Pomegranate-like, carbon-coated Fe3O4 nanoparticle superparticles for high-performance lithium storage

Author’s Accepted Manuscript Pomegranate-like, carbon-coated Fe3O4 nanoparticle superparticles for high-performance lithium storage Dandan Han, Guannan Guo, Yancui Yan, Tongtao Li, Biwei Wang, Angang Dong www.elsevier.com/locate/ensm

PII: DOI: Reference:

S2405-8297(17)30171-X http://dx.doi.org/10.1016/j.ensm.2017.08.003 ENSM196

To appear in: Energy Storage Materials Received date: 5 May 2017 Revised date: 5 August 2017 Accepted date: 5 August 2017 Cite this article as: Dandan Han, Guannan Guo, Yancui Yan, Tongtao Li, Biwei Wang and Angang Dong, Pomegranate-like, carbon-coated Fe 3O4 nanoparticle superparticles for high-performance lithium storage, Energy Storage Materials, http://dx.doi.org/10.1016/j.ensm.2017.08.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Pomegranate-like,

carbon-coated

Fe3O4

nanoparticle

superparticles for high-performance lithium storage Dandan Han, Guannan Guo, Yancui Yan, Tongtao Li, Biwei Wang, Angang Dong* Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, and Department of Chemistry, Fudan University, Shanghai 200433, China *

Corresponding author. [email protected]

ABSTRACT Transition metal oxides such as Fe3O4 have been widely explored as the anode material for lithium-ion batteries (LIBs). The rational design of the architecture of active materials is highly desirable to advance their lithium-storage properties. Herein, we report the construction of pomegranate-like microscale superparticles with an average size of 450 nm, comprising close-packed Fe3O4@C yolk-shell nanoparticles (NPs), by combining bottom-up assembly and top-down etching methods. Combining the merits of yolk-shell NPs, secondary particle configuration, and homogeneous carbon coating, such elaborately designed superparticles show great promise for LIB anodes. Moreover, the ability to fine tune the void space in individual yolk-shell NPs allows for the further improvement in lithium storage. A stabilized capacity as high as 1246 mAh g-1 is achieved at 0.8 A g-1, and even cycled at an ultrahigh current density

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of 5 A g-1, our pomegranate-like Fe3O4 NP superparticles can still maintain a high capacity of 520 mAh g-1 after 1000 cycles. This work establishes a generic strategy for producing pomegranate-like superparticles of transition metal oxides for high-rate and long-cycle lithium storage.

Keywords: Pomegranate-like superparticles, Nanoparticle superlattices, Self-assembly, Lithium-ion batteries

1. Introduction Considering the increasing demand for energy, it is essential to develop reliable energy storage devices including LIBs [1-4]. The energy density and cyclability of current LIBs are insufficient to meet the stringent requirements for next-generation energy storage devices, partly because of the low specific capacity (372 mAh g-1) of graphite anodes [5]. Consequently, a variety of alternative materials, including Si [6,7], Sn [8], metal sulfides (e.g., MoS2 and In2S3) [9,10], and transition metal oxides (e.g., Co3O4 and Fe3O4) [11,12], have been investigated as LIB anodes over the past decade. Among them, Fe3O4 is an attractive anode material because of its high specific capacity (~1000 mAh g-1), low cost, and environmental friendliness [13-15]. The employment of Fe3O4 nanostructures such as NPs as active materials has been very successful in improving the battery performance [16]. However, there are still several issues that hinder the practical applications of Fe3O4-based LIBs. First, the large volume expansion of Fe3O4 (~ 93%) during lithiation can lead to the structural degradation of electrode materials and the loss of electrical contact [17]. In addition,

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the intrinsic low electrical conductivity of Fe3O4 disfavors the efficient utilization of active materials. Finally, Fe3O4 NPs are prone to aggregation and extensive side reactions with electrolytes owing to the large surface area, resulting in thick and unstable layers of solid-electrolyte interphase (SEI) and therefore rapid capacity decay during cycling [18]. Numerous efforts have been devoted to improving the performance of Fe3O4-based anodes in the past few years [19-22]. The construction of hollow spheres can ameliorate the large volume expansion of active materials while facilitating the diffusion kinetics of lithium ions because of the empty interior and the thin shell [23-25]. Despite the greatly enhanced electrochemical performance, the presence of large hollow interiors inevitably reduces the volumetric energy density of the electrodes [26]. The preparation of multi-shelled hollow spheres appears to be feasible for taking advantage of hollow structured electrodes without significantly compromising the volumetric energy density [27]. Another effective strategy is to create yolk-shell structures by encapsulating NPs in a thin carbon shell [28,29]. The void space in yolk-shell structures is beneficial for accommodating the volume changes of NPs, whereas the carbon shell can not only improve electrical conductivity but also suppress NP agglomeration during cycling [30,31]. However, previous methods to form yolk-shell NPs typically involve multistep template-assisted procedures, suffering from a good control over the void space [32,33]. Moreover, most prior studies focus on electrodes comprising discrete or randomly packed yolk-shell NPs, which are undesirable for efficient electron transport and the

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formation of stable SEI layers [34-36]. To improve the comprehensive performance of Fe3O4-based LIBs, it is thus necessary to design new electrode structures with optimized void size, enhanced electrical conductivity, and superior structural stability. Here, we describe our efforts to develop a novel type of pomegranate-like superparticles toward high-rate and long-cycle lithium storage. Our pomegranate-like superparticles, composed of close-packed and interconnected Fe3O4@C yolk-shell NPs, are derived from spherical Fe3O4 NP superlattices by a process involving bottom-up assembly and top-down etching procedures. Such a design can simultaneously address the aforementioned challenging issues associated with Fe3O4-based anodes: (1) the empty space in Fe3O4@C yolk-shell NPs allows the expansion of individual Fe3O4 NPs without changing the size of the entire superparticle, thus efficiently relaxing the mechanical strain induced by repeated lithiation and delithiation; (2) the close-packed carbon shells derived from the native hydrocarbon ligands in situ form an interconnected electron transport network, enabling all the encapsulated Fe3O4 NPs to be electrochemically active; (3) the well-defined spherical morphology of superparticles confines the SEI formation primarily on the outer surface rather than on individual yolk-shell NPs, thus effectively reducing the side reactions between active materials and electrolytes. Moreover, the ability to manipulate the etching degree of Fe3O4 NPs allows for the fine-tuning of the void size in individual yolk-shell NPs. Under optimal etching conditions, our pomegranate-like Fe3O4 NP superparticles deliver an ultrahigh reversible capacity of 1246 mAh g-1 at 0.8 A g-1, superior rate capability (461 mAh g-1

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at 10 A g-1), and prolonged cycling life (520 mAh g-1 at 5 A g-1 after 1000 cycles). 2. Materials and methods 2.1. Materials Oleic acid (OA, 90%) and 1-octadecene (ODE, 90%) were obtained from Aldrich. Sodium oleate was purchased from TCI. Dodecyltrimethylammonium bromide (DTAB, 98%), iron(III) chloride hexahydrate (FeCl3•6H2O, 99.0%), hexane, ethanol, and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. All chemicals were used without further purification. Iron oleate was pre-synthesized by reaction of FeCl3•6H2O and sodium oleate. 2.2. Materials synthesis 2.2.1. Synthesis of monodisperse Fe3O4 NPs Monodisperse, OA-stabilized Fe3O4 NPs were synthesized by the thermal decomposition of iron oleate in the presence of OA [37]. In a typical synthesis of 14 nm Fe3O4 NPs, 18 g of iron oleate and 4.3 g of OA were dissolved in 80 mL of ODE in a three-neck flask. This solution was degassed at 120 oC for 0.5 h and then heated at 320 oC under N2 atmosphere for 1 h. After cooling down to room temperature, the as-synthesized Fe3O4 NPs were purified by precipitation upon the addition of ethanol followed by centrifugation. The precipitated Fe3O4 NPs were re-dispersed in hexane for future use in self-assembly. 2.2.2. Self-assembly of Fe3O4 NP superlattice spheres Microscale Fe3O4 NP superlattice spheres were obtained by an emulsion-based self-assembly process with the assistance of DTAB [38]. In brief, 5 mL of Fe3O4 NP

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solution in hexane (10 mg mL-1) was first mixed with an aqueous solution of DTAB (10 mg mL-1, 50 mL) under vigorous stirring using a homogenizer (6000 rpm). After 10 min of homogenization, the oil-in-water emulsion obtained was heated at 40 oC under continuous N2 flow for 2 h, during which the gradual evaporation of hexane induced the confined assembly of Fe3O4 NPs in oil droplets. The as-assembled superlattice spheres retrieved by precipitation were washed with H2O to remove excess DTAB before drying in a vacuum oven (80 oC for 1 h). 2.2.3. Synthesis of pomegranate-like Fe3O4 NP superparticles Pomegranate-like Fe3O4 NP superparticles, derived from Fe3O4 NP superlattice spheres, were obtained by in situ ligand carbonization followed by controlled acid etching. To carbonize the OA ligands, the dried powder of Fe3O4 NP superlattice spheres was annealed in a quartz tube furnace at 500 oC under Ar atmosphere for 2 h [39]. The resulting carbon-coated Fe3O4 NP superlattice spheres were then subjected to HCl (1 M) treatment at room temperature under mild stirring (~200 rpm). The homogeneous etching of Fe3O4 NPs enabled the formation of pomegranate-like superparticles. The void space in individual yolk-shell NPs was tunable by controlling the etching duration. 2.3. Materials characterization Scanning electron microscopy (SEM) and high-resolution SEM (HRSEM) were carried out on a Zeiss Ultra-55 microscope operated at 5 kV. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), scanning TEM (STEM), and energy-dispersive X-ray spectroscopy (EDS) elemental mapping were conducted on a

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Tecnai G2 20 TWIN microscope operated at 200 kV. The samples were prepared by drop-casting the colloidal particle solution onto carbon-coated copper TEM grids followed by drying under ambient condition. Powder X-ray diffraction (XRD) was performed on a Bruker D4 X-ray diffractometer. Intensity data was collected with Cu K radiation over a 2 range from 20o to 80o at a scanning rate of 0.08 degree/step. Small-angle X-ray scattering (SAXS) patterns were acquired on a Bruker Nanostar U small angle X-ray scattering system. Nitrogen adsorption–desorption isotherms were obtained

by

a

physisorption

analyzer

(Micromeritics,

ASAP

2020).

Thermogravimetric analysis (TGA) was carried out on a Perkin–Elmer Pyris 1 thermogravimetric analyzer with a heating rate of 20 oC min-1 under air flow. The dynamic light scattering (DLS) measurements were performed using a Zetasizer Nano instrument (Malvern Instruments, ZS90). 2.4. Electrochemical measurements The electrochemical performance of Fe3O4 NP superparticles was tested by CR2016 coin cells assembled in an Ar-filled glovebox. The electrode slurry was prepared by mixing 70 wt% active materials, 20 wt% acetylene black, and 10 wt% polyvinylidene fluoride binder in N-methyl-2-pyrrolidinone. The loading mass of active materials is about 1.2 mg cm-2 with a coating amount of ~1.8 mg. The tap density of Fe3O4 NP superparticles is about 3.0 mg cm-3. The electrolyte consisted of dimethyl carbonate, 1.0 M LiPF6 in ethylene carbonate, and ethyl methyl carbonate with a volume ratio of 1:1:1. A polypropylene film (Celgard-2300) was used as the separator. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy

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(EIS) were conducted on an electrochemical workstation (Autolab 204 N). Galvanostatic charge/discharge measurements were carried out on a Neware cell test instrument in a voltage window of 0.005-3.00 V (vs. Li/ Li+). 3. Results and discussion The synthesis of pomegranate-like Fe3O4 NP superparticles is illustrated in Fig. 1, which involves the bottom-up assembly of Fe3O4 NP superlattice spheres, in situ ligand carbonization, and top-down acid etching of the constituent Fe3O4 NPs. Fig. 2a shows the TEM image of 14 nm, OA-stabilized Fe3O4 NPs which were used as building blocks for constructing microscale superlattice spheres by an emulsion-based self-assembly process (Fig. 2b, 2c, and S1) [38]. The average size of Fe3O4 NP superlattice spheres, determined from both statistical analyses and DLS measurements, is around 450 nm (Figure S2). The subsequent heat treatment at 500 oC converted the OA capping layer into a thin and spherical carbon shell, without degrading the close-packed ordering of NPs (Fig. 3a and 3e). The resulting carbon-coated Fe3O4 NP superlattice spheres were subjected to controlled HCl treatment at room temperature, enabling pomegranate-like superparticles upon the partial etching of Fe3O4 NPs. The key to the formation of pomegranate-like superparticles was the homogeneous etching of the constituent Fe3O4 NPs, which could be realized by manipulating the etching conditions. Aside from the HCl concentration, the stirring speed was also found to be a determinative factor for the pomegranate-like structure formation. Control experiments with fast stirring (~ 1000 rpm) typically led to core-shell rather than pomegranate-like superparticles (Fig. S3), in which the

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peripheral NPs were preferentially etched away before the etching of the interior NPs. Lowering the stirring speed to about 200 rpm under otherwise identical conditions significantly reduced the diffusion kinetics of the etchant, such that all the constituent Fe3O4 NPs could be etched at a similar rate, enabling the formation of pomegranate-like superparticles. The etching degree of Fe3O4 NPs and therefore the void size in individual Fe3O4@C yolk-shell NPs could be tailored by controlling the etching time. Samples with the etching duration of 1, 3, and 5 h were denoted as Fe3O4@C-1, Fe3O4@C-3, and Fe3O4@C-5, respectively. For the purpose of comparison, the pristine carbon-coated Fe3O4 NP superlattice spheres without HCl etching were also investigated as LIB anodes and were designated as Fe3O4@C-0. XRD indicated that all the NP superparticle samples exhibited the diffraction peaks indexed to Fe3O4 (Fig. 4a) [40], suggesting the crystal phase of Fe3O4 NPs was retained without introducing any impurities during acid etching. SEM confirmed that the morphology of the initial Fe3O4 NP superlattice spheres was also maintained in the course of HCl treatment (Fig. 3b-d). Prolonging the etching time progressively reduced the size of Fe3O4 NP constituents and therefore created more void space throughout the entire superparticle, as revealed by TEM (Fig. 3f-h and S4). Also evident from TEM was the gradually degraded structural ordering of Fe3O4 NP superparticles with the increase of the etching time, which was further confirmed by the smeared SAXS patterns (Fig. 4b). This result was not surprising because the partially etched Fe3O4 NPs could move freely within carbon shells, thereby deteriorating the overall periodicity of superparticles. Note that the slightly improved

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structural ordering of Fe3O4@C-5 relative to Fe3O4@C-3 originated mainly from the well-defined carbon shells rather than the entrained Fe3O4 NPs due to the greatly decreased NP size. STEM and the corresponding EDS elemental mapping confirmed the homogeneous distribution of Fe3O4 NP residues crossing the entire pomegranate-like superparticle (Fig. S5-S7). The etching of Fe3O4 NPs was accompanied by the creation of void space within superparticles, which was further corroborated by N2 adsorption-desorption measurements. As shown in Fig. 4c, the pristine Fe3O4@C-0 sample exhibited type-IV isotherms with a Brunauer–Emmett–Teller (BET) surface area of 47 m2 g-1. In contrast, the large hysteresis loops emerging at P/P0 of ~0.5 upon acid etching were indicative of the presence of abundant mesopores, in agreement with TEM observations. Prolonging the etching time led to an increase of the surface area from 47 to 197, 783, and 1270 m2 g-1 for Fe3O4@C-1, Fe3O4@C-3, and Fe3O4@C-5, respectively. Likewise, the pore width was progressively widened with the increase of the etching duration (Fig. 4c, inset). The carbon content, determined from TGA (Fig. 4d), was 4.40, 7.76, 30.05, and 53.88 wt% for Fe3O4@C-0, Fe3O4@C-1, Fe3O4@C-3, and Fe3O4@C-5, respectively. Assuming that the carbon content remained unchanged in the course of etching, the size of the residual Fe3O4 NPs in Fe3O4@C-1, Fe3O4@C-3, and Fe3O4@C-5 was estimated to be 11.88, 6.90, and 4.94 nm, respectively. The electrochemical performance of pomegranate-like Fe3O4 NP superparticles was evaluated as LIB anodes based on a half-cell configuration. For comparison, the

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pristine Fe3O4@C-0 sample was also tested under the identical conditions. Fig. 5 shows the galvanostatic charge/discharge voltage profiles at a current density of 0.8 A g-1. Similar to Fe3O4@C-0, both Fe3O4@C-1 and Fe3O4@C-3 exhibited a long and flat plateau at ~ 0.75 V in the first discharge process, which was ascribed to the reduction of Fe3O4 to Fe(0) and the formation of SEI [41]. The discharge plateaus shifted to ~ 1.0 V and remained stable in the following cycles. Meanwhile, a plateau at ~ 1.75 V, which was attributed to the oxidation of Fe(0) to Fe3O4, was observed in the charge processes [46]. These charge/discharge features were fully consistent with the previous results on Fe3O4-based anodes [42], suggesting the reversible conversion reactions between Fe3O4 and Li. It is worth noting that the charge/discharge plateaus of Fe3O4@C-5, although discernible in charge/discharge profiles (Fig. 5d), was less resolved compared with Fe3O4@C-1 and Fe3O4@C-3. We attributed this to the comparatively low Fe3O4 content in Fe3O4@C-5, such that the carbon shells played a nonnegligible role in lithium storage, thereby smearing the redox plateaus of Fe3O4 NPs. This was further confirmed by CV, in which apart from the characteristic peaks for Fe3O4, Fe3O4@C-5 also exhibited a prominent peak near 0 V in the first cathodic sweep (Fig. S8d), which was ascribed to the insertion of lithium ions into the carbon shells [43]. The initial discharge capacity of Fe3O4@C-0, Fe3O4@C-1, Fe3O4@C-3, and Fe3O4@C-5 at 0.8 A g-1 was determined to be 1352, 1377, 2017, and 1542 mAh g-1, respectively. Notably, all pomegranate-like superparticle anodes exhibited an improved initial capacity over the pristine Fe3O4@C-0 sample, with Fe3O4@C-3

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delivering the highest initial discharge capacity. Presumably, the presence of the void space in pomegranate-like superparticles facilitated the accessibility of lithium ions to the constituent Fe3O4 NPs, thereby enhancing the utilization of active materials. The capacity drop from Fe3O4@C-3 to Fe3O4@C-5 was likely caused by the significantly reduced Fe3O4 content. In addition to the highest initial capacity, Fe3O4@C-3 exhibited charge/discharge voltage profiles nearly overlapping from the second cycle (Fig. 5c), suggesting its superior electrochemical reversibility. To further elucidate the role of the void space in improving the battery performance, the superparticle anodes were charged and discharged at a current density of 0.8 A g-1. As shown in Fig. 6a, all pomegranate-like superparticles outperformed the pristine Fe3O4@C-0 sample in cycling performance, revealing the effectiveness of partial acid etching in improving the lithium-storage properties of Fe3O4 NP superlattices. Besides, among the three pomegranate-like superparticle samples, Fe3O4@C-3 exhibited the best cycling performance, retaining a remarkable discharge capacity of 1246 mAh g-1 after 300 cycles. In comparison, the specific discharge capacity of Fe3O4@C-0, Fe3O4@C-1, and Fe3O4@C-5 after 300 cycles was 596, 764, and 879 mAh g-1, respectively, much lower than that of Fe3O4@C-3. Owing to the absence of void space, it is not surprising that Fe3O4@C-0 exhibited the worst capacity retention, as has been commonly observed for Fe3O4-based anodes. In the case of Fe3O4@C-1, the smaller void space was not enough to tolerate the huge volume expansion of Fe3O4 NPs during lithiation, in spite of the slightly increased capacity retention compared with Fe3O4@C-0. It is worth mentioning that

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Fe3O4@C-5 exhibited decent cycling performance, however its comparatively low Fe3O4 content restricted the capacity and was undesirable for achieving high volumetric energy density. As the major difference between various superparticles was the etching degree of the constituent NPs, we thus concluded that the best capacity retention exhibited by Fe3O4@C-3 was attributed to the optimized void size in individual yolk-shell NPs, which enabled the effective accommodation of the volume expansion of Fe3O4 NPs without sacrificing the volumetric energy density. The rate performance of different types of superparticles was evaluated at various current densities ranging from 0.4 to 10 A g-1. As shown in Fig. 6b, the rate capability of Fe3O4@C-3 was far superior to that of other types of superparticles, in accordance with the trend observed in cycling performance. Specifically, the average capacity of Fe3O4@C-3 was 1293, 1167, 1108, 952, 822, and 681 mAh g-1 at 0.4, 0.8, 1, 2, 3, and 5 A g-1, respectively. Even when tested at an ultrahigh rate of 10 A g-1, Fe3O4@C-3 could still deliver a discharge capacity of 461 mAh g-1. Moreover, a stable capacity of 1194 mAh g-1 was recovered when the current density was switched back to 0.4 A g-1. To further investigate the long-term cycling stability, the Fe3O4@C-3 anodes were subjected to cycling at high current densities. As shown in Fig. 6c, Fe3O4@C-3 could deliver a high discharge capacity of 990 mAh g-1 at 2 A g-1 after 300 cycles, while a high capacity of 520 mAh g-1 was maintained at 5 A g-1 even after 1000 cycles (Fig. 6d). It should be noted that the capacity increase during the first tens of cycles in Fig. 6c was attributed to the activation of active materials originating mainly from the lithiation-induced structural rearrangement, as has been

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commonly observed in transition-metal-oxide-based anodes [44-46]. EIS showed that the charge-transfer resistance was reduced during the course of cycling (Fig. S9), which might also contribute to the enhanced capacity. Taken together, the overall electrochemical performance exhibited by Fe3O4@C-3 was superior to that of most Fe3O4-based anodes reported in the literature (Table S1), reinforcing that the formation of pomegranate-like superparticles with optimized void space was indeed efficient for achieving superior lithium storage. To gain deeper insights into the lithiation-induced structural changes for Fe3O4 NP superparticles, SEM and elemental analysis were carried out after 300 cycles at 0.8 A g-1. As shown in Fig. 7, the majority of the pristine Fe3O4@C-0 sample cracked after cycling, while Fe3O4@C-3 preserved the spherical morphology of superparticles without cracking under the identical cycling conditions, with Fe3O4 NPs homogenously confined within superparticles as revealed by elemental mapping (Fig. S10). Even after 1000 cycles at 5 A g-1, Fe3O4@C-3 could still maintain the spherical morphology (Fig. S11), further confirming their structural robustness against repeated lithiation and delithiation. The difference in lithiation-induced structural evolution behaviors for different Fe3O4 NP superparticles was schematically illustrated in Fig. 7e and 7f. For Fe3O4@C-0, owing to the absence of the void space, the large volume expansion of Fe3O4 NPs upon full lithiation would incur huge mechanical strain and eventually break the compact carbon shells (Fig. 7e), resulting in the observed structural collapse. In contrast, the optimized void size in Fe3O4@C-3 provided enough space for the volume expansion of each Fe3O4 NP while simultaneously

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facilitating electrolyte diffusion (Fig. 7f). thereby efficiently ameliorating the mechanical strain induced by repeated lithiation and delithiation. 4. Conclusions In conclusion, we have developed a novel type of pomegranate-like superparticles comprising close-packed, interconnected yolk-shell Fe3O4@C NPs by combining bottom-up assembly and top-down etching methods. Benefiting from the combined merits of yolk-shell NPs, secondary particle configuration, and homogeneous carbon coating, such rationally designed superparticles are well suited for energy storage applications. Moreover, the manipulation of the etching conditions allows for the fine control of the void space in individual yolk-shell NPs, enabling Fe3O4-based anodes with superior electrochemical performance including ultrahigh reversible capacity (1246 mAh g-1 at 0.8 A g-1), exceptional rate capability (461 mAh g-1 at 10 A g-1), and excellent cycling stability (520 mAh g-1 at 5 A g-1 after 1000 cycles). In addition to Fe3O4, we anticipate that this conversion strategy to construct pomegranate-like superparticles is applicable to other transition metal oxides such as Co3O4 and MnFe2O4, offering an opportunity for achieving unprecedented lithium-storage properties.

Acknowledgements The authors acknowledge the financial support from National Basic Research Program of China (2017YFA0207303, 2014CB845602), Natural National Science

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Foundation of China (21373052), and Shanghai International Science and Technology Cooperation Project (15520720100).

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Fig. 1. Schematic illustration of the preparation of pomegranate-like Fe3O4 NP superparticles.

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Fig. 2. (a) TEM image of 14 nm Fe3O4 NPs. (b) Low-magnification SEM and (c) HRSEM images of the as-assembled Fe3O4 NP superlattice spheres.

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Fig. 3. (a-d) SEM images, (e-h) TEM images, and (i-l) the corresponding cross-sectional illustration of Fe3O4@C-0, Fe3O4@C-1, Fe3O4@C-3, and Fe3O4@C-5, respectively.

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Fig. 4. (a) XRD patterns, (b) SAXS patterns, (c) N2 adsorption-desorption isotherms and pore width distribution (inset), and (d) TGA curves of various Fe3O4 NP superparticles.

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Fig. 5. (a-d) Galvanostatic charge/discharge voltage profiles of various Fe3O4 NP superparticles at a current density of 0.8 A g-1.

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Fig. 6. (a) Cycling performance of various Fe3O4 NP superparticles at a current density of 0.8 A g-1. (b) Rate performance of various Fe3O4 NP superparticles at current rates from 0.4 to 10 A g-1. Cycling performance and the corresponding Coulombic efficiency of Fe3O4@C-3 at a current density of (c) 2 and (d) 5 A g-1, respectively.

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Fig. 7. SEM and the corresponding HRSEM images of (a, b) Fe3O4@C-0 and (c, d) Fe3O4@C-3 after 300 cycles at 0.8 A g-1. Cross-sectional illustration depicting the structural evolution of (e) Fe3O4@C-0 and (f) Fe3O4@C-3 during the lithiation process.

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Graphical Abstract

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