Constructing Three-Dimensional Porous Carbon Framework

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Energy, Environmental, and Catalysis Applications

Constructing Three-Dimensional Porous Carbon Framework Embedded with FeSe Nanoparticles as an Anode Material for Rechargeable Batteries 2

Hong Wang, Xia Wang, Qiang Li, Hongsen Li, Jie Xu, Xueying Li, Haifei Zhao, Yinliang Tang, Guoxia Zhao, Hongliang Li, Haiguang Zhao, and Shandong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11479 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018

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ACS Applied Materials & Interfaces

Constructing Three-Dimensional Porous Carbon Framework Embedded with FeSe2 Nanoparticles as an Anode Material for Rechargeable Batteries Hong Wang,1 Xia Wang,1, * Qiang Li,1 Hongsen Li,1 Jie Xu,1 Xueying Li,1 Haifei Zhao,1 Yinliang Tang,1 Guoxia Zhao,1 Hongliang Li,2 Haiguang Zhao,1 Shandong Li1,*

1

College of Physics, National Demonstration Center for Experimental Applied Physics

Education, Qingdao University, Qingdao 266071, P. R. China 2

School of Materials Science and Engineering, Qingdao University, Qingdao 266071, P.

R. China Corresponding Author * Emails:

[email protected], [email protected]

KEYWORDS:

Three-Dimensional Porous Structure, Carbon Framework, Iron

Selenide, Anode Material, Rechargeable Batteries

ABSTRACT: Metal selenides have caused widespread concern due to their high theoretical capacities and appropriate working potential, however, they are suffering from large volume variation during cycling and low electrical conductivity, which limit their practical applications. In this paper, a three-dimensional (3D) porous carbon framework embedded with homogeneous FeSe2 nanoparticles (3D porous FeSe2/C composite) was synthesized by a facile calcined approach, following a selenized method without template. 1

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As the uniformity of FeSe2 nanoparticles and 3D porous structure are beneficial to accommodate volume stress upon cycling and shorten electrons/ions transport path, associated with carbon as a buffer matrix for increasing conductivity, the 3D porous FeSe2/C composite displays excellent electrochemical properties with high reversible capacities of 798.4 and 455.0 mA h g-1 for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), respectively, when the current density is 100 mA g-1 after 100 cycles. In addition, the as-prepared composite exhibits good cycling stability as compared to bare FeSe2 nanoparticles. Therefore, the facile synthetic strategy in the current work provides a new perspective in constructing high-performance anode.

1.

INTRODUCTION

Because of high working voltage and long working life, lithium-ion batteries (LIBs) meet complex requirements of various devices including environmentally friendly vehicles, medical devices and defense equipments. However, the ever-growing demands has prompted people to develop low-cost batteries possessing longer life and higher energy capacities. Consequently, tremendous efforts are concentrated on exploiting alternative anode materials to commercial graphite whose theoretical capacity is only 372 mA h g-1. Meanwhile, sodium-ion batteries (SIBs) draw considerable attention, benefiting from their abundant resources of sodium salts and chemical/physical properties analogous with LIBs. Nevertheless, there are still some 2


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obstacles including low reversible capacities resulting from larger ionic radius of Na+, which limit their widespread practical applications. Until now, various anode materials for SIBs including carbon materials (e.g. graphene1, 2), metal oxides (e.g. TiO2,3-5 SnO26, 7), alloys (e.g. Sb,8, 9 Sn10, 11) and so on were investigated. However, low theoretical capacities of carbon materials and huge volume changes of metal oxides and alloys in charge/discharge process hinder their applications. Accordingly, searching for novel and low-cost anode materials possessing good excellent electrochemical properties is urgent. In recent years, the lithium/sodium storage properties of metal selenides have been investigated, benefiting from their appropriate working potentials and relatively high theoretical capacities, including CoSex,12-14 NiSe2,15,

16

FeSex,17,

18

MoSe2,19, 20 Sb2Se3,21, 22 and Cu2Se23. Among these metal selenides, iron selenide has attracted increasing attention endowed by its abundant resources of Fe, good chemical stability and high theoretical capacity.24-26 However, large volume changes generally exist during cycling process, leading to structural pulverization and formation of unstable solid electrolyte interfaces (SEI). Two efficient strategies have been used to buffer volume changes, including employing materials with nanostrucure, for instance nanoparticles,27-29 yolk-shell,30-32 nanoplates,33, nanospheres,35,

36

34

flower-like particles,37 and formation of composites with other

materials possessing good conductivity. In the latter method, carbon materials including carbon nanotube (CNT),38, 39 multi-walled carbon nanotube (MWCNT),40 3



graphene,41-43 reduced graphene oxides,44, 45 amorphous carbon46, 47, and disordered carbon48 are widely applied to support metal selenides nanoparticles in order to resist the volume changes, because they can prohibit the particle aggregation and enhance electrical conductivity of the composite. In recent years, 3D porous materials have exerted excellent cycling stability and rate capability,49, 50 which were based on the 3D pore allowing a large surface area to generate a full contact of active material with electrolyte. Besides, thin shell of pores offers shortened Li ion diffusion pathways, resulting in a good rate performance. Furthermore, the interconnected porous channel can provide a fast and 3D electronic route in the electrode, leading to fast charge transfer. For example, Yu et al. fabricated 3D macroporous MoxC (x=1 or 2) using carboxylic polystyrene spheres (COOH-PS) as templates, delivering long cycle life and good rate property for LIBs.51As far as we know, the construction of FeSe2 and carbon composite with 3D porous architecture has not been reported previously. Herein, we reported a facile strategy to synthesize a 3D porous FeSe2/C composite. The synthetic procedure is schematically illustrated in Scheme 1. Firstly, polyvinylpyrrolidone (PVP) was mixed with ferric nitrate to form a ferric nitratePVP precursor through the complexation of O and N atoms in the PVP with Fe3+. Subsequently, the ferric nitrate-PVP precursor was calcined at 700 oC in nitrogen (N2) atmosphere to form a Fe3O4/C composite, attested by X-ray diffraction (XRD) in Figure S1. Because organic groups within the precursor can decompose to release 4


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a great deal of gases e.g. CO and CO2 during the heating treatment procedure, the 3D porous structure can be generated for the Fe3O4/C composite. Finally, the 3D porous Fe3O4/C composite was transformed to 3D porous FeSe2/C composite by selenization reaction. In the 3D porous FeSe2/C composite, FeSe2 nanoparticles distributed in carbon matrix. The possible selenization reaction can be expressed by the following chemical equation: 𝐹𝑒3𝑂4 + 4𝐻2 + 6𝑆𝑒→3𝐹𝑒𝑆𝑒2 + 4𝐻2𝑂

(1)

Owing to the synergistic effect between the uniformity of the FeSe2 nanoparticles and the 3D porous framework of the carbon matrix, the final composite demonstrated enhanced electrochemical performance for LIBs/SIBs, compared with the batteries based on the bare FeSe2 nanoparticles. Because of the absence of hard templates, the facile synthetic strategy for the 3D porous FeSe2/C composite with improved electrochemical property indicates wide potential applications in preparation of other types of 3D porous structure based on metal selenides.

2. EXPERIMENTAL SECTION 2.1. Material synthesis 1.5 g of Fe (NO3)·9H2O was dissolved in 30 mL PVP solution with a concentration of 0.017 g/mL. Subsequently, the above solution was dried at 90 oC. Afterwards, the dried mixture was ground and annealed at 700 oC for 1 h in nitrogen atmosphere. Finally, the Fe3O4/carbon precursor was mixed with selenium in a molar ratio of 1:6, 5



and then annealed at 300 oC for 6 h in Ar/H2 atmosphere to obtain the 3D porous FeSe2/C composite. Besides, different amounts of PVP (0, 0.25 and 1 g) were also prepared using the same steps above. 2.2. Characterizations Powder XRD was employed to investigate the crystalline phase of the obtained composite using Rigaku D/Max X-ray diffractometer. Thermogravimetric (TG) analysis was conducted on a Mettler Toledo TGA-2 thermal gravimetric analyser and the temperature range was 30 to 800 oC in O2 atmosphere. Scanning electron microscope (SEM) was carried out with JSM-6700F, JEOL. Transmission electron microscope (TEM) was performed on Tecnai G2F30, FEI. X-ray photoelectron spectroscope (XPS) was conducted on PHI-5702 (Physical Electronics) to study the surface element composition and chemical valence of the obtained composite. Raman spectra were measured using a Renishaw inVia Plus Micro Raman spectroscopy system. The specific surface area measurement was finished on a Quantachrome Autosorb-IQ-MP/XR surface area and pore analyser. The Fe L-edge X-ray absorption spectroscopy (XAS) spectra were tested (Beamline BL08U1A, Shanghai Synchrotron Radiation Facility). 2.3. Electrochemical measurements The assembly, Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) measurements of CR2032 button cells were similar to our previous report.43 Moreover, for SIBs, the electrolyte was NaClO4 (1 mol/L) 6


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dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) solvents at a volume ratio of 1:1 with fluoroethylene carbonate (FEC) (5 wt %).

3. RESULTS AND DISCUSSION

Scheme 1. Schematic illustration for the synthesis of the 3D porous FeSe2/C composite.

The crystalline structure of the obtained product was investigated using XRD, presented in Figure 1a. It is observed that all the XRD peaks of the calcined product are ascribed to FeSe2 (JCPDS No. 12-0291) with orthorhombic structure without relevant impurities, revealing the successful formation of iron selenide by selenization reaction. The characteristic peaks of carbon were not detected, mainly resulting from its very modest crystallinity. Raman peaks centred at 1330 and 1590 cm-1 shown in Figure 1b. Besides, the carbon content of the composite was calculated by TG analysis. Figure 1c shows the weight loss is around 61.2% in the temperature range 200 to 600 oC. The increase of weight percentage can be detected 7



between 200 and 400 oC, on account of the generation of iron oxide and selenium dioxide, confirmed through XRD pattern (Figure S2), which may originate from the oxidation of FeSe2.52,

53

The decrease in mass above 400 oC corresponds to the

sublimation of selenium dioxide and the flaming of carbon. Therefore, the accurate loading of FeSe2 and carbon in the 3D porous FeSe2/C sample was determined to be 51.9% and 48.1%, respectively.

Figure 1. (a-c) XRD patterns, Raman spectrum and TGA curve of the obtained composite, respectively, (d) Fe 2p, (e) Se 3d, and (f) C 1s XPS spectra of the 3D porous FeSe2/C composite, respectively. 8


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The surface composition and chemical state of the 3D porous FeSe2/C composite were investigated by XPS. Figure S3 shows the XPS survey spectrum, in which the characteristic peaks of Fe, Se, and C elements can be observed. Two main peaks centred at 711.0 and 724.8 eV were observed in Fe 2p spectrum (Figure 1d), assigned to Fe 2p3/2 and Fe 2p1/2 of iron selenide, respectively.54 The peaks observed at 707.5 and 729.5 eV are attributed to the metallic Fe,55, 56 and Fe 2p1/2 of Fe3+,57, 58 respectively. The Se 3d spectrum of the 3D porous FeSe2/C composite, shown in Figure 1e, exhibits two significant peaks at 55.4 and 54.6 eV, consistent with Se 3d3/2 and Se 3d5/2 of iron selenide, respectively.59 Besides, Figure 1e exhibits the Se–Se and Se–O bonds, revealing the presence of some elemental selenium and selenium dioxide, which may be originating from the incomplete reaction.24 The characteristic peaks of C-C, C-O and C=O bonds can be observed from the C 1s XPS spectrum (Figure 1f). 26, 60 The morphology and microstrucure of the 3D porous FeSe2/C composite were researched by the SEM and TEM techniques (Figure 2). The obtained composite displays a 3D relatively porous structure, formed by the blowing the ferric nitratePVP precursor and selenization (Figure 2a). Besides, the architecture of the 3D porous FeSe2/C composite is composed of macropores and mesopores with a wide pore size distribution of about 200-400 and 20-30 nm, respectively. Besides, the carbon shell with the thickness of 10-20 nm is interconnected. In a contrast, the morphology of bare FeSe2 (Figure S4a, b) is 9



mainly nanoparticles and the size of nanoparticles is approximately 40 nm. Figure 2c shows the low-magnification TEM image, presenting that the FeSe2

Figure 2. (a, b) Low- and high- magnification SEM images, (c, d) Low- and high- magnification TEM images, (e, f) High-resolution TEM images, and (g) SAED pattern of the 3D porous FeSe2/C composite, respectively. 10


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nanoparticles are well dispersed in the 3D porous carbon framework. Furthermore, the size of FeSe2 nanoparticles is between 10 and 40 nanometers (Figure 2d). Moreover, lattice fringes of 0.190 and 0.203 nm, in accordance with (211) and (121) planes of FeSe2, respectively, were observed in high resolution TEM images (Figure 2e, f), indicating the nanopaticles are wrapped with thin carbon layers. The outer carbon could efficaciously hinder aggregation of the FeSe2 nanoparticles. Additionally, the existence of diffraction rings corresponding to the (110) and (002) planes of carbon and (121) plane of FeSe2 in Figure 2g, which further confirms the composite is composed of FeSe2 and carbon.

Figure 3. (a) N2 absorption/desorption isotherms and (b) Pore size distribution curve of the 3D porous FeSe2/C composite, respectively.

In order to optimize the electrochemical properties of the 3D porous FeSe2/C composite, it is critical to study their specific surface area and pore size. Figure 3a, b present N2 adsorption-desorption isotherms and the corresponding distribution of pore size. The isotherms of the 3D porous FeSe2/C composite belong to typical IV 11



isotherms (Figure 3a).26 There is a large hysteresis loop between the relative pressure of 0.4−1.0, verifying the existence of mesopores. Simultaneously, distribution of pore size measured through Barrett−Joyner−Halenda (BJH) model (Figure 3b) implies the existence of mesopores and micropores. The specific BET surface area is around 214.7 m2 g−1, which is capable of offering a sufficient contact for electrode and electrolyte, and short diffusion channels for ions and electrons. These features are in favour of improving lithium/sodium storage property of the 3D porous FeSe2/C composite.26

Figure 4. (a) Cyclic voltammogram and (b) Galvanostatic charge/discharge voltage profiles of the 3D porous FeSe2/C composite for LIBs.

The electrochemical performance of the 3D porous FeSe2/C composite electrode was evaluated in lithium and sodium cells. Firstly, the CV curves were performed to investigate the electrochemical process for LIBs, shown in Figure 4a. For the 1st cycle, a little reduction peak at approximately 1.7 V was seen,17 12


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corresponding to the formation of LixFeSe2 and two cathodic peaks were also observed at around 1.3 and 0.6 V, indexed to the conversed reaction to form firstly FeSe and Li2Se, and subsequent formation of Fe and Li2Se, respectively.17 These three reduction peaks shift to higher potentials of about 1.9, 1.5 and 0.8 V in subsequent cycles. Moreover, anodic peaks are at approximately 1.9 and 2.3 V, consistent with the formation of LixFeSe2 and FeSe2, respectively.17,

24, 26

It can be

also noted that the double peaks at around 2.0 and 1.9 V for the 2nd, 3rd, 4th and 5th, may also correspond to the reaction between elemental selenium coming from discharge process of the 1st cycle and Li+ to form soluble polyselenides and insoluble Li2Se, respectively.61 Besides, after the first cycle, the CV curves remain steady which suggests the Li+ conversion reaction of the composite electrode keeps reversible as well as steady. Figure 4b shows galvanostatic discharge/charge voltage curves of the 3D porous FeSe2/C composite electrode at 500 mA g-1 in the voltage range of 0.01 to 3 V. In the 1st cycle process, the discharge voltage plateau are at approximately 1.8, 1.4 and 0.6 V while the corresponding charge voltage plateau centred at 1.8 and 2.2 V, in agreement with CV results for LIBs. The related reaction equations are summarized as follows: Discharge process equation: 𝐹𝑒𝑆𝑒2 + 𝑥𝐿𝑖 + + 𝑥𝑒 ― →𝐿𝑖𝑥𝐹𝑒𝑆𝑒2 𝐿𝑖𝑥𝐹𝑒𝑆𝑒2 + (2 ― 𝑥)𝐿𝑖 + + (2 ― 𝑥)𝑒 ― →𝐹𝑒𝑆𝑒 + 𝐿𝑖2𝑆𝑒 𝐹𝑒𝑆𝑒 + 2𝐿𝑖 + + 2𝑒 ― →𝐹𝑒 + 𝐿𝑖2𝑆𝑒 13


(2) (3) (4)


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Charge process equation: 𝐹𝑒 + 2𝐿𝑖2𝑆𝑒→𝐿𝑖𝑥𝐹𝑒𝑆𝑒2 + (4 ― 𝑥)𝐿𝑖 + + (4 ― 𝑥)𝑒 ― 𝐿𝑖𝑥𝐹𝑒𝑆𝑒2→𝐹𝑒𝑆𝑒2 + 𝑥𝐿𝑖 + + 𝑥𝑒 ―

(5) (6)

Figure 5. (a) Ex-situ XRD patterns of the electrodes at the different state (D: state of discharge, C: state of charge for the 1st cycle, (b-d) HRTEM images of the electrodes discharged to 1.8 V, 1.3 V and 0.6 V, respectively, (e, f) HRTEM images of the electrodes charged to 1.9 V and 2.3 V, respectively, for the 1st cycle. The insets in (a-f) show the corresponding SAED patterns.

For better understanding the electrochemical reaction mechanism of the 3D porous FeSe2/C composite, the ex-situ XRD, XAS, TEM, XPS, shown in Figure 5, 14


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Figure S5 and Figure S6, were tested. The ex-situ XRD patterns of the electrodes at set discharged/charged voltages for the 1st cycle are displayed in Figure 5a. When discharged to 1.8 V, XRD peaks of the FeSe2 broaden and move to lower angles, implying lattice expansion of the FeSe2, due to the inserted Li+ into FeSe2. As discharged to 1.3 V, a peak of FeSe located at 37.48 begins to appear, suggesting that FeSe2 is converted to FeSe. With the decrease in discharged voltage to 0.6 V, the characteristic peak of Fe centred at 44.52 was observed, indicating the phase conversion from the FeSe2 to Fe. At the stage of charging, when charged to 1.9 V, the phase of Fe disappears gradually and probably transforms into the LixFeSe2. Besides, while charged to 2.3 V, FeSe2 (36.02o) was discovered, suggesting that FeSe is converted to FeSe2. Accordingly, the reaction process of FeSe2 was determined. There are also peaks of carbon (26.6°) and current collector (Cu) in every pattern of tested samples. The ex-situ XRD patterns are consistent with the results of CV profiles. Figure S5 shows the iron L-edge XAS spectra of the 3D porous FeSe2/C composite electrodes at set voltages during initial stage of lithiation. Remarkably, as discharged voltages drop, the main-edge of peaks, centred at 706.8 and 719.9 eV moves to lower energy region, revealing a gradual reduction of Fe3+ species in the FeSe2 to Fe. The

evolutions

of

the

composition

and

microstructure

at

different

charging/discharging voltages for the 1st cycle have been investigated by ex-situ TEM, 15



HRTEM, SAED and XPS measurements, shown in Figure S6 and Figure 5. The 3D porous structure is maintained when discharged to 1.8 and 1.3 V (Figure S6a, b). As discharged to 0.6 V, the 3D porous architecture can also be maintained but a decrease in the size of macropores was observed from Figure S6c. Subsequently, for the charged processes (1.9 and 2.3 V), both the electrodes depict the similar 3D porous morphologies (Figure S6d, e), retaining the initial structure after the 1st cycle. Figure 5b shows the HRTEM image of the electrode discharged to 1.8 V, which presents lattice pitch of 0.156, 0.277 and 0.203 nm, indexed to Li2Se (JCPDS No. 23-0072), Se (JCPDS No. 24-0714) and FeSe2 (JCPDS No. 12-0291), respectively, in accordance with the SAED result (inset of Figure 5b), indicating that lithium may intercalate into FeSe2. As the electrode are reduced to 1.3 V, lattice spacing of 0.294 and 0.313 nm, consistent with Li2Se (JCPDS No. 230072) and FeSe (JCPDS No.03-0533), respectively, was observed in the locally enlarged HRTEM image (Figure 5c), implying FeSe2 converts to FeSe, combined with its SAED image (inset in Figure 5c). As discharged to 0.6 V, Figure 5d depicts well-resolved lattice fringes coincide with the (101) and (200) reflections of Fe (JCPDS No. 01-1252) and Li2Se, respectively. The corresponding SAED image (inset of Figure 5d) also reveals the FeSe finally converts to Fe and Li2Se, accompanied with selenium that is not fully reacted. Subsequently, for the charged process (1.9 and 2.3 V), the lattice spacing of 0.227 and 0.166 nm, in agreement with (132) and (103) planes of Se (JCPDS No. 24-714) and FeSe, respectively, were observed in HRTEM image of the electrode charged to 1.9 V (Figure 5e), well consistent with the SAED result (inset of Figure 5e). When charged to 2.3 V, the 16


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electrode possesses the resolved lattices with distance of 0.165 nm, corresponding to (221) crystal plane of FeSe2 (Figure 5f) and the corresponding SAED patterns exhibits the presence of FeSe2 (inset of Figure 5f). These results are consistent with the diffraction peaks observed in the ex-situ XRD. The XPS spectra of Fe 2p also further suggest the reversible redox reaction of FeSe2 (Figure S6f). When discharged to 0.6 V, Fe 2p1/2 and Fe 2p3/2 peaks of metallic Fe at around 719.4 and 705.6 eV appear, respectively. While charged to 2.3 V, characteristic peaks of metallic Fe disappear but characteristic peaks of FeSe2 at 711.2 and 724.3 eV can be seen, well consistent with the original electrode.

Figure 6.. (a) Cycling performance and Coulombic efficiency of the 3D porous FeSe2/C composite under the current density of 100 mA g-1 with 0.5 g PVP and the bare FeSe2 for LIBs, (b) Rate capability of the 3D porous FeSe2/C composite with 0.5 g PVP for LIBs, (c) Cycling performance of the 3D porous FeSe2/C composite under the current density of 500 mA g-1 with 0.5 g PVP and the bare FeSe2 for LIBs, (d) Electrochemical performance comparison of various metal selenides electrodes in LIBs, and (e) EIS plots of the 3D porous FeSe2/C composite with 0.5 g PVP and the bare FeSe2. 17



The cyclic property of the 3D porous FeSe2/C composite with 0.5 g PVP at 100 mA g-1 for LIBs was studied (Figure 6a). The initial discharge capacity is 928.6 mA h g-1 and corresponding charge capacity is 647 mA h g-1, accompanied by an initial Coulombic efficiency of 69.67%. The large irreversible capacity loss is caused by formation of SEI film. For porous anode materials, higher pore volume and larger surface area can provide more contact sites for electrolyte and electrode, thus leading to generating a thicker SEI film and lower Coulombic efficiency.62 However, from the 2nd cycle, the Coulombic efficiency gradually rises to 99.5%. Meanwhile, the corresponding reversible capacity also gradually grows, mainly originating from the gradual access of more electrolyte to pores of the 3D porous FeSe2/C composite upon the cycling.63 After 100 cycles, the 3D porous FeSe2/C composite presents a high specific capacity of 798.4 mA h g-1, however, the bare FeSe2 electrode possesses only 351.8 mA h g-1. Considering the capacity contribution of both FeSe2 by reversible conversion reaction and lithium insertion of carbon, the overall theoretical capacity of the composite is around 438.9 mA h g-1 (overall theoretical capacity = 500.9 × 0.519 + 372 × 0.481=438.9 mA h g-1).64 After 100 cycles, the reversible capacity of the 3D porous FeSe2/C composite at 100 mA g-1 is higher than the overall theoretical capacity, which may be resulting from the reversible growing of polymer/gel-like films at low potentials.65 Besides, the rate performance of the 3D porous FeSe2/C composite with 0.5 g PVP electrode for LIBs was also measured, presented in Figure 6b. The reversible 18


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lithium capacities are 602.0, 626.0, 637.0, 649.0, 605.5, 517.0, 379.9, 302.1 mA h g-1 and back to 586.4 mA h g-1 at current densities of 100, 200, 400, 500, 1000, 2000, 4000, 5000 and 100 mA g-1, respectively, higher than those of the bare FeSe2 electrode, indicating the advantage of the combined function of carbon matrix, 3D pore construction, and uniform dispersed FeSe2 nanoparticles. Moreover, the cycling performance of the FeSe2/C with 0.5 g PVP at 0.5 A g-1 (Figure 6c). The 200th discharge capacity is 767.8 mA h g-1, almost twice more than that of the bare FeSe2 (351.8 mA h g-1). Besides, comparison of various other metal selenidescarbon anode materials reported in previous references is listed in Figure 6d. The as-synthesized 3D porous FeSe2/C composite displays a higher reversible capacity than most of other metal selenides.66-73 The discharge capacity of the 3D porous FeSe2/C began to increase gradually from the tenth cycles during cycling and rate performance test. Such behaviour is commonly occurred in transition metal oxides or sulfides/C composites, possessing vacuous or porous construction, on account of the reversible reaction between the electrode and electrolytes and activated process.74,75 In order to investigate the Li+ transfer behaviour, the EIS of the 3D porous FeSe2/C composite and bare FeSe2 was measured (Figure 6e). The compressed semicircle in high-frequency and medium-frequency regions are ascribed to the resistances of SEI film and charge-transfer, respectively. The sloping line in lowfrequency range is associated with the diffusion of Li ions in bulk electrode materials.59, 73 Compared with the bare FeSe2 nanoparticles, the 3D porous FeSe2/C 19



composite with 0.5 g PVP has a lower charge transfer resistance value, which originates from its improved electronic conductivity. Besides, the performances of the 3D porous FeSe2/C composite electrodes for LIBs with different amounts of PVP are shown in Figure S7. All the electrodes were measured at 0.5 A g-1. When amount of PVP is 1.0 g, the capacity of the composite electrode is low (164.5 mA h g-1) after 100 cycles, resulting from high quality of carbon. The decline of the capacities of the 3D porous FeSe2/C composite electrodes with 0 g (351.8 mA h g-1) and 0.25 g PVP (271.8 mA h g-1) are remarkable, precisely because of the insufficient carbon to alleviate the volume expansion. Accordingly, the amount of the addition of PVP is suggested to be 0.5 g for preparing the 3D porous FeSe2/C composite with superior electrochemical property.

Figure 7. (a) Cyclic voltammogram profiles and (b) Cycling performance and Coulombic efficiency of the 3D porous FeSe2/C composite for SIBs.

The sodium-storage property of the 3D porous FeSe2/C composite was also investigated and corresponding CV curves is shown in Figure 7a. Three reduction 20


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peaks centre at 1.04, 0.63, and 0.41 V in initial cycle and shifts to higher potentials of 1.81, 0.74, and 0.43 V in subsequent cycles, which are consistent with generation of NaxFeSe2, FeSe and Fe, respectively.25 Three anodic peaks at 1.53, 2.05 and 2.32 V are corresponding to forming FeSe, NaxFeSe2 and FeSe2, respectively.25 Figure S8 shows that 3D porous FeSe2/C composite electrode exhibits a high charge capacity of 603.6 mA h g-1 and corresponding Coulombic efficiency is 74.2% for the 1st cycle. The reason for initial irreversible capacity loss is similar to that of LIBs. Besides, during the 1st cycling process, three discharge voltage plateau are at approximately 1.16, 0.67 and 0.45 V while the corresponding charge voltage plateau centres at 1.44, 1.90 and 2.28V, which reflect CV results for SIBs. Cyclic property of the 3D porous FeSe2/C composite is displayed in Figure 7b at 100 mA g-1. The specific capacity for the 100th cycle maintains 455 mA h g-1, and the Coulombic efficiency can be stable at 96.3%. The enhanced cycling performances are resulting from the 3D porous carbon framework, which can accommodate volume and stress change during cycling process, shorten Li+/Na+ diffusion pathways and improve the electronic conductivity, associated with the uniformity of the FeSe2 nanoparticles to alleviate the volume change. Comparing features of the 3D porous FeSe2/C composite electrode with other metal selenides for SIBs, listed in Table S1, the

3D

porous FeSe2/C composite electrode presents competitive performance.21,23,26,59,72 Therefore, the PVP functioned as morphology-controlling agent is an effective way

21



to fabricate 3D porous FeSe2/C composite electrode with great Li/Na-storage performance.

Figure 8. (a, b) Low- and high- magnification SEM images of the charged the 3D porous FeSe2/C composite after 200 cycles at 500 mA g-1.

For purpose of revealing the structural stability upon long cycle process, SEM was performed on the 3D porous FeSe2/C composite electrode for the 200th cycle at 500 mA g-1 for LIBs (Figure 8a, b). The 3D porous structure which can be tailored in a range of several tens of nanometers, can still be well preserved. As a result, the robust structural stability can lead to excellent property of the 3D porous FeSe2/C composite. 4. CONCLUSIONS In summary, we design a maneuverable scheme to fabricate a 3D porous FeSe2/C composite without using hard templates through a two-step method, in which ferric 22


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nitrate-PVP compound was calcined under inert atmosphere first and then selenized under Ar/H2 gas. When applied in LIBs and SIBs, the obtained 3D porous FeSe2/C composite presents desired cyclic performance as well as good rate performance. The excellent Li/Na-storage property is mainly originating from the uniformity of the FeSe2 nanoparticles and 3D interconnected carbon framework, which can efficiently enhance conductivity of FeSe2/C composite and accommodate stress change during cycling. As a result, the 3D porous FeSe2/C composite has a great promising application in high-performance LIBs/SIBs.

ASSOCIATED CONTENT Supporting Information Figures depicting XRD patterns of the obtained product after calcining at 700 oC in N2 atmosphere and 3D porous FeSe2/C composite after TG analysis when the temperature is 400 oC, survey XPS spectrum of the 3D porous FeSe2/C composite, SEM images of the bare FeSe2, Fe L-edge XAS spectra, TEM images and ex-situ Fe 2p XPS spectra of the 3D porous FeSe2/C composite electrodes at set discharged potentials during initial stage of lithiation, cyclic property of the 3D porous FeSe2/C composite at 500 mA g-1 with different amounts of PVP for LIBs, galvanostatic charge/discharge voltage curves of the 3D porous FeSe2/C composite for SIBs. A table showing a comparison of the 3D porous FeSe2/C composite and other various metal selenides electrodes in sodium ion. AUTHOR INFORMATION Corresponding Author 23



* E-mails:

[email protected], [email protected]

ORCID Shandong Li: 0000-0001-8105-7612 Xia Wang: 0000-0002-4400-6987 Notes Any additional relevant notes should be placed here. ACKNOWLEDGEMENTS Shandong Li, Jie Xu and Qiang Li thank the National Fund Committee of China for its financial support with Grant No. 11674187, 11604172 and 11504192 (NSFC), respectively. Qiang Li and Xia Wang are grateful to China Postdoctoral Science Foundation under grant No. 2015M570570 and 2017M622138, respectively. Xia Wang acknowledges financial support from Shandong Natural Science Foundation and Science and Technology Board of Qingdao under grant No. ZR2018BEM012 and 18-2-2-7-jch, respectively. REFERENCES (1) Wang, B.; Ryu, J.; Choi, S.; Song, G.; Hong, D.; Hwang, C.; Chen, X.; Wang, B.; Li, W.; Song, H. K.; Park, S.; Ruoff, R. S. Folding Graphene Film Yields High Areal Energy Storage in Lithium-Ion Batteries. ACS Nano 2018, 12, 1739-1746. (2) Wang, G.; Lu, C.; Zhang, X.; Wan, B.; Liu, H.; Xia, M.; Gou, H.; Xin, G.; Lian, J.; Zhang, Y. Toward Ultrafast Lithium Ion Capacitors: a Novel Atomic Layer Deposition Seeded Preparation of Li4Ti5O12/Graphene Anode. Nano Energy 2017, 36, 46-57. (3) Maroni, F.; Carbonari, G.; Croce, F.; Tossici, R.; Nobili, F. Anatase TiO2 as a Cheap and Sustainable Buffering Filler for Silicon Nanoparticles in Lithium-Ion Battery Anodes. ChemSusChem 2017, 10, 47714777. (4) Ren, M.; Xu, H.; Li, F.; Liu, W.; Gao, C.; Su, L.; Li, G.; Hei, J. Sugarapple-Like N-Doped TiO2@Carbon Core-Shell Spheres as High-Rate and Long-Life Anode Materials for Lithium-Ion Batteries. J. Power Sources 2017, 353, 237-244. (5) Senthil, C.; Kesavan, T.; Bhaumik, A.; Yoshio, M.; Sasidharan, M. Nitrogen Rich Carbon Coated TiO2 Nanoparticles as Anode for High Performance Lithium-Ion Battery. Electrochim. Acta 2017, 255, 417-427. (6) Ahmed, B.; Anjum, D. H.; Gogotsi, Y.; Alshareef, H. N. Atomic Layer Deposition of SnO2 on MXene for Li-Ion Battery Anodes. Nano Energy 2017, 34, 249-256. 24


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Constructing Three-Dimensional Porous Carbon Framework

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