Metastable Marcasite-FeS2 as a New Anode Material for

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Metastable Marcasite-FeS2 as a New Anode Material for Lithium-Ion Batteries: CNFs-Improved Lithiation/Delithiation Reversibility and Li-Storage Properties Hong-Hong Fan, Huan-Huan Li, Ke-Cheng Huang, ChaoYing Fan, Xiao-Ying Zhang, Xing-Long Wu, and Jing-Ping Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00578 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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ACS Applied Materials & Interfaces

Metastable Marcasite-FeS2 as a New Anode Material for Lithium-Ion Batteries: CNFs-Improved Lithiation/Delithiation Reversibility and Li-Storage Properties Hong-Hong Fan, Huan-Huan Li, Ke-Cheng Huang, Chao-Ying Fan, Xiao-Ying Zhang,* Xing-Long Wu* and Jing-Ping Zhang*

Faculty of Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Changchun, Jilin 130024, China. * The corresponding authors Email addresses: [email protected] (X.-Y. Zhang); [email protected] (X.-L. Wu); [email protected] (J.-P. Zhang)

KEYWORDS: marcasite, FeS2, carbon nanofibers, anode materials, lithium ion batteries

ABSTRACT: Marcasite (m-FeS2) exhibits higher electronic conductivity than that of pyrite (p-FeS2) because of its lower semiconducting gap (0.4 eV vs. 0.7 eV). Meanwhile, as demonstrates stronger Fe–S bonds and less S–S interactions, the m-FeS2 seems to be a better choice for electrode materials compared to p-FeS2. However, the m-FeS2 has been seldom studied due to its sophisticated synthetic methods up to now. Herein, a hierarchical m-FeS2 and carbon nanofibers composite (m-FeS2/CNFs) with grape-cluster structure was designed and successfully prepared by a straightforward hydrothermal method. When evaluated as an 1 ACS Paragon Plus Environment


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electrode material for lithium ion batteries, the m-FeS2/CNFs exhibited superior lithium storage properties with a high reversible capacity of 1399.5 mAh g−1 after 100 cycles at 100 mA g-1 and good rate capability of 782.2 mAh g−1 up to 10 A g-1. The Li-storage mechanism for the lithiation/delithiation processes of m-FeS2/CNFs was systematically investigated by ex situ powder X-ray diffraction patterns and scanning electron microscopy. Interestingly, the hierarchical m-FeS2 microspheres assembled by small FeS2 nanoparticles in the m-FeS2/CNFs composite converted into a mimosa with leaves open shape during Li+ insertion

process

and

vice

versa.

Accordingly,

a

“CNFs

accelerated

decrystallization-recrystallization” mechanism was proposed to explain such morphology variations and the decent electrochemical performance of m-FeS2/CNFs.

1. INTRODUCTION

In order to satisfy the ever-increasing demands for future energy storage applications, many efforts have been dedicated to the development of advanced anode materials for lithium ion batteries (LIBs), especially those with higher energy density and longer cycle life than those of commercial graphite anode.1-6 In this regard, transition metal sulphides (MxSy, where M represents Mo, W, Fe, Sn, Co, etc.) stand out clearly due to their high theoretical capacity, facile producibility and environmental friendliness.7-13 Among these alternatives, FeS2 with a theoretical capacity of 890 mAh g−1 arising from four electron reaction has attracted considerable interests.14-15 Normally, there are two different polymorphs for FeS2, pyrite (p-FeS2) and marcasite (m-FeS2). p-FeS2 belongs to the cubic crystal structure with Pa3 2 ACS Paragon Plus Environment

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space group, while m-FeS2 possesses the orthorhombic one with space group of Pnnm. Meanwhile, the Wyckoff positions for Fe and S in m-FeS2 are 2a(0,0,0) and 4g(u,v,0), respectively, which are fundamentally different from those of p-FeS2.16-19 On basis of the previous reports, the m-FeS2 exhibits a lower semiconducting gap (0.4 eV) than that of p-FeS2 (0.7 eV), implying a higher electronic conductivity in m-FeS2.18, 20 Furthermore, there are stronger Fe-S bonds and weaker S-S interactions in the crystalline structure of m-FeS2 than those of the p-FeS2 counterpart.16 Hence, both the electrical conductivity and crystal structure of m-FeS2 theoretically promise the greater feasibility of obtaining better electrochemical properties in comparison to p-FeS2. Despite all this, m-FeS2 has been seldom studied because of its low stability of metastable phase and the complicated synthetic procedures. In 2015, the pure m-FeS2 nanoparticles (NPs) were firstly synthesized via the hot-injection protocol.21 Though such m-FeS2 NPs showed a higher Li-storage capacity as the anode materials for LIBs than those of the p-FeS2 counterpart, its specific capacities decreased rapidly from 1200 to 200 mAh g−1 within only 30 cycles at a low current density of 100 mA g−1. Moreover, the initial coulombic efficiency (CE) was only about 48%. So, it is still a big challenge to develop the promising m-FeS2-based electrodes through the structural and compositional engineering. Previous studies had extensively demonstrated that the strategy with carbon incorporation was an effective approach to improve the cycling and rate capabilities of electrode materials.22-30 For examples, Lee et al wrapped p-FeS2 microspheres into the reduced graphene oxide nanosheets, making the obtained composite still deliver a high 3 ACS Paragon Plus Environment


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reversible capacity of 1000 mAh g-1 after 300 cycles at the current density of 890 mA g-1;31 Tao et al employed a biotemplating technology to synthesize nanostructured p-FeS2@S-C composite, which was able to provide a specific capacity of 689 mAh g−1 at 100 mA g−1 after 100 cycles;32 and Yu et al reported a facile self-sacrificing template approach to synthesize the porous p-FeS2 encapsulated by carbon nanocages, which delivered a specific capacity of 500 mAh g-1 after 50 cycles when used as the cathode material for LIBs.33 The pioneer works clearly suggested that the electrochemical properties of FeS2 can be improved via the carbon incorporation. Nevertheless, the reported synthetic methods of FeS2 are comparatively complicated and thus Li-storage properties of FeS2-based electrodes still need to be further improved towards practical application. Herein, we prepared a novel hierarchical m-FeS2 microspheres and carbon nanofibers composite (m-FeS2/CNFs). In this composite, the m-FeS2 microspheres assembled by FeS2 nanoparticles in m-FeS2/CNFs are intertwined closely to CNFs, forming a morphology similar to the grapes growing on the grapevine. This morphology design combines each component advantages of mirco-nano property of m-FeS2 and high conductivity of CNFs at the same time: (1) the interwoven CNFs can not only improve the conductivity of the electrode, but also protect m-FeS2 microspheres from further aggregation; (2) the nanosized building particles obviously shorten the lithium ions transport path; (3) the hierarchical architecture

of

m-FeS2

microspheres

relaxes

the

strains

during

repeated

Li+

insertion/extraction processes. Due to these structural merits, the redox kinetics and cycle stability of m-FeS2/CNFs achieve significantly improvement. As a result, the m-FeS2/CNFs 4 ACS Paragon Plus Environment

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electrode delivered a high reversible capacity of 1399.5 mAh g-1 at a current density of 100 mA g-1 after 100 deep cycles. Moreover, a decent reversible capacity of 573.4 mAh g-1 is still obtained at 5 A g-1 even after 1000 cycles, indicating its good tolerance toward high rate cycling. The morphology variation of m-FeS2/CNFs during (de)lithiation processes was also investigated by ex situ scanning electron microscopy (SEM) characterization. Interestingly, the m-FeS2 microparticles in the m-FeS2/CNFs composite can open gradually to form a mimosa-like intermediate product during discharge process. Moreover, when charging back to 3 V, it returns to the initial state. As this phenomenon has not been reported in the other FeS2-based electrodes, a “CNFs accelerated decrystallization-recrystallization” mechanism (CNFs-D/R) was proposed to explain such morphology variations of m-FeS2/CNFs. That is, m-FeS2 microparticles in the m-FeS2/CNFs composite can react with Li+ more completely compared to pristine m-FeS2 under the enhanced Li-storage mechanism contributed by CNFs during the Li-insertion/extraction processes. We believe that such a unique morphology change can not only enable more Li+ ions insertion but also promote the accessibility of electrolyte, thus effectively improving the electrochemical performance of the m-FeS2/CNFs electrode.

2. EXPERIMENTAL SECTION 2.1 Materials: Iron nitrate nonahydrate (Fe(NO3)3·9H2O, 99.99%), L-cysteine (C3H7NO2S, 99%) and diethylene glycol (>99%) were purchased from Aladdin without any purification. The carbon nanofibers are produced by Sigma-Aldrich. 5 ACS Paragon Plus Environment


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2.2 Synthesis of m-FeS2/CNFs composites and pristine m-FeS2: Fe(NO3)3·9H2O (0.4040 g) was mixed with L-cysteine (0.6058 g), carbon nanofibers (0.0300 g) in distilled water (20 ml). The mixture was vigorously stirred for 20 min to form a black emulsion. Afterwards, diethylene glycol (20 ml) was added slowly into the above solution and continually stirring for 1 hour at room temperature. Then, the mixed solution was transferred into a Teflon bomb and kept at 185 ˚C for 50 hours. After cooling to room temperature naturally, the black product was collected and washed with deionized water and ethanol for several times, finally dried under vacuum at 80 ˚C for 12 hours. For comparison, the pristine m-FeS2 sample was synthesized by the same way with the m-FeS2/CNFs composite except the addition of CNFs. 2.3 Material characterization: The structure and crystallographic data of the as-prepared m-FeS2/CNFs and pristine m-FeS2 were characterized by powder X-ray diffraction (PXRD, Bruker D8 ADVANCE diffractometer) using Cu Kα radiation at room temperature from 10˚ to 90˚. X-ray photoelectron spectra (XPS) were conducted with Al Kα radiation and energy step size of 0.1 eV. The morphology of each sample was studied by transmission electron microscope (TEM, JEOL-2100F, 200 kV) and SEM (JEOL JSM-6700F field emission). Thermo-gravimetric analysis (TGA) was performed on a thermal analyzer at 800 ˚C under an air atmosphere with a heating rate of 10 ˚C min-1. Raman spectra were carried out with a confocal Raman microscope (JYHR-800 Lab Ram) in a backscattering configuration with a 488 nm argon ion for excitation. The Fourier transform infrared spectra (FTIR) were measured with an infrared spectrometer (Nicolet 6700-FTIR) from Thermo-Scientific. Nitrogen adsorption–desorption was tested by Autosorb-iQ2 from Quantachrome. 6 ACS Paragon Plus Environment

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2.4 Electrochemical measurement: For the electrochemical measurements, pristine m-FeS2 and m-FeS2/CNFs active materials were mixed with electrical conductor (acetylene black) and sodium alginate (in a weight ratio of 70:15:15 in distilled water) on copper foil. The CNFs electrode was fabricated by mixing CNFs powders and binder (sodium alginate). The 2032 coin cells were assembled in a dry argon-filled glove box with pure lithium metal as the counter electrode, 1.0 mol L-1 LiPF6 dissolved in ethylene carbonate/dimethyl carbonate mixture (1:1 in volume) as the electrolyte. Cyclic voltammetry (CV) measurements were carried out with a VersaSTAT 3 (Princeton Applied Research) electrochemical workstation over the potential range of 0.01-3 V. Electrochemical impedance spectroscopy (EIS) measurements were performed on VersaSTAT3 (Princeton Applied Research) with an AC amplitude of 10 mV and frequency ranged from 0.1 Hz to 100 kHz. Galvanostatic charge-discharge measurements were carried out using a Land battery (LAND CT2001A) test system from 0.01 to 3 V.

3. RESULTS AND DISCUSSION Grape-like m-FeS2/CNFs was prepared via a one-pot hydrothermal method, as schematically illustrated in Scheme 1. Iron- and sulfur- sources were simultaneously added

Scheme 1. Schematic illustration of synthesis route of grape-like m-FeS2/CNFs. 7 ACS Paragon Plus Environment


into the CNFs suspension under magnetic stirring. Then the mixture was transferred into a Teflon-lined stainless steel autoclave with sealing and maintained at 185 ˚C for 50 hours. Finally, the grape-like m-FeS2/CNFs composite is obtained. For comparison, the pristine FeS2 sample that prepared without carbon nanofibers was also synthesized in the same way. Figure 1a shows the PXRD patterns of the m-FeS2/CNFs composite, pristine m-FeS2 and pure CNFs. The m-FeS2/CNFs composite and pristine m-FeS2 have a similar PXRD diffraction pattern. All diffraction peaks corresponding to pristine m-FeS2 are in good accordance with an orthorhombic crystal phase of marcasite FeS2 (JCPDF card no.24-0074) without other crystalline impurity. The peaks for both materials are sharp, implying that they are well-crystallized. The enlarged version of the selected area is emphasized by the side of PXRD figure. It is clearly demonstrated that for m-FeS2/CNFs, an obvious peak at 26.5˚ readily indexed to the (002) plane of CNFs, indicating that the electrode material has a high graphitic degree. One unit cell of p-FeS2 and m-FeS2 are shown in Figure S1a and S1b, respectively. Both can be described that Fe atoms are octahedrally coordinated with six S (b) m-FeS2/CNFs

Pristine m-FeS2 CNFs

Intensity (a.u.)

(a)

Intensity (a.u.)

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m-FeS2/CNFs

40

60

2θ (degree)

80

D-band

CNFs Pristine m-FeS2

JCPDS: 24-0074

20

G-band

300

600

900

1200

1500 1800

Raman shift (cm -1)

Figure 1. (a) PXRD patterns of CNFs, pristine m-FeS2 and the m-FeS2/CNFs composite. (b) Raman spectra of pristine m-FeS2 and the m-FeS2/CNFs composite. 8 ACS Paragon Plus Environment

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atoms. Nevertheless, in pyrite Fe atoms are located at the face centers and the corners, whereas S atoms are placed in the octahedral vertices. For marcasite, the Fe-S interactions are edge-sharing and Fe atoms are octahedrally coordinated. Figure 1b shows the Raman spectra of the m-FeS2/CNFs composite, pristine m-FeS2 and pure CNFs. For m-FeS2/CNFs composite and pristine m-FeS2 spectra, they all include three peaks (282 cm-1, 320 cm-1 and 385 cm-1), which correspond to the characteristic active modes of m-FeS2. The absence of peak around 400 cm-1 indicates a lack of pyrite.34 Two peaks centered at 1346 and 1591 cm-1 for CNFs can be observed. They can be assigned to the D-band and G-band of CNFs, respectively.31,

35-36

The Intensity ratio of G/D band is 5/1, as demonstrated for the

m-FeS2/CNFs. The large IG/ID value could be ascribed to the high graphitization of carbon in m-FeS2/CNFs. Such a result is consistent with the above PXRD result. It's known that the graphitized carbon could improve the electrical conductivity, thus facilitating electrons transportation. Figure S2 presents TGA results for pristine m-FeS2 and m-FeS2/CNFs electrodes, carried out under an air atmosphere. Based on the residual weight percentage of the pristine m-FeS2 and the m-FeS2/CNFs samples, the content of CNFs was calculated to be 16.7 wt%. The optical properties of m-FeS2 and m-FeS2/CNFs were studied by UV absorption spectra and FT-IR characterization. As revealed in Figure S3a and 3b, both the m-FeS2 and m-FeS2/CNFs exhibit typical characteristics which match well with the FeS2 reported in previous literatures.37-38 The morphology and microstructure of the m-FeS2/CNFs were shown in Figure 2. As shown in Figure 2a and 2b, the homogeneously distributed m-FeS2 microparticles in 9 ACS Paragon Plus Environment


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(b)

(a)

1 µm

2 µm

(c)

(d) (111) 0.241 nm

m-FeS2 (111) 0.241 nm

(002) 0.337 nm

200 nm

5 nm

Figure 2. (a and b) SEM, (c) TEM and (d) HRTEM images of the m-FeS2/CNFs composite, clearly revealing the multiscale and hierarchically grape-like architecture.

m-FeS2/CNFs are intertwined with CNFs, forming a grape-like hybrid morphology. The hierarchical structure has interconnected networks of CNFs and m-FeS2 microparticles, which not only can enhance the electrical conductivity of composite but also mitigate the volume expansion/shrinkage during the lithiation and delithiation processes. The morphology of individual CNFs is shown in Figure S4. It’s composed of one-dimensional linear fiber with diameter of about 100 nm and length of 20~200 µm. In order to further confirm the specific architecture of m-FeS2/CNFs composite, the TEM and high-resolution TEM (HRTEM) images of the prepared electrode were studied. Figure 2c shows that the m-FeS2 microparticles in m-FeS2/CNFs are closely anchored on the CNFs rather than overlay together, thus improving the diffusion kinetics for both lithium ions and electrons. In addition, the HRTEM image shown in Figure 2d reveals that the lattice spacing of 0.241 and

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(a)

(b)

5 µm

(d)

(c)

Iron

(e)

(f)

S Kα

cps/eV

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


S L Fe L CK

Sulfur

Carbon

0.0

1.3

Fe Kα Ѕ Kβ

2.6

Fe Kβ 3.9

5.2

Energy (keV)

6.5

7.8

Figure 3. (a–e) SEM and corresponding elemental mapping images, (f) EDX spectrum of the grape-like m-FeS2/CNFs composite.

0.337 nm are well assigned to the (111) facets of m-FeS2 and the (002) planes of CNFs, respectively. The morphologies of the pristine m-FeS2 are displayed in Figure S5a-5c. Notably, the nanoparticles aggregate seriously in pristine m-FeS2, which may severely restrict redox kinetics. Figure S5d presents HRTEM image of pristine m-FeS2 microspheres. It is clearly observed that two d-spacing values are measured to be around 0.271 and 0.343 nm, those are indexed to the (020) and (110) faces, respectively. To further clarify the hierarchical structure, element mapping was carried out, indicating the existence of Fe, S and C (as shown in Figure 3a–e). The m-FeS2 microspheres are linked uniformly through intertwined CNFs. The intimate contact of CNFs and microparticles and their uniform dispersion allows for fast ions/electrons transfer.39 Figure 3f shows the EDX spectrum of m-FeS2/CNFs, revealing that the product is composed of three elements as well. To further investigate the chemical composition of m-FeS2/CNFs, XPS was also carried out. As shown in Figure S6a, the sharp 11 ACS Paragon Plus Environment


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peaks at 162.3 eV, 286.1 eV and 707.9 eV are assigned to the characteristic peak of S 2p, C 1s and Fe 2p, respectively, indicating the presence of iron, sulfur and carbon elements in the m-FeS2/CNFs composite. Figure S6b displays the high-resolution XPS spectrum of the Fe 2p region. It shows two major peaks centered at 719.8 and 706.1 eV, which can be assigned to Fe 2p1/2 and Fe 2p3/2 peak, respectively. Moreover, the spin energy separation between these two peaks is 13.7 eV, implying the typical characteristic of FeS2 spectra.40 Figure S6c shows the XPS spectrum of S 2p with two major peaks centered at 162.3 and 163.6 eV, which correspond to the S 2p1/2 and S 2p3/2, respectively. The C 1s spectrum of m-FeS2/CNFs in Figure S6d consists of oxygen functional groups (O−C=O, −C−O) and C-C peak. The electrochemical behavior of the m-FeS2/CNFs composite was characterized by CV. Figure 4a shows representative CV curves of the initial five cycles for the m-FeS2/CNFs composite electrode in the potential window of 0.01–3 V versus Li+/Li at a scan rate of 0.1 mV s-1. For the first cycle, it is clearly observed that the CV curve exhibits two oxidation peaks at about 1.89 V and 2.58 V while one sharp reduction peak is at about 1.31 V. The peak at 1.31 V moves notably to around 1.41 V in the following cycles and a new reduction peak appeared at about 2.11 V. Such a phenomenon indicates an irreversible phase transition because of the formation of Li2S and S during the lithium insertion/extraction process.31, 41-42 The sharp peak at about 1.31 V is attributed to the reduction of FeS2 to Fe and the formation of Li2S.43 In the anodic sweep, there are two well defined peaks at around 1.89 V and 2.58 V, those are related to the re-oxidation of Fe to Li2-xFeS2 and the further formation of FeSy and


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0.0 1 st 2 nd 3 rd 4 th 5 th

-0.2

0.0 0.5 1.0

2.0 2.5 +

(c)

1.5 1.0 0.5 0.0 0

+

m-FeS2/CNFs

1200 800

Pristine m-FeS2 Pure CNFs

400 0

20

40

60

Cycle number

80

300

(e)

1600

Current density: A g -1

1200

0.1 0.3 0.5 1.0 1.5 3.0 5.0 10.0

900

0.1

75 50

600

25

300 0

0 0

20

40

60

Cycle number

80

900

1200

10

0.1

2.5 2.0 Current density: A g -1

1.5 1.0 0.5 0.0

0

100 Li-extraction Li-i nsertion

600

Specific capacity (mAh g -1)

(d)

3.0

100

Coulombic Efficiency (%)

1800

1 st 2 nd 3 rd

2.0

3.0

Li-extraction Li-insertion

1600

0

2.5

Potential (V vs. Li /Li) Potential (V vs. Li /Li)

2000

1.5

(b)

3.0

Specific capacity (m Ah g -1)

Current (mA)

+

-0.4


Potential (V vs. Li /Li)

(a)

0.2

S p e c ific c ap a c ity (m A h 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


300

600

900

1200

Specific capacity (mAh g-1) (f)

900

m-FeS2 This work p-FeS2 microsphere/rGO p-FeS2/rGO composite FeS@CNS

Ref. 21 Ref. 31 Ref. 42 Ref. 55

600

FeS/rGO FeS microsheet

Ref. 56 Ref. 57

1200

300 0

0

4

8

12

16

Current density (A g -1)

Figure 4. (a) CV curves at a scan rate of 0.1 mV s-1 of m-FeS2/CNFs. (b) The discharge/charge voltage profiles at 0.1 A g-1 of m-FeS2/CNFs. (c) The long term cycling performance of pristine m-FeS2, pure CNFs and m-FeS2/CNFs composite at 0.1 A g-1. (d-e) High-rate capabilities of m-FeS2/CNFs at various current densities in the voltage range of 0.01–3 V. (f) Comparison of specific capacities of the m-FeS2/CNFs electrode with iron sulphides electrodes reported in the literatures for LIBs.

S, respectively. Intensities and integral areas of this peak are nearly identical, suggesting the good reversibility of Li+ insertion and extraction reactions. For the subsequent cycles, the redox peaks are well-overlapped, implying highly reversible reactions and excellent 13 ACS Paragon Plus Environment


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electrochemical stability. On the basis of above CV analyses, the Li-storage mechanism of m-FeS2/CNFs can be described as following reactions:44 Discharge process:

FeS + 2Li + 2e → Li FeS

(1)

Li FeS + 2Li + 2e → 2Li S + Fe (2) Charge process:

Fe + 2Li S → Li FeS + 2Li + 2e (3) Li FeS → Li FeS + Li + e (4) Li FeS → FeS + 2 − S + 2 − Li + 2 − e (5) Furthermore, the CV curves at different scan rates and corresponding linear relationship between log(i) and log(υ) of m-FeS2/CNFs composite are presented in Figure S7a and 7b, respectively. With the increase of scan rates, the peak currents (i) and the square root of the scan rates (v) are out of proportion, implying the electrochemical process include faradaic and nonfaradaic behaviors.45 Based on the previous reports, peak current (i) and the scan rates (v) satisfy the following equation: i = avb and log(i) = blog(v) + log(a), where a and b are different parameters.46 If b = 1, pseudocapacitance controls the electrochemical process, and if b = 0.5, the system depends on the ionic diffusion. The b values (0.83, 0.62, 0.67 and 0.58) we estimated suggest discharge−charge process include partial pseudocapacitance behavior, which is beneficial to the electrochemical performance of m-FeS2/CNFs composite. Figure 4b shows the galvanostatic charge–discharge profiles of the m-FeS2/CNFs composite electrode at a current density of 100 mA g-1. There exist a long discharge platform near 1.21 V and two 14 ACS Paragon Plus Environment

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charge plateaus in the first cycle, which is in agreement with the CV result. All specific capacities are calculated based on the mass of the whole m-FeS2/CNFs composite. The initial discharge and charge capacities of m-FeS2/CNFs are 1321.8 and 1013.2 mAh g-1, respectively. It is worthy of noting that the CE observed in the first cycle is 76.7%. The low CE may result from the inevitable solid electrolyte interface (SEI) film formed on the surface of m-FeS2/CNFs during the discharge process and the incomplete decomposition of the electrolyte.47-49 In the subsequent cycles, the CEs significantly improve to nearly 100% and the curves are overlapped, which suggests remarkable stability.50 Figure 4c shows the capacity evolutions of m-FeS2/CNFs, pristine m-FeS2 and pure CNFs at the current density of 100 mA g-1. It's clearly seen that after 100 cycles, the reversible capacity of m-FeS2/CNFs is much higher than that of pristine m-FeS2 and pure CNFs. Notably, cycling stability of the m-FeS2/CNFs composite is enhanced by the CNFs. Interestingly, the capacity of m-FeS2/CNFs increases from 1063.5 mAh g-1 to 1399.5 mAh g-1 within 100 cycles, which is normally observed for other transition metal compounds.47, 51-53 This phenomenon is originated from the reversible growth of a polymeric gel-like film deriving from kinetic activation in the electrode.31, 53 Moreover, it also has an activated period to the electrolyte penetration into the inner voids of m-FeS2/CNFs microsparticles.21, 53-54 Far more important is the structural changes of the m-FeS2/CNFs particles during cycling. The m-FeS2/CNFs would gradually turn to a layer structure in the discharge process, which can provide more active sites to store lithium ions. In comparison with the m-FeS2/CNFs composite with contents 16.7% CNFs, we evaluated the electrochemical performance of 15 ACS Paragon Plus Environment


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pristine m-FeS2 merely mixed with 16.7% CNFs by manual coarse grind. Shown in Figure S8, obvious capacity fading is observed in the pristine m-FeS2 mixed with 16.7% CNFs within only 100 cycles at the current density of 5 A g-1. Such results are ascribed to two reasons: (1) the CNFs in the m-FeS2/CNFs composite disperse more uniform than the mixture of m-FeS2 and CNFs. This circumstance can provide intimate contact between the m-FeS2 particles and CNFs, which can hugely shorten the electron/ion transfer pathways. (2) The CNFs in the composite can effectively protect particles from aggregation, thus enabling the m-FeS2/CNFs composite to maintain its structural stability. To evaluate the rate performance of the as-prepared m-FeS2/CNFs electrode, the sample of m-FeS2/CNFs composite was carried out at progressively increased current densities from 100 mA g−1 to 10 A g−1 and the results are shown in Figure 4d and Figure 4e. It is capable of exhibiting decent reversible capacities of 1086.9, 1055.2, 1032.7, 964.1, 922.8, 885.4, 845.5 and 782.2 mAh g−1 at current densities of 0.1, 0.3, 0.5, 1, 1.5, 3, 5 and 10 A g−1, separately. Figure 4f displays the remarkable rate performance of the synthetic m-FeS2/CNFs electrode compared with those of previously reported iron sulphides anode materials.21,

31, 42, 55-57

For the comparison, the

electrochemical performance of pristine m-FeS2 electrode was also measured, as shown in Figure S9a and 9b. The pristine m-FeS2 electrode exhibits comparatively lower specific capacity than that of m-FeS2/CNFs. To stand out the cycling stability of m-FeS2/CNFs electrode, the long-term capacity evolution of m-FeS2/CNFs at a high current density of 5 A g-1 is demonstrated in Figure 5a. After 1000 cycles, a stable reversible capacity of 573.4 mAh g−1 is obtained with the CE of 16 ACS Paragon Plus Environment

1800

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Coulombic Efficiency (%)

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Specific capacity (mAh g -1 )

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Li+ Li+

Figure 5. (a) Cycling performance of the as-prepared m-FeS2/CNFs at a current density of 5 A g-1. (b) Schematic illustration of rapid process during lithiation/delithiation of m-FeS2/CNFs.

100%. The excellent capacity of m-FeS2/CNFs electrode may be attributed to its unique hierarchical grape-like structure, in which Li ions and electrons can transfer rapidly contributed by the conductive CNFs network (shown as Figure 5b). In addition, the hierarchical structure provides free room to ease the volume expansion, thus improving the structural stability. The SEM image after 100 cycles of m-FeS2/CNFs electrode is shown in Figure S10. The grape-like structure of as-prepared m-FeS2/CNFs is maintained quite well after 100 cycles, indicating the excellent structure and cycling stability. The above outstanding electrochemical performance makes m-FeS2/CNFs as a potential high capacity density anode material for future LIBs.



(a)

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0

100

200 300 Z ' (Ω)

400

0

0

150

300

Z ' (Ω)

450

600

Figure 6. EIS spectra before cycling and after 5th, 10th cycle of (a) m-FeS2/CNFs and (b) pristine m-FeS2.

To better study the kinetic properties of m-FeS2/CNFs and pristine m-FeS2, EIS measurements were employed to evaluate the interfacial charge transfer capability. Figure 6a and 6b show Nyquist plots of the fresh cell, after 5, 10 cycles at current density of 100 mA g-1 over a frequency range from 100 mHz to 100 kHz. It can be seen that both impedance spectra consist of a semicircle in the medium frequency range and an approximately 45˚ inclined line in the low frequencies. The semicircle is associated with charge transfer resistance (Rct) of electrode materials, while the sloping long line is bound up with the lithium ion diffusion in the active material.58-65 The major difference of the EIS spectra between m-FeS2/CNFs and pristine m-FeS2 lies in the increase of the semicircle in the medium frequency range. To disclose the differences of kinetic properties of two electrodes, an equivalent circuit (as shown in Fig. S11) was used to fit both EIS plots. In the equivalent circuit, R1 stands for the internal resistance of electrodes and CPE represents the constant phase element. R2 and CPE1 are related with the high frequency region while Rct and CPE2 are associated with the medium frequency semicircle.40 On basis of the fitting results, m-FeS2/CNFs exhibits a lower 18 ACS Paragon Plus Environment

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impedence (R2 = 40.41, Rct = 166.1) than that of pristine m-FeS2 (R2 = 81.98, Rct = 339.7), indicating the superior redox kinetics in the m-FeS2/CNFs composite compared with the pristine m-FeS2 sample as described above.47,

66

Moreover, N2 adsorption-desorption

isotherms has been conducted to further investigate the Brunauer–Emmett–Teller (BET) surface area and pore-size distribution. As shown in Figure S12, m-FeS2/CNFs composite exhibit pore structure with surface areas of 124.9 m2 g-1. Such structure can enlarge the effective contact area between the electrode and electrolyte, which is beneficial to enhance the ion diffusion. In order to further study the possible mechanism for the Li-insertion/Li-extraction reaction, ex situ PXRD and SEM analysis of the m-FeS2/CNFs at various discharge/charge state in the first cycle were performed on electrodes. As shown in Figure 7, when the cell was

G

●E ● D

⃰

D

C● ⃰

C ⃰

B ⃰

‫۝‬

* B●

* ‫۝‬

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Cu

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Cu

2000 2400

discharged to 1.2 V, the FeS2 and Li2FeS2 phase were found. The presence of FeS2 reveals

Intensity (a.u.)

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2.0

1.0

Voltage (V)

0

Figure 7. Ex situ PXRD patterns of m-FeS2/CNFs taken at different states at 100 mA g-1.



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that the electrode material is not completely reacted in the lithium storage reaction, as indicated by the electrochemical performance results. Because the reaction is incomplete thus the capacity will increase gradually in the following cycles. As described in previous studies, m-FeS2 will react with lithium to form Li2FeS2. During the discharge state at a plateau of 0.01 V (stage C), the peak density of Li2FeS2 reduces obviously and a new peak of Li2S appears, corresponding to eqn (2). At charged 1.7 V, the diffraction peak of Li2FeS2 appears again. During the following charge process (1.9 V and 2.3 V), the peak corresponding to Li2FeS2 weakens gradually and the new phase of sulfide generates. Such a result is in accordance with the above conversion reactions (4) and (5). At fully charged state (3 V), the signal of Li2FeS2 nearly vanishes. Compared with the original state, the diffraction peaks of fully charged state

Figure 8. Ex situ SEM images of lithium insertion/extraction process. It can be seen that the hierarchical m-FeS2 microparticles in the m-FeS2/CNFs composite will convert into a novel mimosa with leaves open shape during Li+ insertion process and vice versa. Such changes provide opened porous structure for lithium insertion, thus improving the electrode electrochemical performance. 20 ACS Paragon Plus Environment

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seems almost no change, which further confirms the excellent reversibility of the electrode. Ex situ SEM was performed on electrodes to fully investigate the intermediate morphology of m-FeS2/CNFs (as shown in Figure 8). Unexpectedly, the grape-like m-FeS2/CNFs would gradually turn to a layer structure in the discharge process. In the low resolution SEM image of m-FeS2/CNFs at 0.01 V (Figure S13a), the whole region of the electrode is a layer structure. And when charged back to 3 V, it will gradually recover its original morphology step by step. As this phenomenon was not found in pristine m-FeS2 (as shown in Figure S13b), a “CNFs accelerated decrystallization-recrystallization” mechanism (CNFs-D/R) was proposed to explain the morphology variations of m-FeS2/CNFs. Meanwhile, the morphology variations are thought to be favored for the high reversible specific capacities, good rate capabilities and much improved cycle stabilities of m-FeS2/CNFs.

CONCLUSIONS In summary, hierarchically structured m-FeS2/CNFs composite was obtained via a one-pot hydrothermal process. When evaluated as anode materials for LIBs, the prepared m-FeS2/CNFs electrode exhibited high reversible specific capacities of 1086.9 and 782.2 mAh g−1 at 0.1 and 10 A g-1, respectively. In addition, it also demonstrated superior cycle performance with decent reversible capacity of 573.4 mAh g−1 after 1000 cycles at a high current density of 5 A g-1. The remarkable electrochemical performance is attributed to the unique hierarchical structure and the incorporation of CNFs, which is beneficial to enhance the redox kinetics and structural stability. Moreover, a similar “mimosa phenomenon” for morphology

variation

of

m-FeS2/CNFs

was

firstly

observed

during

lithium 21

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insertion/extraction in metal sulfides. And a CNFs-D/R mechanism was proposed to explain such morphology variations of m-FeS2/CNF. Our results highlight the advantages of grape-like m-FeS2/CNFs as a potential anode candidate for LIBs. What’s more, this work may provide available strategy for the electrode material which has the similar morphology variations.

ASSOCIATED CONTENT Supporting Information Crystal structures of the pyrite and marcasite, TGA analysis for m-FeS2/CNFs and pristine m-FeS2, UV-vis absorption and FT-IR spectra of m-FeS2/CNFs and pristine m-FeS2, SEM image of the CNFs, morphologies of the pristine m-FeS2, XPS spectra of m-FeS2/CNFs, CV curves at different scan rates and corresponding linear relationship of log(i) and log(υ) of m-FeS2/CNFs composite, Cycling performance of the pristine m-FeS2 mixed with 16.7% CNFs, electrochemical performance of pristine m-FeS2, SEM image of m-FeS2/CNFs after 100 cycles, an equivalent circuit suitable of the impedance spectra after 5 cycles, N2 adsorption/desorption isotherm and BJH pore size distribution of m-FeS2/CNFs composite and low resolution SEM images of m-FeS2/CNFs and pristine m-FeS2 at 0.01 V are provided. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email addresses: [email protected] (X.-Y. Zhang); [email protected] (X.-L. Wu); [email protected] (J.-P. Zhang). 22 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial support from the NSFC (21573036, 21274017), the Education Department of Jilin Province (111099108), the Science Technology Program of Jilin Province (20150520027JH) and Jilin Provincial Research Center of Advanced Energy Materials (Northeast Normal University) are gratefully acknowledged.

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Metastable Marcasite-FeS2 as a New Anode Material for

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