Hollow carbon nanosphere embedded with ultrafine Fe3O4 nanoparticles as high performance Li-ion battery anode

Electrochimica Acta 219 (2016) 356–362

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Hollow carbon nanosphere embedded with ultrafine Fe3O4 nanoparticles as high performance Li-ion battery anode Lang Liua , Hua Zhanga , Shuwu Liua , Haimin Yaob , Haoqing Houa , Shuiliang Chena,* a b

Department of Chemistry and Chemical Engineering, Jiangxi Normal University, 330022, Nanchang, China Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

A R T I C L E I N F O

Article history: Received 5 August 2016 Received in revised form 23 September 2016 Accepted 4 October 2016 Available online 4 October 2016 Keywords: hollow carbon nanosphere Fe3O4 nanoparticle Li-ion battery anode

A B S T R A C T

Hollow carbon nanospheres embedded with ultrafine Fe3O4 nanoparticles (Fe3O4@HCNS) were synthesized by using carboxyl functionalized polystyrene latexes as template and poly dopamine as carbon precursor. The ultrafine Fe3O4 nanoparticles (NPs) had a small size of 3  5 nm and the HCNS had a thin shell thickness of 15 nm in the Fe3O4@HCNS. As a promising anode material for lithium-ion batteries, the Fe3O4@HCNS exhibited high reversible capacity, excellent cycling stability (1380 mA h g 1 after 200 cycles at 1 A g 1) and high-rate capability (475 mA h g 1 at 5 A g 1, 290 mA h g 1 at 10 A g 1). The outstanding performance was attributed to the unique structure of the Fe3O4@HCNS, which greatly shorten the path of Li ions intercalation during charging and discharging. ã 2016 Published by Elsevier Ltd.

1. Introduction As one of the leading power sources, rechargeable Li-ion batteries (LIBs) continue to draw our attention [1–3]. Due to the low gravimetric capacity of conventional graphite anodes (372 mA h g 1), great efforts were made to explore new alternatives. With the characteristics of high theoretical capacities (926 mA h g 1), low-cost, environment friendly and natural abundance, Fe3O4 became one of the promising active materials for anode of LIBs [4– 8]. However, its practical application is still restricted by rapid capacity degradation and poor cyclic performance, which results from dramatic volume change during lithiation/delithiation process [9,10]. The advent of nanomaterials gave LIBs a new lease of life [11]. It has been indicated that the size is a crucial factor for improving cell performance and cracking of particles would not occur below a critical size [12,13]. In addition, the nano-sized Fe3O4 could also provide more active reaction sites. However, due to electrolyte decomposition and NPs aggregation, their capacity fade with cycling [14]. To improve the performance of the Fe3O4, carbon coating was applied to fabricate a core/shell structure [6,15–17]. The outer carbon shell in such structures could not only reduce the formation

* Corresponding author. E-mail address: [email protected] (S. Chen). http://dx.doi.org/10.1016/j.electacta.2016.10.023 0013-4686/ã 2016 Published by Elsevier Ltd.

of the SEI but also promote the electron transport [18,19]. Additionally, tension associated with lithiation/de-lithiation could also be relieved to some extent. Therefore, a series of carbon coated magnet materials were prepared, such as nanospheres [20,21], nanorods [22], nanowires [23], nanoflakes [24] and NPs [25]. For example, Jung and coworkers [14] prepared Fe3O4/carbon microspheres by co-precipitation method. Interestingly, the authors found the morphologies of the products varying from the weight ratio of Fe3O4 NPs to polymer matrix. In addition, this material showed a high reversible capacity of 500 mA h g 1 at 1C with an excellent columbic efficiency of 99% even after 300 cycles. Despite recent progress, there is still considerable room for improvement. Recently, hollow structured materials have provoked wide interests and research [26,27]. In this structure, the hollow space could provide enough room for expansion, which greatly relief the volume change as well as shorten the pathway for Li intercalation and electron transfer. For example, Li and coworkers [28] designed a new structure which hollow C/Fe3O4 microspheres were embedded in graphene networks. In this unique structure, the Fe3O4 NPs were uniformly distributed in and also supported by the hollow structured carbon, achieving a strongly coupled C/Fe3O4 hybrid, which resulted in a strong synergetic effect between carbon and Fe3O4 [28] . When used as LIBs anodes, the hybrid material exhibited outstanding longterm cycling performance (1208 mA h g 1 after 200 cycles at

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100 mA g 1). Such an elegant designed structure implied great potential of application in the field of LIBs. The aforementioned studies imply that the ideal Fe3O4 based anode nanomaterials in the LIBs should possess following key characteristics: (a) the Fe3O4 should have a small size to ensure small volume change and short pathway for Li ions interaction, (b) the small Fe3O4 should be confined by an electrically conductive cage to ensure a sufficient electron transfer, (c) the Fe3O4 based composite should be porous and hollow structured to provide sufficient void space for buffering the volume change and facilitate Li transfer. Herein, hollow carbon nanospheres embedded with uniform ultrafine Fe3O4 NPs (Fe3O4@HCNS) were synthesized by using carboxyl functionalized polystyrene (PS-COOH) latexes as templates for high performance LIBs anode. The obtained Fe3O4@HCNS was composed of Fe3O4 NPs with a diameter of 3  5 nm encapsulated in hollow mesoporous carbon nanoshell with a thickness of 15 nm. Owing to the unique structure, the Fe3O4@HCNS exhibited excellent cycling stability (1380 mA h g 1 after 200 cycles at 1 A g 1), high-rate capability (475 mA h g 1 at 5 A g 1, 290 mA h g 1 at 10 A g 1) when used as anode material of LIBs. 2. Experimental 2.1. Materials Styrene (99.0%), potassium persulfate (K2S2O8, 98.0%), methacrylic acid (MAA, 99%), diethylene glycol (DEG, 99%), commercial Fe3O4 NPs (100–200 nm, Alfa Aesar) and anhydrous iron(III) chloride (FeCl3, 98%) were purchased from Sigma-Aldrich. Deionized water was purified with Mili Q ultra-filtration system. 2.2. Synthesis of samples 2.2.1. Synthesis of PS-COOH latexes PS-COOH latexes with an average diameter of 120 nm was prepared by following previous work [29]. Typically, 18 mL styrene, 0.2 g K2S2O8, 2 mL MAA and 160 mL deionized water were added to a 500 mL flask and then heated to 80  C in N2 atmosphere with magnetic stirring. After 24 h, the mixture cooled down naturally, and PS-COOH latexes with weight concentration of 10% was obtained. 2.2.2. Preparation of PS@Fe3O4 0.2 g NaOH and 20 mL DEG were added to a 100 mL flask and then heated to 120  C in N2 atmosphere for 1 h under magnetic stirring. Uniformed yellow NaOH/DEG solution was prepared for use. 0.12 g FeCl3, 20 mL DEG and a certain amount of 10 wt.% PS latexes were added into another flask then heated to 220  C for 30 min under nitrogen flow and constant magnetic stirring; then 5 mL yellow NaOH/DEG solution was quickly added into the above mixture and reacted for another 30 min. The product was centrifuged and washed with distilled water for several times, then dried overnight. The influence of PS dosage on the composite morphology was also investigated. High dosage of PS latexes (4.5 mL) resulted in few Fe3O4 NPs distributed on the PS surface (Fig. S1B), while low dosage of PS (1.5 mL) led to agglomeration of the Fe3O4 NPs (Fig. S1C). Thus, 3 mL 10 wt.% PS was used in the following experiments, PS@Fe3O4 with well dispersed Fe3O4 NPs were obtained (Fig. 1B). 2.2.3. Preparation of PS@Fe3O4@polydopamine Typically, 200 mg as-prepared PS@Fe3O4 was dispersed in Trisbuffer (100 mL, 10 mM; pH 8.5), and then 200 mg dopamine was added to the mixture and stirred for 10 h at room temperature. The

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product was centrifuged and washed for several times and then dried at 50  C for use. 2.2.4. Preparation of Fe3O4@HCNS The PS@Fe3O4@polydopamine (200 mg) was pyrolyzed at a given temperature in a quartz tube for 1 h with a heating rate of 2  C min 1 under N2 atmosphere. Pyrolyzing temperatures of 550, 650 and 750  C were used, and the final products were denoted as Fe3O4@HCNS-550, Fe3O4@HCNS-650 and Fe3O4@HCNS-750, respectively. 2.2.5. Preparation of Fe3O4@C The commercial Fe3O4 NPs with the diameter of 100–200 nm were coated with polydopamine firstly, then pyrolyzed at 550  C for 1 h in N2 atmosphere. 2.2.6. Preparation of hollow carbon nanosphere (HCNS) PS latexes were coated with a layer of polydopamine, then pyrolyzed at 550  C for 1 h in N2 atmosphere. 2.3. Characterization Scanning electron microscopy (SEM) imaging was carried out with Hitachi S4800 field-emission SEM system. TEM imaging was carried out with JEOL 2010 electron microscope operated at 200 kV. TGA was carried out by STA449 F3 Jupiter from room temperature to 800  C in air with a heating rate of 10  C min 1. The element analysis was conducted by Perkin-Elmer PHI 550 spectrometer with Al Ka as the X-ray source. The N2 adsorption/desorption tests were carried out by BET measurements using an ASAP-2010 surface area analyser. XRD measurements were conducted using Shimadzu XRD-7000 instrument with Cu Ka radiation. Zeta potentials measurements were carried out on the dynamic light scattering analysis (Zeta sizer 3000HSA) at room temperature. 2.4. Electrochemical measurements Active material, carbon black and poly (vinyldifluoride) (PVDF) with weight ratio of 60:20:20 were mixed together using Nmethylpyrrolidinone as solvent to form a mixture slurry. After spreading the slurry onto the copper foil and drying, the electrodes were prepared. Subsequently, the electrodes were dried in a vacuum oven at 120  C for 6 h to remove the residue solvent. The loading of the active material was 1.3 mg cm 2. The Celgard 2300 membrane was used as a separator. The cells were finally assembled in an argon-filled glove box with Li foil as counter electrodes. The electrolyte we used consists of 1.0 M LiPF6 dissolved in a 1:1 (v/v) mixture of ethylene carbonate and dimethyl carbonate. CV was measured using VMP3 (Bio-Logic) electrochemical workstation. The discharge-charge measurements were carried out at a Neware BTS test system with a voltage ranging from 0.01 V to 3.0 V (vs. Li/Li+). 3. Results and discussion 3.1. Synthesis of PS@Fe3O4 As illustrated in Scheme 1, uniform PS-COOH latexes with average diameter of 120 nm (Figs. 1 A and S1A) were used as template for preparing the Fe3O4@HCNS. Zeta-potential measurement and IR spectra (Fig. S2) confirmed that these PS latexes were negative charged ( 36.5 mV) and carboxyl functionalized. In the preparation of PS@Fe3O4, Fe3+ was firstly chelated with carboxy groups and then transformed into Fe3O4 NPs in-situ. Therefore, it was believed that the chelation of the carboxy groups with the

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Fig. 1. (A) TEM image of PS-COOH latexes and (B) TEM image of PS@Fe3O4 prepared with 3 mL 10 wt.% PS-COOH latexes; (C and D) TEM images of Fe3O4@HCNS-550; (E) HRTEM image of Fe3O4@HCNS-550, the insets were the SAED pattern of Fe3O4@HCNS-550 and crystal lattice fringes of individual Fe3O4 NP; (F) TEM image of Fe3O4@HCNS-550 after cycling for 200 cycles.

Fe3+ was key to anchor the formed Fe3O4 NPs on the surface of the PS latexes. During the formation of Fe3O4, the Fe3+ was first hydrolyzed to Fe(OH)3, subsequently partially reduced to Fe(OH)2 and eventually dehydrated to form the Fe3O4 NPs [30]. The diethylene glycol (DEG) not only acted as the solvent and reducing agent, but also restricted the growth of Fe3O4 NPs and prevented agglomeration [31,32]. To obtain NPs with narrow size distribution, high temperature (220  C) was very necessary, because it greatly promoted the nucleation process and gave rise to burst of nucleation within several seconds [33]. Transmission electron microscopy (TEM) image of PS@Fe3O4 shown in Fig. 1B revealed that the Fe3O4 NPs with average size of about 3 nm were welldistributed on the surface of the PS latexes. 3.2. Synthesis of Fe3O4@HCNS Since the invention by Dai and co-workers [34], the method of using dopamine as carbon source to construct carbon-based nanomaterials has attracted considerable attention [35,36]. Compared with other carbon precursors (such as glucose [23],

citric acid [37] and oleic acid [38]), it has several significant advantages: (a) it was more versatile and facile; (b) the coating layer contain N species which benefits the cycling performance and rate capacity [39,40]; (c) the thickness of the coating layer could be precisely controlled [41]. In this study, dopamine was chosen as the precursor to fabricate the thin carbon shell. As illustrated in Fig. S1D and E, uniform and continuous polydopamine layer with the thickness of around 20 nm was produced on the surface of the PS@Fe3O4 NPs. After pyrolyzing at 550  C, the PS template was removed and Fe3O4@HCNS-550 was obtained. HCNS also could be prepared through the similar process. The TEM images in Fig. 1C, D and Fig. S1H confirmed the hollow structure of the Fe3O4@HCNS-550 and the HCNS. They all had a uniform inner diameter of around 100 nm and carbon shell thickness of 15 nm (Fig. 1D). Ultrafine Fe3O4 NPs with diameter in the range of 3  5 nm were firmly embedded in the inner wall of the HCNS (Fig. 1E), which was probably attributed to the strong affinity of catechol functional groups in the polydopamine and the ultrafine size of the Fe3O4 NPs [42]. The high-resolution TEM (HRTEM) image (inset of Fig. 1E) displays clear crystal lattice fringes with a

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Scheme 1. Schematic diagram of the process for the preparation of Fe3O4@HCNS-550.

spacing of 0.2967 nm which corresponds to the (220) plane of Fe3O4. The electron diffraction pattern of the selected area (SAED) illustrated in the inset of Fig. 1E gives a set of diffraction rings, corresponding to the (440) and (311) planes of Fe3O4. Fig. 2A is the X-ray powder diffraction (XRD) spectra of the Fe3O4@HCNS-550 (the XRD pattern of Fe3O4@HCNS-650 and Fe3O4@HCNS-750 see Fig. S3). The broadening signal located in the position of 23 was ascribed to the noncrystalline carbon [43], which suggests that the coating polymer layer has been converted to carbon. The signals located at 35.3 , 43.0 , 56.9 and 62.5 are the typical diffractions of face-centered-cubic magnetite (JCPDS No. 19-0629), which confirmed the formation of nanocrystal Fe3O4. The broad peaks at 35.3 suggests the small size of the Fe3O4 NPs. According to the Scherrer’s formula, the average size of the Fe3O4 nanocrystalline is 5.6 nm, which is slightly larger than that measured from the TEM image. Compared to the PS@Fe3O4 (Fig. 1B), the size of the Fe3O4 NPs in the Fe3O4@HCNS-550 (Fig. 1D) increases slightly, which is likely due to the fusion of adjacent Fe3O4 NPs during the pyrolysis [44]. The effect of calcination temperature on size of Fe3O4 NPs were also investigated. As shown in Fig. S1F, when pyrolyzed at 650  C, the size of the Fe3O4 NPs increases to 9 nm. Higher

A

temperature (Fig. S1G 750  C) results in serious agglomeration and therefore larger size more than 20 nm; moreover, most of the Fe3O4 NPs escaped from the carbon shell. 3.3. Characteristics of Fe3O4@HCNS According to the thermo gravimetric analysis (TGA) curves in Fig. S4, after pyrolysis of the Fe3O4@HCNS-550 at over 800  C in air, the weight percentage of the remaining Fe2O3 is 35%. Considering that the conversion of Fe3O4 to Fe2O3 gives rise to about 3% gain in weight when heated in air [45], the actual Fe3O4 content of in the Fe3O4@HCNS-550 is about 34%. The survey spectra of X-ray photoelectron spectroscopy (XPS) (Fig. S5) indicates that the final product contains C, O, N and Fe. Because the Fe3O4 was covered by a 15 nm thick carbon layer, while XPS signal only gives information concerning the outermost 3–4 nm [46], thus the result of the Fe content (only 0.85 at.%) (Fig. S5A) measured by the XPS is much lower than that by the TGA. In the fine spectrum of Fe2p (Fig. S5B), the peak located at 711.0 and 724.5 eV are corresponding to Fe 2p3/2 and Fe 2p1/2 respectively, which is consistent with previous reports [47]. In addition, the charge transfer satellite of Fe 2p3/2 was not

B

Fig. 2. (A) XRD spectrum of the Fe3O4@HCNS-550. (B) Nitrogen adsorption–desorption isotherms of Fe3O4@HCNS-550, the inset shows the distribution of pore size.

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A

B

Fig. 3. (A) Charge-discharge curves at the 1st, 2nd, 50th, 100th and 200th cycles of Fe3O4@HCNS-550 at a current density of 0.5 A g 1. (B) CV curves of the Fe3O4@HCNS-550.

detected around 720 eV, which further confirmed the formation of Fe3O4 [48]. The fitting curve of high-resolution spectrum of N1s (Fig. S5C) displays two peaks located at 398.5 and 400.3 eV, corresponding to pyridinic nitrogen and pyrrolic nitrogen respectively [49,50]. Therefore, the polydopamine has been successfully converted to N-doped carbon. To gain deeper insight into the textural characteristics of the Fe3O4@HCNS-550, nitrogen adsorption–desorption measurement was carried out. As shown in Fig. 2B, the isotherm profile exhibits feature of typical type IV with a big hysteresis loop, which indicates that it is a mesoporous material. The distribution of pore size in the inset of Fig. 2B indicates that the Fe3O4@HCNS-550 has a relatively narrow distribution of size around 4 nm. Compared with the Fe3O4@C, the Fe3O4@HCNS-550 demonstrated higher BET surface area and larger pore volume (Fig. S6 and Table S1), which was beneficial to provide excellent electrical contact and access for electrolyte, respectively [15,19].

and two small peaks at 1.41 V and 1.0 V are observed in the first cathodic sweep which is attributed to the reduction of the Fe3O4 as well as the formation of Li2O [55–57]. Meanwhile, a broad peak at 1.68 V and a shoulder peak at higher potentials are observed in the anodic sweep, corresponding to the oxidation of Fe0 [55–57]. Long-term stability test reveals that the Fe3O4@HCNS-550 exhibits excellent cycling stability. As shown in Fig. 4A, the Fe3O4@HCNS-550 is able to retain a high capacity of 1380 mA h g 1 after 200 cycles at a current density of 1 A g 1. For comparison, commercial Fe3O4 NPs with size of 100–200 nm, Fe3O4@C and

3.4. Li-ion storage performance of the Fe3O4@HCNS The lithium storage of these Fe3O4@HCNS were investigated. Some typical charge/discharge profiles of the Fe3O4@HCNS-550 in the 1st, 2nd, 50th, 100th and 200th cycles at a current density of 500 mA/g are shown in Fig. 3A (the discharge-charge profiles of Fe3O4@HCNS-550, Fe3O4@C, HCNS, and bare Fe3O4 electrodes at 1 A g 1 were provided in Fig. S7). Apparently, in the discharging profile of the first cycle, a distinct plateau at 1.4 V is clearly discerned; subsequently, an unobvious plateau at around 1.1 V is identified; then, the third well-defined plateau at 0.72 V is observed. It is striking to note that there is no obvious plateau from the second cycle, suggesting that this structure could efficiently reduce the polarization of electrode [51]. The first discharge and charge capacities are 1903.7 and 1028 mA h g 1, respectively, leading to a relatively low Coulombic efficiency of 54%. It was said that during the first charge process, the organic electrolyte decomposes and forms a solid electrolyte interphase (SEI) on the anode surface [52,53]. This process consumes lithium from the cathodes, therefore reduces the specific energy and energy density of LIBs. Jusef Hassoun [54] and coworkers found the electrochemical prelithiation could efficiently address this problem. In the second cycle, the Coulombic efficiency dramatically increases to 84.7%, and reaches to 99% at the fifth cycle. The capacity of the Fe3O4@HCNS-550 gradually increases with cycle numbers and reached to 1542 mA h g 1 at the 200th cycle (Fig. 3A) which was 1.5 times of the 1st cycle (1028 mA h g 1). Cyclic voltammetry (CV) tests were conducted to investigate the electrochemical reactions of the Fe3O4@HCNS-550 as anode materials in LIBs. As shown in Fig. 3B, a sharp peak at 0.71 V

Fig. 4. (A) Cycle performance of Fe3O4@HCNS-550, Fe3O4@C, bare Fe3O4 and HCNS at current density of 1 A g 1. (B) The charge capabilities of Fe3O4@HCNS-550 at different current densities between 1 and 10 A g 1.

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hollow carbon nanosphere (HCNS) are also tested and shown in Fig. 4A. For the bare Fe3O4 NPs, without the protection of carbon shell, the formed Fe NPs would lead to continuous rupture and reformation of the SEI, therefore its capacity of 920 mA h g 1 at the first cycle fade to almost zero after 70 cycles. However, in the Fe3O4@C and Fe3O4@HCNS-550, the carbon shell could serve as a barrier and efficiently prevent the Fe0 NPs from contacting with the SEI. Therefore, the Fe3O4@C and Fe3O4@HCNS-550 maintain high capacity of 790 and 1380 mA h g 1, respectively, after 200 cycles at current density of 1 A g 1. This result demonstrates the importance of the carbon protective layer for the stability of the Fe3O4-based anode materials in LIBs. The pure HCNS without Fe3O4 have desirable cycling stability, but could only deliver capacity as low as 212 mA h g 1. Thus, the Fe3O4 NPs are the major contributors of the capacities of the Fe3O4@C and Fe3O4@HCNS-550. Fig. 4A also shows that both the reversible capacities of the Fe3O4@C and the Fe3O4@HCNS-550 decrease first and then gradually increase. The difference is that the capacity increase begins from the 4th cycle in the Fe3O4@HCNS-550 electrode, while from the 30th cycle in the Fe3O4@Cs electrode. The increase of reversible capacities with cycling is believed to be the gradual activation process and reversible reactions between Fe3O4 particles and electrolytes [58–61]. Though the Fe3O4@HCNS-550 has much lower content of Fe3O4 (Fig. S4), it exhibits much higher capacity and better cyclic stability (1380 mA h g 1 after 200 cycles at 1 A g 1) than the Fe3O4@C (790 mA h g 1 after 200 cycles at 1 A g 1) does. Such high capacity is also superior compared to most of previous works as summarized in Table S1. The high capacity is believed to be comprised by the faradic intercalation and capacitive effects. The contributions of faradaic and capacitive processes to the total discharge capacity at different sweep rates for the Fe3O4@HCNS550 were calculated according to reference [63] and the results are shown in table S2 and Fig. S7. The capacitive process contribute over 68% charge storage in the total capacity at a low charge/ discharge rate of 50 mA g 1 and increase with increase in charge/ discharge rate. It had also been reported that thinner particle size of the transition oxide favored higher capacitive contributions in the total charge storage. [63] Thus, the probably reason might be due to the ultrafine diameter of the Fe3O4 particles in the Fe3O4@HCNS-550, which lead to high capacitive charge storage. This results give a good explanation that the higher capacity of the Fe3O4@HCNS-550 than the theoretical capacity of the pure Fe3O4. Apparently, the unique structure plays an important role to the high capacity. In one aspect the ultrafine Fe3O4 NPs in the Fe3O4@HCNS-550 could relieve the volume change and shorten the pathway of Li ions intercalation and electron transfer [62], as

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illustrated in Scheme 2; on the other hand the carbon shell serves as the electrical conductor for efficient electron transport of the Fe3O4 NPs and prevents the ultrafine Fe3O4 NPs from aggregating. As shown in Fig. 1F, the ultrafine Fe3O4 NPs are still welldistributed without any aggregation after 200 cycles charge/ discharge. Secondary, the Fe3O4@HCNS-550 is hollow and contains abundant mesopores in the carbon shell (Fig. 2B), giving rise to much higher surface area than that of the Fe3O4@C with tight core/ shell structure (Fig. S1I). Such hollow structure not only provides sufficient void space and significantly buffers the volume change, but also greatly facilitates the transfer of Li ions to the Fe3O4 NPs. The cyclic performances of the Fe3O4@HCNS-650 and Fe3O4@HCNS-750 prepared at higher pyrolysis temperature were also investigated. The TEM images in Figs. 1 D, S1F and S1G showed that the size of the Fe3O4 NPs in the Fe3O4@HCNS-650 (9 nm) and Fe3O4@HCNS-750 (20 nm) are bigger than that in the Fe3O4@HCNS-550 (5 nm). Fig. S8A displays that the Fe3O4@HCNS-650 and Fe3O4@HCNS-750 exhibit capacities of 1116 and 964 mA h g 1, respectively, after 200 cycles at current density of 1A g 1, which was relatively lower than that of the Fe3O4@HCNS-550. Thus, it could be concluded that the size of the Fe3O4 NPs is a significant factor which affecting the reversible capacity of the Fe3O4@HCNS composites materials. The smaller the size, the higher the reversible capacity. The unique hollow structure of the Fe3O4@HCNS-550 also bring excellent rate capability. As displayed in Fig. 4B. The as-prepared Fe3O4@HCNS-550 exhibits reversible capacities of 1029, 781, 473 mA h g 1 at the rates of 1, 2 and 5 A g 1 respectively. Moreover, it could still achieve a stable capacity of 290 mA h g 1 even at a high current density of 10 A g 1, and most of the capacity is recovered after returning to 1 A g 1. The rate capability of the Fe3O4@HCNS550 was also superior to most of the reported results as summarized in Table S2. In contrast, the Fe3O4@C and HCNS delivered much lower capacities of 151.7 mA h g 1 and 42 mA h g 1 (Fig. S8B) respectively, at the high current density of 10 A g 1. 4. Conclusions In conclusion, Fe3O4@HCNS was synthesized by using carboxyl functionalized PS latexes as template. The obtained Fe3O4@HCNS was composed of Fe3O4 NPs with diameter of 3  5 nm and a hollow mesoporous carbon nanoshell with thickness of 15 nm. The Fe3O4@HCNS exhibited elevated reversible capacity and excellent cycling stability (1380 mA h g 1 after 200 cycles at 1 A g 1), highrate capability (475 mA h g 1 at 5 A g 1, 290 mA h g 1 at 10 A g 1). The outstanding performance is attributed to the unique structure. That is, the carbon encapsulated Fe3O4 NPs could relieve the

Scheme 2. Schematic diagrams illustrating the pathways of Li+ diffusion, interaction and electron transfer in the (A) Fe3O4@C and (B) Fe3O4@HCNS-550.

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volume change and greatly shorten the pathway of Li ions intercalation and electron transfer; the mesoporous thin carbon shell could ensure efficient transfer of electrons and Li ions, and prevent the ultrafine Fe3O4 NPs from aggregating during charging and discharging. This unique structure could be extensively applied to expand to the other metal oxides which will be very promising for other applications such as supercapacitors and fuel cells. Acknowledgements S.C. would like to acknowledge the support the National Natural Science Foundation of China (51202096, 21464008), the Science and Technology Foundation (20121BBE50024) and the Natural Science Foundation of Jiangxi Province (20143ACB21015). H.Y. would like to acknowledge the support by the General Research Funds from Hong Kong RGC (15206415, 529313) Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.023. References [1] B. Scrosati, J. Hassoun, Y.K. Sun, Energy Environ. Sci. 4 (2011) 3287–3295. [2] M. Reddy, G. Subba Rao, B. Chowdari, Chem. Rev. 113 (2013) 5364–5457. [3] P.G. Bruce, B. Scrosati, J.M. Tarascon, Angew. Chem. Int. Edit. 47 (2008) 2930– 2946. [4] W.M. Zhang, X.L. Wu, J.S. Hu, Y.G. Guo, L.J. Wan, Adv. Func. Mater. 18 (2008) 3941–3946. [5] B.P. Hahn, J.W. Long, A.N. Mansour, K.A. Pettigrew, M.S. Osofsky, D.R. Rolison, Energy Environ. Sci. 4 (2011) 1495–1502. [6] E. Kang, Y.S. Jung, A.S. Cavanagh, G.H. Kim, S.M. George, A.C. Dillon, J.K. Kim, J. Lee, Adv. Func. Mater. 21 (2011) 2430–2438. [7] P.L. Taberna, S. Mitra, P. Poizot, P. Simon, J.M. Tarascon, Nat. Mater. 5 (2006) 567–573. [8] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J. Tarascon, Nature 407 (2000) 496– 499. [9] T. Yoon, C. Chae, Y.K. Sun, X. Zhao, H.H. Kung, J.K. Lee, J. Mater.Chem. 21 (2011) 17325–17330. [10] Y. Piao, H.S. Kim, Y.E. Sung, T. Hyeon, Chem. Commun. (Camb.) 46 (2010) 118– 120. [11] A.S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon, W. Van Schalkwijk, Nat. Mater. 4 (2005) 366–377. [12] M.T. McDowell, S.W. Lee, J.T. Harris, B.A. Korgel, C. Wang, W.D. Nix, Y. Cui, Nano Lett. 13 (2013) 758–764. [13] H. Haftbaradaran, H. Gao, Appl. Phys. Lett 100 (2012) 121907. [14] B.Y. Jung, H.S. Lim, Y.K. Sun, K.D. Suh, J. Power Sources 244 (2013) 177–182. [15] G. Zhou, D.W. Wang, F. Li, L. Zhang, N. Li, Z.S. Wu, L. Wen, G.Q. Lu, H.M. Cheng, Chem. Mater. 22 (2010) 5306–5313. [16] Q. Xiong, Y. Lu, X. Wang, C. Gu, Y. Qiao, J. Tu, J. Alloys Comp. 536 (2012) 219– 225. [17] Z. Zeng, H. Zhao, J. Wang, P. Lv, T. Zhang, Q. Xia, J. Power Sources 248 (2014) 15– 21. [18] Y. Jiao, D. Han, L. Liu, L. Ji, G. Guo, J. Hu, D. Yang, A. Dong, Angew. Chem. Int. Edit. 127 (2015) 5819–5823. [19] S. Xin, Y.G. Guo, L.J. Wan, Acc. Chem. Res. 45 (2012) 1759–1769. [20] Z. Zhou, W. Xie, S. Li, X. Jiang, D. He, S. Peng, F. Ma, J. Solid State Electrochem. 19 (2015) 1211–1215.

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