High Lithium Storage Capacity and Long Cycling Life Fe3S4 Anodes

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High Lithium Storage Capacity and Long Cycling Life Fe3S4 Anodes with Reversible Solid Electrolyte Interface Film and Sandwiched Reduced Graphene Oxide Shell Yu-Jiao Zhang, Jin Qu, Shu-Meng Hao, Wei Chang, Qiu-Yu Ji, and Zhong-Zhen Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13558 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

High Lithium Storage Capacity and Long Cycling Life Fe3S4 Anodes with Reversible Solid Electrolyte Interface Film and Sandwiched Reduced Graphene Oxide Shell Yu-Jiao Zhang,1,2 Jin Qu,1,* Shu-Meng Hao,1 Wei Chang,1 Qiu-Yu Ji,1 and Zhong-Zhen Yu1,2* 1

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China 2

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China E-mail: [email protected] (J. Qu), [email protected] (Z.-Z. Yu) ABSTRACT: Increasing demands for lithium-ion batteries (LIBs) with high energy density and high power density require highly reversible electrochemical reactions to enhance the cyclability and capacities of electrodes. As the reversible formation/decomposition of solid electrolyte interface (SEI) film during the lithiation/delithiation process of Fe3S4 could bring about a higher capacity than its theoretical value, in the present work, synthesized Fe3S4 nanoparticles are sandwich wrapped with reduced graphene oxide (RGO) to fabricate highly reversible and long cycling life anode materials for high-performance LIBs. The micron-sized long slit between sandwich RGO sheets effectively prevents the aggregation of intermediate phases during the discharge/charge process and thus increases cycling capacity, because of the reversible formation/decomposition of SEI film driven by Fe nanoparticles. Furthermore, the RGO sheets interconnect with each other by face-to-face mode to construct a more efficiently conductive network and the maximum interfacial oxygen bridge bonds benefit the fast 1 ACS Paragon Plus Environment


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electron hopping from RGO to Fe3S4, improving the depth of the electrochemical reactions and facilitating the highly reversible lithiation/delithiation of Fe3S4. Thus, the resultant Fe3S4/RGO hybrid shows a highly reversible charge capacity of 1324 mA h g-1 over 275 cycles at a current density of 100 mA g-1, even retains 480 mA h g-1 over 500 cycles at 1000 mA g-1, much higher than reported. KEYWORDS: Fe3S4; lithium ion batteries; reduced graphene oxide; anode; solid electrolyte interface film

INTRODUCTION Since the technology of lithium-ion batteries (LIBs) was commercially developed in the early 1990s, layered lithium-ion technology has already marched into the portable electronics and power fields.1 New structures and new materials should be designed and developed to satisfy the demands of advanced anodes for LIBs with both high energy density and high power density. Metal alloys,2 silicon,3 transition metal oxides4-8 and chalcogenides9-11 were explored as anodes to take the place of graphite in LIBs. Compared to silicon or metal oxides, metal sulfides are promising because they have much less volume expansion upon lithiation to get a better stability.12 Greigite (Fe3S4) has a higher theoretical capacity (785 mA h g−1), than that of conventional graphite of 372 mA h g−1,13 and exhibits a better electronic conductivity than other transition metal sulfides due to its inverse spinel structure.14 Nearly all the iron sulfides share the same electrochemical reaction mechanism after their initial discharge process. However, different from other iron sulphides including FeS and FeS2,9, 15, 16 Fe3S4 is poorly reversible and thus has a lower reversible capacity than its theoretical value.17 Li et al reported 2 ACS Paragon Plus Environment

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that the oxidation peak at ~2.5 V just appeared in the first cycle.13 The reversibility of the electrochemical reactions at different current rates directly influences the capacity and cycling stability of an electrode, which are related to its energy density and power density. Such a poor reversibility indeed affects the lithium storage performance of Fe3S4. In other words, retaining a high lithiation/delithiation reversibility is essential for Fe3S4 to satisfy the increasing demands of high power density and high energy density. Improving the depth of lithiation/delithiation reactions could lead to a better electrochemical reversibility and a higher capacity. Downsizing materials to nanoscale is beneficial for achieving high electrochemical performances because nanoscale could improve lithiation depth and transportation rate of lithium ions by shortening their diffusion pathway. Moreover, the transportation rate of electrons should be enhanced by increasing the electronic conductivity of electrodes. As reported, the interfacial oxygen bridge bonds constructed with carbon nanomaterials could facilitate fast electron transfer and thus significantly deepen the lithiation/delithiation reactions, increasing both reversible capacity and rate capability.18-23 Since more interfacial oxygen bridge bonds between active materials and carbon are beneficial,

the

interfacial

structure

should

be

designed

preciously.

Taking

the

two-dimensional graphene sheets as an example, active materials are decorated on graphene to form two different hybrid structures: (1) two active material layers sandwiched a graphene sheets, or (2) two graphene sheets sandwiched an active material layer. Apparently, the latter would lead to more contact interfaces, implying more interfacial oxygen bridge bonds. In addition, sandwiched graphene layers as a protective matrix could alleviate the large volume variation of active materials during the discharge/charge process and the electrode 3 ACS Paragon Plus Environment


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pulverization. More importantly, the sandwiched graphene sheets could more effectively suppress the aggregation of intermediate phases during the lithiation/delithiation process than just graphene sheets. In the structure (1), the unbonded part of active materials might break down from the graphene and aggregate together; while in the structure (2), the intermediate phases could always be wrapped tightly between two graphene layers. It is thus envisioned that constructing sandwich structure by wrapping Fe3S4 with graphene sheets could improve the reversibility of Fe3S4 to get a highly reversible and long cycling life anode for LIBs. Herein, we demonstrate an in-situ solvothermal method to fabricate sandwich Fe3S4 wrapped by reduced graphene oxide (RGO) for the first time. Thanks to the maximum interfacial oxygen bonds and the enhanced conductivity resulted from the RGO-wrapped sandwich structure, the resultant Fe3S4/RGO hybrid has much better lithium storage performances than both neat Fe3S4 and its mixture with RGO. Meanwhile, the Fe3S4 component turns to tinier nanoparticles with cycles, while the RGO sheets effectively sandwich these intermediate phases to prevent their aggregation during the discharge/charge process, further enhancing the electrochemical reversibility of the Fe3S4/RGO hybrid and increasing its reversible capacity owing to the decomposition of the SEI film driven by Fe nanoparticles formed in situ. The Fe3S4/RGO anode material has an excellent reversible capacities of 1324 mA h g-1 over 275 cycles at 100 mA g-1 and 480 mA h g-1 over 500 cycles at 1000 mA g-1. EXPERIMENTAL SECTION Materials. Beijing Chemical Factory provided hydrochloric acid (HCl), hydrogen peroxide (H2O2), Sulfuric acid (H2SO4), ethylene glycol (EG), ferric chloride hexahydrate 4 ACS Paragon Plus Environment

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(FeCl3.6H2O),

and

Huadong

Graphite

Factory

provided

graphite

flakes (13µm).

Thioacetamide (TAA) was bought from Aladdin Chemicals. Synthesis of Fe3S4 and Fe3S4/RGO hybrids. The preparation of graphite oxide was carried out with a Hummers’ approach.24 Fe3S4/RGO hybrid was synthesized by a solvothermal method. 0.48 g of FeCl3.6H2O (1.78 mmol) and 1.052 g of TAA (14 mmol) were respectively dissolved in 25 mL EG, and the two solutions were mixed together with stirring. When the mixture became clear, it was put into a Teflon-lined autoclave (100 mL). After 6 mL of GO suspension (6 mg/mL) was added and dispersed by ultrasonication, the sealed autoclave was heated at 160 °C for 2 h. The resultant Fe3S4/RGO hybrid was collected, washed, and dried at 60 °C in a vacuum oven overnight. For comparison, neat Fe3S4 nanoparticles and RGO sheets went through the similar solvothermal synthesis process. Characterization. Fe3S4 and Fe3S4/RGO hybrids were characterized with a X-ray diffraction diffractometer (XRD, Rigaku D/Max 2500), a Fourier-transform infrared spectroscopy (FT-IR, Nicolet IS5), a Renishaw inVia Raman microscopy, a X-ray photoelectron spectroscopy (XPS, Thermo VG RSCAKAB 250X), and a thermogravimetric analyzer (TGA, STARe TGA/DSC3 1100SF). Scanning electron microscope (SEM, Zeiss Supra 55), transmission electron microscope (TEM, Hitachi H-800), and high-resolution TEM (HRTEM, JEOL J-3010) were used to observe the morphology and microstructures of Fe3S4 and Fe3S4/RGO hybrids. Electrochemical measurements. Coin-type cells were assembled in an argon-filled glove box. The working electrode was prepared by casting the slurry of active material (Fe3S4/RGO hybrid, or Fe3S4, or Fe3S4/RGO mixture), super-P, and polyvinylidenefluoride in 5 ACS Paragon Plus Environment


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a mass ratio of 7:2:1 onto a Ni foam and drying at 60 oC. The loading mass for the cell testing is 1.0-1.5 mg cm-2. The coin cells were fabricated with a glass fabric (GF/D) as the separator and a lithium foil as the counter electrode. The electrolyte was obtained by dissolving 1 M LiPF6 in a mixture of diethyl carbonate, ethylene carbonate, and dimethyl carbonate (1:1:1). Cycling and rate performances were recorded using an electrochemical workstation (Land CT 2001A) at varied current densities (100-2000 mA g-1) and voltage ranges (0.005-3.0 V vs. Li+/Li). Cyclic voltammograms (CV) and electrochemical impedance spectra (EIS) were measured with a CHI 660E electrochemical workstation. RESULTS AND DISCUSSION Figure 1a shows XRD patterns of Fe3S4 and Fe3S4/RGO hybrid. The sharp peaks could be clearly indexed to (220), (311), (400), (511) and (440) reflections, which belong to the cubic phase Fe3S4 (JCPDS 89-1998). No other impurities are observed. There are no obvious differences between neat Fe3S4 and Fe3S4/RGO hybrid, implying that the presence of RGO may not cause any structural changes of Fe3S4. However, SEM images (Figure 1b, inset) of Fe3S4 nanoparticles clearly illustrate the morphological differences between neat Fe3S4 nanoparticles and Fe3S4/RGO hybrid. For the Fe3S4 nanoparticles, some micron-scale agglomerations are observed (Figure 1b inset). Whereas, the Fe3S4 nanoparticles with uniform octahedral shape and a mean size of ~500 nm are uniformly dispersed and well sandwich wrapped by RGO sheets in the Fe3S4/RGO hybrid (Figure 1b), indicating that the presence of RGO effectively prevents the aggregation of Fe3S4 nanoparticles and promotes the formation of uniform octahedral shape. Clearly, the TEM image (Figure 1c) also shows that the octahedral Fe3S4 nanoparticles are tightly sandwich wrapped by the soft and elastic 6 ACS Paragon Plus Environment

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RGO sheets. Such a tight contact and stretchable sandwiched RGO shell would be the key to improve the interfacial electron transportation rate and buffer the volume expansion during the lithiation/delithiation processes for excellent lithium storage performances.25 The HRTEM image of Fe3S4/RGO hybrid shows that the Fe3S4 nanoparticles possess legible crystal lattices and their spacings of the fringes are 2.98 and 5.72 Å, corresponding to the (311) and (111) planes of Fe3S4 crystal with an inverse spinel structure (Figure 1d). Its fast Fourier-transform spectroscopy (Figure 1d inset) confirms the single crystal structure of the Fe3S4 nanoparticles. Since the Fe3S4 nanoparticles are sandwich wrapped by RGO sheets (Figure 1e), the intermediate phases during the lithiation/delithiation process could be effectively confined between the sandwiched RGO sheets to avoid the aggregation, and thus leading to high cycling stability and reversible capacity.

Figure 1. (a) XRD curves of Fe3S4 and Fe3S4/RGO hybrid. (b) SEM image of Fe3S4/RGO hybrid (inset: SEM image of Fe3S4). (c) TEM and (d) HRTEM images of Fe3S4/RGO hybrid 7 ACS Paragon Plus Environment


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(inset: fast Fourier transform spectroscopy of Fe3S4/RGO hybrid). (e) Schematic illustrating the sandwich structure of Fe3S4/RGO hybrid. Figure 2a presents the FT-IR spectra of Fe3S4 nanoparticles, RGO sheets, and Fe3S4/RGO hybrid. The peaks of RGO and Fe3S4/RGO hybrid at 2853 and 2920 cm-1 correspond to symmetric VsCH2 and antisymmetric VasCH2 of the alkyl chains, respectively. Compared to RGO, the C=O stretching vibration peak at 1728 cm-1 disappears in Fe3S4/RGO hybrid, which is an evidence of the reduction of GO.26, 27 The characteristic peak of -OH vibration shifts from 3424 cm-1 of RGO to 3434 cm-1, 28 while the characteristic C-O-C peak shifts from 1040 cm-1 of RGO to a low frequency of 1034 cm-1 for the Fe3S4/RGO hybrid. These results suggest that the -OH groups may connect RGO with Fe3S4.29 It is thus confirmed that the RGO has covalent bonding with the Fe3S4. As above mentioned, the interfacial oxygen bonds could promote interfacial transportation rate of electrons, leading to a better electrochemical performance.

Figure 2. (a) FT-IR spectra of Fe3S4, RGO, and Fe3S4/RGO hybrid. (b) Raman spectra of GO, and Fe3S4/RGO hybrid.

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The Raman spectra are measured to monitor the variation of GO during the solvothermal synthesis process. Compared to GO, Fe3S4/RGO hybrid shows three new Raman modes at 223, 290 and 405 cm-1 (Figure 2b). They can be attributed to the T2g3, Eg and T2g2 modes of Fe3O4, because the incident laser induces the oxidation in air and makes the Raman lines of Fe3S4 nearly disappeared.13 The peaks at 1590 and 1340 cm−1 correspond to G and D bands, respectively.30, 31 The ID/IG ratio represents the graphitization extent of carbon materials.32 It is seen that the ID/IG ratio increases from 0.86 of GO to 1.23 of the hybrid, implying that more storage sites result from increased defects and disorders for lithium ions.33 To evaluate the content of Fe3S4 in the Fe3S4/RGO hybrid, Figure 3a shows TGA results of Fe3S4 and Fe3S4/RGO hybrid in the range of 40-900 oC. The content of Fe3S4 is ~ 61.4 wt.% in the Fe3S4/RGO hybrid. The mass profile of neat Fe3S4 exhibits a major mass loss of 23 %, which is nearly the same as the theoretical value of 21.6 wt.% for converting Fe3S4 to α-Fe2O3 in air as confirmed by XRD pattern (Figure S1).34 Additionally, the mass increase at 250 oC implies that the internal irons are oxidized prior to the sulfur and converted to α-Fe2O3 in air. There is a sharp decline in mass around 400 oC because of the sulfur oxidation to SO2. The composition changes of the hybrid before and after the solvothermal process are characterized with XPS spectra (Figure 3b-d). In addition to the presence of carbon, oxygen, iron, and sulfur elements in the Fe3S4/RGO hybrid (Figure 3b), there is a new peak at 531.8 eV, corresponding to C-O-Fe bond (Figure 3c), indicating a covalent bonding between RGO and Fe3S4. Such an oxygen bridge between the RGO sheets and the active material of Fe3S4 would be beneficial for the lithium storage.31, 35 The oxygen bridge facilitates the fast electron transport from graphene to Fe3S4 and the reversible discharge/charge of Fe3S4.31 In Figure 3d, 9 ACS Paragon Plus Environment


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the peak of ether/epoxide group at 286.8 eV is markedly weakened after the hybridization of GO with Fe3S4, certifying the formation of RGO. Hence, RGO sheets serve as a flexible conducting network for the hybrid.36, 37

Figure 3. (a) TGA curves of Fe3S4 and Fe3S4/RGO hybrid; (b) XPS Survey scans of

Fe3S4/RGO hybrid, Fe3S4, and GO; (c) O 1s and (d) C 1s XPS spectra of GO and Fe3S4/RGO hybrid. The Fe3S4/RGO anode material exhibits high lithium storage performances. CV curves of the Fe3S4/RGO hybrid (Figure 4a), Fe3S4 (Figure 4b) and the Fe3S4/RGO mixture (Figure 4c) are slightly different. In the first cycle of the Fe3S4/RGO hybrid, there are four peaks at ~1.80, 10 ACS Paragon Plus Environment

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~1.60, ~1.32 and ~0.72 V, which is consistent with the literature.17 The first peak at ~1.80 V correlates to the insertion of Li+ to form LixFe3S4; the second peak at ~1.60 V represents the reduction of Fe3+ to Fe2+; and the third peak at ~1.32 V involves a two-phase reaction to form Fe0 and Li2S.38 These reactions could be written as a single reaction Fe3S4 + 8Li+ + 8e- → 4Fe + 3Li2S, because Li+ diffusion in Fe3S4 is relatively slow. Furthermore, the last voltage peak at ~0.72 V disappears in the second and third cycles, due to the formation of SEI films. In subsequent oxidation scans, the Fe0 is oxidized to Li2FeS2 at ~1.97 V and then converted to FeS at ~2.50 V by the reaction of Li2FeS2 with the resulting Fe0, which is also consistent with the two-phase reaction. All these peaks nearly coincide with the voltage plateaus in the galvanostatic charging curve (Figure S2a). Additionally, a weak peak at ~2.39 V is observed for the hybrid (Figure 4a), while it does not appear in neat Fe3S4 (Figure 4b) and the mixture (Figure 4c), due to the oxidation of polysulphide adsorbed on RGO during the solvothermal process.39 However, this electrochemical reaction is irreversible, it could not be observed after 60 cycles (Figure 4d). The main reactions after the first cycle can be described as follows: 2FeS + 2Li+ + 2e− ↔ Fe + Li2FeS2 Li2FeS2 + 2Li+ + 2e− ↔ Fe + 2Li2S In the subsequent reduction/oxidation scans, the current intensity of the Fe3S4/RGO hybrid nearly does not change, accompanied with the voltage position shift due to the structural modification after the first cycle. The third cycle has the same voltage plateaus with the second cycle. Even after 60 cycles, the integral areas of the 61th and 62th cycle CV curves of the Fe3S4/RGO hybrid are almost the same, indicating its excellent cycling reversibility. However, the current intensities of both Fe3S4 (Figure 4b) and Fe3S4/RGO mixture (Figure 11 ACS Paragon Plus Environment


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4c) decrease sharply in the 2nd and 3rd cycles. The sharp oxidation peak of Fe3S4 and the mixture at ~1.97 V become more and more unnoticeable, while the broad peak at ~2.50 V nearly disappears after two cycles. Therefore, the remaining peak of the Fe3S4/RGO hybrid at 2.50 V after 60 cycles confirms its better reversibility than both neat Fe3S4 and the mixture (Figure 4d). By using Fe3S4/RGO-containing coin cells, the charge/discharge cycles are conducted at 100 mA g-1 and room temperature. The initial charge capacity of 851 mA h g-1 is more than the theoretical value of 785 mA h g-1 (Figure S2a). The exceeding capacity just agrees with the broad peak around 0.7-1.4 V, because of the partial decomposition of the SEI film, leading to an increase in charge capacity. The broad reduction peak at 0.8-1.2 V in the subsequent cycles is attributed to the SEI film formation. Interestingly, the couple of the broad reduction/oxidation peaks at 0.8-1.2 V and 0.7-1.4 V have a high reversibility, even up to 60 cycles (Figure 4d). This phenomenon should due to the reversible formation and decomposition of the SEI film on the electrode surface, which is driven by the numerous Fe nanoparticles.7, 13 The couple of the broad reduction/oxidation peaks is also observed in neat Fe3S4 and the mixture (Figure 4b, 4c) even after 250 cycles (Figure S4), but the peaks are much weaker. It implies that the reversible formation and decomposition of the SEI film driven by the Fe nanoparticles is the nature of Fe3S4 used as anodes for LIBs. The reversible formation and decomposition of SEI film are responsible for the increased reversible capacity above its theoretical value.


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Figure 4. CV curves of (a) Fe3S4/RGO hybrid, (b) Fe3S4, (c) Fe3S4/RGO mixture, and (d) 61th and 62th cycles of Fe3S4/RGO hybrid at 0.1 mV s-1. The electrochemical characteristics of Fe3S4/RGO hybrid, neat Fe3S4, and Fe3S4/RGO mixture after 60 cycles are examined by EIS measurements (Figure S2b). For the Fe3S4/RGO hybrid, its semicircle diameter is smaller and its slope value of the inclined line is higher than those of Fe3S4, indicating a faster lithium ion diffusion rate. It is believed that the sandwiched RGO shell corresponds to the lower charge transfer resistance of the hybrid.28 After 60 cycles, however, the diameter of the semicircle increases from 56 to 108 Ohm for the hybrid. It is seen that the cycled Fe3S4 nanoparticles become smaller (Figure S3), which may increase the charge-transfer resistance.40 But the cycled downsizing effect could serve as a power to mix 13 ACS Paragon Plus Environment


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the Fe nanoparticles, Li2S and Li2FeS2 to form a well-distributed system. In other words, more newly formed SEI film could be more efficiently decomposed by Fe nanoparticles, leading to a highly reversible formation/decomposition of SEI film on the particle electrode surface and a higher reversible capacity. As confirmed in Figure 4, this process is the nature of Fe3S4 used as anodes for LIBs, but the sandwich RGO shell could confine the well-distributed system well and avoid their aggregation, thus leading to a better reversibility than unwrapped Fe3S4. Figure 5a shows the rate capabilities of Fe3S4, Fe3S4/RGO hybrid and Fe3S4/RGO mixture. Fe3S4/RGO hybrid has a superior rate performance than both the Fe3S4/RGO mixture and neat Fe3S4. The reversible capacities of Fe3S4/RGO hybrid are 614, 511, 413 and 348 mA h g-1 at 100, 200, 500 and 1000 mA g-1, respectively. Even at 2000 mA g-1, the specific capacity is still 297 mA h g-1, much higher than those of Fe3S4/RGO mixture and neat Fe3S4. After 60 cycles, the reversible capacity recovers to 554 mA h g-1, revealing a superior rate capability at the high current density. When RGO is physically mixed with Fe3S4 at the same mass ratio as the hybrid (38.6 wt% of RGO, determined by TGA in Figure 3a), the electrochemical performance of the mixture is improved as compared to Fe3S4 alone. However, the capacity of the mixture is much lower than that of the Fe3S4/RGO hybrid, which is an indirect evidence of the synergistic effect between RGO and Fe3S4. The cycling and rate performances of the Fe3S4/RGO hybrids with different GO contents are shown in Figure 5b and S5. The Fe3S4/RGO hybrid fabricated with 6 mL GO solution has the highest capacity, indicating that the GO content has an influence on the synergistic effect between Fe3S4 and


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RGO.6 The Fe3S4 nanoparticles are not uniform and regular (Figure S6), which may result in the poor electrochemical performance.

Figure 5. Rate performances of (a) Fe3S4/RGO hybrid, Fe3S4/RGO mixture, and Fe3S4, and (b) the hybrids with different RGO contents. Long cycling tests of the Fe3S4/RGO electrode at (c) 100 mA g-1 over 275 cycles, and (d) 1000 mA g-1 over 500 cycles. Figure 5c and d show the long cyclic performances of the Fe3S4/RGO hybrid anode. It is seen that the hybrid retains an exceptional specific capacity of 1324 mA h g-1 at 100 mA g-1 after 275 cycles. The initial capacity loss may result from the side reaction between Li+ and active material. After the initial 5-cycle decrease in the charge capacity, the Fe3S4/RGO hybrid exhibits a stable capacity and the Coulombic efficiency increases to nearly 100 %. 15 ACS Paragon Plus Environment


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Surprisingly, the specific capacity steady climbs up from the 125th cycle and reaches the plateau of 1324 mA h g-1 after 265 cycles, suggesting a highly reversible capacity increase for the Fe3S4/RGO anode. There is a competition between the capacity increase effect and the capacity decrease effect. The increased capacity should be attributed to the steady reversible formation/decomposition of SEI film driven by the Fe nanoparticles. If the Fe nanoparticles could be well dispersed in the Li2S and SEI film matrix, more newly formed SEI film could be effectively electro-decomposed to increase the capacity. As shown in Figure S3 and S8, the size becomes smaller and smaller with cycles, which makes the dispersion of Fe0 more and more homogeneous in the Li2S and SEI film matrix, benefiting the catalytic decomposition of the SEI film to increase the charge capacity. In addition, the decrease in size leads to an increased surface area with more defects and vacancies, which could also trap lithium-ions and thus benefit the increase in capacity. Meanwhile, the structure collapse of Fe3S4 and the irreversible loss of Li+ due to the new formation of SEI film could lead to a capacity decay. At about the 125th cycle, the increase effect overcomes the decrease effect, so the capacity started shooting. Similar results were also reported in other systems.41-43 Such an increase is not remarkable at 1000 mA g-1. Even so, the hybrid still retains a high capacity of 480 mA h g-1 after 500 cycles (Figure 5d). Similar results were reported in many other electrodes, especially those with sulfides as the electrode materials.33, 44 Figure 6a compares the CV curves of the Fe3S4/RGO hybrid at 61th and 276th cycles. In addition to the highly reversible peaks at 1.40, 1.97 and 2.50 V, the couple of the enlarged broad reduction/oxidation peaks at 0.8-1.2 V and 0.7-1.4 V is beneficial for the increased capacity, which can be explained by the steady reversible formation/decomposition of SEI 16 ACS Paragon Plus Environment

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film driven by the Fe nanoparticles. TEM images of the cycled batteries after discharging to 0.005 V and charging to 1.5 V are shown in Figure S7. A thick SEI film at the edge of the active material is observed after discharging to 0.005 V (Figure S7a). By contrast, the SEI layer is not observed at the edge of the active material after charging to 1.5 V (Figure S7b). The reversible formation/decomposition of the SEI film is hence confirmed. The elemental mapping analysis and morphology after 200 cycles also support the capacity increase phenomenon (Figure S8). Although it is really difficult to distinguish super-P and active material, the elemental mapping analysis shows that all elements are uniformly distributed in the hybrid. It is noted that the size becomes smaller with cycles, which makes the dispersion of Fe0 more and more homogeneous in the Li2S and SEI film matrix, benefiting the catalytic decomposition of the SEI film to increase the charge capacity. During the whole process, all the intermediate phases are sandwich restricted by the wrapped RGO shells to prevent the Fe nanoparticle aggregation. When the nanoparticle size decreases to tiny enough, the charge capacity reaches the plateau. Additionally, the decrease in size leads to an increased surface area with more defects and vacancies, which could also trap lithium-ions and thus benefit the increase in capacity. Figure 6b and Table S1 compare the lithium storage performance of the Fe3S4/RGO hybrid with those reported in the literature.9, 13, 14, 17, 45-49 Clearly, the Fe3S4/RGO hybrid exhibits much higher capacities and longer cycling performances at different current densities than those reported.



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Figure 6. (a) Comparison of the cyclic voltammetry curves of Fe3S4/RGO hybrid at 61th and 276th cycle. (b) Comparison of reversible charge capacities of Fe3S4/RGO hybrid with those reported. CONCLUSIONS We confirm the nature of Fe3S4 used as anodes for LIBs that the SEI film could undergo reversible catalytic decomposition by Fe formed in situ during the charge/discharge process, then leading to a higher capacity than the theoretical value of Fe3S4. Thus, highly reversible and long cycling life Fe3S4 anodes sandwich wrapped by RGO sheets are designed and constructed for the first time. The sandwiched RGO shells efficiently confine the intermediate phases of Fe nanoparticles, Li2S and SEI film, and keep them in a well-distributed system with cycles. The newly formed SEI film undergoes efficient catalytic decomposition to increase the reversible capacity of the Fe3S4/RGO hybrid electrode. Additionally, the sandwich RGO shells help generate the maximum interfacial oxygen bonds with Fe3S4 and construct electronic conduction networks, thus the Fe3S4/RGO hybrid exhibits a much better reversibility in the whole voltage window than both Fe3S4 and the Fe3S4/RGO mixture. 18 ACS Paragon Plus Environment

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Thanks to the sandwiched structure and the cycled activation effect, the Fe3S4/RGO hybrid exhibits a very high reversible capacity of 1324 mA h g-1 over 275 cycles at a low current density of 100 mA g-1 and the capacity is still 480 mA h g-1 over 500 cycles at a high current density of 1000 mA g-1. Such a sandwiched structure could extend to other two-dimensional materials, such as MoS2, BN and MXene to improve the structural stability, catalytic and energy storage performances of nanomaterials. Acknowledgements Financial support from the National Natural Science Foundation of China (51402012, 51533001, 51521062), and the National Key Research and Development Program of China (2016YFC0801302) is gratefully acknowledged. REFERENCES 1. Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices Science 2011, 334, 928-935. 2. Zhang, W. J. A Review of the Electrochemical Performance of Alloy Anodes for Lithium-ion Batteries. J. Power Sources 2011, 196, 13-24. 3. Esmanski, A.; Ozin, G. A. Silicon Inverse-Opal-Based Macroporous Materials as Negative Electrodes for Lithium Ion Batteries. Adv. Funct. Mater. 2009, 19, 1999-2010. 4. Hu, Y. S.; Kienle, L.; Guo, Y. G.; Maier, J. High Lithium Electroactivity of Nanometer-Sized Rutile TiO2. Adv. Mater. 2006, 18, 1421-1426. 5. Reddy, M. V.; Yu, T.; Sow, C. H.; Shen, Z. X.; Lim, C. T.; Subba Rao, G. V.; Chowdari, B. V. R. α-Fe2O3 Nanoflakes as an Anode Material for Li-Ion Batteries. Adv. Funct. Mater. 2007, 17, 2792-2799. 19 ACS Paragon Plus Environment


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TOC

Reversible solid electrolyte interface film of Fe3S4 anodes driven by Fe nanoparticles endows the Fe3S4/RGO hybrid with a highly reversible capacity, which increases to as high as 1324 mA h g-1 over 275 cycles at the current density of 100 mA g-1, much higher than its theoretical value. The sandwiched RGO shells not only efficiently stabilize the reversibility of the solid electrolyte interface film, but also construct a conductive network and interfacial oxygen bridge bonds to enhance the lithium storage performance of the Fe3S4/RGO hybrid.


High Lithium Storage Capacity and Long Cycling Life Fe3S4 Anodes

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