Carbon-coated MnFe2O4 nanoparticle hollow microspheres as high-performance anode for lithium-ion batteries

Carbon-coated MnFe2O4 nanoparticle hollow microspheres as high-performance anode for lithium-ion batteries

Accepted Manuscript Title: Carbon-coated MnFe2 O4 nanoparticle hollow microspheres as high-performance anode for lithium-ion batteries Authors: Yancui...

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Accepted Manuscript Title: Carbon-coated MnFe2 O4 nanoparticle hollow microspheres as high-performance anode for lithium-ion batteries Authors: Yancui Yan, Guannan Guo, Tongtao Li, Dandan Han, Jiahui Zheng, Jianhua Hu, Dong Yang, Angang Dong PII: DOI: Reference:

S0013-4686(17)31257-4 http://dx.doi.org/doi:10.1016/j.electacta.2017.06.020 EA 29657

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

11-4-2017 16-5-2017 3-6-2017

Please cite this article as: Yancui Yan, Guannan Guo, Tongtao Li, Dandan Han, Jiahui Zheng, Jianhua Hu, Dong Yang, Angang Dong, Carbon-coated MnFe2O4 nanoparticle hollow microspheres as high-performance anode for lithium-ion batteries, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.06.020 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 proof before it is published in its final 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.

Carbon-coated MnFe2O4 nanoparticle hollow microspheres as high-performance anode for lithium-ion batteries Yancui Yana,b, Guannan Guoa,b, Tongtao Lib, Dandan Hanb, Jiahui Zhenga,b, Jianhua Hua, Dong Yanga,* and Angang Dongb,*

a

State Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science, Fudan University, Shanghai 200433, China b Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, China *Corresponding authors. E-mail addresses: [email protected](Y. D.), [email protected](A. D.). Graphical Abstract

ABSTRACT In this work, carbon-coated MnFe2O4 nanoparticle (NP) hollow microspheres are fabricated by a facile emulsion-based assembly method followed by in situ ligand carbonization. Specifically, MnFe2O4 NPs stabilized by oleic acid (OA) are the primary building blocks to assemble hollow microspheres, while the subsequent carbonization of OA ligands leads to the formation of uniform carbon coatings without degrading the ordering of NPs. As anode materials for lithium-ion batteries, such MnFe2O4 NP hollow composite microspheres exhibit significantly improved 1

electrochemical performance in comparison with their solid counterparts and most MnFe2O4-based anodes reported to date, retaining a high reversible capacity of 730 mAh g-1 after 300 cycles at a current density of 2 A g-1. Furthermore, even when tested at an ultrahigh rate of 10 A g-1, MnFe2O4 NP hollow microspheres can still deliver a high specific capacity of 433 mAh g-1. The superior performance of MnFe2O4 NP hollow microspheres is attributable to their hollow superstructure, close-packed configuration of the constituent NPs, and uniform carbon coatings, which facilitate lithium-ion and electron transport while simultaneously alleviating the drastic volumetric change during cycling. Our work establishes that the optimized MnFe2O4 anode material offers great promise for high-performance lithium-ion batteries.

Keywords: MnFe2O4 nanoparticle, Hollow superstructure, Self-assembly, Carbon coating, Lithium-ion batteries

1. Introduction Over the past decade, rechargeable lithium-ion batteries (LIBs) have been widely used as the power source for portable electronic devices and electric vehicles because of their high energy density, long cycle life, and good rate capability [1, 2]. However, graphite serving as the predominant anode material for commercial LIBs suffers from a low theoretical capacity (372 mAh g-1), which cannot meet the continuously surging demands for next-generation LIBs [3]. In recent years, considerable efforts have been devoted to searching for competent anode materials to replace graphite [4-6]. Among 2

the alternative candidates, transition metal oxides (TMOs) offering more than twice the capacity of graphite have been widely investigated [7-10]. Compared with simple TMOs,

mixed

transition

metal

oxides

(MTMOs)

usually

exhibit

higher

electrochemical performance owing to their complex chemical compositions and the synergetic effects [11]. As a typical MTMO, MnFe2O4 has aroused much research interest in virtue of its high theoretical capacity (926 mAh g-1), low cost, and environmental friendliness [12-19]. Nevertheless, there are still some challenges associated with MnFe2O4 anodes including poor electronic conductivity, low lithium-ion diffusion coefficient, and drastic volume change during cycling, which significantly hamper the practical applications of MnFe2O4-based LIBs. An effective strategy to solve the above-mentioned problems is to construct hierarchical electrode materials with rationally designed structures [20-24]. In particular, hollow structured materials consisting of functional shells and large interior void space have been extensively explored because of their unique structural advantages for LIBs [25-28]. Specifically, the thin shell thickness can greatly reduce the diffusion pathways for both lithium-ions and electrons, thus leading to enhanced rate performance. Meanwhile, the interior void space can endow more lithium-storage sites and accommodate the huge volume variation induced by repeated lithiation and delithiation, thereby giving rise to enhanced cycling stability. To further improve the electrochemical performance of MnFe2O4 anodes, another feasible strategy is to combine active materials with carbonaceous components such as carbon nanotubes [29], graphene [30], and amorphous carbons [31] to form composite electrodes. The 3

presence of carbon not only enhances the electronic conductivity of active materials, but also provides a physical buffering layer to alleviate the structural strain caused by volume change during cycling, leading to the comprehensively enhanced electrochemical performance. Considering the aforementioned factors, hollow nanostructured materials in combination with proper carbon coatings are anticipated to render greater opportunities for improving the lithium-storage properties of MnFe2O4-based anodes. Herein, we develop a facile method for constructing carbon-coated hollow composite microspheres composed of close-packed MnFe2O4 NPs via an emulsion-based assembly process followed by in situ carbonization of the oleic acid (OA) ligands originally tethered to the NP surface. Compared with other strategies for fabricating hollow structured materials [32], our method involving the self-assembly of the pre-synthesized colloidal NPs allows for the wide tuning of the composition and size of the constituent NPs. When evaluated as anode materials for LIBs, the carbon-coated MnFe2O4 NP hollow microspheres exhibit excellent lithium-storage properties in terms of specific capacity, cycling stability, and rate performance, outperforming those of their solid counterparts and most MnFe2O4-based anodes reported previously. The superior performance of MnFe2O4 NP microspheres is attributed to their advantageous structures. Specifically, the hollow superstructure can effectively buffer the volume change during cycling and shorten the diffusion pathways for both lithium-ions and electrons. Additionally, the close-packed nature of the constituent NPs further facilitates lithium-ion and electron transport between 4

active materials [33]. Moreover, the uniform carbon shell coating derived from the native organic ligands not only improves the electronic conductivity of electrode materials, but also helps to prevent NPs from agglomeration and pulverization during repeated lithiation and delithiation. 2. Experimental 2.1. Chemicals Oleic acid (OA, 90%) and dodecyltrimethylammonium bromide (DTAB, 98%) were purchased from Sigma-Aldrich. Sodium oleate (70%) and 1-octadecene (ODE, 90%) were purchased from Aladdin. Anhydrous Iron(Ⅲ) chloride (FeCl3), manganese chloride tetrahydrate (MnCl2·4H2O), anhydrous ethanol, and hexane were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were used as received without further purification. 2.2. Sample preparation 2.2.1. Synthesis of MnFe2O4 NPs Monodisperse MnFe2O4 NPs stabilized by OA were prepared according to the literature

procedure

with

minor

modifications

[34].

The

mixed

metal

(Mn2+Fe3+)-oleate complex was prepared by the reaction of sodium oleate and the corresponding metal chlorides (the mixture of Mn2+ and Fe3+). In a typical synthesis for 14 nm MnFe2O4 NPs, 36 g of Mn2+Fe3+-oleate complex and 8.5 g of OA were dissolved in 150 g of ODE in a three-neck flask. The mixture was kept under vacuum at 120 °C for 30 min, and then was heated up to 320 °C and maintained at this temperature for 2 h under N2 atmosphere. After cooling down to room temperature, 5

isopropanol and ethanol were added to precipitate MnFe2O4 NPs. After centrifugation, the precipitated MnFe2O4 NPs were re-dispersed in hexane with a concentration of ~ 40 mg mL-1. 2.2.2. Self-assembly of MnFe2O4 NP microspheres MnFe2O4 NP microspheres were fabricated by an emulsion-based assembly method [35-40]. In brief, a hexane dispersion of MnFe2O4 NPs (10 mL) was added to an aqueous solution of DTAB (20 mg mL-1, 100 mL). The resulting mixture was subjected to homogenization with a homogenizer (5000 rpm) for 10 min, producing an oil-in-water (O/W) emulsion system. Under continuous mechanical stirring (400 rpm), the emulsion was then heated to 40 °C under N2 for 2 h, during which the evaporation of hexane led to solid microspheres composed of close-packed MnFe2O4 NPs. The resultant MnFe2O4 NP microspheres suspended in water could be readily collected by a magnet. Hollow microspheres of MnFe2O4 NPs were fabricated by a similar procedure, except that the mechanical stirring speed was increased to 800 rpm. To carbonize the surface-coating OA ligands, both solid and hollow MnFe2O4 NP microspheres were calcined at 500 °C under Ar atmosphere for 2 h, resulting in carbon-coated microspheres while retaining the NP ordering. 2.2.3. Selective removal of SEI layer To investigate the structural change of active materials after cycling, we removed the SEI layer according to the following procedure. Specifically, the electrodes after long-term cycling were immersed in acetonitrile for 48 h in an argon-filled glovebox to remove the electrolyte residues. Afterwards, the electrodes were taken out of the 6

glovebox and washed with water to remove the residual Li2O species. 2.3. Characterization Scanning electron microscopy (SEM) and high resolution SEM (HRSEM) images were obtained using a Zeiss Ultra-55 microscope operated at 5 kV. Energy dispersive X-ray spectroscopy (EDS) and elemental mapping were carried out on the equipped Oxford X-Max 50 detector. Transmission electron microscopy (TEM) images were obtained on a Tecnai G2 F20 S-TWIN microscope operated at 200 kV. Small-angle X-ray scattering (SAXS) was tested on a Nanostar U small-angle X-ray scattering system. X-ray diffraction (XRD) was conducted on a Bruker D4 X-ray diffractometer. Nitrogen adsorption-desorption isotherms were tested on a Quantachrome AUTOSORB-IQ instrument. X-ray photoelectron spectroscopy (XPS) was carried out on an SSI S-Probe XPS spectrometer equipped with a monochromatic Al Kα source (1486.6 eV). Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer Pyris 1 TGA analyzer. 2.4. Electrochemical measurements The electrochemical measurements were carried out using a coin-cell (CR 2016) configuration and the working electrodes were prepared by a slurry coating procedure. The slurry was prepared by mixing active material, conductive carbon (Super P), and poly (vinylidene fluoride) (PVDF) binder with a weight ratio of 7:2:1 and was then uniformly pasted on copper foil. The typical mass loading of the electrode materials was about 1.8 mg cm-2. The electrodes were then dried at 90 °C for 12 h in vacuum. The electrolyte was 1.0 M LiPF6 dissolved in a 1:1 (v/v) mixture of ethylene 7

carbonate (EC) and ethyl methyl carbonate (EMC), and a microporous polypropylene film (Celgard-2300) was used as the separator. Coin cells were assembled in an Ar-filled glovebox with lithium foil as the counter and reference electrode. The galvanostatic charge/discharge measurements were carried out on a Neware cell test instrument at different current densities in a voltage window of 0.01-3.00 V (vs Li+/Li). Cyclic voltammetry (CV) was conducted on an Autolab 204 N electrochemical workstation in a potential range of 0-3.0 V (vs Li+/Li) at a scanning rate of 0.1 mV s-1. 3. Results and discussion The fabrication of carbon-coated MnFe2O4 NP microspheres is illustrated in Scheme 1. MnFe2O4 NPs stabilized by OA are synthesized by the thermal decomposition of Mn2+Fe3+-oleate complex. Fig. S1 shows the TEM image of the as-synthesized MnFe2O4 NPs with a mean size of 14 nm. Solid MnFe2O4 NP microspheres are obtained by an emulsion-based self-assembly process. Briefly, a hexane dispersion of MnFe2O4 NPs is first mixed with an aqueous solution of DTAB, and the following vigorous stirring leads to an oil-in-water (O/W) emulsion system, in which the hydrophobic MnFe2O4 NPs are confined within the oil (hexane) droplets. The subsequent evaporation of hexane drives NPs to assemble within the droplets, yielding solid microspheres composed of close-packed NPs. It is interesting to find that MnFe2O4 NP microspheres undergo a solid-to-hollow structural transformation by simply increasing the mechanical stirring speed while keeping other parameters constant. Presumably, the evolution of W/O/W double emulsions generated upon the 8

increase of the stirring speed is responsible for the confined assembly of supracrystalline shells. For both solid and hollow MnFe2O4 NP microspheres, heat treatment at 500 °C under Ar leads to the carbonization of OA ligands without structural failure, yielding carbon-coated composite microspheres. Fig. 1a shows the low-magnification SEM image of the carbon-coated MnFe2O4 NP solid microspheres and the diameter of which ranges from 200 nm to 1.5 µm with an average diameter of ~700 nm (Fig. S2a). A typical HRSEM image in Fig. 1b reveals

that

each

individual

microsphere

is

composed

of

numerous

hexagonally-packed NPs and possesses a three-dimensional (3D) superlattice nature, which is further supported by TEM (Fig. 1c). SAXS is carried out to analyze the long-range ordering of MnFe2O4 NPs. According to the well-resolved diffraction peaks it can be confirmed that the as-prepared MnFe2O4 NP microspheres adopt a typical face-centered-cubic (FCC) structure (Fig. 1g, black curve). Hollow structured MnFe2O4 NP microspheres are produced by simply increasing the mechanical stirring speed from 400 to 800 rpm with other experimental parameters fixed. As shown in Fig. 1d, the presence of holes on the surface of NP microspheres is indicative of their hollow interiors. The diameter of hollow microspheres ranges from 300 nm to 2.0 µm, with an average diameter of ~900 nm (Fig. S2b). Fig. 1e shows the HRSEM image of a broken microsphere, the shell of which is constructed from multilayer, close-packed MnFe2O4 NPs and has a typical thickness of ~60 nm. Similar to their solid counterparts, hollow structured MnFe2O4 NP microspheres also possess a long-range ordered FCC superlattice structure as evidenced by SAXS (Fig. 1g, red curve). Fig. 1f 9

shows the TEM image of hollow microspheres, further confirming the presence of large void space inside the sphere. TEM survey indicates that the yield of hollow microspheres is around 80%. EDS elemental mapping provides clear evidence of the coexistence of C, Mn, Fe, and O (Fig. 1h and 1i), which are distributed homogeneously across the entire microsphere. The formation of hollow structured microspheres appears to be associated with the morphological variation of the emulsions caused by the different stirring speeds. To further investigate the influence of the stirring speed on the structure of NP microspheres, we increase the stirring speed from 800 to 1200 rpm. As shown in Fig. S3, most hollow NP microspheres crack under this condition, probably due to the unstable emulsions caused by the too high stirring speed. This result also confirms that the mechanical stirring speed indeed has a great influence on the morphology of NP microspheres. We hypothesize that the stirring speed of 800 rpm is beneficial for the inclusion of water into the oil droplets for forming stable W/O/W double emulsions, which are responsible for the evolution of hollow NP microspheres as observed. However, we should emphasize that further work needs to be carried out to elucidate the formation mechanism of hollow structured microspheres. XPS is conducted to measure the composition of hollow microspheres and the oxidation states of metal species. As depicted in Fig. 2a, the peaks of Fe 3p, C 1s, O 1s, Mn 2p, and Fe 2p are clearly obseved in the XPS spectrum, corroborating the existence of Fe, C, O, and Mn, in agreement with EDS results. In addition, the Mn 2p spectrum shows two major peaks at 641 and 653 eV (Fig. 2b), which are ascribed to 10

Mn 2p3/2 and Mn 2p1/2, respectively, consistent with the standard XPS features for Mn2+ [41]. Furthermore, the Fe 2p spectrum exhibits two characteristic peaks (711 and 724 eV) ascribed to Fe 2p3/2 and Fe 2p1/2 (Fig. 2c), respectively, indicating the presence of Fe3+ [42]. The XRD patterns of both solid and hollow microspheres are displayed in Fig. 2d. All the diffraction peaks can be well indexed to cubic spinel-type MnFe2O4 (JCPDS card No.73-1964) and no other peaks are observed, indicating that the carbonization of OA ligands do not change the crystal phase of MnFe2O4 NPs. The carbon content of carbon-coated MnFe2O4 microspheres is evaluated by TGA in air as shown in Fig. 2e. Assuming the conversion of MnFe2O4 to Mn2O3 and Fe2O3 by oxidation [43], the carbon content in hollow microspheres is determined to be ~ 8% (red curve, see Supporting Information for calculation details), close to that in solid microspheres (black curve). Fig. 2f shows the nitrogen adsorption-desorption isotherms of MnFe2O4 NP microspheres. Both samples exhibit a type-VI hysteresis curve, indicative of the presence of mesopores, which are presumably attributed to the interstitial space between neighboring NPs. The BET specific surface area of hollow MnFe2O4 NP microspheres is 39 m2 g-1, slightly higher than that (25 m2 g-1) of their solid counterparts. The lithium-storage properties of carbon-coated MnFe2O4 NP microspheres are evaluated based on a half-cell configuration, with lithium foil as counter and reference electrode. Fig. 3a shows the first three CV curves of hollow MnFe2O4 NP microspheres. The CV features are in accordant with those of solid microspheres (Fig. 3b) and other MnFe2O4-based anodes reported previously [13-15]. Specifically, the 11

large reduction peak around 0.50 V in the first cathodic scan is ascribed to the reduction of MnFe2O4 by Li (MnFe2O4 + 8Li → Mn + 2Fe + 4Li2O) as well as the formation of SEI layer [17]. The anodic peak at 1.65 V in the first scan corresponds to the oxidation of Mn to MnO and Fe to Fe3O4 [17]. Moreover, this anodic peak is positively shifted in the subsequent sweeps because of the polarization of active materials [44]. It should be noted that the CV curves nearly overlap upon further scans, implying excellent electrochemical reversibility of hollow MnFe2O4 NP microspheres. The galvanostatic discharge/charge voltage profiles of MnFe2O4 NP microspheres at a current density of 0.2 A g-1 are presented in Fig. 3c and 3d. For both solid and hollow microspheres, a prominent voltage plateau at about 0.77 V is observed during the first discharge process and it shifts to 1.0 V in the subsequent cycles, consistent with MnFe2O4-based anodes reported previously [13-15]. The cycling performance of carbon-coated MnFe2O4 NP microspheres at a current density of 0.2 A g-1 is given in Fig. 4a. The first discharge capacity of hollow microspheres is 1390 mAh g-1, while a reversible capacity of 1000 mAh g-1 is retained in the subsequent charge process, corresponding to an initial Coulombic efficiency of 71.9%. This capacity loss, which is commonly observed in many TMO-based anode materials [45-47], is presumably caused by electrolyte decomposition and the formation of SEI layer. Despite the irreversible initial capacity loss, hollow MnFe2O4 NP microspheres exhibit outstanding cycling stability, as manifested by the high capacity of 970 mAh g-1 after 100 cycles and stabilized Coulombic efficiency over 98% from the fifth cycle. In comparison, the first discharge and charge capacities of solid 12

MnFe2O4 NP microspheres are 1295 and 879 mAh g-1, respectively. Accordingly, the initial Coulombic efficiency of solid microspheres is 67.9%, much lower than that of their hollow counterparts. Moreover, solid microspheres can only maintain a capacity of 740 mAh g-1 after 100 cycles (Fig. 4a, black curve), which is also much lower than that of hollow microspheres. The rate capability of carbon-coated MnFe2O4 NP microspheres is studied at different current densities ranging from 0.2 to 10 A g-1 (Fig. 4b). Apparently, hollow microspheres exhibit a significantly higher capacity than their solid counterparts at each current density. The average specific capacity of hollow microspheres is 960, 820, 766, 682 and 554 mAh g-1 at 0.2, 0.5, 1, 2, and 5 A g-1, respectively. Remarkably, even when tested at an ultrahigh current density of 10 A g-1, hollow microspheres can still deliver a stable capacity of 433 mAh g-1, which is even higher than the theoretical capacity of graphite (372 mAh g-1). Additionally, upon switching the current density back to 0.2 A g-1, the discharge capacity recovers to 1028 mAh g-1. These results indicate that the hollow interiors indeed contribute significantly to the specific capacity and rate capability of MnFe2O4 NP microspheres. Fig. 4c shows the long-term cycling performance of solid and hollow MnFe2O4 NP microspheres at a high current density of 2 A g-1. For hollow microspheres (red curve), it is noted that the initial capacity drops in the first 40 cycles followed by a gradual recovery until about 200 cycles, maintaining a high reversible capacity of 730 mAh g-1 after 300 cycles, which is much higher than that of their solid counterparts (black curve). The visible capacity growth with cycling is primarily driven by the 13

activation and stabilization of electrode materials. Similar phenomena have been observed for many other TMO-based anodes [7, 48-50]. Based on the above results, it is evident that the carbon-coated MnFe2O4 NP hollow microspheres demonstrate outstanding electrochemical performance in terms of specific capacity, cycling stability, and rate capability, outperforming those of most MnFe2O4-based anodes reported in the literature (Table S1) [12-19]. These results also demonstrate that the elaborate structure design of electrode materials is indeed beneficial for improving their lithium-storage properties. To further understand the superior electrochemical performance exhibited by MnFe2O4 NP microspheres, SEM is employed to investigate the structural evolution of active materials after cycling. Fig. 5a shows that most solid microspheres crack after long-term cycling (2 A g-1 after 300 cycles), perhaps due to the accumulated strain caused by the large volume change during cycling. A typical HRSEM image of a cracked solid microsphere is presented in Fig. 5b. By comparison, MnFe2O4 NP hollow microspheres still possess the well-defined spherical morphology without cracking under the identical cycling conditions (Fig. 5c and 5d). The significantly improved structural integrity of MnFe2O4 NP hollow microspheres upon cycling is attributed to their large hollow interiors, which effectively ameliorate the drastic volume variation during lithiation and delithiation, thereby giving rise to the improved electrochemical performance as described above. 4. Conclusions

14

In summary, carbon-coated MnFe2O4 NP hollow composite microspheres are successfully synthesized by an emulsion-based assembly process followed by thermal treatment. Benefiting from their unique superstructure and excellent structural stability, the as-synthesized MnFe2O4 NP hollow microspheres exhibit greatly enhanced electrochemical performance over their solid counterparts and most MnFe2O4-based anodes developed previously. Notably, a high specific capacity of 730 mAh g-1 is retained after 300 cycles at a high current density of 2 A g-1, and a reversible capacity of 433 mAh g-1 can be reached even at an ultrahigh rate of 10 A g-1. The synthetic strategy proposed in our work is expected to be scalable and applicable to a variety of TMO and MTMO NPs. The resulting NP hollow microspheres may find a wide range of applications such as energy storage, catalysis, water treatment, and drug delivery.

15

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Fig. 1. (a) SEM, (b) HRSEM, and (c) TEM images of carbon-coated MnFe2O4 NP solid microspheres; (d) SEM, (e) HRSEM, and (f) TEM images of carbon-coated MnFe2O4 NP hollow microspheres; (g) SAXS patterns of solid and hollow microspheres; (h) SEM image and (i) the corresponding EDS elemental mapping of a single hollow microsphere.

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Fig. 2. XPS spectra of carbon-coated MnFe2O4 NP hollow microspheres: (a) survey scan, (b) Mn 2p spectrum, (c) Fe 2p spectrum; (d) XRD patterns, (e) TGA curves, and (f) nitrogen adsorption-desorption isotherms of MnFe2O4 NP microspheres.

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Fig. 3. Cyclic voltammograms of (a) hollow and (b) solid microspheres of MnFe2O4 NPs; Galvanostatic discharge/charge voltage profiles of (c) hollow and (d) solid microspheres at 0.2 A g-1.

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Fig. 4. (a) Cycling performance of carbon-coated MnFe2O4 NP microspheres at 0.2 A g-1; (b) Rate capability of MnFe2O4 NP microspheres at various current densities from 0.2 to 10 A g-1; (c) Long-term cycling performance of carbon-coated MnFe2O4 NP microspheres at 2 A g-1.

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Fig. 5. (a) SEM and (b) HRSEM images of the deeply cycled, MnFe2O4 NP solid microspheres. (c) SEM and (d) HRSEM images of MnFe2O4 NP hollow microspheres under the identical cycling conditions.

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Scheme 1. Schematic illustration of the preparation of carbon-coated MnFe2O4 NP composite microspheres.

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