FeSb2S4 anchored on amine-modified graphene towards high-performance anode material for sodium ion batteries

Journal Pre-proofs FeSb2S4 anchored on amine-modified graphene towards high-performance anode material for sodium ion batteries Qimeng Peng, Xuebu Hu, Tianbiao Zeng, Biao Shang, Minglei Mao, Xun Jiao, Guocui Xi PII: DOI: Reference:

S1385-8947(19)33272-3 https://doi.org/10.1016/j.cej.2019.123857 CEJ 123857

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

10 October 2019 3 December 2019 15 December 2019

Please cite this article as: Q. Peng, X. Hu, T. Zeng, B. Shang, M. Mao, X. Jiao, G. Xi, FeSb2S4 anchored on aminemodified graphene towards high-performance anode material for sodium ion batteries, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123857

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FeSb2S4 anchored on amine-modified graphene towards high-performance anode material for sodium ion batteries

Qimeng Penga, Xuebu Hua,*, Tianbiao Zengb, Biao Shanga, Minglei Maoc, Xun Jiaoa, Guocui Xia a

College of Chemistry and Chemical Engineering, Chongqing University of

Technology, Chongqing 400054, China b

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute,

Sichuan University, Chengdu 610065, China c

Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy

Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China *Corresponding author Email: [email protected]; Tel: +86-23-62563250; Fax: +86-23-62563221.

Abstract: Sodium ion batteries (SIBs) have been considered as promising alternatives to lithium ion batteries(LIBs), due to the earth-abundance and low cost of Na metal, as well as the similar chemistry between SIBs and LIBs. For the advancement of SIBs, the challenges are still remained in anode materials. Metal sulfides with high theoretical specific capacity have been explored as SIB anodes adequately. However, the exploration of high-performance SIBs is hindered by the disadvantages of metal sulfides anodes, including low reversible capacity, limited cycling lifespan and 1

inferior rate performance. Herein, we report a new bimetallic sulfide, FeSb2S4 anchored on amine-modified graphene (FeSb2S4/EN-rGO) as an advanced anode for SIBs，which shows a reversible capacity of 782.5 mAh g-1 at 0.1 A g-1 after 100 cycles and remains a high capacity of 515.7 mAh g-1 at 5 A g-1 after 500 cycles with a low capacity loss rate of 0.04% per cycle. Moreover, both experimental and theoretical calculation results reveal that the strong chemical interaction between amine-modified graphene and FeSb2S4 as well as discharge products (Na2S) is the crucial reason for enabling the stability of anode micro-architecture. Furthermore, the evolution mechanism of the reactions between FeSb2S4 and Na ions is disclosed by ex-situ XRD diffraction. This work opens up a new door for the rational design of bimetallic sulfide anodes for sodium ion batteries with high capacity and rate performance as well as superior cycling stability. Keywords:FeSb2S4; Amine-modified graphene; Anode material; Sodium ion batteries.

1. Introduction Sodium ion batteries have been considered to be the most potential candidates for lithium ion batteries owing to natural abundance and low cost of sodium as well as the similar intercalation mechanism [1, 2]. Nevertheless, compared to Li+ (0.76 Å), the larger radius of Na+(1.06 Å) aggravates the diffusion barrier and causes sluggish sodiation/desodiation kinetics. In addition, compared to the commercial LIBs, the less satisfactory energy density of SIBs limits their widespread application from 2

technological and commercial perspectives [3, 4].The key points to achieve high-performance SIBs lie in the advancements of both the anode and cathode. To date, various types of cathode materials for SIBs have been extensively explored, including layer and tunnel type transition metal oxides, transition metal sulfides and oxyanionic compounds, etc [5]. However, lack of sufficient and suitable anode materials are restraining the development of SIBs. For instance, graphite anode, widely used in LIBs, shows an ultralow capacity of less than 35 mAh g−1 for SIBs, which can be mainly attributed to the mismatch between Na ions and graphite layers [6-8]. Compared to the intercalation-type SIB anodes like carbon materials (such as hard carbon), metal sulfides deliver much higher theoretical capacity based on conversion reaction or alloy-dealloying reaction [9-12]. Compared to monometal sulfides (MSs), bimetallic sulfides (MM’Ss) exhibit enhanced electrochemical performances, deriving from their high erelectronic conductivity and more abundant redox active sites [13, 14]. Furthermore, in terms of bimetallic sulfides, there is “synergistic effect” between the two metal cations, originated from the different redox potential of the two metals. In particular, during Na ions insertion/extraction processes, metal M/M’, Na-M/Na-M’ alloy or Na-M-S/Na-M’-S intermetallic can be reciprocally the electronic conductivity matrix instead of low electronic conductivity matrix (Na2S) in the subsequent electrochemical reactions [15]. Bimetallic sulfides can be classified as three types on the basis of Na ions storage mechanism. The first type is the conversion mechanism, in which bimetallic sulfides convert to representative metals and Na2S after Na ions 3

insertion, including CuCo2S4, Ni3Co6S8, NiMo3S4 [16-18]. Krengel et al. reported a bimetallic sulfide CuV2S4, which irreversibly converted to Cu, V and Na2S in the first discharge process. It showed a reversible capacity of about 510 mAh g-1 after 125 cycles at 0.15 A g-1 with an initial coulombic efficiency (ICE) of 72.5% [19]. The second type is the alloy-dealloying mechanism, in which bimetallic sulfides firstly convert to metals with alloy formation ability, followed by the alloying reaction to form Na-rich alloys at lower redox potential, such as SnSbSx, Bi0.94Sb1.06S3, In-Sb-S [20-22]. Jia et al. synthesized carbon-coated ZnSnS3 nanocubes anode for SIBs, which delivers an ICE of 61.5% and a capacity of 661.4 mAhg-1 after 250 cycles at 0.10 A g-1 [23]. Furthermore, the third-type mechanism of bimetallic sulfides are composed of conversion reaction and alloy-dealloying reaction, such as Zn0.33Co0.67S and CuSbS2 [24, 25]. For instance, CuGaS2 has been explored as SIBs anode materials, which might be reduced by Na ions to form Na-Ga alloys, Cu and Na2S during discharging, delivering an ICE of 65.0% and a reversible capacity of 321 mAh g-1 after 100cycles at 1.0 A g-1 [26]. In this bimetallic sulfide system, one metal (M) is active metal component based on alloy-dealloying reaction with Na ions, and the other metal (M’) is inactive metal component that can act as self-conductivity matrix and conductivity auxiliary dispersed into Na2S matrix to further improve the electronic conductivity [27-29]. Thus, the combined-type bimetallic sulfides can take full advantage of the difference in redox potential for electrochemical reactions， which is worth of in-depth research. To the best of our knowledge, sulfides have some active metal elements on ⅣA and ⅤA families such as Ge, Sn, Pb, Sb and Bi [30, 4

31]. Pb is not environmentally friendly, and Ge is too expensive in commercial applications [32, 33]. The large atomic mass of Bi results in are latively low theoretical specific capacity of 385 mAh g-1 [34]. Compared to Sn-S based sulfides, Sb2S3 is kinetically more reversible in sodiation/desodiation process (Fig. S1, Supplementary Information), revealing that Sb is more appropriate as active metal element [35]. For inactive metal element, obviously, the transition metals can achieve such function and Fe is the cheapest one [36]. Therefore, bimetallic sulfide Fe-Sb-S system was selected to construct an advanced SIB anode in terms of sufficient sodium ion storage, stable cycling reversibility and prominent rate capability. Taking account of generally poor electronic conductivity of sulfides, reduced graphene oxide (rGO) with outstanding conductivity, large surface area and superior flexibility can work as matrix to combine with Fe-Sb-S as SIB anode [37]. However, the bonding between pure rGO and metal sulfides is too weak to immobilize metal sulfides onto the rGO sheets. The easy detachment of metal sulfides from the conductive matrix can lead to rapid capacity decay and deterioration of cycling stability [38]. It has been investigated that the chemical interactions between chemically modified graphene and active materials as well as its discharge products can effectively enhance the cycling stability during repeated sodiation/desodiation processes and further lengthen the cycle lifespan [39]. Herein, for the first time, we report a facile one-pot method to achieve a new and durable bimetallic sulfide composite, FeSb2S4 anchored on ethylenediamine-modified reduced graphene oxide composite (FeSb2S4/EN-rGO). As a result, the FeSb2S4/EN-rGO as SIB anode 5

delivers a capacity of 782.5 mAh g-1 at 0.1 A g-1 for 100 cycles, corresponding to a high ICE of 83.4%. Even at 5 A g-1 for 500 cycles, this anode shows a low capacity loss rate only of 0.04% per cycle. Based on the results, the as-prepared FeSb2S4/EN-rGO with high reversible capacity, outstanding cycling stability and excellent rate performance can be a promising next-generation anode for SIBs.

2. Results and discussion

Fig.1. (a) XRD results of FeSb2S4/EN-rGO and FeSb2S4. The green bottom bars are standard diffraction peaks of FeSb2S4. (b) Crystal structure of FeSb2S4. Dark yellow, 6

navy and yellow balls represent Sb, Fe and S atoms, respectively. (c) Raman spectra of the EN-rGO, pure FeSb2S4 and FeSb2S4/EN-rGO composite. (d) FESEM image of FeSb2S4/EN-rGO. (e, f) TEM and (g) HRTEM images of FeSb2S4/EN-rGO. (h) The reduced FFT image of (g).

The phase structure of as-synthesized FeSb2S4/EN-rGO and FeSb2S4 was confirmed by X-ray diffraction (XRD). As shown in Fig. 1a, the diffraction peaks of FeSb2S4/EN-rGO are similar to that of the pristine FeSb2S4, which can match well with orthorhombic FeSb2S4 (JCPDS No. 24-0509). Based on the XRD pattern of EN-rGO (Fig. S2, Supplementary Information), no apparent diffraction peaks of graphene located at 2θ=20-30° can be observed, which can be ascribed to that FeSb2S4 has covered on the surfaces of graphene sheets and further alleviated the restacking of graphene sheets into graphite [40]. Fig. 1b displays the orthorhom bicstructure (Pnam space group, 62) of FeSb2S4, in which FeS6 octahedral and SbS5 orthorhombic pyramids compose the open three-dimensional framework, benefiting for Na+ insertion/extraction. In Raman spectra (Fig. 1c), the peaks located between 100 and 400 cm-1 are characteristic Raman peaks of FeSb2S4 [12]. Two strong Raman peaks positioned at ~ 1350 and ~ 1598 cm-1 correspond to the D- and G-band of the graphene, respectively [41]. In addition, the D band is in connection with sp3 defects and disorders in the graphene layers, and the G band is induced by vibrations in the graphitic sp2 carbon atom planes [60]. FeSb2S4/EN-rGO and EN-rGO show relatively high ID/IG ratios of about 1.13 and 1.02, respectively, suggesting highly defective 7

nature and disordered structure of the amine-modified graphene sheets, which can be ascribed to the partial removal of oxygen-functional groups [46]. Moreover, compared to EN-rGO, the higher ID/IG value of FeSb2S4/EN-rGO indicates the variation of the graphite plane structure, which could originate from the additional defects triggered by the interfacial interaction of EN and FeSb2S4. Furthermore, the content of EN-rGO is measured by TGA to be 11.6 wt % (Fig. S3 and 4, Supplementary Information), which is consistent with 13.4 wt% calculated by ICP-MS (Table S2, Supplementary Information). The morphology and detailed structure of the FeSb2S4/EN-rGO composite were investigated by field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). FeSb2S4/EN-rGO composite displays an interconnected sheet-like structure with porous framework constructed by EN-rGO, while pure FeSb2S4 particles show dense agglomeration (Fig. 1d andFig. S5). TEM images of FeSb2S4/EN-rGO composite (Fig. 1e andf) further exhibit that abundant FeSb2S4 particles (less than 200 nm) are grasped and spatially confined on the surfaces of the functionalized graphene layers. As illustrated in the high-resolution transmission electron microscope (HRTEM) image (Fig. 1g), FeSb2S4 particles are intimately wrapped by EN-rGO, and the lattice fringes of 0.31 nm and 0.53 nm can be attributed to (211) and (210) plane of orthorhombic FeSb2S4, respectively, in accordance with fast Fourier transform (FFT) pattern (Fig. 1h). The corresponding element mapping (Fig. S6b-f, Supplementary Information) demonstrate the uniform distribution of Fe, Sb, S and C, indicating the homogeneous growth of FeSb2S4. 8

Additionally, the existence of N illustrates the successful amine-chemical modification on the surface of graphene. In such a structure, the EN-rGO in the FeSb2S4/EN-rGO composite plays a dual role in providing a fast electronic conductive web and a functionalized carbon matrix to chemically fasten active materials for maintaining the structural integrity.

Fig.2. XPS spectra of FeSb2S4/EN-rGO: (a) S 2p spectrum, (b) Sb 3d spectrum, (c) Fe 2p spectrum, (d) N 1s spectrum and (h) C 1s spectrum. XPS spectra of FeSb2S4: (e) Sb 3d spectrum and (f) Fe 2p spectrum. XPS spectra of EN-rGO: (g) N 1s spectrum and (i) C 1s spectrum.

To

probe

the

element

compositions 9

and

chemical

interactions

in

FeSb2S4/EN-rGO, XPS measurements were further carried out. According to Fig. S7a (Supplementary Information), Fe, Sb, S, C and N elements can be detected in FeSb2S4/EN-rGO. The high-resolution S 2p spectrum (Fig. 2a) can be fitted to four peaks assigned to S 2p1/2 and S 2p3/2 of metal-sulfur bonds, corresponding to Sb-S (161.7 and 163.4 eV) and Fe-S bonds (162.7 and 164.1 eV) [42, 43]. The peak located at 160.8 eV can be ascribed to monovacancy species S2− in low coordination at the surface [44]. Additionally, Two predominant peaks center at 530.1 (Sb 3d5/2) and 539.4 eV (Sb 3d3/2) in Fig. 2b, proving the presence of Sb3+ [43]. In the case of Fe 2p core-level spectrum (Fig. 2c), two peaks at 710.9 eV (Fe 2p3/2) and 724.5 eV (Fe 2p1/2) correspond to Fe2+ cation in FeSb2S4, and a small shoulder at around 716.2 eV is ascribed to Fe2+ satellite peaks [45]. As presented in Fig. 2d, the scanned N 1s spectrum of FeSb2S4/EN-rGO is fitted as pyridinic N (398.7 eV), pyrrolic N (400.3 eV) and amino N (401.6 eV). The N 1s spectrum of EN-rGO (Fig. 2g) shows similar nitrogen-doping types to that of FeSb2S4/EN-rGO except a slight shift to lower binding energy, suggesting the charge transfer from FeSb2S4 to EN-rGO. This is also supported by the positive shift in binding energy for the Sb 3d and Fe 2p spectra of FeSb2S4, which demonstrates possible shifting electron clouds from EN-rGO to FeSb2S4 in FeSb2S4/EN-rGO (Fig. 2e and f). This is caused by the interaction between Fe2+/Sb3+ and nitrogen element after incorporation, attesting that FeSb2S4 particles have been anchored on the EN-rGO sheets via interfacial chemical bonding [46]. Moreover, the C 1s spectrum of FeSb2S4/EN-rGO displays four obvious peaks assigned to C-C (284.6 eV, sp2-hydridized carbon), C-N/C-S (285.4 eV), C-O (286.5 10

eV) and C=O (287.6 eV), which is consistent with the results of EN-rGO (Fig. 2h and i) [47]. The relatively low intensity of oxygen-functional groups manifests the reduction of GO to graphene, and the appearance of C-N peak reveals the functionalization of rGO through the ethylenediamine-thermal treatment.

Fig.3. Discharge/charge voltage profiles of (a) FeSb2S4/EN-rGO and (b) FeSb2S4 anodes at different cycles at 0.1 A g-1. CV curves of (c) FeSb2S4/EN-rGO and (d) FeSb2S4 anodes at 0.1 mV s−1.

The electrochemical performance of FeSb2S4/EN-rGO for SIBs was evaluated (Fig. 3a). FeSb2S4/EN-rGO can deliver an initial discharge and charge capacity of 984.1 mAh g-1 and 820.5 mAh g-1, corresponding to a high ICE of 83.4%. While pure 11

FeSb2S4 shows a high initial discharge and charge capacity of 1115.8 mAh g-1 and 901.0 mAh g-1 with an ICE of 80.7%. The high specific capacity and ICE could be largely originated from the rich redox reactions endowed by the bimetallic sulfide (FeSb2S4) as well as the synergistic effects from the bifunctional metal species. Specifically, benefited from the different redox potential, the Fe and NaxFeS2 (x≤2) can functionalize as electronic conductivity matrix instead of less conductive Na2S matrix during alloying/de-alloying processes between Sb and Na+. In particular, NaxFeS2 (x ≤ 2) possesses better electronic conductivity than Na2S (Fig. S9, Supplementary Information), thus enhancing the electrochemical reaction activity during charge/discharge processes. Furthermore, the Fe nanoparticles, reversibly formed from the conversion reaction, can intimately disperse in the Na2S matrix and further maintain the electronic conductivity of the anode materials. Compared to FeSb2S4/EN-rGO, the cycling stability of pure FeSb2S4 is less satisfactory, and the voltage hysteresis becomes much more serious with the increasing cycle numbers. In Fig. 3c, all of the CV peaks in FeSb2S4/EN-rGO are in good agreement with the obvious voltage platforms in discharge/charge profiles. Moreover, the reduction peaks of FeSb2S4/EN-rGO and FeSb2S4 (Fig. 3d) in the initial scan are different from those in the subsequent scans, suggesting a different sodiation mechanism. In the initial scan, the peak area of FeSb2S4/EN-rGO is smaller than that of FeSb2S4, demonstrating a less initial capacity of FeSb2S4/EN-rGO, being in accord with their initial discharge/charge voltage profiles (Fig. S10). Compared to FeSb2S4, the reduction peaks of FeSb2S4/EN-rGO slightly shift to higher potential, indicating better 12

electrochemical activity and faster Na-ion diffusion kinetics in FeSb2S4/EN-rGO anode. Furthermore, from the second scan cycle onward, the CVs curves of FeSb2S4/EN-rGO almost completely overlap, indicating an enhanced cycling reversibility.

Fig.4. (a) Initial discharge/charge profiles of FeSb2S4 electrode at 0.1 A g−1 in the voltage range of 0.01-2.5 V and (b) Ex-situ XRD patterns in different discharge and charge states.

In order to explore the underlying mechanisms of sodium ion storage in FeSb2S4 anode, ex-situ XRD analysis was employed to track phase changes during the initial sodiation/desodiation processes (Fig. 4). After an apparent discharge plateau (C in Fig. 4b), the pristine FeSb2S4 peaks completely disappear and transform into a series of new peaks, corresponding to Sb2S3 (JCPDS NO. 51-1418), Sb (JCPDS NO. 35-0732) 13

and Na2S (JCPDS NO. 23-0441), illustrating the decomposition of FeSb2S4 (Equation 1). When discharged from E to G (Fig. 4b), the diffraction peaks of related discharge products (Sb2S3 and Sb) fade away, nevertheless, several new peaks can be matched well with Na3Sb phase (JCPDS NO. 04-0724), demonstrating the conversion reaction of Sb2S3 and Na−Sb alloying reaction (Equation 3 and 4) [48, 49]. When charged from G to J (Fig. 4b), the Na3Sb phase disappears, meanwhile, the peaks of Sb come out and then strengthen, disclosing the de-alloying processes of Na3Sb (Equation 6). Accompanied by the gradually prominent peaks of Sb2S3, the Sb peak vanishes at K (Fig. 4b), indicating an exhaustive conversion reaction (Equation 7). After fully charged to L (Fig. 4b), the XRD peaks can be well matched to Sb2S3 instead of FeSb2S4, suggesting that the decomposition reaction of FeSb2S4 in the first discharge process is irreversible, as proven by the CVs. Furthermore, a NaFeS2 phase (JCPDS NO. 34-0935) at J (Fig. 4b) can be assigned, and then it disappears in the following charge states, demonstrating a multi-step desodiation mechanism of NaxFeS2 (Equation 9). There are no characteristic peaks of Fe and FeS phase can be observed in the ex-situ XRD patterns, which may be ascribed to nanosize nature of the formed Fe and FeS during cycling [50]. In view of this, XPS measurements were performed to further investigate the FeSb2S4 electrode after the initial cycle, as shown in Fig. S11 and Fig. S12 (in Supplementary Information). The XPS results prove that the Fe and FeS are bi-products after electrochemical reaction. To sum up, the redox reactions could be formulated as follows. In the initial discharge process: 14

FeSb2S4 + (3+x)Na+ + (3+x)e-→NaxFeS + 1/2Sb2S3 + Sb + 3/2Na2S (x≤2)

(1)

NaxFeS + (2-x)Na+ + (2-x)e-⇔ Fe + Na2S

(2)

1/2Sb2S3 + 3Na+ + 3e- ⇔ Sb + 3/2Na2S

(3)

2Sb+ 6Na+ + 6e- ⇔ 2Na3Sb

(4)

Total sodiation reaction: FeSb2S4 + 14Na+ + 14e-→ 2Na3Sb + Fe + 4Na2S

(5)

In the initial charge process: 2Na3Sb ⇔ 2Sb+ 6Na+ + 6e-

(6)

2Sb + 3Na2S ⇔ Sb2S3 + 6Na+ + 6e-

(7)

1/2Fe + Na2S ⇔ 1/2NaxFeS2+ (4-x)/2Na+ + (4-x)/2e- (x≤2)

(8)

1/2Fe + 1/2NaxFeS2 ⇔ FeS + x/2Na+ + x/2e-

(9)

Total desodiation reaction: 2Na3Sb + Fe + 4Na2S ⇔ Sb2S3 +FeS + 14Na+ + 14e-

15

(10)

Fig.5. (a) Cycling performance and coulombic efficiency of FeSb2S4/EN-rGO and FeSb2S4 at 0.1 A g-1. (b) Rate performance of FeSb2S4/EN-rGO and FeSb2S4. (c) Long cycling stability of FeSb2S4/EN-rGO anode at 0.5 and 5 A g-1. (d) Comparison of sodium storage performance at different current densities for recently reported metal sulfides as SIB anodes. 16

Cycling performance of FeSb2S4/EN-rGO was carried out and depicted in Fig. 5. After 100 cycles, FeSb2S4/EN-rGO anode can retain a reversible capacity as high as around 782.5 mAh g-1 at 0.1 A g-1, corresponding to 90.7% of the second capacity (Fig. 5a). The capacity loss between the initial and subsequent cycles can be largely imputed to the different sodiation mechanism in the first discharge process combined with some irreversible reactions including the formation of SEI film. The coulombic efficiency quickly increases to and then maintains at nearly 100% after the initial five cycles. Moreover, the capacity of FeSb2S4/EN-rGO for the initial several cycles is lower than that of pure FeSb2S4, which can be ascribed to the poor sodium storage property of EN-rGO. The average reversible capacity of EN-rGO is only 21.7 mAh g-1 during 500 cycles at 0.1 A g-1 (Fig. S13, Supplementary Information). In the case of pure FeSb2S4, apparently, the continuous and severe capacity decay can be observed in the next 80 cycles, indicating that the pristine FeSb2S4 electrode could suffer from drastic volume expansion and ultimately succumbed to the pulverization of the electrode structure. To further prove the positive effect of amine-modification, FeSb2S4-rGO was formed by the mixture of FeSb2S4 particles and graphene oxide under polyol-thermal treatment (Fig. S14, Supplementary Information). Compared to FeSb2S4/EN-rGO, FeSb2S4-rGO displays continuous capacity decay, demonstrating that one of the main advancements of this bimetallic sulfide anode is hybridization with amine-modified rGO that fulfils a dual role as a simultaneous electronic conductive matrix and active materials holder. With regard to the rate capability (Fig. 17

5b), the specific capacity of FeSb2S4/EN-rGO is much superior to that of pure FeSb2S4 at all rates, especially at high rates. Particularly, FeSb2S4/EN-rGO exhibits high reversible capacity of 963.7, 894.7, 833.8, 808.1, 763.3, 706.8, 643.5, 561.8 and 461.5 mAh g-1 at 0.1, 0.2, 0.5, 1, 2, 5, 8, 10 and 12 A g-1, respectively. When suddenly switching the current density back to 0.1 A g-1, a reversible capacity of 887.0 mAh g-1 for FeSb2S4/EN-rGO is restored, manifesting an attractive rate performance with the aid of EN-modified rGO. To further validate the micro-architecture benefits of FeSb2S4/EN-rGO and interfacial conciliation effect of EN-modified graphene for the improved sodium ion storage, the long-term cycling stability of FeSb2S4/EN-rGO anode was tested (Fig. 5c). The long-term cycling measurements were first activated at 0.1 A g-1 for 5 cycles. At a rate of 0.5 A g-1, the discharge capacity of 651.6 mAh g-1 can be achieved after 500 cycles, corresponding to 0.03% capacity loss rate per cycle, demonstrating a mild capacity degradation as cycling progressed. When improved to high rate of 5 A g-1, FeSb2S4/EN-rGO still sustains a high reversible capacity of 515.7 mAh g-1at 500th cycle with a low capacity loss rate of 0.04% per cycle. The excellent reversibility of FeSb2S4/EN-rGO anode benefits from the decoration of EN-modified graphene, which can not only offer abundant and efficient electron transfer channels, but also provide strong chemical affinity to anchor active materials onto the graphene interface with suppressed aggregation of them during cycling. In this bimetallic sulfide system, Sb acts as active metal component, therefore, the electrochemical performances of FeSb2S4/EN-rGO were compared with some of the recently reported Sb-based sulfide 18

anodes (Table S3, Supplementary Information). Clearly, FeSb2S4/EN-rGO shows superior sodium storage capacity and high-rate capability especially for long-term cycling. Further compared with other bimetallic sulfide and transition metal sulfide anodes (Fig. 5d), FeSb2S4/EN-rGO has superiority in reversible capacity and rate performance in comparison with other bimetallic sulfides [15-19, 21-24, 26, 51-59].

Fig.6. (a) CV curves of FeSb2S4/EN-rGO anode at different scan rates from 0.2 to 1.0 mVs-1. (b) Corresponding log(i) versus log(v) plots at selected peak currents. (c) The fraction of the pseudocapacitive contribution shown by the green area in CV curve at 1 mV s-1. (d) Bar chart revealing the percentage of the pseudocapacitive contribution at different scan rates. 19

The improvement in cycling stability for FeSb2S4/EN-rGO was investigated by EIS (Fig. S15 and Table S1, Supplementary Information). The Rct of pure FeSb2S4 is 88.9 Ω at the 25th cycle and increases to 1080Ω at 100th cycle. As for FeSb2S4/EN-rGO, the Rct slightly raises from 63.2 Ω (25th cycle) to 75.7 Ω (100th cycle), revealing a more stable cycling reversibility after the embellishment of EN-rGO, which is in accordance with above cycling results. Moreover, the nature of sodium storage of FeSb2S4/EN-rGO anode was revealed through CV measurements at different sweep rates from 0.2 to 1.0 mV s-1 (Fig. 6a). The relationship of the peak current (i) and sweep rates (v) meets the below power-law Equation (11) [61]: i= avb

(11)

where a and b are adjustable parameters. The value of b was employed to study the charge storage mechanism. Specifically, the b value of 0.5 corresponds to a diffusion-controlled

behavior,

whereas

that

of

1.0

indicates

a

surface

capacitive-controlled process. As illustrated in Fig. 6b, the values of b calculated by the slope of the log(v)-log(i) plots for peak Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ, Ⅵ, Ⅶ and Ⅷ are 0.93, 0.94, 0.84, 0.86, 0.82, 0.96, 0.83 and 0.87, respectively. Consequently, the high b-value demonstrates that the kinetics for FeSb2S4/EN-rGO is surface capacitive dominated. In addition, the pseudocapacitive contribution can be quantified by Equation (12): i(V) =k1v + k2v 1/2

(12)

where k1 and k2 are constants for a given potential. The k1v and k2v1/2 stand for 20

capacitive-controlled contribution and diffusion-controlled contribution, respectively. As exposed in Fig. 6c, the capacitive-controlled contribution (the green area) is as high as 89.1% for FeSb2S4/EN-rGO at 1.0 mV s-1. The rich pyridinic/pyrrolic-type nitrogen as well as amino-containing groups and the nanosized reaction products contribute to the pseudocapacitive effect. In addition, Fig. 6d exhibits the increasing proportion of the pseudocapacitive contribution from 70.1% to 89.1% with the incremental scan rates, suggesting that the pseudocapacitive processes play a dominant role of charge storage in FeSb2S4/EN-rGO, thus enabling fast sodiation kinetics and excellent rate performance.

Fig.7. TEM (a) and HRTEM (b) images of FeSb2S4/EN-rGO anode after cycles at 5 A g-1. The models in first principles theoretical calculations, illustrating the interactions of (c) FeSb2S4 with pure graphene, and (d-h) FeSb2S4 with EN-modified graphene. 21

The iron (Fe), antimony (Sb), sulfur (S), carbon (C), nitrogen (N) and hydrogen (H) are denoted by violet, purple, yellow, gray, blue and white balls, respectively.

To gain an in-depth understanding in the enhancement of cycling reversibility even at high rates, the structural stability of the FeSb2S4/EN-rGO anode was further explored. The anode materials after 400 cycles at 5 A g-1 were collected and further investigated using TEM and HRTEM analysis. Fig. 7a illustrates that the structural integrity of the composite is maintained after cycling. As shown in Fig. 7b, the anode materials after cycling are still tightly anchored onto EN-rGO sheets, deducing an effective chemical attraction between them. In addition, the marked interplanar spacing of around 0.35, 0.31 and 0.23 nm can be assigned to the (1 1 2) plane of Sb2S3, (0 1 2) plane of Sb and (2 2 0) plane of Na2S, respectively, which is consistent with the above ex-situ XRD results. Interestingly, compared to the particle size of anode materials before cycling, the size of cycled anode materials decreases to nanosize below 15 nm, thus promoting the cycling reversibility and rate capability of the anode. Overall, the active materials are indeed anchored on the graphene layers, and the aggregation

of

the

generated

nanocrystals

is

suppressed

during

sodium

insertion/extraction, which could benefit from the chemical affinity between active materials and EN-modified rGO. To shed more lights onto the significance of the interfacial chemical bonding in FeSb2S4/EN-rGO from the amine-reduced reaction, we performed first principles theoretical calculations to further verify the strengthened interactions between 22

FeSb2S4 (and its discharge product Na2S) and EN-modified rGO. The calculation details are described in the Experimental section. In the case of EN-rGO in the calculated model system, three kinds of nitrogen atoms, the pyridinic nitrogen, the pyrrolic nitrogen and the nitrogen atom of –NH2 are targeted.Fig. 7c reveals that the binding energy between FeSb2S4 and rGO without functional groups is estimated as low as 0.37 eV. After EN modification, the binding energies between pyridinic nitrogen/pyrrolic nitrogen and FeSb2S4 via C-N-Sb bond can achieve as high as 2.08 eV and 1.23 eV, respectively (Fig. 7d, e). Through the interaction via C-N-Fe bond, the binding energy increases to 1.35 eV (Fig. S16, Supplementary Information). As for the amino-containing functional groups (Fig. 7f, g), the binding energies can reach to 1.43 and 1.92 eV. Moreover, FeSb2S4 can also bind with both nitrogen atoms in –NH2 with binding energies as high as 1.75 eV (Fig. 7h,). After EN functionalization, the binding energy between FeSb2S4 and rGO is significantly strengthened from 0.37 eV to 2.08 eV. Evidently, the above results demonstrate that FeSb2S4 particles can spontaneously adhere to EN-modified rGO sheets by the strong chemical interactions, thus being effectively pinned onto the surfaces of electronic conductive matrix. Furthermore, it should be noted that the discharge product, Na2S, can be also bound on EN-rGO layers as confirmed by calculations (Fig. S17a, Supplementary Information). The results prove that EN-rGO can possess strong chemical affinity for FeSb2S4 as well as its discharge product Na2S, and this chemical affinity contributes significantly to suppressing the aggregation of reaction products and diminishing the detachment of electrode materials from graphene sheets during repeated 23

sodiation/desodiation processes. Combined with experimental results and theoretical calculations, the EN-rGO in FeSb2S4/EN-rGO composite accomplishes a dual role as a simultaneous electronic conductive matrix and active materials anchor, thus resulting in high capacity, enhanced cycling stability and prolonged cyclic lifespan.

3. Conclusions A new bimetallic sulfide, FeSb2S4, chemically immobilized on amine-modified graphene composites (FeSb2S4/EN-rGO), was first reported as anode material for SIBs, in which it can deliver a high discharge capacity of 782.5 mAh g-1 at 0.1 A g-1 for 100 cycles. Even after 500 cycles at high rate of 5 A g-1, it still remains a reversible capacity of 515.7 mAh g-1, corresponding to only 0.04% capacity loss per cycle. The sodium storage mechanism of FeSb2S4 was investigated by ex-situ XRD analysis, CV and TEM. Calculations indicate that the strong chemical affinity of EN-rGO to FeSb2S4 and its discharge products contributes to the stability of anode architecture during cycling, thus improving the sodium storage performance. This study may provide a viable approach to the design and advancement of bimetallic sulfide anodes with high reversible capacity, cycling stability and rate performance.

4. Experimental section 4.1. Materials synthesis All the reagents used in this experiment were analytical grade purity without any further purification. Graphene oxide was prepared from graphite powder through the 24

modified Hummers’ method. The FeSb2S4/EN-rGO composite was synthesized by a facile one-pot method. In a typical synthesis process, 0.50 g of FeCl3·6H2O and 0.90 g of SbCl3 were dissolved in 50 mL of diethylene glycol (DEG) to form clarified solution. Subsequently, the obtained mixture was transferred into a 250 mL three-necked round-bottom flask with 20 mL of graphene oxide (5 mg mL-1) and followed by ultrasonicated for 30 minutes. Next, 4 mL of ethylenediamine was injected into the flask in Ar atmosphere under sustained stirring, and subsequently refluxed at 180 ºC in an oil bath. After half an hour, 0.30 g of sulfur powder was rapidly added into the above solution with vigorous stirring. Then, the obtained black mixture was hydrothermally treated at 180 ºC for 4h under refluxing. After cooling down to room temperature naturally, the precipitate was separated by centrifugation and washed with absolute alcohol and deionized water for several times, followed by drying overnight at 80 °C to obtain FeSb2S4/EN-modified rGO hybrid. Finally, as-prepared FeSb2S4/EN-rGO (dark powders) was further heating at 450 °C for 3 h under argon atmosphere to remove residual sulfur. For comparison, FeSb2S4 was obtained according to the above procedures without the addition of graphene oxide, and only 2 mL EN was added.

4.2. Computational method The binding energy was calculated using SIESTA software. All the models were constructed and obtained from VESTA software. Generalized gradient approximations (GGA) were used in the calculation. The k-points set was 5×5×5 considering the 25

atoms were less than 100 in the models. For calculating the binding energy, all the constructed models were geometry optimized until the displacement iteration less than 0.01 Å. The geometry optimized models were used to calculated the total energy (denoted as Ebased-adsorbed), then the adsorbed molecules were deleted and calculated the total energy (denoted as Ebased). The total energy of the adsorbed molecules were calculated as Eadsorbed at the same time. The binding energy was acquired according to the difference values, that is Eb = (Ebased + Eadsorbed) - Ebased-adsorbed. The value of Ebased-adsorbed must be less than the value of (Ebased + Eadsorbed) because the adsorption is an energy-decreased status.

4.3. Materials characterization The crystalline structure of the as-obtained samples was characterized by X-ray diffraction (XRD，Shimadzu-XRD-7000 X-ray diffractometer, Cu-Kα, λ = 1.54056 Å, 40 kV, 40 mA). The powder samples and the electrodes for ex-situ XRD were all scanned at a rate of 5°min-1and 0.5°min-1 (with a step length of 0.01°), respectively, in XRD measurement. The elemental information of the samples were investigated via X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI). The particle size and morphology of the samples were detected by field emission scanning electron microscope (FESEM, Hitachi S4800) and transmission electron microscope (TEM, FEI TecnaiG2 F20). Raman spectroscopic measurement was performed using a Laser Confocal Micro-Raman Spectroscopy (LabRAM HR800) with an excitation wavelength of 532 nm at room temperature. The content of carbon was analyzed by 26

thermogravimetric analysis (NETZSCH STA2500) in air from 30 ºC to 800 ºC with a heating rate of 10 ºC min-1. The mass contents of element Fe and Sb were tested by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700ce).

4.4. Electrochemical measurements To fabricate the electrodes, active material was mixed with carboxymethyl cellulose sodium and super P in deionized water with a mass ratio of 75:15:10 to form homogeneous slurry. Then the slurry was uniformly coated onto a copper foil with a loading mass of about1.25 mg cm-2of FeSb2S4/EN-rGO or FeSb2S4. Coin-type half-cells were assembled in an argon-filled glove box. Metallic sodium was used as the counter electrode with glass fiber as the separator in a half-cell configuration. The electrolyte was 1 M sodium perchlorate (NaClO4) dissolved in EC and DEC in a volume ratio of 1:1 with 5 wt% fluoroethylene carbonate (FEC) additive. Galvanostatic charge/discharge tests were performed on a multichannel battery testing system (LAND-CT2001) in a potential range of 0.01-2.5 V (vs. Na+/Na) based on the total weight of the composite. Cyclic voltammetry (CV) measurements were investigated on Autolab PGSTAT 128N electrochemical work station at various scanning rate (0.1-1.0 mV s-1) from the open circuit potential to 0.01 V and then back to 2.5 V. Electrochemical impedance spetra (EIS) measurements were carried out on the same work station in a frequency range of 10-2 ~ 105 Hz. All experiments and tests were done at room temperature.

27

Acknowledgments This work was supported by the Key Project of Chongqing Science and Technology Commission of China. (No.KJZD-K201801103).

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Highlights As a novel anode material, sodium storage mechanism of FeSb2S4 is adequately studied. There is strong chemical affinity between FeSb2S4 and EN-rGO. The chemical affinity contributes to the stability of anode micro-architecture. The FeSb2S4/EN-rGO anode exhibits long lifespan, high capacity and rate performance.

35

Fig. 1 Graphical Abstract.

36