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Enabling a high performance of mesoporous #-Fe2O3 anodes by building a conformal coating of cyclized-PAN network Di Wang, Hui Dong, Huang Zhang, Yang Zhang, Yunlong Xu, Chongjun Zhao, Yunong Sun, and Nan Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06096 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 15, 2016

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ACS Applied Materials & Interfaces

Enabling a High Performance of Mesoporous α-Fe2O3 Anodes by Building a Conformal Coating of Cyclized-PAN Network Di Wang1, Hui Dong 1, Huang Zhang2, Yang Zhang1, Yunlong Xu1*, Chongjun Zhao1, Yunong Sun1, Nan Zhou1 1

School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China

2

Department of Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium

KEYWORDS:

α-Fe2O3, cyclized-polyacrylonitrile, mesoporous, lithium ion

batteries, anode materials

Abstract The mesoporous α-Fe2O3/cyclized-polyacrylonitrile (C-PAN) composite was synthesized by a rapid and facile two-step method. The electrode was fabricated without conductive carbon addictive and employed as anode for lithium ion batteries. Results demonstrate that building a conformal coating of C-PAN network can provide a strong adhesion with active materials and contribute excellent electronic conductivity to the electrode, which can relieve the huge volume changes during lithiation/delithiation process and accelerate the charge transfer rate. The material exhibited high reversible capacity of ca. 996mAh g-1 after 100 cycles at 0.2C, 773mAh g-1 at 1C and 655mAh g-1 at 2C, respectively, showing well-enhanced cycling performance and superior rate capacity, and also exhibiting significantly

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improved power density and energy density compared to the traditional graphite materials. Our results provide a facile and efficient way to enhance the performance of α-Fe2O3 anode material, which also can be applied for other oxide anode materials.

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Introduction Along with the increasingly severe environmental pollution and petroleum scarcity, the development and deployment of new energy technologies have been brought to the forefront of attention in recent years. Lithium-ion batteries (LIBs) with the characteristics of high energy density, low cost and excellent safety are considered as one of the most promising devices which can be applied to power portable electronic devices, electric vehicles and plug-in hybrids1-8. However, as a traditional anode material, graphite seems to be unable to meet the ever-growing requirements for electrochemical energy storage due to the relatively low theoretical specific capacity (372mAhg-1 vs. Li+/Li) and safety issues. Therefore, searching for novel high-performance electrode materials has been a consequential task for the next generation of Li-ion batteries9-12. Iron oxides, like α-Fe2O313-14, γ-Fe2O315-16 and Fe3O417-19, have been attracting substantial attention due to non-toxicity, resourceful abundance, environmental friendliness and low cost. Among them, α-Fe2O3 in corundum-type structure, demonstrates a terrific electrochemical stability and much higher theoretical capacity of 1007 mAhg-1 than traditional graphite materials, and it is also comparatively easy to prepare by environmentally friendly means from low-cost resources20-23. However, α-Fe2O3 is the irreversibility of the conversion type and suffers from severe volume expansion during the lithium-ion insertion/desertion process, resulting in a high initial capacity loss and capacity fading in repeated cycling. Moreover, the poor intrinsic electrical conductivity is another defect, which is the inferior factor to limit the

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electrochemical performance at high rate24-26. Therefore, some nanostructured α-Fe2O3 materials, such as nanoparticles27, nanorods28-29, nanotubes30, nanoflakes31, nanosheets19, 32, nanodiscs33, nanowires34, nanoflowers35, nanorings36, nanospindles37, have been pertinently synthesized, which can provide a short path for Li-ion diffusion and effectively accommodate the large strain resulted from volume expansion. Additionally, to construct a 3D conductive framework with functional carbon materials such as graphene38, carbon nanotube (CNT)39, carbon nanofiber (CNF)40, human hair-derived (HHC)41, Ketjen black42 comes out to be a promising approach to improving the overall electrochemical performance, which can be devoted to high-performance power batteries owing to its excellent electronic conductivity and electrochemical stability. Although tremendous remarkable achievements have already been obtained on the exploration of high energy oxide anode materials, the conductive polymers also show promising application for potential anode materials in high power batteries. Polyacrylonitrile (PAN), a polar polymer, which can be cyclized by controlling pyrolysis temperature at 300°C or only cyclization rather than carbonization in an inert atmosphere, namely the cyclized-PAN (C-PAN), has been used as an efficient binder for silicon anode materials. In cyclized-PAN (C-PAN), the delocalized sp2 π bond can make a positive contribution to electronic conductivity, as it working in graphite, which will favor to the rate properties of electrode. Meanwhile, with the inherent mechanical resiliency, C-PAN also can perform a strong adhesion with active materials. This is an important factor to improve the long term performance of

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materials with volume expansion during lithiation/delithiation process. Practically, Daniela et al took advantages of cyclized-PAN both as conductive additive and binder, creating composites with Si metal, and the compositions had good capacity retention with acceptable rate capability and high reversible capacity43-44. This achieved result indicates that the CPAN-based materials can be a promising electrode component for high-power lithium-ion batteries. Herein, we report a surfactant-free solvothermal method combined with heat treatment to synthesize the hierarchically nanostructured mesoporous α-Fe2O3 microspheres. And then the α-Fe2O3/C-PAN composite was obtained by heat-treating the mixture of α-Fe2O3 microspheres and PAN, in which the C-PAN acts as both binder and conductive additive replacing the transitional binder PVDF and conductive carbon. These obtained composite materials exhibit an improved capacity density, superior cycling performance and excellent rate capability compared to the traditional graphite materials.

Experimental Preparation of hierarchically nanostructured mesoporous α-Fe2O3 microspheres The hierarchically nanostructured mesoporous α-Fe2O3 microspheres were prepared through a surfactant-free solvothermal method followed with heat treatment. Typically, 0.404g Fe(NO3)3·9H2O and 0.8g tartaric acid were added into 60ml N,N-dimethylformamide (DMF) under vigorous stirring. Then an adequate amount of solution was transferred into a 100ml Teflon lined stainless steel autoclave, sealed, and heated at 160°C for 10h. After cooling to room temperature naturally, the

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precursor powders were collected by pumping filtration, repeatedly washed with deionized water and ethanol and dried in a vacuum oven at 80°C. At last, the as-prepared precursor was calcined at 500°C in air for 3h, ground and sifted to obtain the hierarchically nanostructured mesoporous α-Fe2O3 microspheres45-46. Preparation of mesoporous α-Fe2O3/C-PAN composite As shown in Figure S1, the synthesized hierarchically nanostructured mesoporous α-Fe2O3 microspheres were mixed with PAN (Mw=15000 g mol-1, Sigma-Aldrich) in a mass ratio of 8:2 in N-methyl-2-pyrrolidone (NMP) by magnetic stirring and ultrasonic dispersion. The obtained slurries were then bladed onto a copper foil as current collector and dried at 80°C for about 10 h in a vacuum oven. In order to prevent curling, the obtained film was covered by glass slide and clamped by foldback clip. Afterwards, it was calcined in a tube furnace at a temperature of 300°C for 12 h in argon atmosphere to obtain the mesoporous α-Fe2O3/ C-PAN composite electrode. Materials characterization X-ray diffraction measurements (XRD，Cu Κα, γ =0.154nm, D/MAX 2550V, Japan) were used to confirm the crystal structure of synthesized samples. Raman spectra (LabRAM HR) were conducted to evaluate the electronic conductivity of C-PAN. Differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) were carried out on a SDT Q600 and ESCALAB 250Xi instruments, respectively, to attest the cyclization of C-PAN. The surface area and pore distributions of the as-prepared samples were determined via nitrogen adsorption

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with the Micromeritics Tristar surface area and porosity analyzer (BET, BJH, TriStar 3000, USA). The morphology and microstructure of the α-Fe2O3 and α-Fe2O3/C-PAN composite were observed by scanning electronic microscopy (SEM, NOVA NanoSEM450, USA) and field emission transmission electron microscopy (TEM, JEM-2100, Japan). The elemental distribution was investigated by energy dispersive spectrometer (EDS, NOVA NanoSEM450, USA). Electrochemical characterization: The electrochemical performance of α-Fe2O3/C-PAN composite was investigated in assembled 2032 coin type cell. The cells were fabricated in an argon-filled glove box using lithium foil as counter/reference electrode, 1M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) as electrolyte, Celgard 2400 microporous polyethylene membrane as separator, and tested after aging for at least 12 h. The charge/discharge cycling test was performed on a battery tester (CT2001A, LAND Battery Program-control Test System, China) over a voltage range of 0.005-3.0V. The electrochemical behaviors of individual composite electrodes were evaluated by cyclic voltammetry (CV) on an Electrochemical Workstation (CHI, 660B, CHENHUA, China) at a scan rate of 0.1 mV s-1 between 0.005 and 3V. Electrochemical impedance spectroscopy (EIS) was also carried out on the Electrochemical Workstation. The EIS spectra were potentiostatically collected by using a DC potential equal to the open circuit voltage (OCV) of the cell and an AC oscillation of 5 mV over a frequency range of 105 Hz-0.01 Hz.

Result and discussion 7



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Figure 1. Schematic diagram of the structure and synthetic process of α-Fe2O3/C-PAN composite.

Figure 1 shows the schematic illustration of the structure and synthetic process of α-Fe2O3/C-PAN composite. The hierarchically nanostructured mesoporous α-Fe2O3 microspheres were obtained via a surfactant-free solvothermal method combined with heat treatment. The synthetic mechanism is shown in Figure S2. Fe(NO3)3·9H2O and tartaric acid were dissolved into DMF under vigorous stirring, and heated at 160°C for 10h to generate ferrous tartrate. DMF played an important role in self-assembly of nanostructures, which can promote the formation of hierarchical microspheres of ferrous tartrate. The as-prepared precursor was calcined at 500°C in air for 3h to construct three-dimensional networks of the nanostructure mesoporous α-Fe2O345. Then the composite mixed with PAN were dissolved in NMP by magnetic stirring and ultrasonic dispersion in which PAN as a kind of adhesive coated on the surface of α-Fe2O3 microspheres. And then the composite heated at a temperature of 300°C for 12 h in argon atmosphere for cyclization43. It can be apparently seen that the cyclized-PAN was coated on the surface of α-Fe2O3 as the magnifying diagram showing. Figure 2(a) shows the XRD patterns of mesoporous α-Fe2O3, α-Fe2O3/C-PAN composite and the standard diffraction pattern of α-Fe2O3, respectively. The

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diffraction peaks of mesoporous α-Fe2O3 and α-Fe2O3/C-PAN composite are in good accordance with the characteristic diffraction of α-Fe2O3 phase (JCPDS No. 33-0664), demonstrating the C-PAN does not transform the crystal structure of α-Fe2O3. It is noticeable that the relative intensity of the diffraction peaks of (104), (110), (214) and (300) has a little different after cyclization, which can be attributed to the grain orientation.

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Figure 2. (a) XRD patterns of α-Fe2O3 and α-Fe2O3/C-PAN composite; (b) Raman spectrum of PAN, C-PAN, α-Fe2O3 and α-Fe2O3/C-PAN composite; (c) TG curve of α-Fe2O3/C-PAN composite.

To prove the fact that C-PAN has intrinsic electronic conductivity, the Raman spectroscopy of PAN, C-PAN, α-Fe2O3 and α-Fe2O3/C-PAN composite are presented in Figure 2(b). It can be clearly seen that the Raman spectrum of C-PAN and α-Fe2O3/C-PAN composite display two peaks at 1360cm-1 and 1590cm-1 in agreement with D band (disorder band) and G band (graphite band), respectively47. The appearance of the D and G bands corresponds to the existence of disordered and ordered structural configurations of C-PAN, and the relative intensity ratio of the D to G bond (ID/IG) means the degree of orderly structure, which can be used to demonstrate the intrinsic electronic conductivity of C-PAN43. Specifically, the ID/IG value of α-Fe2O3/C-PAN composite is 1.86, larger than that of the C-PAN (1.18), which is attributed to the superposition leading to the peak intensity of α-Fe2O3 at 1360cm-1. What’s more, there are inconspicuous peaks between 0-600 cm-1 for α-Fe2O3/C-PAN resulting from C-PAN as a conformal coating on the α-Fe2O3 surface. To further calculate the C-PAN content of α-Fe2O3/C-PAN composite and certify whether the content of the PAN will change after cyclization, the thermogravimetric analysis is demonstrated in Figure 2(c). It is obviously to see that a slight initial weight loss occurs ranging from 0 to 260°C, which can be ascribed to the evaporation of surface adsorbed water. And there is a sharply weight loss about 19.93% between 260 and 550°C because of the decomposition and oxidization of C-PAN.

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Consequently, the C-PAN content of the composite is approximately 19.93%, indicating that the content of PAN is basically unchanged before and after cyclization.

Figure 3. (a) DSC spectra of PAN; (b) XPS patterns of PAN at cyclizing temperature 300 ℃ ;(c) Structural representation of the cyanic group (N1), pyridinic group (N2), and substitutional graphite group (N3)

Figure 3 shows the differential scanning calorimetry (DSC) spectra of PAN and X-ray photoelectron spectroscopy (XPS) patterns of PAN at cyclizing temperature of 300°C in an inert atmosphere. There is an exothermic peak at 281°C, corresponding to the cyclization of PAN. According to the XPS patterns, two different peaks can be deconvoluted, namely the pyridinic group N2 (C-N = C) and substitutional graphite

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group N3 (N coordinated with three C atoms), which can be explained as the cyanic group N1 converting to N2 and N3 at the preparing temperature. The structures are exhibited in Figure 3(c), theoretically demonstrating the cyclization of PAN happened at 300°C but not the carbonation43.

Figure 4. N2 adsorption-desorption isotherms and pore size distributions of (a) α-Fe2O3 and (b) α-Fe2O3/C-PAN composite.

N2 adsorption-desorption isotherms and pore size distributions (insert) of 13



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α-Fe2O3 and α-Fe2O3/C-PAN composite are shown in Figure 4 to confirm the mesoporous structure of these two samples. And the detailed parameters of all the samples are depicted in Table1. The Brunauer-Emmett-Teller (BET) specific surface areas of α-Fe2O3 and α-Fe2O3/C-PAN were calculated to be 23.33m2g-1 and 15.16m2g-1, while the Barret-Joyner-Halendar (BJH) average pore sizes were 39.9nm and 25.1nm, respectively. The decrease in parameters may be explained as the fact of C-PAN conformal coating on the surface of mesoporous α-Fe2O3, and partially filling into the mesoporous structure, which will lead to the decline of both specific surface areas and pore size. Table 1 BET surface area and pore volume and pore size of α-Fe2O3 and α-Fe2O3/C-PAN samples

SBET(m2g-1)

Vtotal(cm3g-1)

DProe(nm)

α-Fe2O3

23.33

0.17

39.9

α-Fe2O3/C-PAN

15.16

0.11

25.1

Figure 5(a-f) present the morphology and structure of α-Fe2O3 (a,b,c) and α-Fe2O3/C-PAN composite (d,e,f) by scanning electron microscopy (SEM). It can be clearly observed that α-Fe2O3 microspheres with a diameter of about 5μm show a typically hierarchically mesoporous structure. Figure 5(d-f) show a conformal coating of C-PAN on the surface of α-Fe2O3 and C-PAN bonding between the α-Fe2O3 microspheres, which can accelerate the electronic transport and improve the electronic conductivity. It is apparent to see from the enlarged SEM image, as shown in the 14


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Figure 5(c,f), that the mesoporous structure is not destroyed by the C-PAN coating, remaining the advantages of such structure.

Figure 5. SEM images of α-Fe2O3 (a,b,c) and α-Fe2O3/C-PAN composite (d,e,f); TEM images of α-Fe2O3/C-PAN composite(g,h,i).

To further identify the constructed mesoporous structure, the α-Fe2O3/C-PAN nanocomposite was characterized by field emission transmission electron microscopy (TEM). From Figure 5(g) and Figure 5(h), it can be seen that the surface of mesoporous α-Fe2O3 was coated with C-PAN uniformly. From the HRTEM image in Figure 5(i), the well parallel crystal lattice fringes from the photograph can be distinctly recognized, which suggests the prepared α-Fe2O3 nanoparticles are highly 15



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crystallized. Meanwhile, we can find that the amorphous structure of the nanoparticle boundary is C-PAN with a thickness of about 2.5nm, which is marked in the dotted box, vividly indicating the conformal coating of C-PAN network on the surface of α-Fe2O3 nanoparticles.

Figure 6. Elemental mapping for the particles of α-Fe2O3/C-PAN composite. (a) SEM image; (b) overall elemental; (c) Fe elements; (d) O elements; (e) C elements.

The elemental distribution of α-Fe2O3/C-PAN composite is shown in Figure 6, in which green is Fe element, blue is O element, and red is C element, respectively. SEM image of the measured region is exhibited in Figure (a). Figure 6(c,d,e) present the individual distribution of Fe, O, C elements in selected area. Figure 6(b) presents the overall elemental distribution, which adequately demonstrates the homogeneous dispersion of C-PAN on α-Fe2O3 nanoparticle and linking the contiguous α-Fe2O3

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microspheres, as shown in the tagged dashed box.

Figure 7. (a) Cyclic voltammogram curves of the α-Fe2O3/C-PAN composite for the initial three cycles; (b) Discharge and charge curves of the α-Fe2O3/C-PAN composite for the 1st, 2nd, 3rd cycles at 0.1C; (c) Cycling performance of α-Fe2O3/C-PAN, α-Fe2O3/PAN/AB, α-Fe2O3/PVDF/AB composite and coulomb efficiency of α-Fe2O3/C-PAN composite; (d) Rate performance of α-Fe2O3/C-PAN, α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB; (e) Cycling performances of α-Fe2O3/C-PAN, α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composite at large currents density of 1C and 2C for 100 cycles.

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Figure 7(a) shows the typical cyclic voltammetry (CV) curves of α-Fe2O3/C-PAN composite measured between 0.005 and 3V at a scan rate of 0.1mVs-1 for the first three cycles. In the first scan, a sharp cathodic current peak is discovered at approximately 0.5V (vs. Li+/Li), corresponding to the transformation of metallic lithium into Li2O and the irreversible reduction reaction of Fe3+ to Fe0. Meanwhile, there is a peak presented at about 1.7V, which is ascribed to the reversible oxidation of Fe0 to Fe3+. Obviously, there is a higher cathodic peak during the first cycle, resulting from the formation of a solid electrolyte interface (SEI) on the mesoporous α-Fe2O3 and C-PAN interface during the first lithium intercalation process and the polarization of the active materials, which can lead to irreversible capacity decay and a low coulomb efficiency in the first cycle41, 48. Besides, with the process of repeated cycle, the curves of the subsequent two cycles are basic coincident, indicating a good reversibility and excellent cycling stability of the α-Fe2O3/C-PAN composite. Reaction equations are as follows: Fe2 O3 + 6Li+ + 6𝑒 − ↔ 2 Fe + 3Li2 O

（1）

The relevant initial discharge and charge curves of the α-Fe2O3/C-PAN electrode for the 1st, 2nd, 3rd cycles are demonstrated in Figure 7(b), which were measured at a constant current density (0.1C) and fixed voltage window (0.005V to 3V). The first discharge capacity is 1376mAh g-1, higher than the specific theoretical capacity of α-Fe2O3. This can be attributed to the hierarchically nanostructured mesoporous α-Fe2O3 microspheres providing more active sites for lithium insertion/desertion. Furthermore, the SEI film also makes a contribution to the high capacity for the first

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cycle42. There were two apparent voltage plateaus in the first discharge curves. The first plateau occurs at 1.1V (vs Li/Li+), which is related to the lithium insertion into nanostructure α-Fe2O3. The second long plateau appears at 0.8V, which mirrors the reduction of Fe3+ to Fe0 and the irreversible formation of SEI film on the electrode surface, contributing to the dominating source of capacities. Likewise, in agreement with the CV results, the second and third cycle are nearly overlapped, which implies a distinguished reversibility and good cycling stability under the protection of SEI film. For comparative purpose, we choose the acetylene black (AB) as the conductive additive and the polyvinylidene fluoride (PVDF) as the binder to feature the dual functionality of C-PAN. As shown in Figure S3(a,b), the α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composites exhibit quit different electrochemical performance after the first cycle in peak area, indicating the inferior cycling stability compared with α-Fe2O3/C-PAN composite, which also can be found in discharge and charge curves (Figure S3(c,d)). The

cycling

performance

of

α-Fe2O3/C-PAN,

α-Fe2O3/PAN/AB

and

α-Fe2O3/PVDF/AB composites measured at 0.2C (200mA g-1) in 100 cycles and coulomb efficiency of α-Fe2O3/C-PAN composite are shown in the Figure 7(c). The discharge capacity of α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composites decreases to 368mAh g-1 and 567mAh g-1 after 100 cycles, respectively. Particularly, the cyclability of α-Fe2O3 materials was substantially improved after C-PAN coating, delivering a stabilized capacity of about 996mAh g-1 after 100 cycles, and the high coulomb efficiency after the first few cycles represents the best reversibility of the

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synthesized

α-Fe2O3/C-PAN

composites.

Consequently,

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the

α-Fe2O3/C-PAN

composite shows superior cycle performance with good capacity retention than the other two electrode materials, which can be ascribed to the mesoporous structure of α-Fe2O3 and conformal coating of C-PAN network, effectually accommodating the volume expansion during the lithiation/delithiation process and enhancing the electronic conductivity. It should be noted that the capacities of the α-Fe2O3/C-PAN composite exhibit a trend of increasing after 80th cycle, which may be attributed to the gradual activation and infiltration of the electrodes. Thus it can be concluded that C-PAN can accelerate the electron transport in electrodes and effectively keep the active film adhered to the current collector stably, improving the cycling stability of α-Fe2O3 anodes. Figure 7(d) presents the rate capabilities of α-Fe2O3/C-PAN, α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composite at various rates of 0.2 (200mA g-1), 0.5 (500mA g-1), 0.8 (800mA g-1), and 1C (1000mA g-1), with each rate for 20 cycles. It can be distinctly seen that the α-Fe2O3/C-PAN composite displays the discharge capacity of 1067mAh g-1 at 0.2C, 901mAh g-1 at 0.5C, 841mAh g-1 at 0.8C, 797mAh g-1 at 1C, respectively, much higher than other two samples, and reverts to the original value while the current density steps back to 0.2C, which reveals the fact that the mesoporous α-Fe2O3 coated by C-PAN could maintain the stabilized structure and excellent electrochemical properties. In order to further confirm the performance achieved, the high-rate cycling performance of α-Fe2O3/C-PAN, α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composite

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examined at high rate of 1C (1000mA g-1) and 2C (2000mA g-1) are as shown in Figure 7(e). The initial capacities of α-Fe2O3/C-PAN composite are 920mAh g-1 at 1C and 724mAh g-1 at 2C, and remain 773and 655mAh g-1 after 100 cycles, which exhibit a superior cycling performance and rate capability compared with α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composite at both ambient and high current densities, whose reversible capacities were less than 350mAh g-1 after 100 cycles. This phenomenon should be readily explained to the increased electrical conductivity of the C-PAN. Figure S4 shows the electrochemical performance of generally nanosized α-Fe2O3 coated with C-PAN. It only maintains a capacity of about 532mAh g-1 after 100 cycles at 0.2C, suggesting the superiority of hierarchically nanostructured mesoporous structure. Electrochemical

impedance spectroscopy (EIS)

and

Nyquist

plots

of

α-Fe2O3/C-PAN, α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composites are illustrated in Figure 8(a), in order to make a better understanding for the electrochemical kinetics of prepared electrodes. All the EIS spectra are fitted with the equivalent circuit inserting in the Figure 8(a) and the Nyquist plots are typically composed of a depressed semicircle in high frequency region corresponding to the charge transfer impedance and an inclined line in low frequency region which represents the Li-ion diffusion. The derived results are listed in Table 2. The lithium ion diffusion coefficient (DLi+) and relationship of Warburg factor σw with Zre can be calculated from the following formulas:

Zre = R e + R ct + σ w ω-1/2

(2)

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R 2T 2 (3) DLi   2 4 4 2 2 2 A n F C W Re representatives the electrolyte resistance, Rct stands for the charge transfer resistance, Zre is related to the real part of the impedance, σw reflects the Warburg coefficient. Normally, the slope of the Zre-ω-1/2 (Figure 8(b)) can be used to evaluate the lithium-ion diffusion coefficient. In the Eq.(3), the DLi+ value is inversely proportional to σw and the other letters represent the different constants31, 49-50. It is evidently seen that the σw values of three samples appear without any remarkable difference because these samples are all in mesoporous structure, which play the same role on the lithium ion diffusion. Notably, compared with α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB anodes, the α-Fe2O3/C-PAN electrode possesses a smaller diameter semicircle which can be always defined as the smaller Rct, indicating the lower charge transfer resistance. As shown in Figure S5, with the increasing of cycle number, both the Rct and σw of α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composites increase rapidly, while α-Fe2O3/C-PAN electrode maintains in a relatively low value. This can be illustrated by the C-PAN bonding with contiguous α-Fe2O3 microspheres so as to maintain the better charge transfer velocity and higher electronic conductivity. Table 2 Impedance parameters of the three samples Samples

Re(Ω)

Rct(Ω)

σw (Ω cm2 s-1/2)

α-Fe2O3/cyclized-PAN

3.536

81.8

13.07

α-Fe2O3/PAN/AB

21.44

259

21.79

α-Fe2O3/PVDF/AB

20.58

185

21.96

Re: electrolyte resistance; Rct:: charge transfer resistance; σw: Warburg impedance 22


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Figure 8. (a) EIS spectra in a frequency range between 0.01 Hz and 100 kHz and (b) the relationship between Zre and ω-1/2 at low frequency of α-Fe2O3/C-PAN, α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composites

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Figure 9. The surface images of α-Fe2O3/C-PAN (a) and α-Fe2O3/PVDF/AB (b) electrodes after 100 cycles. The cross sectional images of α-Fe2O3/C-PAN (c) and α-Fe2O3/PVDF/AB (d) electrode after 100 cycles.

In order to verify the structure stability in the process of cycling test, Figure 9 shows

the

surface

and

cross

sectional

images

of

α-Fe2O3/C-PAN

and

α-Fe2O3/PVDF/AB electrodes after 100 cycles. It is clearly seen that α-Fe2O3 microspheres coated with C-PAN remain the superior structural integrity and stability compared with α-Fe2O3/PVDF/AB electrode, indicating that C-PAN with its inherent mechanical resiliency can accommodate the huge volume changes and maintain the structural stability of electrodes, which can facilitate the overall electrochemical properties.

Conclusions 24


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Overall, we reported an interesting facile two-step method to synthesize mesoporous α-Fe2O3/C-PAN microspheres as a superior anode material for LIBs. The fabricated electrode based on the mesoporous α-Fe2O3/C-PAN shows significantly enhanced electrochemical performance in comparison with α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composite, which delivers a high reversible capacity of approximately 996mAh g-1 after 100 cycles at 0.2C, even maintain 773mAh g-1 at 1C and 655mAh g-1 at 2C. The superior electrochemical performance of this material is believed to associated to the following points: (1) hierarchically nanostructured mesoporous α-Fe2O3 microspheres providing shorter path for Lithium-ion diffusion and sustaining the large volume changes during charge/discharge process; (2) the contribution of cyclized PAN with sp2π bonding like graphite to excellent electronic conductivity; (3) the binder ability of C-PAN showing a strong adhesion with active materials to enhance the mechanical resiliency of electrode. In addition to the electrochemical performance, our intriguing results suggest a novel strategy for the furtherance of high performance oxides anode materials.

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AUTHOR INFORMATION Corresponding Author Email: [email protected].

ASSOCIATED CONTENT Supporting Information Reaction scheme of α-Fe2O3/C-PAN electrode; synthetic mechanism of mesoporous α-Fe2O3 microspheres; CV and discharge/charge curves of the α-Fe2O3/PAN/AB composite; TEM and cycling performance of nanoparticle α-Fe2O3/C-PAN composite; the changes of Rct (a) and σw (b) for α-Fe2O3/C-PAN, α-Fe2O3/PAN/AB and α-Fe2O3/PVDF/AB composite during cycling.

ACKNOWLEDGMENT This work is supported by Shanghai Leading Academic Discipline Project (B502), Shanghai Nanotechnology Special Foundation (No. 11nm0500900) and Shanghai Key Laboratory Project (08DZ2230500).

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Enabling a High Performance of Mesoporous Î± ... - ACS Publications

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