Pulverization Control by Confining Fe3O4 Nanoparticles Individually

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Pulverization Control by Confining Fe3O4 Nanoparticles Individually into Macropores of Hollow Carbon Spheres for High-performance Li-ion Batteries Zhijun Yan, Xiaobin Jiang, Yan Dai, Wu Xiao, Xiangcun Li, Naixu Du, and Gaohong He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16530 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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ACS Applied Materials & Interfaces

Pulverization Control by Confining Fe3O4 Nanoparticles Individually into Macropores of Hollow Carbon Spheres for High-performance Li-ion Batteries Zhijun Yan, Xiaobin Jiang, Yan Dai, Wu Xiao, Xiangcun Li,* Naixu Du, Gaohong He State Key Laboratory of Fine Chemicals, Chemical Engineering Department, Dalian University of Technology, Linggong Road 2#, Dalian 116024, China *E-mail: [email protected] Abstract In this paper, double carbon shell hollow spheres which provide macropores (mC) for ultrasmall Fe3O4 nanoparticle (10-20nm) encapsulation individually were first prepared (Fe3O4@mC). The well-constructed Fe3O4@mC electrode materials offer the feasibility to study volume change, aggregation and pulverization process of the active Fe3O4 nanoparticles for Li-ion storage in a confined space. Fe3O4@mC exhibits excellent electrochemical performances, and delivers a high capacity of 645 mA h g−1 at 2 A g-1 after 1000 cycles. Even at 10 A g−1 or after 1000 cycles at 2 A g-1, the porous carbon structure was well maintained and no obvious aggregation and pulverization of the Fe3O4 nanoparticles was observed, though volume of the active Fe3O4 particles was expanded to 40-60 nm compared with the original particles (10-20nm). This can be due to the in situ embedment of one Fe3O4 nanoparticle into one macropore individually. The uniform dispersion and confinement of the Fe3O4 nanoparticles in the macropores of carbon shell could effectively accommodate severe volume variation upon cycling and prevent self-aggregation and spreading out from the carbon shell during expansion process of the nanoscale Fe3O4 particles, leading to improved capacity retention. Our work confirms the effectiveness for pulverization control by confining Fe3O4 nanoparticles individually into macropores to improve its Li-ion storage properties, providing a novel strategy for the design of new-structured anode materials for LIBs. Keywords: Fe3O4; Li-ion battery; Macropores; Pulverization; Carbon 1

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1. Introduction To improve the energy storage properties of Li-ion batteries (LIBs), a great deal of efforts has been devoted to the exploration of new anode materials with high power density, energy density, and excellent cycle stability.1-3 Recently, Fe3O4 has exhibited great potential as the next generation redox-based anode materials for LIBs owing to its high theoretical capacity (926 mA h g-1), low cost, and environmental benignity.4,5 However, similar to the general transition metal oxides, particle reorganization and huge volume change (ca. 200%) during the repeated Li+ insertion/extraction process would lead to pulverization of the Fe3O4 anodes, resulting in fast capacity loss and poor cycle stability.6,7 Therefore, design and preparation of Fe3O4-based anodes with high capacity and extremely stable cycle property is urgent. Nowadays, the strategies to improve Li-ion storage performances of Fe3O4-based anodes include carbon coating or nanostructure engineering.7-10 Carbon coating/encapsulating the active Fe3O4-based materials can effectively improve their electrical conductivity, alleviate volume change of Fe3O4 particles upon cycling. However, cycle stability of Fe3O4-based electrode materials especially at high current densities is still reduced by pulverization and reorganization of the active Fe3O4 particles. Zhu and co-workers reported a porous reduced graphene oxide supported Fe3O4@C composite, the size of Fe3O4@C is about 200nm, merely delivers around 800 mAh g-1 at 100 mAg-1 after 100 cycles.11 Lu et al. reported that graphene-encapsulated hollow Fe3O4 nanoparticles displayed an unsatisfactory cycling life. Meantime, TEM examination of particles shows significant pulverization only after 10 cycles.12 Most of articles published are about the contributions of the carbon coating to the conductivity of electrode materials. Therefore, design of Fe3O4-based anodes with unique and stable structure that prevent negative effect of pulverization is a challenging.13,14 Fabricating various nanostructured electrode materials is another effective method to improve their electrochemical performance, especially porous or hollow spheres that can buffer the stress resulting from volume variation. 2


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Lou and co-workers have prepared series of hollow metal oxide anodes including Fe3O4 hollow microspheres and made excellent lithium ion storage achievements.8 In this work, we report a hierarchical macroporous Fe3O4@mC anode materials based on the both advantages of high conductive property and porous structure of carbon matrix. Particularly, the active ultrasmall Fe3O4 nanoparticles (10-20nm) were dispersed and confined individually in macropores of the graphitic carbon hollow spheres. The macropores could provide sufficient internal void space for large volume change of the active Fe3O4 particles and the porous carbon framework could provide high electron transport route and facilitate diffusion of electrolyte and lithium ions. Importantly, the unique structure could prevent aggregating and spreading out from the carbon shell of the Fe3O4 nanoparticles during the expansion process due to cage confining effect of the macropores on the dispersed Fe3O4 nanoparticles. The unique structured Fe3O4@mC composites exhibit excellent electrochemical performances, such as improved reversible capacity, rate capability, and long-term cycle stability. This work provides a novel method and new structure for carbon based anode materials for LIBs. 2. Results and discussion In this work, macroporous Fe3O4@mC anode materials were fabricated firstly based on the Fe3O4@C-SiO2 core-shell hollow spheres prepared by a facile aerosol spray method. In Figure 1a, Fe3O4@C-SiO2 hollow microsphere templates with a Fe3O4 core, silica and carbon shells were obtained.15-17 First, an aerosol solution was prepared by adding main components such as TEOS (tetraethyl orthosilicate), sucrose and Fe3O4 nanoparticles (~200nm, Figure S 1) into absolute ethanol at appropriate ratios under a magnetic stirring. Liquid droplets were obtained when the aerosol solution were sprayed to pass through a heating tube zone for solvent evaporation. During this process, TEOS was firstly hydrolyzed to form a silica layer on the surface of the droplets and simultaneously sealed other species in the droplet interior. Subsequent high temperature pyrolysis can result in a high internal pressure in the silica shell, 3



promoting the sucrose to form a carbon shell against inner surface of the silica shell.18,19 Thus, Fe3O4 core was encapsulated into the double shells to form Fe3O4@C-SiO2 hollow microspheres (Figure S 2). To prepare Fe3O4@mC anode materials, Fe3O4@C-SiO2-Resin hollow spheres were obtained by coating a phenol resin layer on surface of the Fe3O4@C-SiO2 templates with in situ polymerization reaction. In this work, 2, 4-dihydroxybenzoic acid-formaldehyde (RF-COOH) was selected here. Thus, the resin layer containing lots of -COOH functional groups could chelatively adsorb more CTAB molecules by an electrostatic attraction (-COOgroup and cationic CTA+Br-), resulting in formation of CTAB micelle clusters in the resin layer (Figure 1a).20,21 These CTAB micelle clusters could template large macropores in the carbon shell with the resin layer carbonization at a high pyrolysis temperature.16,22,23 Cheng et al. reported that the Fe3O4 core could be disappeared and fractured into smaller magnetic nanoparticles with pyrolysis process of Fe3O4@PDA (polydopamine) core-shell particles at 700 °C in Ar, and the fragments were dispersed into the porous carbon layer resulted from pyrolysis of PDA coating.24 Inspired by this, we speculated that the Fe3O4 core composed of subunits (~10nm, Figure S1) could be fractured into ultrasmall Fe3O4 nanoparticles with pyrolysis process of the Fe3O4@C-SiO2-Resin hollow microspheres at high temperature. As expectedly, the Fe3O4 core (~200nm) were fractured into ultrasmall ones (10-20nm), and these Fe3O4 fragments were individually dispersed into macropores of the carbon shell, where the macropores were templated by the CTAB clusters with pyrolysis of the resin layer (Figure 1a). For this process, it was supposed that high pyrolysis pressure is produced in the C-SiO2-Resin triple shells which could impel cracking of the Fe3O4 core into smaller ones, and force these smaller Fe3O4 fragments to transfer to outer carbon shell of Fe3O4@mC spheres. Some pores in the inner carbon shell of Fe3O4@mC microspheres (broken hollow spheres, Figure S 3a) may result from the transfer routes of the Fe3O4 debris. It can be observed that double carbon shell hollow spheres were obtained with the removal of the middle silica layer, and ultrasmall Fe3O4 4


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fragments (10-20nm) are individually encapsulated into macropores of outer carbon shell (Figure S 3b, c). When applied as anode materials for Li-ion batteries, the Fe3O4@mC hollow spheres show a large capacity, rate capability and cycle stability (Figure 1b).

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Fe3O4@mC for LIBs anode materials

Figure 1.(a) Preparation of Fe3O4@mC hollow spheres from the Fe3O4@C-SiO2core-shell templates, (b) Fe3O4@mC as an anode material for LIBs with long-term cycle stability.

Figure 2a shows the Fe3O4@C-SiO2 core-shell particles prepared by an aerosol method, which has a Fe3O4 core (~200nm), silica and carbon double shells (Figure S2). The distance between two adjacent lattice planes is ~0.298nm for the core particle (inset in Figure 2a), corresponds to (220) lattice of Fe3O4 crystal.25,26 Subsequently, Fe3O4@C hollow particles were obtained by etching the out silica shell of the Fe3O4@C-SiO2 aerosol particles (Figure S 3d). For comparison, only C-SiO2 double shell hollow spheres were obtained without the addition of Fe3O4 particles in the aerosol solution (Figure S 3e, f), showing the Fe3O4 cores did not interfere with formation of silica and carbon shells during the preparation process of Fe3O4@C-SiO2 hollow spheres. From the control experiments, the method reported here should be a general 5



strategy to prepare M@C-SiO2 hollow core-shell particles, where M represents the metal particle cores. Figure 2b exhibits TEM image of Fe3O4@mC anode materials obtained with the aerosol Fe3O4@C-SiO2 spheres as the templates. The ultrasmall Fe3O4 nanoparticles (10-20nm) are individually encapsulated into macropores of the carbon shell, as proved by the HETEM image, SAED (selected area electron diffraction) and XRD pattern (Figure 2c, Figure S 4a).8,26 High-angle annular dark-field scanning TEM (HAADF-STEM), TEM, and electronic differential system (EDS) element mapping images in Figure 2d-g clearly demonstrate the uniform distribution of the ultrasmall Fe3O4 nanoparticles in the porous carbon framework and their well confinement in the macropores. Interestingly, transfer of Fe3O4 core from the interior of Fe3O4@C-SiO2 spheres to the outer porous carbon shell of Fe3O4@mC largely increases the saturation magnetization from 2.6 emu g-1 to 12.1 emu g-1, this may be due to that the shielding effect from the silica and carbon shells is largely attenuated and high pyrolysis temperature (800℃) improve crystallinity and magnetic response of the Fe3O4 species greatly (Figure S 4a, b). The HRTEM image (Figure 2h) and SEM images (Figure 2i, j) further confirm the presence of open and interconnected macropores in the outer carbon shell of Fe3O4@mC, forming a three-dimensional porous framework loaded with ultrasmall Fe3O4 nanoparticles in the pores (red arrows in Figure 2i, j). High BET surface area and pore volume (177.1m2/g, 0.26cm3/g, Figure S 4c) indicate that most of the macropores are connected and have openings to outer side of the spheres, which facilitates solution and ion transport during the charge/discharge process for Li-ion batteries.27-29 Fe3O4 species was evaluated to be about 40% by a thermogravimetric technique in the composites (Figure S 4d). HRTEM in Figure 2k shows the presence of ordered lattice fringes in the outer carbon shell of Fe3O4@mC spheres, which can be ascribed to graphitization of the carbon shell with the catalytic effect of the iron species.30 It is reported that iron species can improve graphitization degree of carbon materials under high-temperature pyrolysis process due to the possible 6


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chemical interactions between carbon layer and iron oxides. And the electrical conductivity of Fe3O4@mC materials increases along with the expansion of graphitization degree.10,31 The high IG/ID value of 1.06 in Raman spectra (Figure S 5a) also confirms an elevated graphitization degree of carbon shell, which is due to metal-ion (Fe3+) catalytic graphitization during carbonization process of the resin layer.30 The X-Ray photoelectron spectroscopy (XPS) of Fe3O4@mC spheres (Figure S 5b) clearly proves the presence of C, O and Fe elements. Two peaks at about 712.2eV and 725.3eV are attributed to Fe2p3/2 and Fe2p1/2 respectively (Figure S 5c).5,32 The Fe3+/Fe2+ ratio from the resolved peaks is about 2.01, closing to 2:1 in Fe3O4. After being fitted of C1S spectra (Figure S 5d), four peaks denoting sp2 C bonded carbon (284.3eV), sp3 C(285.3eV), carbonyl (C-O, 287.1ev) and carboxyl (O-C=O, 288.5eV) were obtained, the results are consistent with the previous C1S spectra carbonized from the resin precursor.10 (a)

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Figure 2. (a) TEM image of Fe3O4@C-SiO2 with a Fe3O4 core, silica and carbon double shells, (b) TEM 7



image of Fe3O4@mC hollow spheres, (c) HRTEM image and SAED proving that the ultrasmall fragments are Fe3O4, (d) STEM image and EDS element mapping images of carbon, oxygen and iron, (e-g) HRTEM image and EDS prove uniform distribution of Fe3O4 fragments in macropores of outer carbon layer, (h) HRTEM image shows connected macroporous structure, (i, j) SEM images show hierarchical porous structure of carbon layer, (k) graphitic carbon property of the outer carbon layer

Electrochemical performance of the hierarchical porous Fe3O4@mC spheres for Li-ion batteries was evaluated in a half-cell configuration. The initial three and the 81st, 282nd charge-discharge profiles of the anode material at current densities of 0.2A g-1 and 0.5A g-1 in the voltage range of 0.005-3.0 V versus Li/Li+ are shown in Figure 3a. For the first cycle at 0.2 A g-1, the Fe3O4@mC electrode exhibit an initial discharge and charge capacities of 1846 and 1092 mA h g-1 respectively, with an initial Coulombic efficiency (CE) of ~60%. Refer to the research, the initial CE of Fe3O4/grapheme electrode (66.8%) is similar to bare Fe3O4 (64.3%), which both show the irreversible part of capacity loss at about 0.6V during first discharge process, means a low CE mainly from the formation of SEI film due to the large surface area of the Fe3O4 nanoparticles.33 The discharge and charge capacities are stabilized at about 1110 and 1039 mA h g-1 respectively from the second cycle, with a high CE of ~94% based on the total mass of Fe3O4 and carbon. The results are high than the previous reported Fe3O4-based anode materials for Li-ion batteries.4,6,8,14,32,34,35 After 80 cycles from 0.2 to 10A g-1 (Figure 3c), the discharge and charge capacities are increased to 958 and 937 mA h g-1 respectively with the current density returning to 0.5A g-1, and the capacities can still achieve as high as 857and 852 mA h g-1 after continuous 200 cycles at 0.5A g-1, demonstrating super cycle stability of Fe3O4@mC electrode materials. Especially, it should be noted that the cycle performance at 0.5A g-1 were carried out after the rate performance test at 0.2, 0.5, 1, 2, 4, 8 and 10 A g-1 respectively (Figure 3c), proving that the as-synthesized electrode materials can withstand high current surge without structure destroy and loss of cycle performance, which is more superior to 8


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those of recent reported Fe3O4-based electrodes for Li-ion storage.6,7 Further cycling tests at 0.2-10 A g−1 show that the Fe3O4@mC hollow spheres also exhibit excellent rate capability as anode materials for Li-ion batteries (Figure 3b). Remarkably, at a high current density of 10 A g−1, the electrode still deliver a reversible capacity of about 300 mA h g−1 (Figure 3c). Form Figure 3c, the discharge capacity can recover to ~950mA h g−1 with decreasing the current density to 0.5A g−1, and still keep a high value of about 850mA h g−1 with a continuous 200 cycles at 0.5 A g−1. In contrast, Fe3O4@C core-shell hollow spheres show a lower specific capacity and poor cycle stability, compared with Fe3O4@mC (Figure 3c). In addition to rate capability, long-term cycle stability was also investigated at 2.0 A g-1. As shown in Figure 3d and e, a high capacity of about 645 mA h g−1 still can be obtained even after 1000 cycles, suggesting good cycle stability. Superior electrochemical performance of the electrode material can be ascribed to the following advantages of the well-designed Fe3O4@mC hollow structures. First, the large Fe3O4 core in Fe3O4@C-SiO2 spheres was fractured into ultrasmall Fe3O4 fragments in Fe3O4@mC hollow spheres, thus highly improving reaction sites of the active materials and reaction efficiency (Figure 3f).36 The nanosized subunits could shorten transport length for Li +ions and electrons, resulting in enhanced rate capability.8,35 Moreover, uniform dispersion and confinement of the Fe3O4 fragments in macropores of the carbon shell could effectively accommodate volume variation upon cycling and prevent self-aggregation and spreading out from the carbon shell of the nanoscale Fe3O4 particles, thus leading to improved capacity retention of Fe3O4@mC. Finally, the graphic carbon framework could provide high electron transport routes and the hierarchical porous structure would facilitate diffusion of electrolyte and lithium ions.4 This can be further confirmed by the electrochemical impedance spectroscopy (EIS) measurements (Figure S 6a), both of Fe3O4@mC and Fe3O4@C electrodes present a semicircle and a sloped straight line in the middle-high frequency region and low frequency region respectively. The smaller semicircle means Fe3O4@mC has lower 9



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electrochemical reaction resistance and charge transfer. While the large sloped straight line is related to diffusion of lithium ions in the active material, and Fe3O4@mC delivers faster transfer rate of Li+ ions.37-40 (a)

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Figure 3.(a) the initial three and 81th, 282th charge-discharge profiles at current densities of 0.2A g-1 and 0.5 10


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A g-1 in 0.005-3.0 V versus Li/Li+, (b) charge-discharge profiles of Fe3O4@mC from 0.2 to 10 A g-1, (c) rate capacity and cycle stability for 200 cycles at 0.5 A g-1, (d) (e) long-term cycle stability at 2.0 A g-1 and charge-discharge profiles of Fe3O4@mC, (f) large Fe3O4 particles in Fe3O4@C fracturing into smaller Fe3O4 fragments in Fe3O4@mC spheres, improving active sites and reaction efficiency.

To further understand the excellent cycle stability of Fe3O4@mC electrode materials, porous structure of the carbon shell and state of the Fe3O4 nanoparticles were analyzed after one charge-discharge cycle at 0.5A g-1. As shown in Figure 4a and c, after discharging to 0.01V, TEM and EDS element mapping show that the porous carbon structure is well remained and the ultrasmall Fe3O4 nanoparticles are still homogeneously dispersed into the macropores without aggregation. However, the average particle size is estimated to be ~40nm, and the large volume expansion compared with the original Fe3O4 particles (10-20nm) can be ascribed to the Li+ ion insertion into the active Fe3O4 nanoparticles during the discharge process. In the first cycle of CV (cyclic voltammetry) curves (Figure S 6b), the cathodic peak at about 0.4V can be ascribed to reduction reaction of Fe3O4 to Fe0 with the lithiation of Fe3O4, generation of amorphous Li2O, and the formation of SEI based on the irreversible reaction with the electrolyte.5,14 HRTEM image, SAED pattern (Figure 4 b) and XRD spectra (Figure S 6c) shows formation of Fe0 with a weak crystallinity from reduction of Fe3O4.24 Subsequently, the peaks shifts to higher potential (~0.75V) and CV curves nearly overlap, suggesting good reversible lithiation-delithiation properties, and cycle stability of Fe3O4@mC electrode. For the charge process, two oxidation peaks at 1.52V and 1.85V can be ascribed to oxidation of Fe0 to Fe2+ and Fe3+ during the anodic process.4,32 Figure 4d-f shows uniform distribution of the active Fe3O4 particles (mainly 30-40nm in diameter) in the carbon framework after changing to 3V of one cycle. No pulverization and self-aggregation are observed thanks to the confining effect of the macropores on Fe3O4 nanoparticles. However, after one cycle of charge-discharge, the peaks of Fe 3O 4 became very difficult to be observed (Figure S 6c). The reason was that crystal structure of Fe3O4 was damaged in charge-discharge process due to the volume expansion, as documented in 11



the literatures.10,41 Moreover, morphology of the Fe3O4@mC electrode materials after 282 charge-discharge cycles with current density up to 10 A g−1 (Figure 3c) are shown in Figure 4g-i. As shown, the overall porous carbon structure of the anode material was well maintained, without obvious aggregation of Fe3O4 nanoparticles after such a harsh cycling process. Upon deep cycling, volume of the active Fe3O4 particles was largely expanded (Figure 4g, 40-60nm in diameter), electrical connection between the carbon porous structure and the active materials remained relatively intact owing to the in situ embedment of Fe3O4 nanoparticles into the macropores individually. Under a closer examination (Figure 4h), each Fe3O4 particle actually is consisted of many smaller nanodots (loose structure), suggesting some pulverization of the Fe3O4 nanoparticles after cycling at high current density but still confinement in the pores and carbon matrix. Therefore, the rationally designed 3D porous architecture is effective in preventing aggregation and loss, buffering volume expansion of the dispersed Fe3O4 nanocrystals, resulting in structural and interfacial stabilization of the Fe3O4 nanoparticles during the charge-discharge process. Thus, Fe3O4@mC electrode exhibits excellent cycle performance at high rates. Figure 4j and k show the porous carbon framework and distribution of Fe3O4 nanoparticles after 1000 discharge-charge cycles at 2 Ag-1. Compared with the original sample (Figure 4l), aggregating trend of the Fe3O4 nanoparticles (red arrows) in the connected macrospores and their large volume expansion can be due to repeated Li-ion insertion and extraction process. Nevertheless, the individual Fe3O4 nanoparticles can be clearly distinguished with well distribution in the porous carbon support. From the XRD patterns (Figure S 6d), the peaks of Fe3O4 cannot be observed for the samples after 282 and 1000 cycles. All peaks can be ascribed to Li2CO3 and LiF, formed during charge-discharge process.10,42,43 Considering its excellent electrochemical performance and optimized structural features, the Fe3O4@mC hollow spheres are believed to become potential high-performance anode materials for long cycle life and high-rate Li-ion 12


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batteries. (a)

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Figure 4.(a-c) TEM and EDS element mapping images of Fe3O4@mC electrode material after discharge to 0.01 V for the first cycle, (d-f) TEM and EDS element mapping of active Fe3O4particles in carbon framework after changing to 3.0V of one cycle, (g-i) distribution of Fe3O4 particles after 282 charge-discharge cycles 13



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with current density up to 10 A g−1, (j, k) porous carbon framework and distribution of Fe3O4 nanoparticles after 1000 discharge-charge cycles at 2Ag-1, (l) TEM image and distribution of Fe3O4 particles of the original Fe3O4@mC electrode material.

For comparison, morphology and Fe3O4 distribution of Fe3O4@C hollow spheres after one cycle and 282 cycles were also investigated. In Figure 5a-d, after one charge-discharge cycle, the large Fe3O4 particle of ~200nm become loose and tends to crack into smaller ones. Especially, after 282 charge-discharge cycles with current density up to 10 A g−1 (Figure 3c), the original Fe3O4 particle in Figure 5e have been severely pulverized and cracked into ultrafine powders (Figure 5 f, g), and the Fe3O4 debris disperse in the whole hollow carbon spheres with other impurities. The repeated charge-discharge cycles would result in loss of the active materials and inferior electrochemical performance (Figure 3c for Fe3O4@C). The results further confirm the necessity of breaking the large Fe3O4 particle in Fe3O4@C hollow spheres into smaller ones to improve its utilization rate for high reversible capacity, proving high effectiveness of the unique structure Fe3O4@mC in improving electrochemical performance of Fe3O4-based electrode material. Conversely, the corresponding Fe3O4@C spheres (Figure S 3d) with a large Fe3O4 core and solid carbon shell exhibit inferior Li-ion storage property due to pulverization of the active Fe3O4 core (Figure 5f). The results further confirm the effectiveness for pulverization control of Fe3O4 nanoparticles individually into macropores to improve its Li-ion storage properties. Our work provides a novel strategy for the design of Li-ion storage anode materials with high specific capacity, rate capability and long-term cycle stability.

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Figure 5. (a-d) STEM image and EDS element mapping images of carbon, oxygen and iron of Fe3O4@C hollow spheres after one cycle, (e) TEM image of Fe3O4@C hollow spheres before cycling, (f, g) morphology and Fe3O4 particle distribution after 282 cycles, showing the Fe3O4 particle in Figure 5e have been severely pulverized and cracked into ultrafine powders, (h) pulverization and rearrangement of the Fe3O4@C as an anode material for LIBs

3. Conclusions Based on aerosol Fe3O4@C-SiO2 hollow spheres, hierarchical porous Fe3O4@mC anode materials were designed for the first time in this work. Particular interest, the active small Fe3O4 nanoparticles (10-20nm) were dispersed individually into macropores of graphitic carbon hollow spheres. The small size of Fe3O4 could improve reaction sites and reaction efficiency of the active materials, and shorten transport length for Li

+

ions and electrons, resulting in

enhanced rate capability. Moreover, the macropores could accommodate large volume change of the active Fe3O4 particles and the porous carbon framework could provide high electron transport route and facilitate diffusion of electrolyte and lithium ions. Importantly, the unique structure could prevent pulverization and aggregation of the Fe3O4 nanoparticles due to cage 15



confining effect of the macropores on the Fe3O4 nanoparticles. When being applied as an anode material for Li-ion batteries, Fe3O4@mC exhibit excellent electrochemical performances, such as improved large reversible capacity, rate capability, and long-term cycle stability.

4. Experimental section

Fe3O4@C-SiO2hollow particles Fe3O4 particles with diameters of ~200nm were prepared by a solution reaction method. Typically, 150 ml of deionized water was placed in a bottom round flask and was deoxygenated by bubbling N2 gas for 60 min. Then, 2.1g FeSO4•7H2O, 2.7g FeCl3•6H2O and 1g PEG 4000 were added respectively, and the mixture solution was then stirred at 800 rpm for 10 min. The stirring was continued for another 60 min with the addition of 10 ml ammonia solution (~28%) to the above mixture solution. The black precipitates were separated by a magnet and washed with distilled water and ethanol several times before drying at 60 oC in air. For preparation of Fe3O4@C-SiO2 hollow particles, the aerosol precursor aerosol solution was prepared by adding 1.1g of CTAB (cetyltrimethylammonium bromide, Sigma-Aldrich) and 1.0g of FeCl3·6H2O into 15ml of ethanol solution under sonication. And then a homogeneous dispersion was obtained with the addition of 120mg of the as-synthesized Fe3O4 nanoparticles. The resulting aerosol solution was obtained with subsequent addition of 4.5ml of TEOS (tetraethyl orthosilicate), 2.0 ml of 0.1M HCl with the dissolution of 1 g of sucrose respectively. A commercial atomizer (model HD-130, HOLDER) was used to spray the aerosol solution and to form aerosol droplets in a heating zone (200 °C). The collected particles were then calcined at 500℃ for 3 h in N2 to obtain Fe3O4@C-SiO2 hollow spheres. The FeCl3 added in the aerosol solution was re-dispersed mainly into the silica SiO2 layer of Fe3O4@C-SiO2 hollow spheres (Figure S 2b), which was removed with the removal of the SiO2 layer when Fe3O4@C-SiO2 was used as a template to prepare Fe3O4@mC electrode materials. C-SiO2 hollow microspheres without Fe3O4 core was 16


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also prepared by the same process (no Fe3O4 particles was added in the aerosol solution).

Preparation of macroporousFe3O4@mC electrode materials The as-synthesized Fe3O4@C-SiO2 hollow particles and CTAB were dispersed in a mixture solution

of

deionized

water

and

ethanol

by

ultrasonication.

Subsequently,

2,

4-dihydroxybenzoic acid and formaldehyde were added to the dispersion respectively with continuous stirring for 10min. With the addition of ammonia solution (~28%), the weight ratio of Fe3O4@C-SiO2, CTAB, 2,4-dihydroxybenzoic acid, formaldehyde, H2O, ethanol and ammonia solution was 1 : 1.5 : 0.64 : 1.0 : 600 : 200 : 3.66. The resulting solution was further stirred at 25℃ for 24h to obtain Fe3O4@C-SiO2-Resin spheres. Finally, pyrolysis process was carried out at 800℃ for 4h in Ar with a heating rate of 2℃/min to carbonize the resin layer of Fe3O4@C-SiO2-Resin spheres and broken Fe3O4 core. Simultaneously, the Fe3O4@C-SiO2-mC spheres were obtained. Then, Fe3O4@mC hollow spheres were prepared by removing the silica layer in a 2.0M NaOH solution for 24h.

Electrochemical performance of Fe3O4@mC electrode materials The Li-ion storage properties of Fe3O4@mC electrode materials were evaluated by using CR2025 coin-type half cells, consisting of a cathode and a lithium metal anode, separated by a porous polyethylene film (Celgard 2325). The working cathode contains 80% of active material, 10% of super P as conductive additive, and 10% of polyvinyllidene fluoride (PVDF-5130) as a binder. The loading mass of the active materials (including Fe3O4and carbon) on the copper foil was about 2.0 mg/cm2. The capacity was based on the total mass of Fe3O4 and carbon because of the contribution of carbon materials in the voltage range of 0.005-3.0V. Then CR2025-type coin cells were assembled in a glove box after the working electrodes were dried at 100 °C in a vacuum oven over night, and a nonaqueous solution (1:1 for ethylene carbonate and dimethyl carbonate) with 1 M of LiPF6was used as the electrolyte. Galvanostatic charge-discharge studies of the fabricated lithium-ion cells were carried out on a LAND CT2001A multi-channel battery 17



Page 18 of 22

test system in the potential range of 0.005-3.0 V (vs. Li/Li+). For the rate capability test, the charge-discharge currents gradually increased from 0.2 to 0.5, 1, 2, 4, 8 and 10 A g-1, and then restored to 0.5 A g-1. The long-term cycle stability of the cells was tested by charge-discharge at 2A g-1for 1000 cycles.

Measurements and characterizations Microstructures of the electrode materials was observed by using emission scanning electron microscope (FE-SEM, (FE-SEM, Nova Nano SEM 450) and JEOL-2010F transmission electron microscope (TEM) respectively. The element compositions were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). X-ray di℃raction patterns (XRD) were obtained

with

a

adsorption-desorption

D/MAX-2400 isotherms

were

di℃ractometer measured

at

(Cu 77.35

Kα K

radiation). by

a

Nitrogen

Micromeritics

AUTOSORB-1-M. CV measurements were carried out on an Ivium Powerstat (Ivium, Netherlandsat, 0.1mV s-1) electrochemical workstation. Magnetic measurements of samples were performed on a Quantum Design SQUID (MPMS XL-7) magnetometer.

Supporting Information Available: TEM images of the Fe3O4 particles; TEM image of Fe3O4@C-SiO2 hollow particles; SEM image of Fe3O4@mC hollow spheres; XRD patterns and magnetic saturation curves; Raman spectra and XPS spectra; EIS measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements Natural Science Foundation of China (21476044, 21676043), Fundamental Research Funds for the Central Universities (DUT15QY08), and the financial support from Changjiang Scholars Program (T2012049) are greatly appreciated. References: 18


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(1) Xu, K.; Li, W.; Liu, Q.; Li, B.; Liu, X.; An, L.; Chen, Z.; Zou, R.; Hu, J. Hierarchical Mesoporous NiCo2O4@MnO2 Core-Shell Nanowire Arrays on Nickel Foam for Aqueous Asymmetric Supercapacitors. J. Mater. Chem. A 2014, 2, 4795-4802. (2) Li, X.; Wang, L.; Shi, J.; Du, N.; He, G. Multishelled Nickel-Cobalt Oxide Hollow Microspheres with Optimized Compositions and Shell Porosity for High-Performance Pseudocapacitors. Acs Appl. Mater. Interfaces 2016, 8, 17276-17283. (3) Liu, H.; Li, W.; Shen, D.; Zhao, D.; Wang, G. Graphitic Carbon Conformal Coating of Mesoporous TiO2 Hollow Spheres for High-Performance Lithium Ion Battery Anodes. J. Am. Chem. Soc.2015, 137, 13161-13166. (4) Ding, C.; Zeng, Y.; Cao, L.; Zhao, L.; Zhang, Y. Hierarchically Porous Fe3O4/C Nanocomposite Microspheres via a CO2 Bubble-Templated Hydrothermal Approach as High-rate and High-capacity Anode Materials for Lithium-ion Batteries. J. Mater. Chem. A 2016, 4, 5898-5908. (5) Chen, M.; Shen, X.; Chen, K.; Wu, Q.; Zhang, P.; Zhang, X.; Diao, G. Nitrogen-doped Mesoporous Carbon-encapsulation Urchin-like Fe3O4 as Anode Materials for High Performance Li-ions Batteries. Electrochim. Acta 2016, 195, 94-105. (6) Wang, C.; Shao, G.; Ma, Z.; Liu, S.; Song, W.; Song, J. Constructing Fe3O4@N-rich Carbon Core-Shell Microspheres as Anode for Lithium Ion Batteries with Enhanced Electrochemical Performance. Electrochim. Acta 2014, 130, 679-688. (7) Liu, J.; Xu, X.; Hu, R.; Yang, L.; Zhu, M. Uniform Hierarchical Fe3O4@Polypyrrole Nanocages for Superior Lithium Ion Battery Anodes. Adv. Energy Mater. 2016, 6, 1-8. (8) Ma, F.-X.; Hu, H.; Wu, H. B.; Xu, C.-Y.; Xu, Z.; Zhen, L.; Lou, X. W. Formation of Uniform Fe3O4 Hollow Spheres Organized by Ultrathin Nanosheets and Their Excellent Lithium Storage Properties. Adv. Mater. 2015, 27, 4097-4101. (9) Nithya, V. D.; Arul, N. S. Progress and Development of Fe3O4 Electrodes for Supercapacitors. J. Mater. Chem. A 2016, 4, 10767-10778. (10) Chen, Y.; Li, X.; Zhou, X.; Yao, H.; Huang, H.; Mai, Y.-W.; Zhou, L. Hollow-tunneled Graphitic Carbon Nanofibers Through Ni-diffusion-induced Graphitization as High-performance Anode Materials. Energy Environ. Sci. 2014, 7, 2689-2696. (11) Ma, C.; Shi, J.; Zhao, Y.; Song, N.-j.; Wang, Y. A Novel Porous Reduced Microcrystalline Graphene Oxide Supported Fe

3

O

4

@C Nanoparticle Composite as Anode Material with Excellent Lithium Storage

Performances. Chem. Eng. J. 2017, 326, 507-517. (12) Chen, D.; Ji, G.; Ma, Y.; Lee, J. Y.; Lu, J. Graphene-Encapsulated Hollow Fe3O4 Nanoparticle Aggregates as a High-performance Anode Material for Lithium Ion Batteries. ACS. Appl. Mater. Interfaces 2011, 3, 3078-3083. (13) Chen, Z.; Zhou, J.; Wang, X.; Liao, X.; Huang, X.; Shi, B. Natural Collagen Fiber-enabled Facile Synthesis of Carbon@Fe3O4 Core-shell Nanofiber Bundles and Their Application as Ultrahigh-rate Anode Materials for Li-ion Batteries. Rsc Advances 2016, 6, 10824-10830. (14) Xu, F.; Kang, W.; Wang, X.; Liu, R.; Zhao, C.; Shen, Q. A Chemical Aomposition Evolution for the Shape-controlled Synthesis and Energy Storage Applicability of Fe3O4-C nanostructures. Crystengcomm 2013, 15, 4431-4437. (15) Li, X.; John, V. T.; Zhan, J.; He, G.; He, J.; Spinu, L. The Synthesis of Mesoporous TiO2/SiO2/Fe2O3 Hybrid Particles, Containing Micelle-induced Macropores through an Aerosol Based Process. Langmuir 2011, 27, 6252-6259. (16) Li, X.; He, G.; Han, Y.; Xue, Q.; Wu, X.; Yang, S. Magnetic Titania-silica Composite-Polypyrrole Core-shell Spheres and Their High Sensitivity Toward Hydrogen Peroxide as Electrochemical Sensor. J. Colloid 19



Interface Sci.2012, 387, 39-46. (17) Luo, F.; Li, X.; He, G.; Li, M.; Zhang, H. Preparation of Double-shelled C/SiO2 Hollow Spheres with Enhanced Adsorption Capacity. Ind. Eng. Chem. Res. 2015, 54, 641-648. (18) Li, X.; Luo, F.; He, G. Activation of the Solid Silica Layer of Aerosol-Based C/SiO2 Particles for Preparation of Various Functional Multishelled Hollow Microspheres. Langmuir 2015, 31, 5164-5173. (19) Wang, Y.; Sunkara, B.; Zhan, J.; He, J.; Miao, L.; McPherson, G. L.; John, V. T.; Spinu, L. Synthesis of Submicrometer Hollow Particles with a Nanoscale Double-Layer Shell Structure. Langmuir 2012, 28, 13783-13787. (20) Qi, X.; Li, X.; He, G.; Zhu, Y.; Diao, Y.; Lu, H. Preparation of Hollow Submicrospheres with Ordered Porous SiO2 Shell and Multiple AuPt Nanoalloy Cores and Their High Catalytic Activity. Chem. Eng. J.2016, 284, 351-356. (21) Qi, X.; Zheng, W.; Li, X.; He, G. Multishelled NiO Hollow Microspheres for High-performance Supercapacitors with Ultrahigh Energy Density and Robust Cycle Life. Sci. Rep. 2016, 6, 33241-33246. (22) Kim, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C. H.; Park, J. G.; Kim, J.; Hyeon, T. Magnetic Fluorescent Delivery Vehicle Using Uniform Mesoporous Silica Spheres Embedded with Monodisperse Magnetic and Semiconductor Nanocrystals. J. Am. Chem. Soc. 2006, 128, 688-689. (23) Wang, Y.; Gu, H. Core-shell-type Magnetic Mesoporous Silica Nanocomposites for Bioimaging and Therapeutic Agent Delivery. Adv. Mater.2015, 27, 576-585. (24) Cheng, G.; Zhou, M.-D.; Zheng, S.-Y. Facile Synthesis of Magnetic Mesoporous Hollow Carbon Microspheres for Rapid Capture of Low-Concentration Peptides. Acs Appl. Mater. Interfaces 2014, 6, 12719-12728. (25) Wang, L.; Ji, H.; Wang, S.; Kong, L.; Jiang, X.; Yang, G. Preparation of Fe3O4 with High Specific Surface Area and Improved Capacitance as a Supercapacitor. Nanoscale 2013, 5, 3793-3799. (26) Liu, M.; Sun, J. In situ Growth of Monodisperse Fe3O4 Nanoparticles on Graphene as Flexible Paper for Supercapacitor. J. Mater. Chem. A 2014, 2, 12068-12074. (27) Zhang, F.-F.; Huang, G.; Wang, X.-X.; Qin, Y.-L.; Du, X.-C.; Yin, D.-M.; Liang, F.; Wang, L.-M. Sulfur-Impregnated Core-Shell Hierarchical Porous Carbon for Lithium-Sulfur Batteries. Chem.-Eur. J.2014, 20, 17523-17529. (28) Liu, C.; Wang, J.; Li, J.; Zeng, M.; Luo, R.; Shen, J.; Sun, X.; Han, W.; Wang, L. Synthesis of N-Doped Hollow-Structured Mesoporous Carbon Nanospheres for High-Performance Supercapacitors. Acs Appl. Mater. Interfaces 2016, 8, 7194-7204. (29) Grinbom, G.; Duveau, D.; Gershinsky, G.; Monconduit, L.; Zitoun, D. Silicon/Hollow gamma-Fe2O3 Nanoparticles as Efficient Anodes for Li-Ion Batteries. Chem. Mater. 2015, 27, 2703-2710. (30) Yu, Z.-L.; Xin, S.; You, Y.; Yu, L.; Lin, Y.; Xu, D.-W.; Qiao, C.; Huang, Z.-H.; Yang, N.; Yu, S.-H.; Goodenough, J. B. Ion-catalyzed Synthesis of Microporous Hard Carbon Embedded with Expanded Nanographite for Enhanced Lithium/Sodium Storage. J. Am. Chem. Soc.2016, 138, 14915-14922. (31) Wu, Y.; Reddy, M. V.; Chowdari, B. V. R.; Ramakrishna, S. Long-Term Cycling Studies on Electrospun Carbon Nanofibers as Anode Material for Lithium Ion Batteries. Acs Appl. Mater. Interfaces 2013, 5, 12175-12184. (32) Meng, X.; Xu, Y.; Sun, X.; Wang, J.; Xiong, L.; Du, X.; Mao, S. Graphene Oxide Sheets-induced Growth of Nanostructured Fe3O4 for a High-performance Anode Material of Lithium Ion Batteries. J. Mater. Chem. A 2015, 3, 12938-12946. (33) Fu, Y.; Wei, Q.; Wang, X.; Zhang, G.; Shu, H.; Yang, X.; Tavares, A. C.; Sun, S. A Facile Synthesis of Fe3O4 Nanoparticles/Graphene for High-performance Lithium/Sodium-ion Batteries. RSC Advances 2016, 6, 16624-16633. 20


Page 20 of 22

Page 21 of 22 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


(34) He, D.; Li, L.; Bai, F.; Zha, C.; Shen, L.; Kung, H. H.; Bao, N. One-Pot Synthesis of Pomegranate-Structured Fe3O4/Carbon Nanospheres-Doped Graphene Aerogel for High-Rate Lithium Ion Batteries. Chem.-Eur. J.2016, 22, 4454-4459. (35) Ren, M.; Yang, M.; Liu, W.; Li, M.; Su, L.; Qiao, C.; Wu, X.; Ma, H. Ultra-small Fe3O4 Nanocrystals Decorated on 2D Graphene Nanosheets with Excellent Cycling Stability as Anode Materials for Lithium ion Batteries. Electrochim. Acta 2016, 194, 219-227. (36) Zhang, G.; Wu, H. B.; Song, T.; Paik, U.; Lou, X. W. TiO2 Hollow Spheres Composed of Highly Crystalline Nanocrystals Exhibit Superior Lithium Storage Properties. Angew. Chem. Int. Ed. 2014, 53, 12590-12593. (37) Qu, G.; Cheng, J.; Li, X.; Yuan, D.; Chen, P.; Chen, X.; Wang, B.; Peng, H. A Fiber Supercapacitor with High Energy Density Based on Hollow Graphene/Conducting Polymer Fiber Electrode. Adv. Mater. 2016, 28, 3646-3652. (38) Peng, S.; Han, X.; Li, L.; Zhu, Z.; Cheng, F.; Srinivansan, M. U.; Adams, S.; Ramakrishna, S. Unique Cobalt Sulfide/Reduced Graphene Oxide Composite as an Anode for Sodium-Ion Batteries with Superior Rate Capability and Long Cycling Stability. Small 2016, 12, 1359-1368. (39) Shen, L.; Song, H.; Yang, G.; Wang, C. Hollow Ball-in-Ball CoxFe3-xO4 Nanostructures: High-Performance Anode Materials for Lithium-Ion Battery. Acs Appl. Mater. Interfaces 2015, 7, 11063-11068. (40) Liu, Z.; Yu, X.-Y.; Paik, U. Etching-in-a-Box: A Novel Strategy to Synthesize Unique Yolk-Shelled Fe3 O4@Carbon with an Ultralong Cycling Life for Lithium Storage. Adv. Energy Mater. 2016, 6, 1502318. (41) Fang, S.; Shen, L.; Tong, Z.; Zheng, H.; Zhang, F.; Zhang, X. Si Nanoparticles Encapsulated in Elastic Hollow Carbon Fibres for Li-ion Battery Anodes with High Structural Stability. Nanoscale 2015, 7, 7409-7414. (42) Guo, W.; Sun, W.; Wang, Y. Multilayer CuO@NiO Hollow Spheres: Microwave-Assisted Metal Organic-Framework Derivation and Highly Reversible Structure-Matched Stepwise Lithium Storage. Acs Nano 2015, 9, 11462-11471. (43) Lv, Q.; Liu, Y.; Ma, T.; Zhu, W.; Qiu, X. Hollow Structured Silicon Anodes with Stabilized Solid Electrolyte Interphase Film for Lithium-Ion Batteries. Acs Appl. Mater. Interfaces 2015, 7, 23501-23506.

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Pulverization Control by Confining Fe3O4 Nanoparticles Individually

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