Carbon Nanocages with Hierarchical Porosities as Anode Materials for Lithium-ion Batteries

Carbon Nanocages with Hierarchical Porosities as Anode Materials for Lithium-ion Batteries

Accepted Manuscript Title: Hollow Nitrogen-doped Fe3 O4 /Carbon Nanocages with Hierarchical Porosities as Anode Materials for Lithium-ion Batteries Au...

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Accepted Manuscript Title: Hollow Nitrogen-doped Fe3 O4 /Carbon Nanocages with Hierarchical Porosities as Anode Materials for Lithium-ion Batteries Author: Li Wang Jiafeng Wu Yaqin Chen Xiaohong Wang Rihui Zhou Shouhui Chen Qiaohui Guo Haoqing Hou Yonghai Song PII: DOI: Reference:

S0013-4686(15)30717-9 EA 25937

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

3-8-2015 1-10-2015 23-10-2015

Please cite this article as: Li Wang, Jiafeng Wu, Yaqin Chen, Xiaohong Wang, Rihui Zhou, Shouhui Chen, Qiaohui Guo, Haoqing Hou, Yonghai Song, Hollow Nitrogen-doped Fe3O4/Carbon Nanocages with Hierarchical Porosities as Anode Materials for Lithium-ion Batteries, Electrochimica Acta 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.

Hollow Nitrogen-doped Fe3O4/Carbon Nanocages with Hierarchical Porosities as Anode Materials for Lithium-ion Batteries

Li Wang, Jiafeng Wu, Yaqin Chen, Xiaohong Wang, Rihui Zhou, Shouhui Chen, Qiaohui Guo, Haoqing Hou and Yonghai Song *

Key Laboratory of Functional Small Organic Molecule, Ministry of Education, Key Laboratory of Chemical Biology, Jiangxi Province, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, 330022, China.

 Corresponding author: Tel/Fax: +86 791 88120862. E-mail: [email protected] (Y. Song).


Graphical abstract


Highlights Hollow N-doped Fe3O4/C nanocages were prepared by carbonizing [email protected] PDA directly. The nanocomposites exhibit superior electrochemical performance. Hierarchically porous structure improves the transfer of Li+. The N-doped carbon matrix buffers the volume variation of Fe3O4.

Abstract Hollow nitrogen (N)-doped Fe3O4/carbon (C) nanocages with hierarchical porosities have been successfully synthesized by carbonizing polydopamine (PDA)-coated Prussian blue (PB). Scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy and N2 adsorption/desorption isotherms were employed to characterize the hollow N-doped Fe3O4/C superstructures. Owing to their well-defined hierarchical porous structure and high electric conductivity originated from N-doped carbon matrix, the unique hollow nanocages as anode materials for Lithium ion batteries (LIBs) exhibited a specific capacity of 878.7 mA h g-1 after 200 cycles at the specific current of 200 mA g-1, which is much better than that of N-doped Fe3O4/C derived from pure PB (merely 547 mA h g-1). The novel construction with hollow interior nanoarchitecture, hierarchical porosity, high electrical conductivity, N-doped carbon, and the Fe3O4 nanoparticles incorporated into porous interconnected carbon framework make the special nanocomposites be very promising Fe3O4-based anode materials. Keywords: Hierarchical porosity; Fe3O4; N-doped carbon; Anode; Lithium ion batteries


1. Introduction Lithium ion batteries (LIBs) have attracted tremendous attention owing to their widespread application, such as smartphone, smart watches, notebook computer, other portable electronic equipments and so on [1-6]. Nevertheless, the current commercial anode materials, graphite, limited the further application of LIBs due to its poor theoretical capacity (only 372 mA h g-1) and inferior rate performance at lager specific current [2,4,7]. As the electronics consumption age is rapidly coming, higher demands on the battery performance for various electronic devices are becoming intensive. As a result, study on developing new anode materials with high performance is urgently required [8-10]. Among multitudinous new anode materials, transition metal oxides (TMO)/carbon (C) nanocomposites (TMOC) were very promising and potential anode materials (e.g., Fe3O4/C [11,12], MnO2/C [13], Co3O4/C [14-19], etc). TMOC with multi-compositions and various nanostructures have been investigated extensively due to the increased theoretical lithium storage, excellent electrical conductivity and superior cyclability. What’s more, the strong elastic carbon framework could buffer the volume variation of TMO during circulating period (which is a common problem in TMO electrodes) [20-22]. Accordingly, various synthetic methods were proposed to prepare high-performance TMOC for LIBs. For instance, diverse carbon-based materials were used as supporting to anchor or encapsulate TMO nanoparticles (e.g., carbon nanotubes (CNTs)/TMO [18,23], graphene oxide (GO)/TMO [12,14,16,24,25] and porous carbon (PC)/TMO [21,26]). In these works, the synthetic procedures of TMOC were tedious and complicated. Thus the design of novel TMOC is still highly challenging. Metal organic frameworks (MOFs) with unique porosities and desirable versatile functionalities 4

have been extensively used in various fields, including gas catalysis and separation, sensing, energy storage and so on [27-31]. As one kind of MOFs, Prussian blue (PB) was used as precursor to prepare TMOC for LIBs. However, the specific capacity of TMOC as anode materials decreased rapidly to 547 mA h g-1, indicating a coating process was necessary to improve its performance. Herein, the [email protected] (PDA) was developed as TMOC precursor via pH-induced polymerization of dopamine on PB in this work. The synthesis procedures and experiment condition were mild and feasible thus possible for mass production. Then the [email protected] precursor were thermal treated to achieve hierarchical porous hollow nitrogen (N)-doped Fe3O4/C (HPHNF) nanocages. As compared with the reversible capacity of 547 mA h g-1 of N-doped Fe3O4/C originated from pure PB, the HPHNF could deliver a good capacity of 878.7 mA h g-1 at a specific current of 200 mA g-1 even after 200 cycles due to the N-doped carbon network possess improved electrical conductivity and novel interior hierarchical porous hollow structure which are able to relieve the volume change during the discharge/charge processes and enhance the transfer of Li+. 2. Experiment 2.1 Reagents and chemicals. Polyvineypirrolydone (PVP), K3[Fe(CN)6], graphite and HCl were obtained from Sinopharm Chemical Reagent (Shanghai, China). Dopamine hydrochloride (DAH) was obtained from Sigma-Aldrich. Other reagents were purchased from Beijing Chemical Reagent Factory (Beijing, China). Polyvinylidene fluoride (PVDF, Lefu Shanghai Chemicals), carbon black (Kaisai Shanghai Chemicals), copper foil (10 μm thickness, Jiayuan Guangzhou Company) and metallic Li foil (0.6 mm thickness, 99.9%, Zhongneng Tianjin Company) were used without further treatment. Other chemicals used in this study are analytical grade. All solutions were prepared with ultrapure water, 5

purified by a Millipore-Q System (18.2 MΩ cm). 2.2 Preparation of PB. In a typical synthesis, PVP (3 g) and K3[Fe(CN)6] (113.3 mg) were successively dissolved in HCl solution (0.01 M, 40 mL) under magnetic stirring to obtain a clear solution. The beaker was placed into an electric oven and heated at 80 °C for 20 h. After aging, the precipitates were collected by centrifugation and washed in ultrapure water and absolute ethanol for several times. After drying in a vacuum oven at 60 ºC for 12 h, PB crystals with around 150-200 nm were obtained. 2.3 Synthesis of HPHNF nanocages nanocomposites. In a typical procedure, 400 mg PB and 400 mg DAH was added into 100 mL Tris-HCl (10 mM pH=8.5). Then the mixture was sonicated for 30 min. After that, the mixture was stirred for 24 h at room temperature. Then, the product was centrifuged and washed with ethanol and water to obtain the [email protected] nanocomposites. Finally, the [email protected] nanocomposites were calcined at 550 ºC with a heating rate of 10 ºC min-1 and held for 1 h under N2. The HPHNF nanocomposites were then obtained. The preparation procedure was illustrated in Scheme 1. Pure PB without PDA coating was calcined at 550 ºC with a heating rate of 10 ºC min-1 and held for 1 h under N2 to prepare the N-doped Fe3O4/C nanocages. 2.4 Characterization X-ray powder diffraction (XRD) data were collected on a D/Max 2500 V/PC X-ray powder diffractometer using Cu Kα radiation (λ= 1.54056 Å, 40 kV, 200 mA). Scanning electron microscopy (SEM) analysis was acquired on a HITACHI S3400 operated at an acceleration voltage of 20 kV equipped with a Phoenix energy dispersive X-ray analyzer (EDXA). X-ray photoelectron spectroscopy (XPS) was performed using an ESCA-LAB-MKII spectrometer (VG Co., United Kingdom) with AlKα X-ray radiation as the source for excitation. N2 adsorption/desorption 6

isotherms were measured at 77 K in a liquid nitrogen atmosphere using a Tristar 3000 volumetric adsorption analyzer (Micromeritics Instrument Corporation, USA) after the samples were pretreated at 200 °C for 12 h under vacuum. Transmission electron microscopy (TEM) images were acquired on JEOL JEM-2100 microscopes operated at an acceleration voltage of 200 KV. 2.5 Electrochemical measurements The working electrodes were made by a slurry coating procedure. The as-prepared samples were mixed with acetylene black and carboxymethyl cellulose in a weight ratio of 8:1:1 in N-methyl pyrrolidinone to form homogeneous slurry and then spread uniformly on copper foil which was acted as a current collector. The active material loading on copper foil was approximately 0.8 mg cm-2. The fabricated working electrodes were dried in a vacuum oven at 60 ºC for overnight. A celgard 2300 microporous polypropylene film was used as the separator. The cells were assembled in an argon-filled glove box using Li foil as counter electrodes. The electrolyte was made of 1.0 M LiPF6 dissolved in the mixture of ethylene carbonate, diethyl carbonate and dimethyl carbonate (1: 1: 1 in volume). Cyclic voltammograms (CVs) was carried out with a CHI 760D electrochemical workstation (CH Instruments, Shanghai, China) using a conventional three-electrode system with Li metal as the counter and reference electrode, active materials as the working electrode. Other electrochemical experiments were carried out in a two-electrode coin cells. The electrochemical discharge/charge tests of the samples were performed on a Neware BTS test system (Shenzhen, China) at voltage limits of 3.0~0.01 V versus Li/Li+. 3. Results and discussion PB nanocubes were firstly prepared as the raw material to synthesize [email protected] through a simple strategy. The PB nanocubes with smooth surface and mean size of 150-250 nm are observed clearly 7

in the SEM image as shown in Fig. 1a, indicating the successful synthesis of PB nanocubes. Moreover, the TEM image (Fig. 1b) demonstrated its nanocube morphology and solid feature obviously. All existed elements in energy dispersive X-ray spectroscopy (EDS) of PB nanocubes (Fig. 1d) contained C, N, O, K and Fe further confirmed the synthesis of PB. The K element mainly originated from the residual K+ in K3[Fe(CN)6]. The XRD was performed to detect the crystal structure and phase purity of PB nanocubes. As shown in Fig. 1d, all diffraction peaks from the XRD pattern could be assigned to the standard Fe4[Fe(CN)6]3 (JCPDS no. 73-0687) [32], suggesting the extreme purity of the prepared PB. Then PDA was coated on the above PB surface to prepare [email protected] The low-magnification and high-magnification SEM images of as-prepared [email protected] nanocubes are shown in Fig. 2a and b, respectively. Interestingly, the surface of [email protected] became rough and was covered with many grooves. The average size of [email protected] was larger than that of PB correspondingly. TEM was utilized to study the interior structure of [email protected] As revealed in Fig. 2c and d, the PB nanocubes were covered by the uniform PDA layer with a thickness of 10 nm. The [email protected] nanocubes were subsequently carbonized under 550 ºC with a heating rate of 10 ºC min-1 and held for 1 h under N2 to obtain the well-defined HPHNF. It was clearly observed that the grooves on the surface of HPHNF became more apparent and many cavities appeared after the carbonization as revealing by the SEM images of as-prepared HPHNF (Fig. 3a and b). We could even observe interior hollow structure from the broken edges of HPHNF after mechanical grinding (Fig. 3b). TEM image clearly indicated the porous hollow nanocages (Fig. 3c). According to the HRTEM image (Fig. 3d), the PDA layer was transformed into N-doped porous carbon network and at the same time the Fe3O4 nanoclusters were encapsulated completely by multilayer graphitic 8

carbon. The interplane spacing of 0.21 nm was assigned to the (400) crystalline planes of Fe3O4. In order to highlight the superiority of the PDA coating, N-doped Fe3O4/C nanocages without PDA coating were also prepared by calcining pure PB under the identical condition. SEM images (Fig. S1, Supporting Information) revealed that Fe3O4/C possessed the similar morphology to HPHNF after carbonization, but the cube surface inward shrinked to some extent. As shown in Fig. 4a, the XRD pattern of as-synthesized HPHNF reveals detailed information relating to crystalline and purity. The corresponding diffraction peaks at 30.1º, 35.4º, 37.1º, 43.1º, 53.4º, 56.9º, 62.5º and 74.9º are assigned to the (220), (311), (222), (400), (422), (511), (440) and (533) crystalline planes of Fe3O4 (JCPDS no. 19-0629), respectively [33]. No additional diffraction peaks were observed, revealing the high purity of Fe3O4 nanoparticles. The XPS (Fig. 4b) confirmed the presence of C, N, O, K and Fe. The high-resolution XPS profile of Fe 2p (Fig. 4c) showed two obvious peaks at 724.8 eV and 711.3 eV which corresponded to the binding energies of Fe2p1/2 and Fe2p3/2, matching well with the previous reports of Fe3O4 [34,35]. The high-resolution XPS spectrum of C 1s (Fig. S2, Supporting Information) showed four obvious peaks at 284.3 eV, 285.2 eV, 285.8 eV and 288.6 eV belonging to C–C, C–N, C–O and O−C=O groups, respectively [21]. The amount of N and C elements of HPHNF were estimated to be 8.3 % and 43.5 % which were higher than of the N-doped Fe3O4/C (2.9 % and 26.0 %) (Table S1, Supporting Information). The Brunauer–Emmett–Teller (BET) surface area of HPHNF was calculated to be 58.2 m2 g−1 through N2 adsorption/desorption measurements. However, the BET surface area of Fe3O4/C nanocages was measured to be only 12.7 m2 g-1. The larger BET was mainly attributed to the surface modification of PDA layer which resulted in more micro/mesoporosites. According to the results of XRD, XPS and BET, the [email protected] 9

nanocomposites were totally converted into HPHNF after the thermal treatment. . To study the electrochemical mechanism, the first three CVs of the electrode made of the HPHNF (Fig. 5a) were performed over potential window from 0.01 V to 3.00 V at a scan rate of 0.2 mV s-1. In the first discharge process, an unobvious peak centered at 1.50 V might be caused by an irreversible lithium loss, corresponding to the decomposition of the electrolyte and formation of solid electrolyte interphase (SEI) [22,36]. A prominent broad peak locating at 0.50 V was ascribed to the reduction of Fe2+ and Fe3+ to Fe0 as well as the formation of Li2O. In the following discharge curves, the cathodic peak located at 0.50 V was shifted to 0.80 V due to the polarization of the electrode material in the first discharge process [16,37,38]. In the initial charge process, a small peak centered at 1.10 V was detected and it was resulted from the interaction between Li+ and external oxygenic functional groups [21,22]. Furthermore, one broad peak appeared at 1.75 V which was ascribed to the reoxidation of Fe metal [33,37,39]. Obviously, the CV curves nearly overlapped after capacity fading of the initial discharge/charge process. The galvanostatic discharge/charge profiles of the HPHNF for the 1st, 50th, 100th, 150th and 200th cycles at a specific current of 200 mA g-1 over voltage range from 0.01 to 3.00 V are presented in Fig. 5b. During the first discharge procedure, HPHNF exhibited an obvious voltage plateau at approximate 0.75 V owing to the decomposition of Fe3O4 and the formation of Li2O. The electrochemical reaction mechanism of Li+ with Fe3O4 could be depicted as [23,40]: Fe3O4 + 8Li+ + 8e- ↔ 3Fe0 + 4Li2O. The reversible capacities of 1st, 50th, 100th, 150th and 200th were obtained as 944.2 mA h g-1, 787.1 mA h g-1, 739.9 mA h g-1, 753.8 mA h g-1 and 878.7 mA h g-1, respectively. The trend of the specific capacity for metal oxide-based anode materials decreasing firstly and then rising gradually was attributed to the full activation as circulation proceeding. 10

The compare of the cycle performance of the HPHNF, Fe3O4/C derived from pure PB and graphite were illustrated in Fig. 5c. The HPNHF and Fe3O4/C delivered initial discharge capacities of 944.2 mA h g-1 and 917.8 mA h g-1, respectively. From the second cycle, the HPHNF delivered a Coulombic efficiency of almost 100%. The HPHNF electrode could maintain the reversible discharge capacity of 878.7 mA h g-1 after 200 cycles at a specific current of 200 mA g-1, while the N-doped Fe3O4/C electrode only manifested the reversible discharge capacity of 547.1 mA h g-1 and the typical anode material-graphite dropped to 324 mA h g-1 after 100 cycles. The cycling performance of the two Fe3O4-based materials was much better than graphite due to the higher theoretical capacity and unique structure. Two main factors might contribute to the significant difference of the electrochemical performance between HPHNF and N-doped Fe3O4/C. First, the smaller surface area (only 12.7 m2 g-1) of the N-doped Fe3O4/C resulted in longer Li+ diffusion distance and insufficient electronic contact between the electrolyte and the active material as compared with HPHNF. Second, the elastic and robust N-doped carbon network derived from the PDA coating layer not only improved the conductivity of the active material, but also created more defective site for lithium insertion. Therefore, the HPHNF possessed outstanding cycling stability and superior rate performance. The rate performances of the HPHNF and N-doped Fe3O4/C nanocomposites at various current densities were tested as one of the most significant parameters. As shown in Fig. 6, the HPHNF released the reversible capacities of 775.5 mA h g-1, 683.4 mA h g-1, 518.5 mA h g-1, 386.5 mA h g-1 and 254.3 mA h g-1 under the current densities of 100 mA g-1, 200 mA g-1, 500 mA g-1, 1 A g-1 and 2 A g-1, much higher than that of the N-doped Fe3O4/C nanocomposites (only 712.3 mA h g-1, 520.2 mA h g-1, 275.6 mA h g-1, 153.7 mA h g-1 and 78.8 mA h g-1). As the specific current backed to 200 11

mA g-1, the specific capacity of 759.1 mA h g-1 and 560.1 mA h g-1 were kept for HPHNF and N-doped Fe3O4/C nanocomposites, respectively. The nanoarchitecture of HPHNF provided more and shorter Li+ transport channels to enhance the performance. The flexible carbon shells encapsulating Fe3O4 not only minimized the capacity fading due to nanoparticles agglomeration, but also helped to keep the mechanical integrity of active material under large specific current. A detailed comparison of the cycle performance of HPHNF with other previous reported Fe3O4-based materials was listed in Table 1, the as-synthesized HPHNF nanocomposites exhibited a better electrochemical performance. Due to the hierarchical porous hollow structure, the HPHNF nanocomposites not only have more vacant volume to bear huge volume change of Fe3O4, but also accelerate the diffusion rate of Li+.

4. Conclusions In summary, a simple strategy was proposed to synthesize HPHNF nanocages through calcining [email protected] The hierarchical porous structure promoted fast Li+ diffusion rate, the internal hollow construction moderated volume change during lithium insertion/extraction and the robust N-doped carbon network derived from PDA layer enhanced high conductivity, which contributed to the good battery performance together. Both the good enough cycle capacity and well rate performance demonstrate that HPHNF is a promising high-performance anode material.


Acknowledgements This work was financially supported by National Natural Science Foundation of China (21465014 and 21465015), Science and Technology Support Program of Jiangxi Province (20123BBE50104 and 20133BBE50008), Natural Science Foundation of Jiangxi Province (20142BAB203101), The Ministry of Education by the Specialized Research Fund for the Doctoral Program of Higher Education (20133604110002) and the Ground Plan of Science and Technology Projects of Jiangxi Educational Committee (KJLD14023), the Open Project Program of Key Laboratory of Functional Small Organic Molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF-201410; KLFS-KF-201416).


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Figure Caption Figure 1. (a) SEM and (b) TEM image of as-prepared PB, (c) EDS and (d) XRD pattern of PB. Figure 2. Low-magnified and high-magnified SEM images (a,b) and TEM images (c,d) of [email protected] Figure 3. Low-magnified and high-magnified SEM images (a,b) and TEM images (c,d) of the HPHNF nanocomposites. Figure 4. (a) XRD pattern and (b) XPS spectrum of the HPHNF nanocomposites. (c) High-resolution XPS spectrum of Fe 2p. (d) Nitrogen adsorption/desorption isotherm for the HPHNF. The inset is the pore size distribution of the HPHNF sample. Figure 5 (a) CVs of the HPHNF during the first three cycles at 0.2 mV s-1, (b) Galvanostatic charge/discharge profiles of the HPHNF electrodes for the 1st, 50th, 100th, 150th and 200th cycle at a specific current of 200 mA g-1. (c) Cycling performance of the HPHNF nanocomposites, N-doped Fe3O4/C nanocomposites and graphite at a specific current of 200 mA g-1. (d) Coulombic efficiency of HPHNF. Figure 6. Rate performance of HPHNF nanocomposites and N-doped Fe3O4/C nanocomposites from pure PB. Scheme 1. Schematic illustrating the preparation process of the HPHNF nanocomposites.


Figure 1


Figure 2


Figure 3


Figure 4


Figure 5


Figure 6

Scheme 1


Table 1. Comparison of the electrochemical performance of Fe3O4-based materials as anode materials for LIBs. Specific current

Initial coulombic efficiency


mA g−1


mA h g−1

Hollow graphene/ Fe3O4



900 (50)





836 (100)


Graphene/ Fe3O4



650 (100)


Graphene/ Fe3O4



600 (100)


Carbon-decorated Fe3O4



830 (50)


3D Hierarchical Fe3O4/Graphene



605 (50)







[email protected]



560 (80)


Uniform Fe3O4 Hollow Spheres





Porous hollow Fe3O4 beads





Hollow Fe3O4/graphene





Hollow Fe3O4 microsphere





Hollow Fe3O4/C spheres








878 (200)

This work

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