Fe3S4 Nanoparticles Wrapped in an rGO Matrix for Promising Energy

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Fe3S4 Nanoparticles Wrapped in rGO Matrix for Promising Energy Storage: Outstanding Cyclic and Rate Performance Sheng-Ping Guo, Jia-Chuang Li, Jin-Rong Xiao, and Huaiguo Xue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10406 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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ACS Applied Materials & Interfaces

Fe3S4 Nanoparticles Wrapped in rGO Matrix for Promising Energy Storage: Outstanding Cyclic and Rate Performance Sheng-Ping Guo,∗ Jia-Chuang Li, Jin-Rong Xiao and Huai-Guo Xue∗ School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, China

ABSTRACT: Iron sulfides/oxides/fluorides have been profoundly investigated as electrodes for rechargeable batteries recently in view of their high theory capacities, low cost, and environmental benign. Here, Fe3S4 nanoparticles wrapped in reduced graphene oxide (Fe3S4 NPs@rGO) has been obtained using a simple one-pot hydrothermal approach, which is characterized using various techniques. As the anode for LIBs, Fe3S4 NPs@rGO displays a reversible discharge capacity of 950 mAh/g after 100 cycles at 0.1 A/g, and 720 mAh/g capacity can be left after 800 cycles even at 1 A/g. Even at 10 A/g, 462 mAh/g capacity can be maintained. The excellent electrochemical properties for Fe3S4 NPs@rGO can be ascribed to a collaborative effect between Fe3S4 NPs and rGO matrix, which possess high Li-ion storage ability and excellent conductivity, respectively.

KEYWORDS:

Fe3S4

NPs@rGO,

one-pot

hydrothermal

approach,

LIBs,

anode,

electrochemical properties

1. INTRODUCTION Energy crisis pushes researchers to develop various energy storage and conversion devices,

ACS Paragon Plus Environment


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referring to LIBs, SIBs, Li-S batteries, supercapacitors, fuel cell, solar cell, et al.1−5 As a type of important energy storage device, LIBs are extensively used in many aspects in our daily life, including laptops, wrist watches, mobile phones, etc.6,7 Specifically, LIBs, when used as power suppliers for electric vehicles (EVs) and hybrid EVs/plug-in EVs (HEVs/PHEVs), have the potential to replace the traditional fossil energies. It is highly expected that the EVs or HEVs/PHEVs can run as long distance as possible after charging for one time with low cost, however, the available capacity of known LIBs can hardly meet these requirements.8−10 Commercial anode graphene for LIBs with low capacity (372 mAh/g) hinders LIBs’ further application. Hence, it is necessary to look for novel anodes or optimize known anodes to obtain anodes with much better electrochemical performance. Transition metal chalcogenides (TMCs) have been paid much attention because of their much higher theoretical capacities than commercial graphite. Among these TMCs, iron sulfides are extensively investigated as the anodes in view of their rich sources, cheap prices and no environmental pollution. In the past, many good results have been achieved successfully on FeS and FeS2.6,11−13 Another one binary iron sulfide, Fe3S4 (greigite), a kind of significant half-metallic magnetic material, has been investigated its application for energy storage batteries, electrochemical H2 storage and paleomagnetism.9,14,15 Compared with FeS and FeS2, Fe3S4 has a better electrical conductivity, and there are much less studies for its application as anode materials.9,15-18 Fe3S4 also has a theory capacity as high as 725 mAh/g, and the investigations have reported that Fe3S4 anode demonstrated good electrochemical performance. For example, T. T. M. Palstra, et al. reported the Fe3S4 microcrystals exhibited the capacity of 562.9 mAh/g (0.1 A/g) after one hundred of cycles.16 H. Zhang, et al studied


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that the Fe3S4 nanoplates had the 1st discharge capacity of 1207 mAh/g, while only 495 mAh/g (0.1 A/g) could be left after 100 cycles.17 J. Zheng, et al. investigated that mesoporous hollow-sphere Fe3S4 could deliver the capacity of 750 mAh/g (0.2 A/g) after 100 cycles.18 Later, J. Zheng, et al. prepared Fe3S4 nanosheets had the 1st discharge capacity of 1691 mAh/g, and around 32.4 % capacity could be maintained after 120 cycles at 0.2 A/g.15 From these results, it can be noticed that the known electrochemical performance of Fe3S4 is promising but not better optimized, much more work should be done to improve its electrochemical performance. To date, there is no report on the electrochemical performance of Fe3S4@rGO composite, which is only recently investigated to remove Pb2+ ions from water.19 In this work, the Fe3S4 NPs dispersed in reduced graphene oxide, namely, Fe3S4 NPs@rGO, has been successfully prepared using a simple in-situ hydrothermal reaction. It has a first discharge capacity of 1367 at 100 mA/g and 950 mAh/g can be remained after 100 cycles. Moreover, 720 mAh/g capacity (1 A/g) can be remained after 800 cycles, and 462 mAh/g capacity could be reached at 10 A/g.

2. EXPERIMENTAL Preparation of Fe3S4 NPs@rGO and Fe3S4. Graphene oxide (GO) has been obtained employing Hummer’s method with modification, which was scattered in the deionized water to form a certain concentration of homogeneous solution (6 mg/ml) under ultrasonic condition.20 In a representative synthetic process, 10 ml GO solution was firstly added into 70 ml ethylene glycol, stirring for 30 minutes. Secondly, 596.4 mg (3 mmol) FeCl2·4H2O and



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489.7 mg (4 mmol) L-cysteine were then poured to the GO solution, and followed stirring for more 30 minutes. Third, the solution was moved to an autoclave, then maintained for one day under 200 °C. Last, the products were obtained using centrifuging (5000 rpm for 5 minutes) after naturally cooled down to environmental temperature, cleaned for five times using water and ethanol in turn, then vacuum dried under 70 °C for half a day to obtain desired Fe3S4 NPs@rGO composite. The pure Fe3S4 sample was obtained by using the same method without adding GO. The rGO can be also obtained by using the same method. Material Characterizations. The samples’ purities have been analyzed employing a Bruker D8 Advance powder X-ray diffractometer. Pyris 1 Thermogravimetric analyzer has been employed to analyze the weight percentage for Fe3S4 in Fe3S4 NPs@rGO at 10 ºC per minute in environmental atmosphere up to 700 °C. The chemical compositions of Fe3S4 NPs@rGO were characterized employing a Bruker Quantax EDS instrument. The distributions of Fe, S, C elements in Fe3S4 NPs@rGO can be monitored using EDS element mapping analysis. Zeiss-Supra55 SEM, Tecnai G2 F30 S-TWIN HRTEM, and Philips Tecnai12 TEM instruments have been used to get the surface morphologies for Fe3S4 NPs@rGO. Thermofisher Scientific ESCALAB250Xi XPS and Renishaw inVia Raman spectrum were introduced to characterize the structures and composition of Fe3S4 NPs@rGO, respectively. Electrochemical Measurements. Fe3S4 NPs@rGO electrode slurry for CR 2032-type cell was produced using 8 : 1 : 1 Fe3S4 NPs@rGO, carbon black, and PVDF, together with some NMP by continuous mechanical stirring for eight hours. A current collector-Cu foil was used to uniformly hold the slurry to get the electrode plate. After dried at 80 and 120 °C for 8 and


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12 hours in a vacuum oven, respectively, the plated was cut to Φ 1.6 cm disks. The active material’s mass was weighted to ~1.4 mg. Pure Fe3S4 was also employed as active material to prepare the electrode slurry using the same method. The half-cells have been assembled in a glovebox. Li-foil was employed as the counter electrode, and Celgard 2325 film was used as the separator. The electrolyte was 1 M LiPF6 in DEC/EC with the molar ratio of 1 : 1. C–V curves between 0.005–3.0 V at 0.1 mV/s and EIS curves at 2.8 V in the frequency range 0.01–105 kHz were measured by a CHI660D electrochemical workstation. The cyclic behavior was tested at 0.1 A/g from 0.005 to 3.0 Volts employing a NEWARE CT-3008 battery test instrument. 3. Results and discussion

(a)

(b)

Figure 1. (a) The XRD results for the pure Fe3S4 (red) and Fe3S4 NPs@rGO (blue) samples. (b) The TGA curves for pure Fe3S4, Fe3S4 NPs@rGO, and rGO samples, respectively.

Structure and Characterizations. The PXRD patterns for Fe3S4 NPs@rGO and pure Fe3S4 are demonstrated in Figure 1a. The main peaks for Fe3S4 NPs@rGO and pure Fe3S4 match well with the standard one of Fe3S4 (JCPDS No. 89-1998, cubic phase), indicating that the obtained samples are pure Fe3S4 NPs@rGO and Fe3S4, respectively. The broad diffraction



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peaks indicates that the Fe3S4 particles in both of samples have small crystallite sizes. Moreover, the content of Fe3S4 in Fe3S4 NPs@rGO is calculated to be around 84 % according to the results of TGA analysis (Figure 1b). As presented in Figure 2a, EDS analysis of Fe3S4 NPs@rGO indicates that S and Fe have the molar ratio closed to 4 : 3, in accordance with the PXRD results. The EDS element mapping (Figure 2b) analysis of Fe3S4 NPs@rGO was further preformed and showed the distribution of carbon (blue), iron (red) and sulfur (green). The highly color overlay proves that the Fe3S4 NPs are wrapped well in rGO matrix.

(a)

(b)

Figure 2. (a) The EDS result for the Fe3S4 NPs@rGO sample. (b) The EDS elemental mapping results for Fe, S and C in Fe3S4 NPs@rGO.


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The Raman spectra of the Fe3S4 NPs@rGO (red), pure Fe3S4 (blue) and rGO (black) samples were measured to better understand the structure of Fe3S4 NPs@rGO (Figure 3). For pure Fe3S4, only two visible characteristic peaks observed, corresponding to the characteristic peaks of Fe3S4.16 Meanwhile, this result also suggests the existence of Fe3S4 in Fe3S4 NPs@rGO. For the Fe3S4 NPs@rGO and rGO samples, there are two sharp peaks at 1590 and 1350 cm–1, meaning crystalline (G-band) and disordered (D-band) graphite, respectively. The intensity of D-band is around 1.1 times that of G-band in Fe3S4 NPs@rGO, which is higher than that (0.94) of rGO. The increase of the D-band intensity can be ascribed to the Fe3S4 NPs wrapped in rGO, which is possibly caused by the interaction between C (graphene) and S.20,21 Moreover, lots of defects on the surface (ID : IG>1) could not only provide more transport channels for Li-ions during the cyclic process, but also make the connectivity between Fe3S4 NPs and rGO matrix more effectively.22,23

Figure 3. Raman spectra for the Fe3S4 NPs@rGO, pure Fe3S4 and rGO samples.

To continue ascertain the chemical compositions and valence states for Fe3S4 NPs@rGO, XPS measurement (Figure 4) was performed. Fe, S, C and O elements, expected for



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Fe3S4NPs@rGO, were clearly detected (Figure 4a). In the C-1s spectrum, there are five peaks located at 289.93, 286.4, 285.9, 284.82, and 284.78 eV, which are fitted to O=C–O, C=O, C–O, C=C, and C–C bonds, respectively (Figure 4b).23–25 In addition, the O=C–O and C–O bonds indicate GO is not completely reduced after the hydrothermal reaction. The narrow peaks at 713.2 and 726.5 eV, matched with the diagnostic peaks for Fe3+ ions, and the other two peaks at 711 and 724.5 eV belong to the Fe2+ species (Figure 4c).26,27 The smooth peaks of S 2p spectrum (Figure 4d) at 163.5 and 165.1 eV are assigned to the S2– ions.10,26 The molar ratio of Fe and S is around 3 : 4, which is obtained from the calculated peaks’ areas and employed atomic sensitivity factors.

(a)

(b)

(c)

(d)

Figure 4. XPS spectra for the Fe3S4 NPs@rGO sample.

The morphologies and sizes of pure Fe3S4 and Fe3S4 NPs@rGO were analyzed via


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observing the SEM, TEM, and HRTEM images (Figure 5). The pure Fe3S4 particles are unevenly distributed and aggregated (Figure 5a–b). Compared with pure Fe3S4, the SEM image of Fe3S4 NPs@rGO shows that the Fe3S4 NPs are wrapped in rGO layers, and a few ones are attached on the surface and edges of the rGO matrix (Figure 5c). A number of O-based functional groups exist in the structure of rGO, such as COOH, C-OH and COH, which can easily coordinate with Fe2+ ions and make Fe2+ ions adhered to the rGO surface and edges.22 Therefore, the distribution of Fe3S4 NPs is selective on rGO, depending on the positions of functional groups. In addition, the interaction of functional groups and selective nucleation of Fe3S4 NPs on rGO can effectively inhibit Fe3S4 NPs from further growing and aggregating, which can be proved by the TEM images (Figure 5d). Figure 5d shows that Fe3S4 NPs have a good dispersion on rGO matrix, and have sizes in the range of 60–140 nm. The HRTEM image (Figure 5e) indicates that Fe3S4 NPs are wrapped with GO layers and the lattice distances are around 0.35, 0.19, 0.17, 0.25, and 0.30 nm (Figure 5f), corresponding to the (220) , (511), (440), (400), and (311) planes for the cubic Fe3S4 phase, respectively. The adhesion of Fe3S4 NPs to rGO matrix is beneficial for the improvement of electrical conduction for Fe3S4 [email protected],22–24 (a)

(b)


(c)


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(d)

(e)

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(f)

Figure 5. SEM (a) and TEM (b) images for Fe3S4. SEM (c), TEM (d), and HRTEM (e, f) images for Fe3S4 NPs@rGO.

Electrochemical Performance. The C–V curves of Fe3S4 NPs@rGO and pure Fe3S4 were measured (Figure 6). During the initial discharge curve, three peaks can be found at 1.57, 1.16 and 0.75 V in Figure 6a. The shoulder at 1.57 V corresponds to form Li2FeS2, and a sharp peak at 1.16 V indicates that Li2FeS2 is converted to Fe and Li2S.10,12,16,18 (1)

Fe3S4

(2)

Li2FeS2

4Li+

+ +

4e–

+

2Li+

+

→

2e–

→

2Li2FeS2

+

2Li2S

+

Fe Fe

1.57 V 1.16 V

The wide peak at 0.75 V corresponds to form SEI film, which mainly induce the loss of irreversible capacity.13,16 A large peak at 1.97 V corresponds with a oxidation process from Fe to Li2FeS2.12,16,28–31 The small peak at 2.48 V suggests that the beginning of the delithiation process (Li2FeS2 to Li2-xFeS2).12,29,32 For the first three curves, the positions of two cathodic peaks don’t shift and become stable, which can be explained by the following reversible reactions: (3)

Fe

+

(4)

Li2FeS2

2Li2S

+

Li2-xFeS2

Li2FeS2 +

xLi+

2Li +


+ xe–

2e–

1.97 V 2.48 V

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(a)

(b)

Figure 6. C–V plots for Fe3S4 NPs@rGO (a) and pure Fe3S4 (b) in 0.005–3 V.

At the beginning of the 2nd cycle, the cathodic peak at 0.75 V vanishes, and other two cathodic ones move to 1.4 and 1.87 V, respectively.16 Fe3S4 NPs@rGO has the similar C–V curves with that of pure Fe3S4 (Figure 6b). However, the area of the closed curves for pure Fe3S4 are getting more and more small with the increase of scanning laps, indicating the decrease of capacity. While for Fe3S4 NPs@rGO, the curves are almost overlapping for the second and third cycles, indicating its good reversible performance.

(a)

(b)



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(d) (c)

(e)

(g)

(f)

(h)

Figure 7. Charge and discharge curves of Fe3S4 NPs@rGO (a) and Fe3S4 (b) anodes for the first three cycles. (c) Cycling data of Fe3S4 NPs@rGO, Fe3S4, and rGO electrodes for 100 cycles at 0.1 A/g. (d) Cyclic data at 1 A/g for Fe3S4 NPs@rGO. (e) Rate capabilities and (f) Discharge/charge behaviors of Fe3S4 NPs@rGO at various current densities. (g) Compared with the rate performance in the case of Fe3S4 anode materials. (h) Nyquist plots for Fe3S4


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NPs@rGO and Fe3S4 at the 1st cycle between100 kHz and 0.01 Hz.

Figure 7a and 7b show the capacities of Fe3S4 NPs@rGO and pure Fe3S4 electrodes for the first to third cycles at 0.1 A/g at room temperature. Some distinct voltage plateaus can be observed at 1.57, 1.4, 0.75, 1.9 and 2.45 V in the first cycle for Fe3S4 NPs@rGO and pure Fe3S4, which correspond well to the results of C-V test (Figure 6), respectively. The Fe3S4 NPs@rGO (Figure 7a) composite delivers a first discharge 1367 and charge 916 mAh/g capacities, respectively, showing an irreversible 451 mAh/g loss. The high irreversible capacity should ascribe to form a SEI layer in the electrolyte surface and the reaction happened at the phase boundary between metal and Li2S induced an exceptional Li-storage.33,34 In addition, some part of active oxygen-containing functional groups on rGO matrix can also consume Li and lead to the irreversible capacity.22,35,36 However, the discharge capacity of 965 and 964 mAh/g can be maintained in the 2nd and 3rd cycles, respectively. In comparison, pure Fe3S4 just deliver a 1st discharge capacity of 1090 mAh/g, then it quickly drops to 782 mAh/g in the third-cycle (Figure 7b). These results are consistent with the results of C-V curves. To further investigate the cycling stability, Figure 7c shows the discharge capacities of Fe3S4 NPs@rGO and pure Fe3S4 at 0.1 A/g for 100 cycles, and Figure 7d displays the cyclic data of Fe3S4 NPs@rGO at 1 A/g after the rate performance measurement for 800 cycles. The capacity of 950 mAh/g for Fe3S4 NPs@rGO can be left after 100 cycles at 0.1 A/g, which is more than two times of 382 mAh/g for pure Fe3S4. Additionally, the reversible capacity of Fe3S4 NPs@rGO higher than the theory one can be attributed to the some capacity contribution of rGO and multiple positive effects between Fe3S4 NPs and rGO.37−39



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Considering the capacity retention of 98.4 % for the Fe3S4 NPs@rGO from the 2nd to 100th cycles, it can be inferred that the capacity contribution of rGO in Fe3S4 NPs@rGO is almost unchanged. So, the capacity contribution of rGO is around 134 mAh/g according to the 16 wt% rGO in Fe3S4 NPs@rGO and 840 mAh/g reversible capacity of rGO at the second discharge process. The multiple positive effects can be described as follows. First, the introduction of Fe3S4 can expand the intervals of rGO layers, which can bring more Li storage and double-layer capacitance between graphene and electrolyte.39−41 Second, the inclusion of rGO can make Fe3S4 NPs have smaller sizes, which provide more electroactive sites for Li storage.18,42 Third, the formation of polymeric/gel-like film on the surface of electrode materials can generate pseudocapacitive effect after the first discharge, which makes some contributions to enhance the reversible capacity.12,43 A capacity of 720 mAh/g (1 A/g) can be left after 800 cycles. Besides, a Coulombic efficiency of Fe3S4 NPs@rGO can be kept at 99.7% after the 100th cycle, indicating a perfect reversible performance. The superior electrochemical performance of Fe3S4 NPs@rGO arises from its specific complex structure. On one hand, the layered structure of rGO is beneficial for Li+ ions to intercalate and deintercalate, which can provide some part of the reversible capacity; on the other hand, Fe3S4 NPs wrapped in rGO matrix can effectively restrict the volume change of Fe3S4 during cycling. The rate performance of Fe3S4 NPs@rGO is further characterized (Figure 7e-g). A series of capacities of 953, 812, 727, 667, 623, 520, and 462 mAh/g can be obtained when the rates are 0.1, 0.2, 0.5, 1, 2, 5, and 10 A/g, respectively. The discharge capacity of Fe3S4 NPs@rGO was reduced along with the increasing rate, as large currents can induce the polarization and


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reduce the diffusion process.44,45 The capacity of Fe3S4 NPs@rGO could be recovered to 939 mAh/g after reverted to 100 mA/g, which reveals a superior rate capability and reversible performance at high rates. Compared with the rate capabilities of Fe3S4 anode materials, Fe3S4 NPs@rGO anode is superior to the reported ones.15,18 EIS curves of Fe3S4 NPs@rGO and pure Fe3S4 are shown in Figure 7h, which were measured using a half cell after one cycle at 0.1 A/g. Apparently, two different semicircles exist at a high-frequency part, corresponding with charge transferred at the active electrodes and the electrolyte interface. The radius of pure Fe3S4 is much larger than that of Fe3S4 NPs@rGO, meaning the higher charge transfer and contact resistance of pure Fe3S4. This result suggests that the combination of pure Fe3S4 and rGO can effectively improve the migration speed of Li+ ions and electron conductivity, which can enhance the cyclic and rate performance for Fe3S4. Additionally, at low frequency region, the two Nyquist plots show straight lines, indicating the Warburg impedance induced by Li-ions diffusion within the electrodes.46

(a)

(b)

Figure 8. (a) SEM and (b) TEM images for Fe3S4 NPs@rGO after 100 cycles at 0.1 A/g.

To better understand the collaborative effect between Fe3S4 NPs and rGO matrix, the SEM



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and TEM images (Figure 8) of Fe3S4 NPs@rGO after 100 cycles at 0.1 A/g are untaken. It is easily observed that the Fe3S4 NPs on the surface of rGO matrix have pulverized, indicating that the structure of Fe3S4 NPs is unstable in the process of intercalation/deintercalation behavior of Li+ ion (Figure 8a). On the contrary, the Fe3S4 NPs wrapped in rGO layers can be well protected, which is attributed to that the rGO matrix can effectively buffer the volume expansion during charge/discharge.47,48 Moreover, Figure 8b further shows that the whole morphology of rGO matrix can remain well, and no broken particles are observed. The above results show that the stable structure of Fe3S4 NPs@rGO is contributed to its improved electrochemical properties.

4. Conclusions In summary, the Fe3S4 NPs@rGO was obtained employing a simple in-situ hydrothermal approach. The Fe3S4 NPs are wrapped in rGO matrix, which can increase the conductivity of Fe3S4 NPs, and restrict Fe3S4 electrode’s volume expansion during charging/discharging. The Fe3S4 NPs@rGO composite exhibits 950 mAh/g capacity (0.1 A/g) after 100 cycles, and 720 mAh/g capacity can be maintained after the 800th cycle even at 1 A/g. As far as we know, it is the best performance for Fe3S4 so far. Moreover, the capacity of Fe3S4 NPs@rGO can be kept at 953, 812, 727, 667, 623, 520, and 462 mAh/g at 1/10, 1/5, 1/2, 1, 2, 5, and 10 A/g, respectively, indicating its nice rate capabilities. All these data indicates that Fe3S4 NPs@rGO is one type of potential anode choice for LIBs with outstanding cyclic stability and rate performance. Specially, the easy synthetic method may be expanded to prepare other transition metal sulfide@rGO composites to explore new potential electrodes for energy


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storage.

AUTHOR INFORMATION Corresponding Author *E-mail for S.P.G.: [email protected]; H.G.X.: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT We gratefully acknowledge the financial support by the NSF of China (21673203, 21771159), the Higher Education Science Foundation of Jiangsu Province (15KJB150031), Natural Science Foundation of Yangzhou (YZ2016122), Qing Lan Project and the Priority Academic Program Development of Jiangsu Higher Education Institutions. REFERENCES (1) Liu, X.; Huang, J. Q.; Zhang, Q.; Mai, L. Nanostructured Metal Oxides and Sulfides for Lithium-Sulfur Batteries. Adv. Mater. 2017, 29, 1601759. (2) Yu, X. Y.; Yu, L.; Lou, X. W. Metal Sulfide Hollow Nanostructures for Electrochemical Energy Storage. Adv. Energy Mater. 2016, 6, 1501333. (3) Li, B.; Gu, P.; Feng, Y.; Zhang, G.; Huang, K.; Xue, H.; Pang, H. Ultrathin Nickel-Cobalt Phosphate 2D Nanosheets for Electrochemical Energy Storage under Aqueous/Solid-State Electrolyte. Adv. Funct. Mater. 2017, 27, 1605784. (4) Ni, L.; Wu, Z.; Zhao, G.; Sun, C.; Zhou, C.; Gong, X.; Diao, G. Core-Shell Structure



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Graphical Abstract Fe3S4 nanoparticles wrapped in rGO shows the optimal electrochemical performance for Fe3S4. It can deliver a reversible capacity of 720 mAh/g after 800 cycles at 1 A/g and 462 mAh/g can be achieved even at 10 A/g.


Fe3S4 Nanoparticles Wrapped in an rGO Matrix for Promising Energy

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