A Scalable Strategy to Develop Advanced Anode for Sodium-Ion

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A Scalable Strategy to Develop Advanced Anode for Sodium-Ion Batteries: Commercial Fe3O4 Derived Fe3O4@FeS with Superior Full-Cell Performance Bao-Hua Hou, Ying-Ying Wang, Jin-Zhi Guo, Yu Zhang, Qiu-Li Ning, Yang Yang, Wen-Hao Li, Jing-Ping Zhang, Xinlong Wang, and Xing-Long Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16580 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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ACS Applied Materials & Interfaces

A Scalable Strategy to Develop Advanced Anode for Sodium-Ion Batteries: Commercial Fe3O4 Derived Fe3O4@FeS with Superior Full-Cell Performance Bao-Hua Hou,† Ying-Ying Wang,† Jin-Zhi Guo,† Yu Zhang,‡ Qiu-Li Ning,† Yang Yang,† WenHao Li,† Jing-Ping Zhang,† Xin-Long Wang† and Xing-Long Wu*,† †

National & Local United Engineering Laboratory for Power Batteries, Faculty of Chemistry,

Northeast Normal University, Changchun, Jilin 130024, P. R. China ‡

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

Avenue, Singapore 639798, Singapore Corresponding Author *Xing-Long Wu, E-mail: [email protected] KEYWORDS: sodium ion batteries, anode materials, full cell, scalable preparation, Fe3O4

ACS Paragon Plus Environment

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ABSTRACT: A novel core-shell Fe3O4@FeS composed of Fe3O4 core and FeS shell with the morphology of regular octahedra has been prepared via a facile and scalable strategy via employing commercial Fe3O4 as the precursor. When used as anode material for sodium-ion batteries (SIBs), the prepared Fe3O4@FeS combines the merits of FeS and Fe3O4 with high Nastorage capacity and superior cycling stability, respectively. The optimized Fe3O4@FeS electrode shows ultra-long cycle life and outstanding rate capability. For instance, it remains a capacity retention of 90.8 % with a reversible capacity of 169 mAh g-1 after 750 cycles at 0.2 A g-1, and 151 mAh g-1 at a high current density of 2 A g-1, which is about 7.5 times in comparison to the Na-storage capacity of commercial Fe3O4. More importantly, the prepared Fe3O4@FeS also exhibits excellent full-cell performance. The assembled Fe3O4@FeS//Na3V2(PO4)2O2F sodium-ion full battery gives a reversible capacity of 157 mAh g-1 after 50 cycles at 0.5 A g-1 with a capacity retention of 92.3 % and the Coulombic efficiency of around 100 %, demonstrating its applicability for sodium-ion full batteries as a promising anode. Furthermore, it is also disclosed that such superior electrochemical properties can be attributed to the pseudocapacitive behavior of FeS shell as demonstrated by the kinetics studies as well as the core-shell structure. In view of the large-scale availability of commercial precursor and ease of preparation, this study provide a scalable strategy to develop advanced anode materials for SIBs.


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INTRODUCTION Sodium-ion batteries (SIBs) have recently attracted a great interest as a promising alternative to lithium-ion batteries (LIBs) for large-scale energy storage applications owning to their low cost and abundant Na resource.1-7 However, the larger diameter of Na ion than that of the Li one hampers the electrochemical kinetics of SIBs electrodes, making it difficult to achieve the reversible insertion/extraction of Na ions into/out the crystalline host.8,

9

For example, the

market-dominant graphite anode materials of LIBs are almost unsuitable for SIBs.10, 11 Hence, it's still a huge challenge to find the appropriate and scalable anode materials with high specific capacity, outstanding Na-storage reversibility and excellent rate capability.12, 13 To data, various materials have been intensively studied as the anode of SIBs. The main Nastorage mechanism of the anode materials include the de-/intercalation, alloying, and conversion reactions.14-16 Among them, the materials of alloying and conversion mechanism deliver the higher initial Na-storage capacity than those dominated by the de-/intercalation ones, which can achieve the higher energy density of SIBs, although they usually exhibit poor cycling stability.1720

Considering the cost of conversion reaction materials are usually much cheaper compared to

the alloy-type ones (such as Sb anode),

21, 22

making the conversion materials are more suitable

for practical applications.23, 24 Specifically, Fe3O4 has been extensively reported as an anode in LIBs in previous studies due to its high Li-storage capacity, cheapness, safety and environmental friendliness.25-28 Nevertheless, the pure Fe3O4 delivers very low Na-storage capacity when applied for SIBs.29 Although some previous reports have shown that the carbon-incorporated Fe3O4 nanocomposites exhibit the improved Na-storage performance, the cost of production is usually expensive due to the complicated preparation procedures.30,

31

Surprisingly, the cheap commercial Fe3O4 (C-


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Fe3O4) can give an excellent cycling stability despite the low capacity according to our studies, which suggests us it is particularly attractive to develop a facile strategy to improve the sodium storage activity of these cheap C-Fe3O4 resources while maintaining their excellent cycling stability. On the contrary, conversion-type FeS as an easily prepared mental sulfide using Fe3O4, has been proposed as a promising anode for SIBs because of its high theoretical capacity, better conductivity and environmental friendliness, but suffers the rapid capacity loss owing to the large volume change during sodium ion insertion/extraction lead to the rapid agglomeration and pulverization of the electrode, resulting in the poor cycling stability.32-34 To address this problem, a traditional method is the introduction of conductive carbon networks into the composite to buffer the large volume change of FeS.17, 35-39 Although some previous reports have shown that the carbon-incorporated FeS/C composites exhibit the improved Na-storage performance,34, 40, 41 the preparation procedures are usually very complex.42,

43

Thus, from the view of practical

application, the better choice for the FeS-based anode is to develop a facile and scalable approach to prepare the carbon-free materials with superior cycling stability when used for SIBs. Considering the above situations as well as Fe3O4 can be easily converted to FeS, inspiring us to prepare a Fe3O4/FeS composite material without any extra carbon to combine the long cycle life of C-Fe3O4 and the high capacity of FeS. Herein, a novel core-shell Fe3O4@FeS composite with the morphology of regular octahedra has been prepared via a facile and scalable strategy by using commercially available Fe3O4 (CFe3O4) as the low-cost precursor. When investigated as anode material for SIBs, the prepared Fe3O4@FeS exhibits the much improved Na-storage performance, which combines the high capacity of FeS and the excellent cycling stability of Fe3O4. It delivers a reversible Na-storage capacity of 169 mAh g-1 after 750 cycles at 0.2 A g-1 with a capacity retention of 90.8 % and a


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Coulombic efficiency of around 100 %. Furthermore, it is disclosed that such outstanding electrochemical properties should be attributed to the partially pseudocapacitive behaviors in the kinetic of FeS shell and the synergies between Fe3O4 and FeS in the core-shell structure. Furthermore, the full cells are assembled by coupling the prepared Fe3O4@FeS anode with our previously reported Na3V2(PO4)2O2F (NVPF) cathode. The Fe3O4@FeS//NVPF full cell delivers a reversible capacity of 157 mAh g-1 after 50 cycles at 0.5 A g-1 with a capacity retention of 92.3 %. The good performance, simple preparation and low cost raw, carbon-free, make Fe3O4@FeS become a promising anode material for SIBs and it is expected to large-scale commercial applications. EXPERIMENTAL SECTION Preparation of Fe3O4@FeS. Fe3O4@FeS was prepared via a simple annealing process using C-Fe3O4 (Aladdin, 98 %) and sulfur powders (Aladdin, 99.95 %). Fe3O4 and S powder should be uniformly mixed by grinding and then annealing at 500 °C for 4 h in a quartz tube under the inert N2 atmosphere with a heating rate of 2 °C min−1. The mass ratio of Fe3O4 and S powder was 1:1. Material Characterization. The morphology of all samples were characterized by scanning electron microscopy (SEM, HITACHI-SU8010, 10 kV), transmission electron microscopy (TEM, JEOL-2100 F, 200 kV) with an EDX analysis. X-ray diffraction (XRD) patterns were collected on a Rigaku D/max200PC diffractometer using Cu Kα radiation (λ=1.5406 Å). X-ray photoelectron spectroscopy (XPS) data were obtained by ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. Electrochemical Measurements. The Fe3O4@FeS electrode were prepared by mixing 80 wt % Fe3O4@FeS, 10 wt % acetylene black and 10 wt % carboxymethylcellulose sodium (CMC) in deionized water. Then the as-prepared slurry was coated on the copper foil followed by vacuum-


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drying at 60 oC for 12 hours. The electrolyte solution was 1.0 mol/L NaClO4 in ethylene carbonate (EC)/ propylene carbonate (PC) (1:1 by volume), plus 5 wt % FEC. Glass fiber (GF/D) from Whatman were used as a separator and metallic sodium foils were used as a counter electrode. The CR2032 coin-type cells were assembled in an Ar-filled glovebox. For full-cell assembly, the Na3V2(PO4)2O2F was used instead of the metallic sodium. The Na3V2(PO4)2O2F electrodes were prepared under the same procedures as the preparation of Fe3O4@FeS anodes. In order to match the cathode/anode capacity, there is a slight excess capacity of anode compared with cathode, and the mass ratios of Fe3O4@FeS to Na3V2(PO4)2O2F were about 1:1.45. Prior to the fabrication of full cells, chemical pre-sodiation for the Fe3O4@FeS anode was performed to activate the material and stabilize the electrode surface. Cyclic voltammograms (CVs) were measured on the CHI 600E electrochemical workstation. The galvanostatic charge/discharge tests were carried out on the LAND-CT2001A battery-testing system. All the cells were test in the voltage range of 0.01-2.8 V vs. Na+/Na. Electrochemical impedance spectroscopy (EIS) was performed with an amplitude voltage of 5 mV in the frequency range from 1MHz to 100 mHz by using a PMC2000 (Princeton Applied Research). The reversible capacity of half cell is calculated according to the mass of Fe3O4@FeS composite rather than pure Fe3O4 or FeS component. RESULTS AND DISCUSSION The Fe3O4@FeS was controllably prepared by directly annealing the mixture of C-Fe3O4 and S power, and superficial Fe3O4 would convert to FeS in this process. As a result, a novel core-shell Fe3O4@FeS with the morphology of regular octahedra will be obtained. It is a facile and scalable strategy which can easily prepare in large quantities of samples at one batch as shown in Figure S1, and the detailed process is given in the Experimental Section. Figure 1a shows the schematic of the structure change from the C-Fe3O4 to Fe3O4@FeS. C-Fe3O4 exhibits the morphology of


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regular octahedra with an average particle size about 200 nm as shown in SEM images (Figure 1b) (a)

(b)

C-Fe3O4

(c)

(d)

C-Fe3O4

d111=0.484nm Sulfurization

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200nm

(e)

Fe3O4@FeS

100nm

(f)

5nm

(g)

Fe3O4 (Core) d101=0.264nm

Fe3O4@FeS FeS (Shell)

(h)

200nm

(i)

O

100nm

(j)

S

5nm

(k)

Fe

100nm

Figure 1. (a) The illustration of preparation procedures for Fe3O4@FeS. (b) SEM, (c) TEM and (d) HRTEM images of C-Fe3O4. (e) SEM, (f) TEM and (g) HRTEM images of Fe3O4@FeS. (h) STEM image of Fe3O4@FeS and the corresponding mappings showing the elemental distribution of (i) O, (j) S and (k) Fe elements in it. and TEM image (Figure 1c). The high-resolution TEM (HRTEM) image shows clear lattice fringe spacing of 0.484 nm corresponding to the (111) lattice plane of Fe3O4 (Figure 1d). After annealing, the regular octahedron shape of the particles has no significant changes, only the surface of the particles become rough as shown in Figure 1e, f compared to the C-Fe3O4. HRTEM image on the surface of Fe3O4@FeS (Figure 1g) shows clear lattice fringes separated by 0.264 nm corresponding to the (101) lattice plane of FeS. It illustrates that Fe3O4@FeS is a coreshell structure consists of Fe3O4 core and FeS shell, which is helpful to improve the charge transfer owning to the better conductivity of FeS.44 TEM element mapping images show that the Fe, O, S are uniformly distributed in the particle which proves the FeS shell on the particle


7


surface were uniform (Figure 1h-k). Another, the SEM element mapping images (Figure S2) show that the Fe, O, S are uniformly distributed throughout the materials which reveals the homogeneity of the materials. The contents of FeS and Fe3O4 in Fe3O4@FeS were determined by the EDS as shown in Figure S3, the material contains 30 % FeS and 70 % Fe3O4. Figure 2a shows the X-ray diffraction (XRD) patterns of C-Fe3O4 and Fe3O4@FeS. All the main diffraction peaks of two materials can be indexed to the Fe3O4 (PDF#65-3107),27 but an extra peak located in 44 degree can be obviously observed in Fe3O4@FeS which can be indexed to the standard card of FeS (PDF#65-9124).45 Thus, it is further proved that we have successful obtained a novel core-shell Fe3O4@FeS composite with the morphology of regular octahedra. The spectra

Fe3O4@FeS

*

C-Fe3O4 Fe3O4-PDF#65-3107

Fe 2p Fe3+

Intensity (a. u.)

511

440

(b)

102

311 400

Intensity (a. u. )

220

(a)

Fe2+ Fe3O4@FeS Fe3+ Fe2+ C-Fe3O4

* FeS-PDF#65-9124 20

30

40

50

60

70

80

716

S 2p

712

710

708

706

S 2p1/2 S22S2-

C-Fe3O4

(d) -CO

Intensity (a. u.)

(c) Fe3O4@FeS

714

Binding Energy (eV)

2θ (degree)

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O 1s

O-S Fe3O4

-CO2

-CO2

Fe3O4@FeS Fe3O4

-CO

C-Fe3O4

172

168

164

160

536

534

Binding Energy (eV)

532

530

528

526

Binding Energy (eV)

Figure 2. (a) XRD patterns of C-Fe3O4 and Fe3O4@FeS. High-resolution XPS spectra of (b) Fe 2p, (c) S 2p, (d) O 2p for C-Fe3O4 and Fe3O4@FeS.


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of X-ray photoelectron spectroscopy (XPS) are employed to compare the elemental composition and chemical states of C-Fe3O4 and Fe3O4@FeS (Figure S4, Figure 2b-d). In the Fe2p spectra (Figure 2b), peaks present at binding energy of 711.6 eV can be ascribed to Fe3+ and 710 eV can be ascribed to Fe2+.24, 41 After annealing, Fe3+ ratio is obviously decreased, and Fe2+ ratio is obviously increased indicating that some Fe3+ have convert to Fe2+ caused by the conversion from Fe3O4 to FeS. Moreover, the characteristic peaks of S indicate the presence of FeS in Fe3O4@FeS (Figure 2c). Interestingly, in the O1s spectra (Figure 2d), the peak at 529.7 eV corresponding to the Fe3O4 species is significantly weakened,46 but a new peak presents at 530.2 eV ascribed to -SO. Such a -SO suggests that there is a chemical bond interaction between Fe3O4 and FeS in Fe3O4@FeS which would be helpful to stabilize the highly active FeS in the surface of the (a) 0.13

(b)

C-Fe3O4

1.50

Current (mA)

Current (mA)

0.04 0.00 -0.04

0.48

-0.08 -0.12

1st 2nd 3rd 4th 5th

0.56

0.48 0.0

0.5

1.0

1.5

2.0

2.5

1.43

0.12 0.06

∆V1

0.00

∆V2 -0.06

1st 2nd 3rd 4th 5th

1.74

0.010.30

0.95

-0.12

0.82 -0.18

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Potential (V vs.Na+/Na)

Potential (V vs.Na /Na) (c)

(d) 0.10

Fe3O4@FeS 1.82

0.11

+

C-Fe3O4 Fe3O4@FeS

0.05 0.00 -0.05

3.0 2.5

Potential (V)

Current Density (mA g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 1.5

C-Fe3O4 Fe3O4@FeS

1.0 0.5 0.0

-0.10 0.0

0.5

1.0

1.5

2.0 2.5 + Potential (V vs.Na /Na)

3.0

0

50

100

150

200

250

Capacity (mAh g-1)

Figure 3. CV curves of initial 5 cycles at 0.1 mV s-1 of (a) C-Fe3O4 and (b) Fe3O4@FeS. The comparison between C-Fe3O4 and Fe3O4@FeS of (c) CVs curves of the second circle at 0.1 mV s-1 and (d) the charge/discharge curves for the second cycle at 0.05 A g-1.


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Fe3O4@FeS and obtain a good cycling stability.47, 48 It should be noted that the peaks at 533 and 531.5 eV in two materials are assigned to -CO2, and -CO species from the additional conductive tape using in the test process.46, 49 The electrochemical performance of both C-Fe3O4 and Fe3O4@FeS in the Na-ion half cells are compared in Figure 3. Figure 3a shows cyclic voltammograms (CVs) of C-Fe3O4 of the initial 5 cycles collected at a scan rate of 0.1 mV s-1. However, it presents a typical CVs of conductive carbon black but it is almost impossible to observe the characteristic peaks of Fe3O4 (only a weak anodic peak can be noticed at ~1.5 V corresponding to the oxidation process of Fe to Fe3O4 and decomposition of Na2O, the reduction process of Fe3O4 to Fe should be merged in the peaks of Na insert to the carbon black),50 which illustrate a lower activity of C-Fe3O4.24 Figure 3b shows the CVs of Fe3O4@FeS with the same test conditions, which presents a typical CVs of FeS ruling out the peaks of carbon black at 0.5 V, 0.01V in sodiation process and 0.11 V in desodiation process. During the initial cathodic scan, the peak at 0.82 V relate to the formation of solid-electrolyte interface (SEI) on the surface of the materials and the conversion from FeS to Fe and Na2S.34, 41 For the anodic scan, the sharp peak at ~1.43 V and the broad peak at ~1.82 V correspond to the desodiation reaction, where Na2FeS2 and NaxFeS2 species are probably formed, respectively. The desodiation processes can be expressed as follows according to the previous reports:34 2ܰܽଶ ܵ + ‫ܽܰ → ݁ܨ‬ଶ ‫ܵ݁ܨ‬ଶ + 2ܰܽା + 2݁ ି

(1)

ܰܽଶ ‫ܵ݁ܨ‬ଶ → ܰܽ௫ ‫ܵ݁ܨ‬ଶ + ሺ2 − ‫ݔ‬ሻܰܽା + ሺ2 − ‫ݔ‬ሻ݁ ି

(2)

However, for the subsequent anodic scans, there are three peaks for the sodiation processes. Obviously, the peak at 1.74 V is a reversible process of (2): ܰܽ௫ ‫ܵ݁ܨ‬ଶ + ሺ2 − ‫ݔ‬ሻܰܽା + ሺ2 − ‫ݔ‬ሻ݁ ି → ܰܽଶ ‫ܵ݁ܨ‬ଶ

(3)


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The peaks at 0.95 and 0.3 correspond the reversible process of process (1), and the charge/discharge curves of Fe3O4@FeS show the similar process (Figure S5). For the subsequent cycles, all the peaks are completely overlapped, indicating the good reversibility and cycling stability of [email protected], 52 In addition, the voltage differences (both ∆V1 and ∆V2 shown in Figure 3b) of the redox peaks are stable, indicating the high electrode reactioin kinetics and excellent reversibility of the Fe3O4@FeS electrodes for Na storage.53 Figure 3c shows a more intuitive comparison of CVs between C-Fe3O4 and Fe3O4@FeS. The CVs shape of as-prepared Fe3O4@FeS has a huge change which are completely consistent with FeS. Another, the CVs of Fe3O4@FeS shows the larger area than C-Fe3O4 correspond to the larger charge/discharge capacity which is agreement with its charge/discharge curves (Figure 3d). The above results indicated that the electrochemical activity of Fe3O4@FeS was significantly improved compared to C-Fe3O4.

100

Unit: A g-1

300 0.05 200

0.5

1

1.5

40

2

20

100

0

0.05 60

Fe3O4@FeS 0.1 0.2

80

C-Fe3O4 0

10

0

20

30

40

-20

(b) Potential (V vs. Na+/Na)

Charge Capacity (mAh g-1)

(a) 400

Efficiency (%)

3.0 Fe3O4@FeS 2.0 0.05 A g-1 0.1 A g-1 0.2 A g-1 0.5 A g-1 1 A g-1 1.5 A g-1 2 A g-1

1.0

0.0 0

50

Cycle number (N)

100

150

200

250

Capacity (mAh g-1)

Charge Capacity (mAh g-1)

(c) 400

100

300

80

Fe3O4@FeS

0.2 A g-1

90.8% 60

200 40 100

0

C-Fe3O4

0

50

100

150

200

20

250

300

350

400

450

500

550

600

650

700

Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 750

Cycle number (N)


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Figure 4. (a) The comparison of rate performance between C-Fe3O4 and Fe3O4@FeS, (b) the charge/discharge curves of Fe3O4@FeS at various current densities, and (c) the comparison of long-term cycling stability between C-Fe3O4 and Fe3O4@FeS, in the Na-ion half cells. Furthermore, the rate performance and cycling stability are plotted in Figure 4. Fe3O4@FeS shows an elevated rate performance, delivering a high reversible capacity of 215 mAh g-1 at 0.05 A g-1, and a reversible capacity of 151 mAh g-1 can still be retained even at a current as high as 2 A g-1, and the capacity can be recovered to over 230 mAh g-1 as the current is returned to 0.05 A g-1 (Figure 4a). Moreover, the voltage plateaus of charge/discharge curves at 2 A g-1 are perfectly retained (Figure 4b). The reversible capacity of half cell is calculated based on the mass of Fe3O4@FeS composite rather than pure Fe3O4 or FeS component. However, C-Fe3O4 only delivers a reversible capacity of 56 mAh g-1 at 0.05 A g-1 and 20 mAh g-1 at 2 A g-1. Cycling stability of the materials is another important indicator

Table 1. The comparison of cycling stability of Fe3O4@FeS with reported FeS-based anode materials for Na storage. Numbers

Samples

Current density

Cycle numbers

（A g-1）

（n）

Retention

Full-cell test

Reference

（%）

name

1

FeS nanofibers

0.5

150

127

N.A.

[54] Nano Res. 2017

2

FeS/CA

0.5

200

99.6

N.A.

[55] Angew. Chem. Int. Ed. 2016

3

FeS@C-N

0.1

500

47.3

N.A.

[56] J. Alloys Compd. 2016

4

FeS/RGO

0.5

50

103

N.A.

[41] Chem. Eur. J. 2016

0.09

50

86.9

5

FeS@C

6

FeS@C

N.A. 0.7

200

47.6

0.1

300

67.6

N.A.

[57] ACS Appl. Mater. Inter. 2015 [34] Nat. Commun. 2015


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This work

Fe3O4@FeS

0.2

750

90.8

Yes

--

for the practical applications which is also investigated under the same conditions (Figure 4c). After 750 cycles at 0.2 A g-1, a reversible capacity of 169 mAh g-1 is kept for Fe3O4@FeS, corresponding to capacity retention of 90.8 %. This is the best cycle stability of the FeS-based sodium anode material right now as shown in Table 1.34, 41, 54-57 It is noteworthy that, there is an obvious boost effect at the intial ~150 cycles during cycling (Figure 4c and S9), which should be attributed to the activation process due to large Na-ions as well as the reversible growth of a polymeric gel-like film coming from kinetically activated electrolyte degradation.25 The exact causes of such boost effect are still on the way of studies. Such an excellent cycling stability and rate performance of Fe3O4@FeS main due to the core-shell structure and the interaction between Fe3O4 core and FeS shell. All representations indicate that it is a simple and feasible strategy to combine the long cycle life of Fe3O4 and the high capacity of FeS to develop an excellent anode material of SIBs. In addition, ex situ XRD tests were used to investigate the reasons for excellent sodium storage performance of Fe3O4@FeS as shown in Figure 5a. It is obviously that the Fe3O4 is always exists whether in charged and discharged states, even after 100 cycles. It indicates that Fe3O4 core in Fe3O4@FeS is stable and not participate in the electrochemical reactions in the charge/discharge processes, but it has played a crucial role in stabilizing FeS shell on the surface of Fe3O4@FeS. Figure 5b shows the SEM image of Fe3O4@FeS after 100 cycles, it can be observed that the octahedral structure of Fe3O4@FeS is still clearly visible, and HRTEM image on the surface the


13


(a)

* 100th

Intensity (a. u. )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

*

* Cu

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(b)

2.8 V 0.01 V Fresh Fe3O4-PDF#65-3107

10

20

30

40

50

60

500nm

70

2θ (degree)

(d)

(c)

(e)

O

(g)

Fe

d101=0.264nm 5µm

(f)

S

5nm

Figure 5. (a) The comparison of XRD patterns of Fe3O4@FeS at different states, (b) SEM images and (c) HRTEM images of Fe3O4@FeS after 100 cycles in the Na-ion half cells. (d) SEM images of Fe3O4@FeS electrode after 100 cycles in the Na-ion half cells, and the corresponding elemental mappings showing the distribution of (e) O, (f) S and (g) Fe elements in (d). Fe3O4@FeS still shows the clear lattice fringes with d-spacing of 0.264 nm corresponding to the (101) lattice plane of FeS (Figure 5c). Another, element mappings of Fe3O4@FeS show uniform distribution of C, O and S after 100 cycles (Figure 5d-g). All the results support that the structure of Fe3O4@FeS is stable in the charge and discharge process resulting in an excellent cycling stability. The effect of the FeS ratio in the Fe3O4@FeS on the electrochemical performance is also investigated by adjusting the ratio of C-Fe3O4 and S powers before annealing. XRD (Figure S6) was used to compare the Fe3O4@FeS with different feeding mass ratio of C-Fe3O4 and S power.


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It can be found that the characteristic diffraction peak of FeS becomes stronger and stronger with increasing mass radio of S and C-Fe3O4 from 1:1 to 1:3, while the characteristic diffraction peak of Fe3O4 become weaker and weaker. We can infer that the thickness of the FeS shell increases with increasing mass radio of S and C-Fe3O4 corresponding to the schematic of Figure S7. With

Current (mA)

(a)

0.8

peak 1

0.0 0.1 mV s-1 0.2 mV s-1 0.3 mV s-1 0.5 mV s-1 1 mV s-1

-0.4 peak 2 0.0

0.5

1.0

1.5

120 100

Fe3O4@FeS

0.4

-0.8

(b)

Pseudocapacitive contribution (%)

the ratio of S increasing, the reversible capacity of Fe3O4@FeS is improved (Figure S8), but the

2.0

2.5

3.0

81%

83%

85%

87%

93%

0.1

0.2

0.3

0.5

1

80 60 40 20 0

Potential (V)

Scan rate (mV S-1)

(c) 0.6

(d) 1200

C-Fe3O4 fresh C-Fe3O4 after 3 cycles Fe3O4@FeS fresh Fe3O4@FeS after 3 cycles

900

0.3

-Z" (ohm)

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

600

300

-0.3 1 mV s-1

-0.6

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

Voltage (v)

300

600

900

1200

Z' (ohm)

Figure 6. (a) CV curves and (b) the pseudocapacitive contribution of Fe3O4@FeS at different scan rates, (c) CV curve with the pseudocapacitive fraction shown by the atrovirens of Fe3O4@FeS at a scan rate of 1 mV s-1, (d) Nyquist impedance plots of fresh and after the first cycle of Fe3O4@FeS and C-Fe3O4, in the Na-ion half cells. cycle retention is abruptly reduced from 90.8 % to 28.7 % after 750 cycles at the current density of 0.2 A g-1 (Figure S9). It initially proves that the range of interaction is limited between Fe3O4 core and FeS shell. When the thickness of FeS increases, the outermost layer of FeS will slowly lose its interaction with Fe3O4 core, leading to rapid decay of the capacity. Thus, the ratio of 1:1 is a more appropriate feeding mass ratio of C-Fe3O4 and S power to prepare Fe3O4@FeS.


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To further investigate the electrode process kinetics of Fe3O4@FeS, the CV tests at different scan rates were carried out (Figure 6a). However, the peak current is not well proportional to the square root of the scan rate, which indicates that the charge/discharge process is composed both of faradaic and non-faradaic behaviors.43,

58

The peak current (i) and scan rate (v) have a

relationship like the following equations: ݅ = ܽ‫ ݒ‬௕

(4)

݈‫݃݋‬ሺ݅ሻ = ܾ × ݈‫݃݋‬ሺ‫ݒ‬ሻ + ݈‫݃݋‬ሺܽሻ

(5)

where a and b are the adjustable parameters. If b = 0.5, the electrochemical process is controlled by pseudocapacitance. As b = 1, the process mainly relies on ionic diffusion. Figure S10 displays the log (i) vs. log (v) plot of peaks 1 and 2, the b-values are 0.81 and 0.91, respectivly. It suggests that the redox processes of Fe3O4@FeS include partial pseudocapacitive behaviors.59 Figure 6b summarizes the pseudocapacitive contribution at various scan rates which is calculated by the following equation: ݅ = ݇ଵ ‫ ݒ‬+ ݇ଶ ‫ ݒ‬ଵ/ଶ

(6)

where k1v and k2v0.5 represent the pseudocapacitive and the inserted contributions, respectively. The pseudocapacitive contributions are 81 %, 83 %, 85 %, 87 % and 93 %, respectively which indicates that the electrochemical reactions of Fe3O4@FeS are mainly controlled by the pseudocapacitive behaviors, leading to a fast kinetic process and resulting in a high rate performance.14, 58 The detail pseudocapacitive fraction at 1 mV s-1 is presented in Figure 6c. Electrochemical impedance spectroscopy (EIS) was also used to explain the electrochemical behaviors of C-Fe3O4 and Fe3O4@FeS (Figure 6d). The semicircle at high frequency is derived from the charge transfer process and the linearity at low frequency is caused by the Na+ diffusion process.60-62 Fe3O4@FeS displays a lower charge-transfer resistance than C-Fe3O4 whether


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before or after the cycles, and the charge-transfer resistance becomes small after three cycles than initial. This phenomenon proves that Fe3O4@FeS has the better conductivity and the faster kinetics. The aforementioned excellent electrochemical properties of Fe3O4@FeS encourages us to measure its practicability in the full cell. Recently, our group successfully prepares a promising cathode material of NVPF,63 which takes an outstanding sodium storage performance as shown in (a)

(b)

(c) 4

NVPF cathode Separator Fe3O4@FeS anode

Anode case

Voltage (V)

Cathode case

3

2 Fe3O4@FeS//NVPF 1 0

40

80

120

160

200

Capacity (mAh g-1) (e) 300

100

250 80

Unit: A g-1 200 0.05 150

0.1

0.2

0.05 0.5 1

60 1.5 2

5

100 Fe3O4@FeS//NVPF

50 0

0

5

10 15 20 25 30 35 40 45

Cycle number (N)

40 20 0

Disharge Capacity (mAh g-1)

300

(f) 100

250

80

200

0.5 A g-1

60

150 40

100

Fe3O4@FeS//NVPF

20

50 0 0

10

20

30

40

50

Efficiency (%)

Disharge Capacity (mAh g-1)

(d)

Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

Cycle number (N)

Figure 7. (a) Schematic illustration of the Na-ion full battery with NVPF as cathode and Fe3O4@FeS as anode. (b) Charge/discharge curves of Fe3O4@FeS//NVPF full cell at 0.05 A g-1. (c) A photo shows that a full cell can light up three LED bulb. (d) Rate performance and (e) cycling stability of Fe3O4@FeS//NVPF full cell. (f) A photo shows that a pouch-type full cell can light up 40 LED bulbs. Figure S11. So it is used as a cathode to couple with Fe3O4@FeS anode, and the Fe3O4@FeS//NVPF button full cell is assembled according to the schematic illustration as shown in Figure 7a. It should be noted that the reversible capacity of the full cell is calculated from the mass of Fe3O4@FeS anode material. Figure 7b shows the charge/discharge curves of the full cell


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at 0.05 A g-1, and the average output voltage is about 2.8 V, which can light up three LED bulbs (Figure 7c). Figure 7d shows the rate performance of the full cell, it delivers a reversible capacity of 182 mAh g-1 at 0.05 A g-1, even at 5 A g-1, a reversible capacity of 99.4 mAh g-1 is kept. The full cell also delivers a good cycling stability, which delivers a reversible capacity of 157 mAh g1

at 0.5 A g-1 after 50 cycles corresponding to capacity retention of 92.3 % (Figure 7e).

Moreover, a pouch-type full cells are successfully assembled, which can light up 40 LED bulbs as shown in Figure 7f for half an hour. The results indicate that Fe3O4@FeS has potential for practical applications. CONCLUSIONS In summary, a novel core-shell Fe3O4@FeS with the morphology of regular octahedra has been prepared by a facile and scalable strategy method from cheap C-Fe3O4 for the first time and investigated as an anode material for SIBs. The optimized Fe3O4@FeS sample exhibits an excellent sodium storage performance which combines the high capacity of FeS and the good cycling stability of Fe3O4. Fe3O4@FeS delivers a high reversible capacity of 215 mAh g-1 at 0.05 A g-1 and 169 mAh g-1 after 750 cycles at 0.2 A g-1, corresponding to a capacity retention of 90.8 % between 0.01-3 V. Such a durable cycling stability is attributed to the interaction between Fe3O4 core and FeS shell in the core-shell structure and the pseudocapacitive behavior during the charge and discharge processes. Furthermore, the Fe3O4@FeS//NVPF full cell was assembled which delivers a reversible capacity of 157 mAh g-1 after 50 cycles at 0.5 A g-1 with a capacity retention of 92.3 % and a Coulombic efficiency of about 100 %. The superior electrochemical performance, facile strategy and low-cost raw materials make the prepared Fe3O4@FeS promising anode material for practical SIBs. In view of the large-scale availability of commercial


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precursor, ease of preparation and universality of sulfurization, this study provide a scalable and general strategy to develop advanced anode materials for SIBs. ASSOCIATED CONTENT Supporting Information. More SEM, XPS, XRD and electrochemical dates are included. AUTHOR INFORMATION Corresponding Author *Xing-Long Wu, E-mail: [email protected] ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51602048), and the Fundamental Research Funds for the Central Universities (2412017FZ013). REFERENCES (1).

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A Scalable Strategy to Develop Advanced Anode for Sodium-Ion

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