comparison, FeS2 and MoS2 were synthesized individually as follows. FeS2 powder was synthesized through solvothermal method. In this process, 4 mmol of FeSO4·7H2O (ferrous sulfate heptahydrate), 20 mmol of sulfur powder and 20 mmol of urea were dissolved into mixed solvent of dimethyl formamide and ethylene glycol in the volume ratio of 5
4:3. The dispersion was then transferred into the Teflon lined autoclave and it was kept at 180 oC for 12 h. Finally, the black colored FeS2 powder was obtained. For MoS2 synthesis, 0.1619 g of H2MoO4 (molybdic acid) and 0.1502 g of CH3CSNH2 were dissolved in 40 mL of deionized water. Then the as-prepared precursor solution was transferred into the Teflon-lined stainlesssteel autoclave and maintained at 180 °C for 24 h. After the hydrothermal process, the autoclave was gradually cooled down to room temperature and the MoS2 powder was obtained by centrifugation. The obtained powder was washed several times with deionized water and then dried at 60 °C for a 12 h.
2.2. Synthesis of Na3V2(PO4)2F3 Na3V2(PO4)2F3 (NVPF) was synthesized via sol-gel technique. In the typical synthesis process, NaF (15 mmol), NH4VO3 (10 mmol), NH4H2PO4 (10 mmol), citric acid (8 mmol) were used as the starting precursors to get the NVPF particles. NH4VO3 and citric acid were dissolved in 20 ml of H2O under magnetic stirring at 50 oC for 1 h. Then, NH4H2PO4 and NaF solution were added into the above solution and the process continued at 80 oC. After evaporation of water, the blue-colored sol was obtained. It was dried at 120 oC for 12 h, and the dried sample was grounded and calcinated at 700 oC for 5 h in an Ar atmosphere to finally obtain high purity Na3V2(PO4)2F3 particles.
2.3. Characterization and measurements The crystalline structure was identified through Rigaku Mini Flex X-ray diffractometer (XRD) operating at 40 KV, 15 mA with Cu Kα radiation (1.5418 Å). Thermogravimetric analysis (TGA) was performed by using a thermal analyzer (SDT Q600, TA Instrument) in the 6
temperature range of 30 to 800 °C at heating rate of 5 °C min-1. The surface area was measured through BET isotherms using micrometrics 3Flex version 3.01 instruments. The particle morphologies were investigated by transmission electron microscopy (TEM, JEOL, JEM 2100F). X-ray photoelectron spectroscopy (XPS) was carried out using XPS (VG multilab ESCA system, 220i) with Mg/Al Ka radiation. Morphological analysis of the electrodes was performed using a scanning electron microscope (JEOL JSM 6701F).
2.4. Electrode preparation and cell assembly The electrode slurry was prepared using FeS2/MoS2-rGO as an active material, super P carbon and poly(amide imide) binder in the weight ratio of 70:15:15 in N-methyl pyrrolidine solvent. The slurry was grounded well, then uniformly coated on the Cu current collector using doctor blade and dried at 80 oC for 12 h under vacuum. The CR-2032 coin-cell was fabricated by sandwiching a glass fiber membrane as a separator between sodium foil and the prepared FeS2/MoS2-rGO electrode. A liquid electrolyte with consisting of 1.0 M NaClO4 in EC/PC (1:1 in vol.) containing 10.0 wt.% fluoroethylene carbonate (FEC) as an additive. The full cell was assembled with FeS2/MoS2-rGO anode and Na3V2(PO4)2F3 cathode using same electrolyte. When assembling the sodium-ion full cell, the mass ratio of anode to the cathode was optimized to 1.0:5.5. Cyclic voltammetry and electrochemical impedance spectroscopy were performed using a CH instrument (CHI660D electrochemical workstation). Galvanostatic charge-discharge experiment was carried out using battery testing equipment (WBCS 3000, WonATech) at 25 oC.
3. Results and discussion The FeS2/MoS2 composite was synthesized through one-step hydrothermal method by using 7
FeCl2, (NH4)6Mo7O24·H2O and CH3CSNH2 as starting precursors. The phase purity of the prepared samples was examined using XRD analysis, is shown in Fig. 1a. In XRD pattern of the FeS2/MoS2 composite, the sharp crystalline peaks could be observed at 28.5, 32.9, 37.1, 40.6, 47.2, 56.3, 61.4 and 64.5o, which confirms the crystalline structure of cubic phase in pyrite-FeS2 (JCPDS file No-89-3057). One broad diffraction peak was also observed at 14.3o, which corresponds to crystalline peak of MoS2 (JCPDS-77-1716). There was no change in XRD pattern of the FeS2/MoS2-rGO composite. For comparison, the XRD patterns of individual FeS2 and MoS2 particles are shown in Fig. S1. Each XRD pattern was well consistent with the cubic phase of FeS2 and hexagonal crystal structure of MoS2. The weight percentage of rGO in the FeS2/MoS2-rGO composite was measured from the TGA analysis. Fig. 1b shows the TGA curves of the FeS2/MoS2 and FeS2/MoS2-rGO composites, respectively. The pristine FeS2/MoS2 composite shows the weight gain in the temperature range of 250 to 360 oC due to the oxidation of FeS2 to Fe2(SO4)3, followed by weight loss in multi steps due to conversion of MoS2 to MoO3 and Fe2(SO4)3 to Fe2O3 [33,34]. Compared to pristine FeS2/MoS2 composite, the FeS2/MoS2rGO composite exhibits two additional weight losses. One weight loss (~2.6 wt.%) at 200 to 260 o
C is associated with thermal decomposition of the functional groups of hydroxyl, carbonyl and
epoxy groups in rGO [35].
Second weight loss (~14.1 wt.%) at around 410 to 500 oC
corresponds to the elimination of carbon in the composite. Accordingly, TGA results reveal that about 16.7 wt.% of carbon with functional groups presents in the FeS2/MoS2-rGO composite. Surface area and pore size distribution of the FeS2/MoS2 and FeS2/MoS2-rGO composites are shown in Fig. 1c and 1d, respectively. The N2 adsorption-desorption curves demonstrate the type-IV isotherm, indicating the existence of mesopores in the samples. The FeS2/MoS2-rGO composite exhibits surface area (95.44 m2 g-1) and pore volume (0.2979 cm3 g-1), which are
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higher than those of pristine FeS2/MoS2 composite (62.59 m2 g-1 and 0.1844 cm3 g-1). High surface area and large pore volume can assist to fast transport of sodium ions within the electrode and provide the high reactive sites for electrochemical reaction. TEM images of the pristine FeS2/MoS2 composite are given in Fig. 2a and 2b. As shown in figures, the nano-sized FeS2 and MoS2 particles are uniformly distributed in the FeS2/MoS2 composite. It also reveals that the FeS2 particles are well encapsulated by layered MoS2 structure with polycrystalline nature in the composite (Fig. 2c). The FeS2/MoS2 composite was well wrapped by transparent web-like graphene nanosheets in the FeS2/MoS2-rGO composite, as depicted in Fig. 2d and 2e. Its HR-TEM image (Fig. 2f) also shows that FeS2 and MoS2 particles are embedded in the rGO nanosheets. The calculated interlayer lattice spacings of about 0.17 and 0.60 nm correspond to the (311) plane of FeS2 and (002) plane of MoS2 with amorphous nature of rGO nanosheets. Selected area diffraction pattern (SAED) of the FeS2/MoS2-rGO composite in Fig. 2g reveals that it remains polycrystalline nature as similar to the FeS2/MoS2 composite. EDS element mapping was performed to investigate the distribution of various elements in the FeS2/MoS2-rGO composite, and the results are presented in Fig. 2h2m. As shown in figures, the Fe elements are mainly observed in the core, indicating FeS2 is wrapped by MoS2 and rGO nanosheets. It can provide the double-layer protection that can suppress the dissolution of polysulfide species into the electrolyte solution. Fig. S2 presents the elemental composition in the FeS2/MoS2-rGO composite. The composition of Mo, Fe, S, C and N elements in the FeS2/MoS2-rGO composite are 20.8, 17.2, 44.6, 17.1 and 0.3 wt.%, respectively. The chemical composition and valence state in the FeS2/MoS2-rGO composite were investigated by XPS analysis. In Fe 2P XPS spectrum of Fig. 3a, the binding energy levels of
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2p1/2 (720.5 eV) and 2p3/2 (707.4 eV) can be attributed to the existence of Fe2+ oxidation states in FeS2 compound [36,37]. The Mo 3d spectrum presents three peaks at 231.6, 228.4 and 225.5 eV (Fig. 3b). The first two peaks can be assigned to the Mo 3d3/2 and Mo 3d5/2 energy levels of Mo4+ oxidation state in MoS2 and later one is associated with the S 2s core-level [38]. The S 2p spectrum in Fig. 3c shows two binding energies at 162.2 and 161.2 eV, which corresponds to the splitting energy level of S 2p1/2 and S 2p3/2 in S2- ligand in FeS2/MoS2 compound [39]. The C 1s spectrum in Fig. 3d reveals the functional groups attached to the rGO. Two peaks at 284.1 and 285.3 eV can be assigned to the sp2 C and sp3 C groups, respectively. The remaining peaks at 286.4 and 288.2 eV are associated with the functional groups of C-O/C-N and O-C=O groups, respectively [40-43]. It implies the very feeble presence of oxygen containing groups that elucidate the reduction of GO into rGO in the composite. Moreover, the nitrogen doping on the graphene sheets helps to enhance the electronic conductivity of the composite material. Cyclic voltammetry of the FeS2/MoS2 and FeS2/MoS2-rGO composite electrodes are performed in the potential range of 0.01-3.0 V at scan rate of 0.2 mV s-1, and the resulting cyclic voltammograms are shown in Fig. 4. As compared to FeS2/MoS2 electrode, the FeS2/MoS2-rGO composite electrode exhibits the well distinguished oxidation and reduction peaks, which indicates faster electrode reaction kinetics in the FeS2/MoS2-rGO electrode compared to the pristine FeS2/MoS2 electrode. In the FeS2/MoS2-rGO electrode, three cathodic peaks observed at 1.43, 0.97 and 0.26 V are associated with the intercalation of Na+ ions into the FeS2/MoS2 (NaxFeS2 and NaxMoS2), solid electrolyte interphase (SEI) layer formation by electrolyte decomposition, and conversion reaction of NaxFeS2/NaxMoS2 into metallic Fe/Mo with the formation of Na2S [20,24,26]. During the anodic scan, the oxidation peaks at 1.31 and 1.91 V are corresponding to the oxidation of metallic Mo and Fe elements, which form the FeS2 and
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MoS2 species, and two broad oxidation peaks at 2.20 and 2.51 V are related with side reactions of polysulfides [44,45]. In the second cycle, the cathodic peaks observed at 2.05, 1.78 and 0.51 V are related with the step wise reversible reductions of FeS2 and MoS2 into metallic Fe and Mo nanoparticles with Na2S formation, which is well consistent with the previous reports [33,44,45]. In the cyclic voltammogram of the FeS2/MoS2-rGO electrode, the anodic and cathodic peaks shifted to lower and higher potential compared to pristine FeS2/MoS2 electrode, respectively, which was mainly caused by the reduced overpotential in the FeS2/MoS2-rGO electrode. In this electrode, the intertwined rGO layers act as an electronic wiring between the active materials and current collector, resulting in faster reaction kinetics than pristine FeS2/MoS2 electrode. This phenomenon usually occurs when the active materials are coated or composited with conducting matrix such as carbon and rGO [46]. Charge and discharge cycling test of the prepared electrodes are carried out at 100 mA g-1, and the representative cycling curves are shown in Fig. 5a and 5b. The sodiation process presents three sloping regions at 1.5-1.2, 1.2-0.4 and 0.4-0.01 V, which can be associated with the Na+ ion insertion into the crystal structure of FeS2/MoS2, electrolyte decomposition followed by NaxFeS2/NaxMoS2 into metallic Fe/Mo and Na2S formation. This result is well consistent with the cyclic voltammetry results discussed above, and can be also confirmed from the charge and discharge curves of pristine FeS2 and MoS2 electrodes shown in Fig. S3. During the desodiation cycle, the metallic Fe/Mo particles are converted to FeS2 and MoS2, which occur in multi-step processes at 0.25-1.60, 1.60-2.25 and 2.25-2.80 V. In the first cycle, the FeS2/MoS2 and FeS2/MoS2-rGO electrodes delivered the reversible capacities of 391.2 and 468.0 mAh g-1 with coulombic efficiencies of 63.0 and 60.1%, respectively. The low initial coulombic efficiency is associated with formation of SEI layer on the electrode surface during initial
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sodiation process. Fig. 5c presents the cycling performance of the FeS2/MoS2 and FeS2/MoS2rGO electrodes as a function of cycle number at 100 mA g-1. After initial few cycles, the coulombic efficiency is increased and then reached the steady state value over 99% throughout cycling. It is noticeable that the discharge capacity of pristine FeS2/MoS2 electrode is gradually increased to 459.1 mAh g-1, suddenly dropped after the 105th cycle and finally reached to 91.4 mAh g-1 at 150 cycles. The severe capacity fading can be explained by two factors. One is dissolution of polysulfide species into the electrolyte from the active materials during continuous cycling. The other is volume expansion that leads to detachment of active materials from the current collector and loss of interconnectivity between active materials, conducting carbon and polymer binder. It is thought that the capacity fading in the FeS2/MoS2 composite electrode is mainly originated from the FeS2 particles, because the cycling stability of pristine MoS2 electrode is better that pristine FeS2 electrode, as presented in Fig. S3 and S4. It has been well known that FeS2 electrode underwent sodium polysulfide dissolution into the electrolyte solution, resulting in loss of sulfur species from the active materials and large capacity fading with cycling [47-50]. The FeS2/MoS2-rGO composite electrode delivered a high de-sodiation capacity of 432.5 mAh g-1 after 150 cycles. The interconnected rGO layers with high surface area can effectively absorb the soluble polysulfide species during electrochemical reaction and thus inhibit the polysulfide dissolution which was confirmed by ex-situ analysis shown in Fig. S5. Fig. S5 depicts the ex-situ XRD patterns of the FeS2/MoS2-rGO electrode before and after cycling. After the repeated cycling, it shows three sharp peaks that correspond to Na2S and NaS2 phases. There is no signature of pristine metal sulfides due to amorphous nature or existence of other phases (NaxFeSy).
It reveals more than two Na-ions in the electrode, indicating the
existence of polysulfides in the composite. Also the formed polysulfide compounds are not
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dissolved into the electrolyte due to the wrapping by the rGO.
High mechanical flexibility of
rGO can also offer good electrode stability by alleviating the volume strain during sodiation and de-sodiation processes [47,48]. The rate capability of the electrodes was investigated at different current densities from 100 to 3000 mA g-1, and the results are compared in Fig. 5d. The FeS2/MoS2-rGO composite electrode delivered a high discharge capacity of 346.5 mAh g-1 even at high current density of 3000 mA g-1, which is much higher than those of the pristine FeS2/MoS2 composite (116.1 mAh g-1), pristine FeS2 (78.5 mAh g-1) and MoS2 (48.8 mAh g-1) electrodes (Fig. S6a and S6b). The interconnected rGO layer in the FeS2/MoS2-rGO composite enhances the electronic conductivity of the FeS2/MoS2 particles, which allows to fast electronic transport during electrochemical reaction. The FeS2/MoS2-rGO composite electrode showed the high reversible capacities at high current rates compared to those reported in the previous works, such as Fe1-xS@porous carbon, FeS2@rGO, FeS2@C nanorods, FeS2 microsphere/rGO and FeS2 nanocrystals [47-53]. Such a good cycling stability and rate capability of the FeS2/MoS2-rGO composite electrode can be achieved through the synergistic effects between the FeS2, MoS2 and rGO layers. Here, the FeS2 delivers a high reversible capacity, MoS2 and rGO layers well confine the FeS2 particles in the composite thereby suppress the polysulfide dissolution, and rGO maintains the electrode integrity without any deterioration from volume expansion as well as enhances the electronic conductivity [54]. We investigated the morphology changes of the electrodes after cycling. Fig. S7 shows the FE-SEM images of the FeS2/MoS2 and FeS2/MoS2rGO electrodes before and after cycling, respectively. Before cycling, all the active materials are uniformly coated on the current collector, as shown in Fig. S7a and S7c. After cycling, there are large cracks on the surface of the FeS2/MoS2 electrode due to large volume change during cycling (Fig. S7b). In case of the FeS2/MoS2-rGO electrode, the relatively dense electrode with
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small cracks can be observed on its surface, which is well attached to the current collector (Fig. S7d). This result is because the interconnected rGO layers act as a buffer matrix as well as polysulfide reservoir, keeping the integrity of the electrode. The electrochemical behavior of the electrodes are further investigated by electrochemical impedance spectroscopy. Fig. 5e and 5f present AC impedance spectra of the FeS2/MoS2 and FeS2/MoS2-rGO composite electrodes before and after cycling. They gave an overlapped semicircle at the high to medium frequency region, and sloped line at the low frequency region. The real axis intercept at high frequency can be ascribed to the electrolyte resistance (Re), and the depressed semicircle is associated with ionic resistance in SEI layer formed on the electrode surface (RSEI) and charge transfer resistance between the electrode and electrolyte interface (Rct), and straight line at low frequency corresponds to the sodium ion diffusion in the electrode (Warburg impedance, Zw). Accordingly, AC impedance spectra could be fitted using equivalent circuit in Fig. S8 and the fitting results are given in Table S1. The electrolyte resistance is slightly increased after cycling. In case of FeS2/MoS2 composite electrode, the interfacial resistance including RSEI and Rct is significantly increased after cycling, which can be attributed to the loss of electronic conductivity of the electrode by severe volume expansion. In contrast, the interfacial resistance is slightly increased after cycling in the FeS2/MoS2-rGO composite electrode. As a result, the interfacial resistance of FeS2/MoS2-rGO electrode is less than that of FeS2/MoS2 electrode after cycling, which elucidates fast charge transfer kinetics and the robust electrode integrity.
These results
demonstrated that the embedding of FeS2/MoS2 particles in the rGO nanosheets enhanced the electronic conductivity of the electrode and cycling stability. In order to investigate the reaction kinetics of the FeS2/MoS2-rGO composite electrode, cyclic voltammetry was performed at different scan rates ranging from 0.2 to 1.0 mV s-1, and the
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resulting cyclic voltammograms are presented in Fig. 6a. As expected, the peak current is increased with scan rate.
There are no remarkable changes in the shape of the cyclic
voltammogram with respect to the scan rate, suggesting the fast charge transfer and good reversible reactions. Sodium storage mechanism can be explained by the following relationship, as previously reported [55,56], ip = a × vb log ip = log a + b log v where ip and v are peak current and scan rate, and a and b are the adjustable parameters. When b equals 1.0, the electrochemical process is a capacitive-controlled reaction, whereas b is 0.5, the process is a diffusion-controlled reaction. Fig. 6b and Fig. S9a depict the linear relationship between the log ip vs. log v for cathodic and anodic peaks, respectively. From the cathodic peaks, “b” values are calculated to be 0.96, 0.80 and 0.92 for A, B and C peaks, respectively. On the other hand, the calculated “b” values are 0.94, 0.76, 0.95 and 0.93 for D, E, F and G in the anodic peaks, respectively. These values are near to 1.0, which reveals the domination in pseudocapacitive nature. Further quantitative analysis was carried out to distinguish the relative contribution of diffusion- and capacitive-controlled nature. The current response can be expressed at the fixed potential as follow [57,58], i = k1 v + k2 v1/2 where k1 v stands for the surface capacitive contribution and k2 v1/2 represents the contribution of the diffusion-controlled process. The k1 and k2 values are determined by using linear relationship between the v1/2 and i v-1/2 plot shown in Fig. S9b. According to the equation, the total charge is stored by the combination of diffusion-controlled and surface-capacitive controlled behavior. Fig. 6c presents the relative contribution to the total charge accumulation as a function of a scan
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rate, which illustrates that the relative capacitive contribution is increased with scan rate. Fig. 6d shows the typical cyclic voltammogram with separating the capacitive contribution from the total current at a scan rate of 0.2 mV s-1. It is clearly seen that the electrochemical reactions of the FeS2/MoS2-rGO composite electrode are mainly controlled by the pseudocapacitive reactions, which leads to a fast electrode kinetics resulting in excellent high rate performance. In addition, the diffusion coefficient is calculated from the peak current versus square root of scan rate using Randles-Sevcik equation [59-61], as presented in Fig. S10. The calculated diffusion coefficients are 4.0×10-12 and 5.3×10-11 cm2 s-1 for the FeS2/MoS2 electrode and FeS2/MoS2-rGO electrode, respectively, indicating that fast Na+ ion diffusion occurs in the FeS2/MoS2-rGO composite electrode. To further investigate the feasibility of FeS2/MoS2-rGO composite as an active anode material for sodium-ion battery, a sodium-ion full cell is assembled with the FeS2/MoS2-rGO anode and Na3V2(PO4)2F3 cathode with optimized mass ratio. Before assembling the full cell, the FeS2/MoS2-rGO anode is pre-sodiated for 5 cycles to prevent the side reactions between the electrode and electrolyte. Cycling performance of the Na/Na3V2(PO4)2F3 half-cell is shown in Fig. 7a and 7b. The Na3V2(PO4)2F3 cathode is delivered an initial discharge capacity of 92.1 mAh g-1 with good capacity retention at current density of 100 mA g-1. Fig. 7c shows the charge-discharge curves of Na-ion full cell at a constant current density of 200 mA g-1. The cell initially delivered a discharge capacity of 400.3 mAh g-1 based on the mass of anode. Good cycling stability was attained with low capacity loss during 100 cycles. It delivered a discharge capacity of 322.3 mAh g-1 after 100 cycles with capacity retention of 80.5% (Fig. 7d). These results are guaranteed that FeS2/MoS2-rGO composite will be considered as an anode material for sodium-ion batteries.
16
4. Conclusions The FeS2/MoS2-rGO composite material was successfully synthesized by hydrothermal synthesis and its electochemmical performance was investigated for sodium-ion battery application. The FeS2/MoS2-rGO compoiste electrode exhibited a high initial discharge capacity of 468.0 mAh g-1 with a capacity retention of 92.4% after 150 cycles at current density of 100 mA g-1. It delivered a high reversible capacity of 346.5 mAh g-1 at high current density of 3000 mA g-1, which was much higher than those of the FeS2/MoS2 composite, FeS2 and MoS2 electrodes. Good cycling stability and excellent rate capability could be achieved through the synergistic effects between the FeS2, MoS2 and rGO layers. Here, the FeS2 delivers a high reversible capacity, MoS2 and rGO layers well confine the FeS2 particles in the composite thereby suppress the polysulfide dissolution, and rGO maintains the electrode integrity without any deterioration from volume expansion as well as enhances the electronic conductivity. The Na-ion full cell assembled with FeS2/MoS2-rGO anode and Na3V2(PO4)2F3 cathode exhibited high reversible capacity with good capacity retention. In our study realized that FeS2/MoS2-rGO composite can be a good anode candidate for sodium-ion battery applications.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) funded by the
Korea
government
(Ministry
of
Science,
ICT
and
Future
Planning)
(2017R1A2A2A05020947 and 2019R1A4A2001527).
17
Appendix A. Supplementary data Supplementary
data
related
to
this
article
can
be
found
at
http://
dx.doi.org/10.1016/j.jallcom.XXX.
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25
Figure Captions
Fig. 1. (a) XRD patterns, (b) TGA curves, (c) N2 adsorption/desorption isotherms and (d) pore size distributions of FeS2/MoS2 and FeS2/MoS2-rGO composites.
Fig. 2. (a) TEM image and its magnified image (insert), (b) HR-TEM image and (c) SAED pattern of FeS2/MoS2 composite, (d) TEM image and (e) its magnified part, (f) HR-TEM image and (g) SAED pattern of FeS2/MoS2-rGO composite, (h-m) EDS element mapping images of Fe (yellow), Mo (pale blue), S (red), C (green) and N (orange) of FeS2/MoS2-rGO composite.
Fig. 3. (a) Fe 2p, (b) Mo 3d, (c) S 2p and (d) C 1s XPS spectra of FeS2/MoS2-rGO composite.
Fig. 4. Cyclic voltammograms of (a) FeS2/MoS2 and (b) FeS2/MoS2-rGO composite electrodes.
Fig. 5. Charge and discharge curves of (a) FeS2/MoS2 and (b) FeS2/MoS2-rGO electrodes. (c) Cycling stability and (d) rate capability of FeS2/MoS2 and FeS2/MoS2-rGO electrodes, and (e, f) AC impedance spectra of FeS2/MoS2 and FeS2/MoS2-rGO electrodes before and after cycles.
Fig. 6. (a) Cyclic voltammograms of FeS2/MoS2-rGO composite electrode at different scan rates and (b) linear plot between log ip and log v, (c) relative contribution and (d) graphical representation of capacitive area to total current.
26
Fig. 7. (a) Charge-discharge profile and (b) cycling stability of NVPF electrode at 100 mA g-1, (c) charge-discharge curves and cycling stability of Na-ion full cell assembled with FeS2/MoS2-rGO anode and NVPF cathode.
27
Fig. 1
110
FeS2/MoS2-rGO
Weight loss (%)
(311) (222) (023) (321)
(220)
(210) (211)
(200) (111)
(002)
Intensity (arb.unit)
(a)
FeS2/MoS2
(b)
FeS2/MoS2-rGO
100 2.6 wt.%
90 80
14.1 wt.%
70
rGO 16.7 wt.%
FeS2/MoS2
60 10
20
30
40
50
60
100
70
400
500
600
700
Temperature ( C)
200
-1
1.6 (c)
dV/dD (cm 3 g -1nm-1)
3
300
o
2 Theta (degree) Volume adsorbed (cm g )
200
FeS 2/MoS2 FeS 2/MoS2/rGO
160 120 80 40
(d)
1.4
FeS2/MoS2 FeS2/MoS2/rGO
1.2 1.0 0.8 0.6 0.4 0.2 0.0
0 0.0
0.2
0.4
0.6
0.8
Relative pressure (P/Po)
1.0
0
10
20
30
40
50
60
70
80
90
Pore diameter (nm)
28
Fig. 2
29
(a) Fe 2p
730
Fe 2p3/2
Intensity (arb. unit)
Intensity (arb. unit)
Fig. 3
Fe 2p1/2
725
720
715
710
705
700
Mo 3d3/2
S 2s
236 234 232 230 228 226 224 222 220
Binding energy (eV) 2
(c) S 2p
Intensity (arb. unit)
Intensity (arb. unit)
Binding energy (eV)
166
S 2p3/2 S 2p1/2
164
Mo 3d5/2
(b) Mo 3d
162
160
158
Binding energy (eV)
156
sp C 284.1
(d) C 1s
292
3
sp C 285.3 C-O/C-N 286.4 O-C=O 288.2
290
288
286
284
282
280
278
Binding energy (eV)
30
Fig. 4
1.0
1.0 (a) FeS2/MoS2
2.60 V
Current (mA)
Current (mA)
(b) FeS2/MoS2-rGO 1.31 V 1.91 V 2.20 V 2.51 V 0.5
1.41 V
0.5 0.0 -0.5
1.40 V 1.78 V
0.34 V 0.89 V
-1.0 0.20 V
1st cycle 2nd cycle
-1.5 -2.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0 +
Applied potential (V vs. Na/Na )
0.0 -0.5
1.43 V 2.05 V 1.78 V
0.51 V
-1.0 -1.5
0.26 V
1st cycle 2nd cycle
0.97 V
-2.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0 +
Applied potential (V vs. Na/Na )
31
Fig. 5
3.5
3.5
2.5 2.0 1.5 1.0
2.5 2.0 1.5 1.0
0.5
0.5
0.0
0.0 0
100
200
300
400
500
600
700
0
-1
100
500
80
400 60 300 40
200 FeS2/MoS2 FeS2/MoS2-rGO
100
20
0 0
25
50
75
100
125
0 150
-1
Discharge capacity (mAh g )
600
Coulombic efficiency (%)
-1
Discharge capacity (mAh g )
Specific capacity (mAh g ) 120
@ 100 mA g-1
(c)
100 200 300 400 500 600 700 800 -1
Specific capacity (mAh g ) 700
600
(d) 500
100 mA/g 200
1800
FeS2/MoS 2 FeS2/MoS 2-rGO 300
500
1000 1500 2000 2500 3000
400 300 200 100 0 0
10
20
30
40
Cycle number
Cycle number 1800
(e) FeS2/MoS2
Before cycles After 150 cycles
1500 1200
(f) FeS2/MoS2-rGO
Before cycles After 150 cycles
1500 1200
-Z''(ohm)
-Z''(ohm)
1st cyle 2nd cycle 5th cycle
3.0
Voltage (V)
Voltage (V)
3.0
(b) FeS2/MoS2-rGO
1st cyle 2nd cycle 5th cycle
(a) FeS2/MoS2
900 600 300
900 600 300
0 0
300
600
900
1200
Z' (ohm)
1500
1800
0 0
300
600
900
1200
1500
1800
Z' (ohm)
32
Fig. 6
2.0
0.4 F
-1
G
D
0.0 -1.0 -2.0
A
B
0.2 0.4 0.6 0.8 1.0
C
log current (A g )
E
1.0
-1
Current (A g )
(a)
-1
mV s -1 mV s -1 mV s -1 mV s -1 mV s
-3.0 0.0
0.5
1.0
1.5
2.0
2.5
B
0.0
A -0.2 -0.4 -0.6 -0.8 -1.0 -0.8
3.0
+
-0.6
Potential (V vs. Na/Na )
0.0
-1
Total Capacitive
(d) 0.4 -1
100
-0.2
0.6
Diffusion Capacitive
(c)
-0.4
log scan rate (mV s )
Current (A g )
Relative contribution (%)
120
C
(b)
0.2
80 60 68.8
79.2
81.7
86.4
87.8
40 20 0 0.2
0.4
0.6
0.8
-1
Scan rate (mV s )
1.0
0.2 0.0 -0.2 -0.4 -0.6 0.0
0.5
1.0
1.5
2.0
2.5
3.0
+
Potential (V vs. Na/Na )
33
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Highlights
▶ FeS2/MoS2 composite embedded in rGO nanosheets is synthesized as an anode material. ▶ The FeS2/MoS2-rGO delivers a high discharge capacity of 346.5 mAh g-1 at 3000 mA g-1. ▶ Pseudo capacitive nature of FeS2/MoS2-rGO leads to a fast electrode kinetics. ▶ Sodium-ion full cell assembled with FeS2/MoS2-rGO exhibits good cycling performance.
We would like to state authors’ contributions to the paper using the relevant CRediT roles.
Author
Contribution
Subramanian Yuvaraj
Conceptualization, Methodology, Writing-Original Draft
Ganesh Kumar Veerasubramani
Visualization, Investigation
Myung-Soo Park
Formal Analysis, Resources
Pandiarajan Thangavel
Formal Analysis
Dong-Won Kim
Supervision, Writing-Reviewing and administration, Funding acquisition
Editing,
Project
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34468-8
DOI:
https://doi.org/10.1016/j.jallcom.2019.153222
Reference:
JALCOM 153222
To appear in:
Journal of Alloys and Compounds
Received Date: 9 August 2019 Revised Date:
14 November 2019
Accepted Date: 29 November 2019
Please cite this article as: S. Yuvaraj, G.K. Veerasubramani, M.-S. Park, P. Thangavel, D.-W. Kim, Facile synthesis of FeS2/MoS2 composite intertwined on rGO nanosheets as a high-performance anode material for sodium-ion battery, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.153222. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Facile synthesis of FeS2/MoS2 composite intertwined on rGO nanosheets as a high-performance anode material for sodium-ion battery
Subramanian Yuvaraja, Ganesh Kumar Veerasubramania, Myung-Soo Parka, Pandiarajan Thangavelb, Dong-Won Kima,*
a
Department of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea
b
Department of Chemistry, Ulsan National Institute of Science and Technology, Ulsan 44919,
Republic of Korea
*Corresponding author Tel.: +82 2 2220 2337; Fax: +82 2 2298 4101 E-mail address: [email protected] (D.-W. Kim)
1
Abstract In this work, we demonstrate the FeS2/MoS2 composite embedded in rGO nanosheets (FeS2/MoS2-rGO composite) as an anode material for sodium-ion battery. The FeS2/MoS2 composite material was synthesized by one step hydrothermal method, and the prepared particles were wrapped by reduced graphene oxide (rGO) nanosheets, which facilitated the electronic transport between the particles, and suppressed the volume expansion of active materials as well as the polysulfide dissolution into the electrolyte during cycling. The FeS2/MoS2-rGO composite electrode delivered a high initial discharge capacity of 468.0 mAh g-1 and exhibited good cycling stability at a current density of 100 mA g-1. Due to the pseudocapacitive behavior of FeS2/MoS2rGO electrode, a high reversible capacity of 346.5 mAh g-1 was achieved at 3000 mA g-1, which was much higher than those of FeS2, MoS2 and FeS2/MoS2 composite electrodes. The sodiumion full cell assembled with FeS2/MoS2-rGO composite anode and Na3V2(PO4)2F3 cathode exhibited a high reversible capacity with good cycling stability, which demonstrates that the prepared FeS2/MoS2-rGO composite can be used as a promising anode material for sodium-ion batteries.
Keywords: FeS2/MoS2-rGO composite, Reduced graphene oxide, Anode material, Sodium-ion battery, Cycling performance
2
1. Introduction The discovery of energy storage devices with high energy density is one of the most important research topics in the recent years. In the last three decades, lithium-ion batteries have been considered as the superior electrochemical storage device due to their high energy density and long cycle life [1-8]. However, lithium sources are limited in the earth crust, which lay down to find the alternative energy storage devices. In this respect, sodium-ion batteries have been attracted great interest as the cost-effective energy storage system due to largely abundant sodium sources. However, the ionic radius of sodium is slightly larger than the lithium, which may affect the cycling performance of electrode materials [9-12]. A lot of studies have been dedicated to discover the novel efficient electrode materials for sodium-ion batteries. In case of cathode materials, the sodium layered materials (NaMO2, M= Mn, Co, Ni and Fe), Nasicon-type sodium vanadium phosphates and fluorophosphates have been investigated for Na-ion battery application [13-17]. As the anode materials, hard carbon can be a good candidate owing to its low potential and high reversibility [18]. However, its low capacity and poor rate capability are hurdles to its practical implementation. Recently, pyrite FeS2 material has been considered as a suitable anode candidate due to its highly abundance in earth surface [19,20]. During the electrochemical reaction, it adopts four electron transfer reaction (FeS2 + 4Na ↔ Fe + 2Na2S), which results in high specific capacity (894 mAh g-1). However, it has some demerits such as poor electronic conductivity, large volume expansion during sodiation/de-sodiation process and polysulfide dissolution into the electrolyte, leading to large capacity fading and low rate capability [20-22]. Many research groups have paid attention to enhance the electrochemical performance of FeS2 materials. Walter et al. mitigated the volume expansion of FeS2 material by reducing the particle size [20]. It exhibited a high capacity of 410 mAh g-1 after 600 cycles, 3
however, the capacity loss was observed in nano-sized FeS2 particles due to polysulfide dissolution. Liu et al. reported the FeS2@C yolk-shell nanoboxes, which delivered a high capacity of 511 mAh g-1 with moderate capacity retention and good rate capability, since yolkshell structure can accommodate the volume strain and enhance the electronic conductivity [23]. Wang et al. synthesized the FeS2 and rGO composite, which exhibited the excellent electrochemical performance [24]. In addition, the studies on electrolyte optimization have been reported to increase the cycling performance of FeS2. Zhu et al. reported the ether-based electrolyte in Na/FeS2 system, which demonstrated the superior performance than carbonatebased electrolyte due to the suppression of polysulfide dissolution [25]. However, they reduced the cut-off voltage to avoid the conversion reaction, which decreased the reversible capacity of the FeS2 material. Binary metal chalcogenides are also attractive anode materials for Na-ion batteries due to their excellent sodium storage capability and long term cyclability [26-28]. Zhao et al. reported the Lychee-like FeS2@FeSe2 core-shell microspheres, which showed the excellent electrochemical performance due to the synergistic effects between FeS2 and FeSe2 [26]. Geng et al. also investigated the Co9S8/MoS2 yolk-shell spheres that showed the remarkable enhancement in cycling stability and rate capability than its individual counterparts [29]. To concern the above issues and advantages of binary chalcogenides, we investigate the FeS2/MoS2 composite embedded in rGO nanosheets (FeS2/MoS2-rGO composite) as an anode material for Na-ion batteries. Here, MoS2 is an electrochemically active material for Na-ion storage and can also effectively adsorb the polysulfide species during electrochemical reaction [30-32]. rGO nanosheets in the composite can facilitate fast electron transport in the electrode and accommodate the volume strain due to its high mechanical strength. In this respect, it is believed that the FeS2/MoS2-rGO composite can be a suitable anode material for sodium-based
4
energy storage applications.
In this work, we synthesized the FeS2/MoS2-rGO composite
through single step hydrothermal method and investigated its electrochemical performance. The FeS2/MoS2-rGO electrode exhibited a high initial discharge capacity of 468.0 mAh g-1 with a capacity retention of 92.4% after 150 cycles and delivered the high capacity of 346.5 mAh g-1 even at current density of 3000 mA g-1. The sodium-ion full cell assembled with FeS2/MoS2-rGO anode and Na3V2(PO4)2F3 cathode exhibited high reversible capacity with good cycling stability. Such a superior cycling performance can be attributed to the synergistic effect of FeS2, MoS2 and rGO, and our results demonstrate that the FeS2/MoS2-rGO composite can be a promising anode material for sodium-ion batteries.
2. Experimental section 2.1. Synthesis of FeS2/MoS2 and FeS2/MoS2-rGO composite The FeS2/MoS2 composite was synthesized using hydrothermal method. In this process, 2 mmol of FeCl2, 2 mmol (NH4)6Mo7O24·4H2O (ammonium molybdate tetra hydrate) and 12 mmol of CH3CSNH2 (thioacetamide) were dissolved in 50 mL of distilled water under magnetic stirring. The solution was transferred to the Teflon lined autoclave and it was kept at 180 oC for 24 h. The obtained black colored FeS2/MoS2 powder was washed with distilled water/ethanol and dried at 80 oC for 12 h. The sample was finally heated at 400 oC for 2 h under Ar atmosphere. The FeS2/MoS2-rGO composite was prepared using same way with addition of 120 mg of graphene oxide. For comparison, FeS2 and MoS2 were synthesized individually as follows. FeS2 powder was synthesized through solvothermal method. In this process, 4 mmol of FeSO4·7H2O (ferrous sulfate heptahydrate), 20 mmol of sulfur powder and 20 mmol of urea were dissolved into mixed solvent of dimethyl formamide and ethylene glycol in the volume ratio of 5
4:3. The dispersion was then transferred into the Teflon lined autoclave and it was kept at 180 oC for 12 h. Finally, the black colored FeS2 powder was obtained. For MoS2 synthesis, 0.1619 g of H2MoO4 (molybdic acid) and 0.1502 g of CH3CSNH2 were dissolved in 40 mL of deionized water. Then the as-prepared precursor solution was transferred into the Teflon-lined stainlesssteel autoclave and maintained at 180 °C for 24 h. After the hydrothermal process, the autoclave was gradually cooled down to room temperature and the MoS2 powder was obtained by centrifugation. The obtained powder was washed several times with deionized water and then dried at 60 °C for a 12 h.
2.2. Synthesis of Na3V2(PO4)2F3 Na3V2(PO4)2F3 (NVPF) was synthesized via sol-gel technique. In the typical synthesis process, NaF (15 mmol), NH4VO3 (10 mmol), NH4H2PO4 (10 mmol), citric acid (8 mmol) were used as the starting precursors to get the NVPF particles. NH4VO3 and citric acid were dissolved in 20 ml of H2O under magnetic stirring at 50 oC for 1 h. Then, NH4H2PO4 and NaF solution were added into the above solution and the process continued at 80 oC. After evaporation of water, the blue-colored sol was obtained. It was dried at 120 oC for 12 h, and the dried sample was grounded and calcinated at 700 oC for 5 h in an Ar atmosphere to finally obtain high purity Na3V2(PO4)2F3 particles.
2.3. Characterization and measurements The crystalline structure was identified through Rigaku Mini Flex X-ray diffractometer (XRD) operating at 40 KV, 15 mA with Cu Kα radiation (1.5418 Å). Thermogravimetric analysis (TGA) was performed by using a thermal analyzer (SDT Q600, TA Instrument) in the 6
temperature range of 30 to 800 °C at heating rate of 5 °C min-1. The surface area was measured through BET isotherms using micrometrics 3Flex version 3.01 instruments. The particle morphologies were investigated by transmission electron microscopy (TEM, JEOL, JEM 2100F). X-ray photoelectron spectroscopy (XPS) was carried out using XPS (VG multilab ESCA system, 220i) with Mg/Al Ka radiation. Morphological analysis of the electrodes was performed using a scanning electron microscope (JEOL JSM 6701F).
2.4. Electrode preparation and cell assembly The electrode slurry was prepared using FeS2/MoS2-rGO as an active material, super P carbon and poly(amide imide) binder in the weight ratio of 70:15:15 in N-methyl pyrrolidine solvent. The slurry was grounded well, then uniformly coated on the Cu current collector using doctor blade and dried at 80 oC for 12 h under vacuum. The CR-2032 coin-cell was fabricated by sandwiching a glass fiber membrane as a separator between sodium foil and the prepared FeS2/MoS2-rGO electrode. A liquid electrolyte with consisting of 1.0 M NaClO4 in EC/PC (1:1 in vol.) containing 10.0 wt.% fluoroethylene carbonate (FEC) as an additive. The full cell was assembled with FeS2/MoS2-rGO anode and Na3V2(PO4)2F3 cathode using same electrolyte. When assembling the sodium-ion full cell, the mass ratio of anode to the cathode was optimized to 1.0:5.5. Cyclic voltammetry and electrochemical impedance spectroscopy were performed using a CH instrument (CHI660D electrochemical workstation). Galvanostatic charge-discharge experiment was carried out using battery testing equipment (WBCS 3000, WonATech) at 25 oC.
3. Results and discussion The FeS2/MoS2 composite was synthesized through one-step hydrothermal method by using 7
FeCl2, (NH4)6Mo7O24·H2O and CH3CSNH2 as starting precursors. The phase purity of the prepared samples was examined using XRD analysis, is shown in Fig. 1a. In XRD pattern of the FeS2/MoS2 composite, the sharp crystalline peaks could be observed at 28.5, 32.9, 37.1, 40.6, 47.2, 56.3, 61.4 and 64.5o, which confirms the crystalline structure of cubic phase in pyrite-FeS2 (JCPDS file No-89-3057). One broad diffraction peak was also observed at 14.3o, which corresponds to crystalline peak of MoS2 (JCPDS-77-1716). There was no change in XRD pattern of the FeS2/MoS2-rGO composite. For comparison, the XRD patterns of individual FeS2 and MoS2 particles are shown in Fig. S1. Each XRD pattern was well consistent with the cubic phase of FeS2 and hexagonal crystal structure of MoS2. The weight percentage of rGO in the FeS2/MoS2-rGO composite was measured from the TGA analysis. Fig. 1b shows the TGA curves of the FeS2/MoS2 and FeS2/MoS2-rGO composites, respectively. The pristine FeS2/MoS2 composite shows the weight gain in the temperature range of 250 to 360 oC due to the oxidation of FeS2 to Fe2(SO4)3, followed by weight loss in multi steps due to conversion of MoS2 to MoO3 and Fe2(SO4)3 to Fe2O3 [33,34]. Compared to pristine FeS2/MoS2 composite, the FeS2/MoS2rGO composite exhibits two additional weight losses. One weight loss (~2.6 wt.%) at 200 to 260 o
C is associated with thermal decomposition of the functional groups of hydroxyl, carbonyl and
epoxy groups in rGO [35].
Second weight loss (~14.1 wt.%) at around 410 to 500 oC
corresponds to the elimination of carbon in the composite. Accordingly, TGA results reveal that about 16.7 wt.% of carbon with functional groups presents in the FeS2/MoS2-rGO composite. Surface area and pore size distribution of the FeS2/MoS2 and FeS2/MoS2-rGO composites are shown in Fig. 1c and 1d, respectively. The N2 adsorption-desorption curves demonstrate the type-IV isotherm, indicating the existence of mesopores in the samples. The FeS2/MoS2-rGO composite exhibits surface area (95.44 m2 g-1) and pore volume (0.2979 cm3 g-1), which are
8
higher than those of pristine FeS2/MoS2 composite (62.59 m2 g-1 and 0.1844 cm3 g-1). High surface area and large pore volume can assist to fast transport of sodium ions within the electrode and provide the high reactive sites for electrochemical reaction. TEM images of the pristine FeS2/MoS2 composite are given in Fig. 2a and 2b. As shown in figures, the nano-sized FeS2 and MoS2 particles are uniformly distributed in the FeS2/MoS2 composite. It also reveals that the FeS2 particles are well encapsulated by layered MoS2 structure with polycrystalline nature in the composite (Fig. 2c). The FeS2/MoS2 composite was well wrapped by transparent web-like graphene nanosheets in the FeS2/MoS2-rGO composite, as depicted in Fig. 2d and 2e. Its HR-TEM image (Fig. 2f) also shows that FeS2 and MoS2 particles are embedded in the rGO nanosheets. The calculated interlayer lattice spacings of about 0.17 and 0.60 nm correspond to the (311) plane of FeS2 and (002) plane of MoS2 with amorphous nature of rGO nanosheets. Selected area diffraction pattern (SAED) of the FeS2/MoS2-rGO composite in Fig. 2g reveals that it remains polycrystalline nature as similar to the FeS2/MoS2 composite. EDS element mapping was performed to investigate the distribution of various elements in the FeS2/MoS2-rGO composite, and the results are presented in Fig. 2h2m. As shown in figures, the Fe elements are mainly observed in the core, indicating FeS2 is wrapped by MoS2 and rGO nanosheets. It can provide the double-layer protection that can suppress the dissolution of polysulfide species into the electrolyte solution. Fig. S2 presents the elemental composition in the FeS2/MoS2-rGO composite. The composition of Mo, Fe, S, C and N elements in the FeS2/MoS2-rGO composite are 20.8, 17.2, 44.6, 17.1 and 0.3 wt.%, respectively. The chemical composition and valence state in the FeS2/MoS2-rGO composite were investigated by XPS analysis. In Fe 2P XPS spectrum of Fig. 3a, the binding energy levels of
9
2p1/2 (720.5 eV) and 2p3/2 (707.4 eV) can be attributed to the existence of Fe2+ oxidation states in FeS2 compound [36,37]. The Mo 3d spectrum presents three peaks at 231.6, 228.4 and 225.5 eV (Fig. 3b). The first two peaks can be assigned to the Mo 3d3/2 and Mo 3d5/2 energy levels of Mo4+ oxidation state in MoS2 and later one is associated with the S 2s core-level [38]. The S 2p spectrum in Fig. 3c shows two binding energies at 162.2 and 161.2 eV, which corresponds to the splitting energy level of S 2p1/2 and S 2p3/2 in S2- ligand in FeS2/MoS2 compound [39]. The C 1s spectrum in Fig. 3d reveals the functional groups attached to the rGO. Two peaks at 284.1 and 285.3 eV can be assigned to the sp2 C and sp3 C groups, respectively. The remaining peaks at 286.4 and 288.2 eV are associated with the functional groups of C-O/C-N and O-C=O groups, respectively [40-43]. It implies the very feeble presence of oxygen containing groups that elucidate the reduction of GO into rGO in the composite. Moreover, the nitrogen doping on the graphene sheets helps to enhance the electronic conductivity of the composite material. Cyclic voltammetry of the FeS2/MoS2 and FeS2/MoS2-rGO composite electrodes are performed in the potential range of 0.01-3.0 V at scan rate of 0.2 mV s-1, and the resulting cyclic voltammograms are shown in Fig. 4. As compared to FeS2/MoS2 electrode, the FeS2/MoS2-rGO composite electrode exhibits the well distinguished oxidation and reduction peaks, which indicates faster electrode reaction kinetics in the FeS2/MoS2-rGO electrode compared to the pristine FeS2/MoS2 electrode. In the FeS2/MoS2-rGO electrode, three cathodic peaks observed at 1.43, 0.97 and 0.26 V are associated with the intercalation of Na+ ions into the FeS2/MoS2 (NaxFeS2 and NaxMoS2), solid electrolyte interphase (SEI) layer formation by electrolyte decomposition, and conversion reaction of NaxFeS2/NaxMoS2 into metallic Fe/Mo with the formation of Na2S [20,24,26]. During the anodic scan, the oxidation peaks at 1.31 and 1.91 V are corresponding to the oxidation of metallic Mo and Fe elements, which form the FeS2 and
10
MoS2 species, and two broad oxidation peaks at 2.20 and 2.51 V are related with side reactions of polysulfides [44,45]. In the second cycle, the cathodic peaks observed at 2.05, 1.78 and 0.51 V are related with the step wise reversible reductions of FeS2 and MoS2 into metallic Fe and Mo nanoparticles with Na2S formation, which is well consistent with the previous reports [33,44,45]. In the cyclic voltammogram of the FeS2/MoS2-rGO electrode, the anodic and cathodic peaks shifted to lower and higher potential compared to pristine FeS2/MoS2 electrode, respectively, which was mainly caused by the reduced overpotential in the FeS2/MoS2-rGO electrode. In this electrode, the intertwined rGO layers act as an electronic wiring between the active materials and current collector, resulting in faster reaction kinetics than pristine FeS2/MoS2 electrode. This phenomenon usually occurs when the active materials are coated or composited with conducting matrix such as carbon and rGO [46]. Charge and discharge cycling test of the prepared electrodes are carried out at 100 mA g-1, and the representative cycling curves are shown in Fig. 5a and 5b. The sodiation process presents three sloping regions at 1.5-1.2, 1.2-0.4 and 0.4-0.01 V, which can be associated with the Na+ ion insertion into the crystal structure of FeS2/MoS2, electrolyte decomposition followed by NaxFeS2/NaxMoS2 into metallic Fe/Mo and Na2S formation. This result is well consistent with the cyclic voltammetry results discussed above, and can be also confirmed from the charge and discharge curves of pristine FeS2 and MoS2 electrodes shown in Fig. S3. During the desodiation cycle, the metallic Fe/Mo particles are converted to FeS2 and MoS2, which occur in multi-step processes at 0.25-1.60, 1.60-2.25 and 2.25-2.80 V. In the first cycle, the FeS2/MoS2 and FeS2/MoS2-rGO electrodes delivered the reversible capacities of 391.2 and 468.0 mAh g-1 with coulombic efficiencies of 63.0 and 60.1%, respectively. The low initial coulombic efficiency is associated with formation of SEI layer on the electrode surface during initial
11
sodiation process. Fig. 5c presents the cycling performance of the FeS2/MoS2 and FeS2/MoS2rGO electrodes as a function of cycle number at 100 mA g-1. After initial few cycles, the coulombic efficiency is increased and then reached the steady state value over 99% throughout cycling. It is noticeable that the discharge capacity of pristine FeS2/MoS2 electrode is gradually increased to 459.1 mAh g-1, suddenly dropped after the 105th cycle and finally reached to 91.4 mAh g-1 at 150 cycles. The severe capacity fading can be explained by two factors. One is dissolution of polysulfide species into the electrolyte from the active materials during continuous cycling. The other is volume expansion that leads to detachment of active materials from the current collector and loss of interconnectivity between active materials, conducting carbon and polymer binder. It is thought that the capacity fading in the FeS2/MoS2 composite electrode is mainly originated from the FeS2 particles, because the cycling stability of pristine MoS2 electrode is better that pristine FeS2 electrode, as presented in Fig. S3 and S4. It has been well known that FeS2 electrode underwent sodium polysulfide dissolution into the electrolyte solution, resulting in loss of sulfur species from the active materials and large capacity fading with cycling [47-50]. The FeS2/MoS2-rGO composite electrode delivered a high de-sodiation capacity of 432.5 mAh g-1 after 150 cycles. The interconnected rGO layers with high surface area can effectively absorb the soluble polysulfide species during electrochemical reaction and thus inhibit the polysulfide dissolution which was confirmed by ex-situ analysis shown in Fig. S5. Fig. S5 depicts the ex-situ XRD patterns of the FeS2/MoS2-rGO electrode before and after cycling. After the repeated cycling, it shows three sharp peaks that correspond to Na2S and NaS2 phases. There is no signature of pristine metal sulfides due to amorphous nature or existence of other phases (NaxFeSy).
It reveals more than two Na-ions in the electrode, indicating the
existence of polysulfides in the composite. Also the formed polysulfide compounds are not
12
dissolved into the electrolyte due to the wrapping by the rGO.
High mechanical flexibility of
rGO can also offer good electrode stability by alleviating the volume strain during sodiation and de-sodiation processes [47,48]. The rate capability of the electrodes was investigated at different current densities from 100 to 3000 mA g-1, and the results are compared in Fig. 5d. The FeS2/MoS2-rGO composite electrode delivered a high discharge capacity of 346.5 mAh g-1 even at high current density of 3000 mA g-1, which is much higher than those of the pristine FeS2/MoS2 composite (116.1 mAh g-1), pristine FeS2 (78.5 mAh g-1) and MoS2 (48.8 mAh g-1) electrodes (Fig. S6a and S6b). The interconnected rGO layer in the FeS2/MoS2-rGO composite enhances the electronic conductivity of the FeS2/MoS2 particles, which allows to fast electronic transport during electrochemical reaction. The FeS2/MoS2-rGO composite electrode showed the high reversible capacities at high current rates compared to those reported in the previous works, such as Fe1-xS@porous carbon, FeS2@rGO, FeS2@C nanorods, FeS2 microsphere/rGO and FeS2 nanocrystals [47-53]. Such a good cycling stability and rate capability of the FeS2/MoS2-rGO composite electrode can be achieved through the synergistic effects between the FeS2, MoS2 and rGO layers. Here, the FeS2 delivers a high reversible capacity, MoS2 and rGO layers well confine the FeS2 particles in the composite thereby suppress the polysulfide dissolution, and rGO maintains the electrode integrity without any deterioration from volume expansion as well as enhances the electronic conductivity [54]. We investigated the morphology changes of the electrodes after cycling. Fig. S7 shows the FE-SEM images of the FeS2/MoS2 and FeS2/MoS2rGO electrodes before and after cycling, respectively. Before cycling, all the active materials are uniformly coated on the current collector, as shown in Fig. S7a and S7c. After cycling, there are large cracks on the surface of the FeS2/MoS2 electrode due to large volume change during cycling (Fig. S7b). In case of the FeS2/MoS2-rGO electrode, the relatively dense electrode with
13
small cracks can be observed on its surface, which is well attached to the current collector (Fig. S7d). This result is because the interconnected rGO layers act as a buffer matrix as well as polysulfide reservoir, keeping the integrity of the electrode. The electrochemical behavior of the electrodes are further investigated by electrochemical impedance spectroscopy. Fig. 5e and 5f present AC impedance spectra of the FeS2/MoS2 and FeS2/MoS2-rGO composite electrodes before and after cycling. They gave an overlapped semicircle at the high to medium frequency region, and sloped line at the low frequency region. The real axis intercept at high frequency can be ascribed to the electrolyte resistance (Re), and the depressed semicircle is associated with ionic resistance in SEI layer formed on the electrode surface (RSEI) and charge transfer resistance between the electrode and electrolyte interface (Rct), and straight line at low frequency corresponds to the sodium ion diffusion in the electrode (Warburg impedance, Zw). Accordingly, AC impedance spectra could be fitted using equivalent circuit in Fig. S8 and the fitting results are given in Table S1. The electrolyte resistance is slightly increased after cycling. In case of FeS2/MoS2 composite electrode, the interfacial resistance including RSEI and Rct is significantly increased after cycling, which can be attributed to the loss of electronic conductivity of the electrode by severe volume expansion. In contrast, the interfacial resistance is slightly increased after cycling in the FeS2/MoS2-rGO composite electrode. As a result, the interfacial resistance of FeS2/MoS2-rGO electrode is less than that of FeS2/MoS2 electrode after cycling, which elucidates fast charge transfer kinetics and the robust electrode integrity.
These results
demonstrated that the embedding of FeS2/MoS2 particles in the rGO nanosheets enhanced the electronic conductivity of the electrode and cycling stability. In order to investigate the reaction kinetics of the FeS2/MoS2-rGO composite electrode, cyclic voltammetry was performed at different scan rates ranging from 0.2 to 1.0 mV s-1, and the
14
resulting cyclic voltammograms are presented in Fig. 6a. As expected, the peak current is increased with scan rate.
There are no remarkable changes in the shape of the cyclic
voltammogram with respect to the scan rate, suggesting the fast charge transfer and good reversible reactions. Sodium storage mechanism can be explained by the following relationship, as previously reported [55,56], ip = a × vb log ip = log a + b log v where ip and v are peak current and scan rate, and a and b are the adjustable parameters. When b equals 1.0, the electrochemical process is a capacitive-controlled reaction, whereas b is 0.5, the process is a diffusion-controlled reaction. Fig. 6b and Fig. S9a depict the linear relationship between the log ip vs. log v for cathodic and anodic peaks, respectively. From the cathodic peaks, “b” values are calculated to be 0.96, 0.80 and 0.92 for A, B and C peaks, respectively. On the other hand, the calculated “b” values are 0.94, 0.76, 0.95 and 0.93 for D, E, F and G in the anodic peaks, respectively. These values are near to 1.0, which reveals the domination in pseudocapacitive nature. Further quantitative analysis was carried out to distinguish the relative contribution of diffusion- and capacitive-controlled nature. The current response can be expressed at the fixed potential as follow [57,58], i = k1 v + k2 v1/2 where k1 v stands for the surface capacitive contribution and k2 v1/2 represents the contribution of the diffusion-controlled process. The k1 and k2 values are determined by using linear relationship between the v1/2 and i v-1/2 plot shown in Fig. S9b. According to the equation, the total charge is stored by the combination of diffusion-controlled and surface-capacitive controlled behavior. Fig. 6c presents the relative contribution to the total charge accumulation as a function of a scan
15
rate, which illustrates that the relative capacitive contribution is increased with scan rate. Fig. 6d shows the typical cyclic voltammogram with separating the capacitive contribution from the total current at a scan rate of 0.2 mV s-1. It is clearly seen that the electrochemical reactions of the FeS2/MoS2-rGO composite electrode are mainly controlled by the pseudocapacitive reactions, which leads to a fast electrode kinetics resulting in excellent high rate performance. In addition, the diffusion coefficient is calculated from the peak current versus square root of scan rate using Randles-Sevcik equation [59-61], as presented in Fig. S10. The calculated diffusion coefficients are 4.0×10-12 and 5.3×10-11 cm2 s-1 for the FeS2/MoS2 electrode and FeS2/MoS2-rGO electrode, respectively, indicating that fast Na+ ion diffusion occurs in the FeS2/MoS2-rGO composite electrode. To further investigate the feasibility of FeS2/MoS2-rGO composite as an active anode material for sodium-ion battery, a sodium-ion full cell is assembled with the FeS2/MoS2-rGO anode and Na3V2(PO4)2F3 cathode with optimized mass ratio. Before assembling the full cell, the FeS2/MoS2-rGO anode is pre-sodiated for 5 cycles to prevent the side reactions between the electrode and electrolyte. Cycling performance of the Na/Na3V2(PO4)2F3 half-cell is shown in Fig. 7a and 7b. The Na3V2(PO4)2F3 cathode is delivered an initial discharge capacity of 92.1 mAh g-1 with good capacity retention at current density of 100 mA g-1. Fig. 7c shows the charge-discharge curves of Na-ion full cell at a constant current density of 200 mA g-1. The cell initially delivered a discharge capacity of 400.3 mAh g-1 based on the mass of anode. Good cycling stability was attained with low capacity loss during 100 cycles. It delivered a discharge capacity of 322.3 mAh g-1 after 100 cycles with capacity retention of 80.5% (Fig. 7d). These results are guaranteed that FeS2/MoS2-rGO composite will be considered as an anode material for sodium-ion batteries.
16
4. Conclusions The FeS2/MoS2-rGO composite material was successfully synthesized by hydrothermal synthesis and its electochemmical performance was investigated for sodium-ion battery application. The FeS2/MoS2-rGO compoiste electrode exhibited a high initial discharge capacity of 468.0 mAh g-1 with a capacity retention of 92.4% after 150 cycles at current density of 100 mA g-1. It delivered a high reversible capacity of 346.5 mAh g-1 at high current density of 3000 mA g-1, which was much higher than those of the FeS2/MoS2 composite, FeS2 and MoS2 electrodes. Good cycling stability and excellent rate capability could be achieved through the synergistic effects between the FeS2, MoS2 and rGO layers. Here, the FeS2 delivers a high reversible capacity, MoS2 and rGO layers well confine the FeS2 particles in the composite thereby suppress the polysulfide dissolution, and rGO maintains the electrode integrity without any deterioration from volume expansion as well as enhances the electronic conductivity. The Na-ion full cell assembled with FeS2/MoS2-rGO anode and Na3V2(PO4)2F3 cathode exhibited high reversible capacity with good capacity retention. In our study realized that FeS2/MoS2-rGO composite can be a good anode candidate for sodium-ion battery applications.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) funded by the
Korea
government
(Ministry
of
Science,
ICT
and
Future
Planning)
(2017R1A2A2A05020947 and 2019R1A4A2001527).
17
Appendix A. Supplementary data Supplementary
data
related
to
this
article
can
be
found
at
http://
dx.doi.org/10.1016/j.jallcom.XXX.
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25
Figure Captions
Fig. 1. (a) XRD patterns, (b) TGA curves, (c) N2 adsorption/desorption isotherms and (d) pore size distributions of FeS2/MoS2 and FeS2/MoS2-rGO composites.
Fig. 2. (a) TEM image and its magnified image (insert), (b) HR-TEM image and (c) SAED pattern of FeS2/MoS2 composite, (d) TEM image and (e) its magnified part, (f) HR-TEM image and (g) SAED pattern of FeS2/MoS2-rGO composite, (h-m) EDS element mapping images of Fe (yellow), Mo (pale blue), S (red), C (green) and N (orange) of FeS2/MoS2-rGO composite.
Fig. 3. (a) Fe 2p, (b) Mo 3d, (c) S 2p and (d) C 1s XPS spectra of FeS2/MoS2-rGO composite.
Fig. 4. Cyclic voltammograms of (a) FeS2/MoS2 and (b) FeS2/MoS2-rGO composite electrodes.
Fig. 5. Charge and discharge curves of (a) FeS2/MoS2 and (b) FeS2/MoS2-rGO electrodes. (c) Cycling stability and (d) rate capability of FeS2/MoS2 and FeS2/MoS2-rGO electrodes, and (e, f) AC impedance spectra of FeS2/MoS2 and FeS2/MoS2-rGO electrodes before and after cycles.
Fig. 6. (a) Cyclic voltammograms of FeS2/MoS2-rGO composite electrode at different scan rates and (b) linear plot between log ip and log v, (c) relative contribution and (d) graphical representation of capacitive area to total current.
26
Fig. 7. (a) Charge-discharge profile and (b) cycling stability of NVPF electrode at 100 mA g-1, (c) charge-discharge curves and cycling stability of Na-ion full cell assembled with FeS2/MoS2-rGO anode and NVPF cathode.
27
Fig. 1
110
FeS2/MoS2-rGO
Weight loss (%)
(311) (222) (023) (321)
(220)
(210) (211)
(200) (111)
(002)
Intensity (arb.unit)
(a)
FeS2/MoS2
(b)
FeS2/MoS2-rGO
100 2.6 wt.%
90 80
14.1 wt.%
70
rGO 16.7 wt.%
FeS2/MoS2
60 10
20
30
40
50
60
100
70
400
500
600
700
Temperature ( C)
200
-1
1.6 (c)
dV/dD (cm 3 g -1nm-1)
3
300
o
2 Theta (degree) Volume adsorbed (cm g )
200
FeS 2/MoS2 FeS 2/MoS2/rGO
160 120 80 40
(d)
1.4
FeS2/MoS2 FeS2/MoS2/rGO
1.2 1.0 0.8 0.6 0.4 0.2 0.0
0 0.0
0.2
0.4
0.6
0.8
Relative pressure (P/Po)
1.0
0
10
20
30
40
50
60
70
80
90
Pore diameter (nm)
28
Fig. 2
29
(a) Fe 2p
730
Fe 2p3/2
Intensity (arb. unit)
Intensity (arb. unit)
Fig. 3
Fe 2p1/2
725
720
715
710
705
700
Mo 3d3/2
S 2s
236 234 232 230 228 226 224 222 220
Binding energy (eV) 2
(c) S 2p
Intensity (arb. unit)
Intensity (arb. unit)
Binding energy (eV)
166
S 2p3/2 S 2p1/2
164
Mo 3d5/2
(b) Mo 3d
162
160
158
Binding energy (eV)
156
sp C 284.1
(d) C 1s
292
3
sp C 285.3 C-O/C-N 286.4 O-C=O 288.2
290
288
286
284
282
280
278
Binding energy (eV)
30
Fig. 4
1.0
1.0 (a) FeS2/MoS2
2.60 V
Current (mA)
Current (mA)
(b) FeS2/MoS2-rGO 1.31 V 1.91 V 2.20 V 2.51 V 0.5
1.41 V
0.5 0.0 -0.5
1.40 V 1.78 V
0.34 V 0.89 V
-1.0 0.20 V
1st cycle 2nd cycle
-1.5 -2.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0 +
Applied potential (V vs. Na/Na )
0.0 -0.5
1.43 V 2.05 V 1.78 V
0.51 V
-1.0 -1.5
0.26 V
1st cycle 2nd cycle
0.97 V
-2.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0 +
Applied potential (V vs. Na/Na )
31
Fig. 5
3.5
3.5
2.5 2.0 1.5 1.0
2.5 2.0 1.5 1.0
0.5
0.5
0.0
0.0 0
100
200
300
400
500
600
700
0
-1
100
500
80
400 60 300 40
200 FeS2/MoS2 FeS2/MoS2-rGO
100
20
0 0
25
50
75
100
125
0 150
-1
Discharge capacity (mAh g )
600
Coulombic efficiency (%)
-1
Discharge capacity (mAh g )
Specific capacity (mAh g ) 120
@ 100 mA g-1
(c)
100 200 300 400 500 600 700 800 -1
Specific capacity (mAh g ) 700
600
(d) 500
100 mA/g 200
1800
FeS2/MoS 2 FeS2/MoS 2-rGO 300
500
1000 1500 2000 2500 3000
400 300 200 100 0 0
10
20
30
40
Cycle number
Cycle number 1800
(e) FeS2/MoS2
Before cycles After 150 cycles
1500 1200
(f) FeS2/MoS2-rGO
Before cycles After 150 cycles
1500 1200
-Z''(ohm)
-Z''(ohm)
1st cyle 2nd cycle 5th cycle
3.0
Voltage (V)
Voltage (V)
3.0
(b) FeS2/MoS2-rGO
1st cyle 2nd cycle 5th cycle
(a) FeS2/MoS2
900 600 300
900 600 300
0 0
300
600
900
1200
Z' (ohm)
1500
1800
0 0
300
600
900
1200
1500
1800
Z' (ohm)
32
Fig. 6
2.0
0.4 F
-1
G
D
0.0 -1.0 -2.0
A
B
0.2 0.4 0.6 0.8 1.0
C
log current (A g )
E
1.0
-1
Current (A g )
(a)
-1
mV s -1 mV s -1 mV s -1 mV s -1 mV s
-3.0 0.0
0.5
1.0
1.5
2.0
2.5
B
0.0
A -0.2 -0.4 -0.6 -0.8 -1.0 -0.8
3.0
+
-0.6
Potential (V vs. Na/Na )
0.0
-1
Total Capacitive
(d) 0.4 -1
100
-0.2
0.6
Diffusion Capacitive
(c)
-0.4
log scan rate (mV s )
Current (A g )
Relative contribution (%)
120
C
(b)
0.2
80 60 68.8
79.2
81.7
86.4
87.8
40 20 0 0.2
0.4
0.6
0.8
-1
Scan rate (mV s )
1.0
0.2 0.0 -0.2 -0.4 -0.6 0.0
0.5
1.0
1.5
2.0
2.5
3.0
+
Potential (V vs. Na/Na )
33
5.0 (a)
Voltage (V)
4.5 4.0 3.5 3.0
1st cycle 10th cycle 50th cycle 100th cycle
2.5 2.0 1.5 0
20
40
60
80
100
Specific capacity (mAh g-1)
Fig. 7
120 (b) 100 80 60 40 Charge Discharge
20 0 0
-1
20
(c)
Voltage (V)
4.0
1st cycle 10th cycle 50th cycle 100th cycle
1.0 0.0 0
100
200
300
400
-1
Specific capacity (mAh g )
500
Specific capacity (mAh g-1)
5.0
2.0
60
80
100
Cycle number
Specific capacity (mAh g )
3.0
40
500 (d) 400 300 200 100
Charge Discharge
0 0
20
40
60
80
100
Cycle number
34
Highlights
▶ FeS2/MoS2 composite embedded in rGO nanosheets is synthesized as an anode material. ▶ The FeS2/MoS2-rGO delivers a high discharge capacity of 346.5 mAh g-1 at 3000 mA g-1. ▶ Pseudo capacitive nature of FeS2/MoS2-rGO leads to a fast electrode kinetics. ▶ Sodium-ion full cell assembled with FeS2/MoS2-rGO exhibits good cycling performance.
We would like to state authors’ contributions to the paper using the relevant CRediT roles.
Author
Contribution
Subramanian Yuvaraj
Conceptualization, Methodology, Writing-Original Draft
Ganesh Kumar Veerasubramani
Visualization, Investigation
Myung-Soo Park
Formal Analysis, Resources
Pandiarajan Thangavel
Formal Analysis
Dong-Won Kim
Supervision, Writing-Reviewing and administration, Funding acquisition
Editing,
Project
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: