Superior electrochemical properties of micron-sized aggregates of (Co0.5Fe0.5)3O4 hollow nanospheres and graphitic carbon

Accepted Manuscript Superior Electrochemical Properties of Micron-Sized Aggregates of (Co0.5Fe0.5)3O4 Hollow Nanospheres and Graphitic Carbon Young Jun Hong, Seung-Keun Park, Kwang Chul Roh, Jung-Kul Lee, Yun Chan Kang PII: DOI: Reference:

S1385-8947(18)30606-5 https://doi.org/10.1016/j.cej.2018.04.040 CEJ 18840

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

30 January 2018 23 March 2018 6 April 2018

Please cite this article as: Y.J. Hong, S-K. Park, K.C. Roh, J-K. Lee, Y.C. Kang, Superior Electrochemical Properties of Micron-Sized Aggregates of (Co0.5Fe0.5)3O4 Hollow Nanospheres and Graphitic Carbon, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.04.040

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Superior Electrochemical Properties of Micron-Sized Aggregates of (Co0.5Fe0.5)3O4 Hollow Nanospheres and Graphitic Carbon

Young Jun Honga,b, Seung-Keun Parka, Kwang Chul Rohc, Jung-Kul Leed,*, and Yun Chan Kanga,*

a

Department of Materials Science and Engineering, Korea University, Anam-Dong,

Seongbuk-Gu, Seoul 136-713, Republic of Korea b

Electric Powertrain Core Technology Team, Division of Electric Powertrain, Hyundai Mobis

Co. Ltd., Yongin-si, Gyeonggi 16891, Republic of Korea c

Energy and Environmental Division, Korea Institute of Ceramic Engineering and

Technology (KICET), Jinju, Gyeongnam 52851, Republic of Korea d

Department of Chemical Engineering, Konkuk University, Hwayang-dong, Gwangjin-gu,

Seoul 143-701, Republic of Korea

*Corresponding author. E-mail addresses: [email protected] (Yun Chan Kang)

1

[email protected]

(Jung-Kul

Lee),

Abstract Morphology-controlled micron-sized aggregates consisted of hollow nanospheres and graphitic carbon are considered to be efficient electrode materials for lithium-ion batteries because the advantages of hollow nanospheres are combined with those of micron-size powders with easy processability. In this study, carbon microspheres with extremely large surface area of 3350 m2 g-1 are successfully used as templates to synthesize (Co0.5Fe0.5)3O4graphitic carbon (CoFeO-GC) composite microspheres, which in turn, are composed of hollow nanospheres. The CoFe alloy nanospheres act as catalyst in formation of graphitic carbon during reduction process and transform into metal oxide hollow nanospheres after oxidation by nanoscale Kirkendall diffusion. Owing to their unique structure, CoFeO-GC composite microspheres show lithium-ion storage performances superior to those of the CoFeO-amorphous carbon composites with ultrafine nanocrystals and dense structure. The CoFeO-GC composite microspheres have extremely high capacities of 1072 and 681 mA h g-1 at current densities of 1 and 3 A g-1, respectively, after 350 cycles. This hybrid structure employs synergistic effect of the hollow nanosphere aggregate and high content of graphitic carbon with high electrical conductivity, resulting in superior cycling and rate performances, when tested as anode materials for lithium-ion batteries.

Keywords: carbonaceous microspheres, nanostructured material, Kirkendall diffusion, carbon composite, lithium-ion batteries

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1. Introduction Nanostructured transition metal compounds with controlled morphology and internal architecture have received much attention because of their widespread applications in different areas including energy storage [1-12]. In particular, transition metal compounds with hollow structures have been shown to be efficient anode materials for rechargeable batteries bother due to their ability to accommodate stress created during repeated cycling and the short Li+ diffusion distance, which resulted in improved cycling and rate performances [13-16]. Various synthesis methods with or without removable templates, have been used to prepare single and mixed transition metal compounds with hollow structure [1721]. The preparation of hollow ultrafine nanospheres with sizes below several tens of nanometers was made possible mainly by making use of the Kirkendall effect [22-24]. However, due to the strong van der Waals forces between the nanoparticles, stable dispersions to enable the printing of electrodes, as well as a high tap density, were difficult to obtain. Recently, morphology-controlled micron-sized aggregates of hollow nanospheres or nanoplates were synthesized by applying the Kirkendall effect to combine the advantages of a hollow nanosphere morphology with those of micron-sized powders [25-29]. Liu et al. introduced a strategy using metal organic framework (MOF) to fabricate sulfide-based electrodes for Li-ion batteries (LIBs) and obtained superior performances [26]. The composite structure with hollow Co 9S8 nanoparticles embedded in graphitic carbon nanocages possessed several distinct advantages including ultra-small hollow building units, a porous hollow interior in the hybrid structure, and the presence of a compliant conductive carbon matrix. Gao et al. designed a hierarchical composite structure using an iron-based MOF (MIL-88) as the template to integrate hollow nanostructures in carbon [27]. A unique hierarchical structure composed of NiFe2O4 nanospheres anchored on amorphous carbon 3

nanorods was evaluated as an anode material for LIBs, where a superior electrochemical performance with a stable average capacity of 1045 mA h g-1 after 400 cycles at 1 C was obtained. Cho et al. synthesized hollow nanosphere aggregates by introducing the Kirkendall effect in a spray pyrolysis process [28]. The hollow Fe2O3 nanosphere aggregates exhibited better electrochemical properties when used as anode material for LIBs, when compared to anodes fabricated from solid Fe2O3 powder. Park et al. designed and synthesized hierarchically structured SnO2 microspheres formed from hollow SnO2 nanoplates as an efficient anode material for LIBs [29]. The ultrafine nanosheets contained empty voids, which allowed excellent lithium-ion storage performance even at high current densities. Carbonaceous microspheres have been commonly used as sacrificial templates to prepare uniform metal oxide microspheres with unique structures such as hollow, yolk-shell, and multi-shell [4,30,31]. However, the use of carbonaceous microspheres in the fabrication of micron-sized aggregates consisted of hollow primary particles via Kirkendall process has rarely succeeded. It is due the rapid heat evolution that occurs at the burning stage of carbon materials during an oxidation process, which results in sintering of primary particles. Thus, it is highly desirable to develop a new synthetic process that can precisely control the structures of primary particles without a rapid burning of carbon templates. In this study, carbon microsphere templates with an extremely high surface area of 3000 m 2 g-1 were successfully used for the first time, to synthesize mixed transition metal oxide microspheres containing Co and Fe, which in turn, are composed of hollow nanospheres. Carbon microspheres with high loadings of metal salts of Co and Fe were transformed into microspheres with or without graphitic carbon (GC) coating layer by a two-step posttreatment process. The CoFe alloy nanospheres formed during the reduction process were transformed by nanoscale Kirkendall diffusion into their corresponding hollow metal oxide

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nanospheres. The electrochemical properties of these uniquely structured microspheres with or without a GC layer coating, were investigated for lithium-ion storage.

2. Experimental section Synthesis of CoFeO-GC and CoFeO-C Microspheres: Porous carbon microsphere templates were synthesized by the carbonation and activation of commercial phenol resin spheres [32]. 1 g of the carbon microsphere template, which was dried overnight in a vacuum oven at 60 °C, was placed in a dry mortar. Next, 0.15 g (0.5 mmol) of cobalt nitrate hexahydrate and 0.202 g (0.5 mmol) of iron nitrate nonahydrate were dissolved into 3 ml of ethanol. Drops of the solution were added to the porous carbon template microspheres to allow impregnation by capillary force, followed by drying to evaporate the ethanol solvent. The "drop & dry" procedure was repeated until the total volume of solution was completely impregnated into the porous carbon microspheres. The first-step post-treatment consisted of heating at 500 and 1000 °C for 10 and 3 h, respectively, under 10 % H2/Ar atmosphere, which resulted in the formation of CoFe-C-GC composite microspheres. Next, CoFeO-GC composite microspheres were formed by a second-step post-treatment at 350 °C in air for 5 h. CoFeO-C microspheres were produced by a one-step post-treatment of the carbon template microspheres impregnated with metal salts at 350 °C in air for 5 h. Materials Characterization: The crystal structures of the microspheres were investigated using X-ray diffraction (XRD, X’Pert PRO MPD) with Cu Kα radiation (λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). The morphologies of the nanofibers were characterized using scanning electron microscopy (SEM, TESCAN VEGA3-SB) and field emission transmission electron microscopy (FE-TEM, JEOL, JEM-2100F) at a working voltage of 200 kV. The surface areas of the microspheres were measured using the Brunauer– Emmett–Teller (BET) method with N2 as the adsorbate gas. Thermogravimetric analysis 5

(TGA, TA Instrument Q600, PH407 PUSAN KBSI) was performed in air at a heating rate of 10 °C min-1. Anode Preparation, Battery Assembly and Testing: The electrochemical properties of the microspheres were analyzed by constructing a 2032-type coin cell. The anode was prepared by mixing the active material, carbon black, and sodium carboxymethyl cellulose (CMC) in the weight ratio of 7:2:1. Li metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was 1 M LiPF6 dissolved in a 1:1 vol % mixture of fluoroethylene carbonate/dimethyl carbonate (FEC/DMC). The charge/discharge characteristics of the samples were determined by cycling in the potential range 0.01–3.00 V at fixed current densities. Cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s-1; the diameter of the anode is 1.4 cm, and the mass loading was ~ 1.2 mg cm-2. Electrochemical impedance spectra of the nanofibers were analyzed over the frequency range 0.01 Hz–100 kHz at room temperature with a signal amplitude of 1 mV.

3. Results and discussion The formation of the uniquely structured CoFeO-GC composite microsphere using a micron-sized carbonaceous template is shown in Scheme 1. During the repeated “drop and dry” process, the meso- and macropores of the carbonaceous microsphere with ultrahigh surface area and uniform morphology, were filled with cobalt and iron metal salts. The first-step post-treatment at 1000 °C under 10 % H 2/Ar atmosphere formed CoFe-C-GC composite microspheres. The decomposition of the metal salts followed by simultaneous reduction and alloying reactions resulted in the formation of CoFe alloy nanocrystals. The reduction process under 10 % H2/Ar atmosphere produced uniformly-sized CoFe nanocrystals, which were dispersed uniformly inside the carbonaceous microsphere. Amorphous carbon covering the CoFe nanocrystals 6

transformed into graphitic carbon due to the presence of the metal nanocatalyst. The growth of larger CoFe nanocrystals was achieved by consuming smaller ultrafine metal nanocrystals, and resulted in ultrafine GC hollow balls. The second-step of the post-treatment of CoFe-C-GC composite microsphere at 350 °C under air atmosphere produced CoFeO-GC composite microspheres. The complete conversion of CoFe alloy nanocrystals by nanoscale Kirkendall diffusion resulted in forming hollow CoFeO nanospheres. Oxidation under controlled oxygen content led to the slow combustion of amorphous carbon without burning. Consequently, CoFeO-GC composite microspheres with a uniform distribution of CoFeO hollow nanospheres and GC layers were produced by a simple two-step heat treatment of carbonaceous microspheres impregnated with metal salts. The morphologies of the carbonaceous microspheres before and after the impregnation of metal salts are shown in Fig. S1. The similar morphologies of the microspheres before and after metal salts loading verify the complete impregnation of the metal salts into the carbonaceous microspheres (Fig. S1a and b). N2 gas adsorption and desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution of carbonaceous microspheres are shown in Fig. S1c and d. The surface area and volumes of micro-, meso- and macropores in the microspheres were 3350 m 2 g-1, and 1.17, 0.15, and 0.10 cm 3 g−1, respectively (Fig. S1c and d). In addition, their total pore volume is as high as 1.93 cm 3 g−1, providing sufficient spaces for the complete impregnation of the metal salts. Correspondingly, the amount of metal salts impregnated into 1g of carbonaceous microspheres was 0.35 g. The calculated volume of the impregnated metal salts was 0.2 cm 3 g-1 and the corresponding filling rate of the metal salt into the empty pores of the carbonaceous template was 14.1 %. The metal

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salts were impregnated mainly within the meso- and macropores of the carbonaceous microspheres. The morphology and crystal structure of the composite microspheres (denoted as CoFe-GC-C-10) obtained by heating at 1000 °C under 10 % H 2/Ar atmosphere, are shown in Fig. 1 and S2, respectively. The broad XRD peaks in Fig. S2 confirm that ultrafine CoFe alloy nanocrystals and graphic carbon layers are formed during the first step of the post-treatment process. The SEM and low resolution TEM images shown in Fig. 1a and b, respectively, confirm the formation of CoFe-GC-C-10 composite microspheres with uniform external and internal morphologies. The fractured surface of the microsphere shown in Fig. S3a shows uniformly distributed ultrafine nanocrystals all over the composite microsphere. The TEM images in Fig. 1c and S3b show the distribution of CoFe nanocrystals, amorphous carbon, and GC layers. The high resolution TEM image shown in Fig. 1d reveals clear lattice fringes separated by 0.20 and 0.34 nm, which correspond to the interplanar spacing in (110) and (002) crystal planes of CoFe alloy and graphitic carbon, respectively. The SAED pattern and elemental mapping images shown in Fig. 1e and f, respectively, further confirm the formation of CoFe alloy nanocrystals and GC layers, and their uniform distribution in CoFe-GC-C-10 composite microsphere. The morphologies and crystal structures of the composite microspheres obtained following oxidation of CoFe-GC-C-10 microsphere at 350 °C in air are shown in Fig. 2 and S4, respectively. The flow rate of air was fixed at 30 mL min-1 to eliminate amorphous carbon and prevent burning of carbon because burning of carbonaceous material causes quick combustion of graphitic carbon as well as amorphous carbon and produces carbon-free metal oxide microspheres. The complete oxidation of the CoFe alloy nanocrystals in Co/Fe mole ratio of 1/1 produces ultrafine nanocrystals of CoFe oxides. The densely structured CoFe8

GC-C-10 composite microspheres are transformed by the heat treatment into porous CoFeOGC (denoted as CoFeO-GC-10) microspheres as shown by SEM and TEM images in Fig. 2a and b, respectively. The complete oxidation of the CoFe alloy nanocrystals by nanoscale Kirkendall diffusion and combustion of amorphous carbon results in porous composite microspheres comprising ultrafine hollow nanospheres [25,33,34]. The TEM images shown in Fig. 2c and d reveal the presence of hollow metal oxide nanospheres and empty GC layers. The high resolution TEM image shown in Fig. 2d shows clear lattice fringes separated by 0.29 and 0.34 nm, which correspond, respectively, to the interplanar distances in (220) and (002) crystal planes in cubic (Co0.5Fe0.5)3O4 and GC. The XRD and SAED pattern shown in Fig. S2a and 2e, respectively, confirm the complete conversion of the CoFe alloy to form the hollow (Co0.5Fe0.5)3O4. Furthermore, the enlarged XRD pattern (Fig. S4b) showed that all diffraction peaks of CoFeO-GC-10 microspheres shifted to lower angles compared to those of CoFe2O4, which means the formation of (Co 0.5Fe0.5)3O4 solid-solution. The elemental mapping images shown in Fig. 2f show that a high amount of carbon is uniformly distributed over the CoFeO composite microsphere. The XPS spectra of CoFeO-GC-10 composite microspheres are given in Fig. 3. The XPS survey spectrum confirm the presence of Fe, Co, C and O (Fig. 3a). In the Fe 2p spectrum (Fig. 3b), main peaks are observed at binding energies of 711.0 eV (Fe 2p3/2) and 724.7 eV (Fe 2p1/2), and the peaks are well fitted into Fe2+ and Fe3+ together with shake-up satellite. Similarly, in the Co 2p spectrum (Fig. 3c), the Co 2p3/2 and 2p1/2 peaks at 780.4 and 795.9 eV, respectively, are deconvoluted into three peaks, which correspond to Co3+, Co2+ and satellite. These results underline the formation of (Co0.5Fe0.5)3O4 solid-solution [35,36]. Furthermore, in the C 1s XPS spectrum (Fig. 3d), along with peaks corresponding to sp2-bonded carbon (C–C), epoxy and alkoxy groups (C–O) are observed at 284.5 and 285.9, respectively [37]. The highest intensity peak corresponds to the C–C bond, indicating the formation of graphitic carbon. The TGA curve of CoFeO-GC-10 9

composite microspheres (Fig. S5) shows a two-step weight loss. The first and second weight loss steps occurring around 322 and 464 ℃ are attributed to the combustion of amorphous and graphitic carbon, respectively. The combustion temperature of graphitic carbon is higher than that of amorphous carbon owing to its high thermal stability. This different thermal behavior has been reported in previous reports [38]. Thus, from the TG analysis, the amorphous and graphitic carbon contents are estimated to be 3.2 and 15.3 wt%, respectively. The morphologies of the composite microspheres (denoted as CoFe-GC-C-5) obtained by post-treatment at 500 °C under 10% H 2/Ar atmosphere, of carbonaceous microspheres impregnated with metal salts, are shown in Fig. S6. The corresponding XRD pattern shown in Fig. S2 confirms the formation of CoFe alloy nanocrystals and GC layers at a relatively low post-treatment temperature of 500 °C. The composite microspheres post-treated under 10% H 2/Ar atmosphere at 500 and 1000 °C show similar morphologies as deduced from SEM images. The overall morphology and crystal structure of CoFeO-GC composite microspheres (denoted as CoFeO-GC-5) obtained by the oxidation of CoFe-GC-C-5 composite shown in Fig. 4 and S4, respectively, are similar to those of CoFeO-GC-10 composite microspheres. CoFeGC-C-5 composite microsphere are made of ultrafine hollow nanospheres.

Here too,

it is seen that ultrafine CoFe nanocrystals formed at 500 °C transform into CoFeO hollow nanospheres by nanoscale Kirkendall diffusion. The mean sizes of the hollow nanospheres calculated from the TEM images of CoFeO-GC-5 and CoFeO-GC-10 are found to be 27 and 30 nm, respectively. The high resolution TEM image (Fig. 4c and d) and the SAED pattern (Fig. 4e) confirm the formation of microspheres of (Co0.5Fe0.5)3O4-GC composite. However, the elemental mapping images and TG curve shown in Fig. 4f and S5, respectively, reveal a low carbon content of 5.1 %. Thus, at a lower post-treatment temperature of 500 °C, a smaller amount of amorphous carbon is 10

transformed into graphitic carbon. Subsequent combustion of this amorphous carbon by oxidation at 350 °C results in CoFeO-GC composite microspheres with low GC content. The morphology and crystal structure of the composite microspheres (denoted as CoFeO-C) obtained by a one-step post-treatment of the carbonaceous microspheres impregnated with metal salts at 350 °C in air are shown in Fig. 5 and S4, respectively. It is seen that the conversion of amorphous carbon into graphitic carbon does not occur during this one-step post-treatment process in air. Consequently, the resulting CoFeO-C composite microspheres have a low amorphous carbon content of 3.2 wt%, as confirmed by TG analysis shown in Fig. S5. The step-wise weight loss by the combustion of amorphous carbon below 670 °C observed in this case is due to the dense structure of the composite microspheres. The CoFeO-C composite microspheres however, have external morphology and crystal structure similar to those of CoFeO-GC composite microspheres. The CoFeO-C composite microspheres also show a porous morphology due to the elimination of amorphous carbon during combustion. The TEM images shown in Fig. 5b and c show the presence of the ultrafine nanocrystals with sizes below 7 nm. Thus, the oxidative elimination of amorphous carbon without burning enables the formation CoFeO-C composite microspheres with ultrafine nanocrystals. The elemental mapping images shown in Fig. 5d confirm the uniform distribution of a low amorphous carbon content within the CoFeO-C composite microspheres. The Raman spectra of three samples are shown in Fig. S7. Two major peaks located at 1345 cm–1 and 1587 cm–1 are common observed in the Raman spectrum, which can be assigned to the D and G bands of carbon, respectively. The G band is characteristic of sp2 hybridized graphitic carbon [39,40]. The peak intensity ratios (ID/IG) of the D to G bands of CoFeOGC-10, CoFeO-GC-5, and CoFeO-C microsphere, which is closely related to the degree of graphitization, is approximately 0.74, 0.94, and 1.27, respectively. The lowest value 11

of CoFeO-GC-10 microsphere indicates a large amount of GC in the composite, which is in well agreement with other results. The N2 adsorption and desorption isotherms and BJH pore size distributions of the composite microspheres are shown in Fig. S8. CoFeO-GC-10, CoFeO-GC-5, and CoFeO-C composite microspheres have mesopores with a pore size distribution with maximum peaks corresponding to pore sizes of 4, 12, and 6 nm, respectively. In all the three samples, the mesopores mainly originate either from the CoFeO hollow nanospheres or from nanoparticles. Mesopores with a maximum peak intensity pore size of 4 nm, as well as micropores observed in CoFeO-GC-10 composite microspheres, can be attributed to the high GC content. The BET surface areas of CoFeO-GC-10, CoFeO-GC-5, and CoFeO-C composite microspheres are calculated to be 279, 111, and 110 m2 g-1, respectively. The electrochemical properties of CoFeO-GC-10 composite microspheres were compared with those of CoFeO-GC-5 and CoFeO-C composite microspheres to evaluate their respective capacities for lithium-ion storage. The cyclic voltammetry (CV) curves of the three samples over the potential range of 0.01–3.00 V (vs. Li/Li+) at a scan rate of 0.1 mV s-1 for the first four cycles are shown in Fig. 6. The CV curves show one broad reduction peak at around 0.75 V in the first cathodic scan, principally corresponding to the reduction of CoFe oxides to form metallic nano-grains of Co0 and Fe0 and amorphous Li2O [33,41,42]. The two broad oxidation peaks observed at ~1.5 and 2.1 V for the three samples during the first charging process are attributed, respectively, to the oxidation of Fe0 to Fe3+ and Co0 to Co2+ [41,43]. The reduction peaks observed at higher potentials of ~0.8 and 1.5 V from the second cycle onwards, arise from the conversion of ultrafine Fe2O3 and CoO nanocrystals, respectively [33,41,43]. Meanwhile, the CV curves of CoFeO-GC-10 and CoFeO-GC-5 microspheres showed an extra pair of redox peaks located at 0.03 and 0.1 V, which 12

correspond, respectively, to the lithiation and delithiation of GC layers [44,45]. The low intensity redox peaks located at 0.03 and 0.1 V point to the low GC content in CoFeO-GC-5 microspheres. The initial charge and discharge curves of the three samples at a constant current density of 1 A g-1 are shown in Fig. S9. The initial discharge capacities of CoFeO-GC-10, CoFeO-GC-5, and CoFeO-C composite microspheres are found to be 1389, 1376, and 1232 mA h g-1, respectively, and their corresponding initial Coulombic efficiencies are 72, 70, and 63 %, respectively. CoFeO-GC-10 composite microspheres with high GC content show the highest initial Coulombic efficiency and the dense structure of CoFeO-C composite microspheres results in low initial capacities at the high current density of 1 A g-1. The cycling performances of the three samples at 1 A g-1 current density are shown in Fig. 7a. The capacities of CoFeO-GC-5 and CoFeO-C composite microspheres decrease steadily to 568 and 427 mA h g-1 during the first 60 and 40 cycles, respectively. However, CoFeO-GC-10 composite microspheres retain extremely high capacities of 1072 and 681 mA h g-1 even for the 350th cycle at current densities of 1 and 3 A g-1 as shown in Fig. 7a and b, respectively. CoFeO-GC-10 composite microspheres also show a superior rate performance (Fig. 7c), where the reversible discharge capacities decrease slightly from 1003 to 685 mA h g-1 when the current density is increased from 1 to 8 A g-1. The graphitic carbon formed around metal oxides can facilitate electron transfer and suppress the huge volume expansion of metal oxides during cycling. In addition, hollow interiors of (Co0.5Fe0.5)3O4 in the composites can shorten the transport pathway for ion and electrons as well as accommodate the volume changes. These unique features of CoFeO-GC10 microsphere result in improved electrochemical properties such as high capacity and good cycling stability and excellent rate capability.

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The electrochemical impedance spectroscopy (EIS) results of the three electrodes obtained after 1 and 50 cycles are shown in Fig. 8. CoFeO-GC-10 composite microspheres show stable charge transfer resistance (Rct) values even after 50 cycles. In addition, Rct values of CoFeO-GC-10 composite microspheres are lower than those of CoFeO-GC-5 and CoFeO-C composite microspheres. The stable and low Rct values of CoFeO-GC-10 composite microspheres result from their highly stable structure resulting from the high GC content, which also leads to high electrical conductivity [14,38]. Fig. 9 shows the morphologies of the three electrodes after 200 cycles. CoFeO-GC-10 composite microspheres maintained their spherical morphologies without cracking; in contrast, cracks due to the high volumetric changes during cycling are observed in CoFeO-GC-5 and CoFeO-C composite microspheres as indicated by arrows in Fig. 9b and c.

4. Conclusions Carbon microspheres have been used for the first time as a template material to synthesize a unique composite structure comprising mixed transition metal oxide microspheres comprising hollow nanospheres. The meso- and macropores of the carbonaceous microsphere template were filled with cobalt and iron metal salts by repeating the “drop and dry” process. Reduction at 1000 °C produced uniformly sized CoFe nanocrystals, which catalyzed the transformation of amorphous carbon into graphitic carbon. The complete conversion of CoFe alloy nanocrystals by nanoscale Kirkendall diffusion resulted in hollow CoFeO nanospheres. Oxidation in atmosphere with controlled oxygen content led to the slow combustion of amorphous carbon without burning. Consequently, CoFeO-GC composite microspheres with excellent lithium-ion storage performances were formed. This novel synthesis strategy can be generalized to the preparation of composite microspheres consisting of ultrafine metal oxide 14

hollow nanospheres and graphitic carbon, which can be potentially used in many applications including energy storage.

AUTHOR INFORMATION Corresponding Author * E-mail addresses: [email protected] (Jung-Kul Lee), [email protected] (Yun Chan Kang).

ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2017R1A2B2008592). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1A4A1014806).

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Figure Captions

Scheme 1. Formation scheme of the unique structured CoFeO-GC composite microsphere from the micron-sized carbonaceous template. Fig. 1. Morphologies of CoFe-C-GC-10 composite microspheres obtained at 1000 °C under 10% H2/Ar atmosphere: (a) SEM, (b,c) TEM, (d) high resolution TEM images, (e) SAED pattern, and (f) elemental mapping images. Fig. 2. Morphologies of CoFeO-GC-10 composite microspheres: (a) SEM, (b,c) TEM, (d) high resolution TEM images, (e) SAED pattern, and (f) elemental mapping images. Fig. 3. XPS spectra of CoFeO-GC-10 composite microspheres: (a) survey scan, (b) Fe 2p, (c) Co 2p, and (d) C 1s. Fig. 4. Morphologies of CoFeO-GC-5 composite microspheres: (a) SEM, (b,c) TEM, (d) high resolution TEM images, (e) SAED pattern, and (f) elemental mapping images. Fig. 5. Morphologies of CoFeO-C composite microspheres: (a) SEM, (b) TEM, (c) high resolution TEM images, and (d) elemental mapping images. Fig. 6. Cyclic voltammetry (CV) curves of (a) CoFeO-GC-10, (b) CoFeO-GC-5, and (c) CoFeO-C composite microspheres. Fig. 7. Electrochemical properties of CoFeO-GC-10, CoFeO-GC-5, and CoFeO-C composite microspheres: cycling performances at (a) 1 A g-1 and (b) 3 A g-1, and (c) rate performances. 21

Fig. 8. Nyquist plots of (a) CoFeO-GC-10, (b) CoFeO-GC-5, and (c) CoFeO-C electrodes after 1 and 50 cycles. Fig. 9. Morphologies of (a) CoFeO-GC-10, (b) CoFeO-GC-5, and (c) CoFeO-C microspheres obtained after 200 cycles.

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Scheme 1. Formation scheme of the unique structured CoFeO-GC composite microsphere from the micron-sized carbonaceous template.

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Fig. 1. Morphologies of CoFe-C-GC-10 composite microspheres obtained at 1000 °C under 10% H2/Ar atmosphere: (a) SEM, (b,c) TEM, (d) high resolution TEM images, (e) SAED pattern, and (f) elemental mapping images.

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Fig. 2. Morphologies of CoFeO-GC-10 composite microspheres: (a) SEM, (b,c) TEM, (d) high resolution TEM images, (e) SAED pattern, and (f) elemental mapping images.

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Fig. 3. XPS spectra of CoFeO-GC-10 composite microspheres: (a) survey scan, (b) Fe 2p, (c) Co 2p, and (d) C 1s.

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Fig. 4. Morphologies of CoFeO-GC-5 composite microspheres: (a) SEM, (b,c) TEM, (d) high resolution TEM images, (e) SAED pattern, and (f) elemental mapping images.

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Fig. 5. Morphologies of CoFeO-C composite microspheres: (a) SEM, (b) TEM, (c) high resolution TEM images, and (d) elemental mapping images.

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Fig. 6. Cyclic voltammetry (CV) curves of (a) CoFeO-GC-10, (b) CoFeO-GC-5, and (c) CoFeO-C composite microspheres.

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Fig. 7. Electrochemical properties of CoFeO-GC-10, CoFeO-GC-5, and CoFeO-C composite microspheres: cycling performances at (a) 1 A g-1 and (b) 3 A g-1, and (c) rate performances.

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Fig. 8. Nyquist plots of (a) CoFeO-GC-10, (b) CoFeO-GC-5, and (c) CoFeO-C electrodes after 1 and 50 cycles.

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Fig. 9. Morphologies of (a) CoFeO-GC-10, (b) CoFeO-GC-5, and (c) CoFeO-C microspheres obtained after 200 cycles.

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Graphitic abstract

Superior Electrochemical Properties of Micron-Sized Aggregates of (Co0.5Fe0.5)3O4 Hollow Nanospheres and Graphitic Carbon Young Jun Hong, Seung-Keun Park, Kwang Chul Roh, Jung-Kul Lee*, and Yun Chan Kang*

Morphology-controlled micron-sized aggregates of hollow nanospheres and graphitic carbons are found to be efficient electrode materials for lithium-ion batteries. The synergy between the (Co0.5Fe0.5)3O4 hollow nanospheres aggregate and the high content of graphitic carbon with high electrical conductivity, result in superior cycling and rate performances, when tested as anode materials for lithium-ion batteries.

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Morphology-controlled micron-sized aggregates of hollow nanospheres and graphitic carbons are synthesized.

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Carbon microspheres with extremely large surface area of 3000 m2 g-1 have been successfully used as templates.

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CoFe2O4-Co3O4-graphitic carbon composite microspheres show excellent lithium-ion storage performances.

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