Accepted Manuscript Title: Fe3 [email protected]
nanocapsules/expanded graphite as anode materials for lithium ion batteries Author: You-Guo Huang Xi-Le Lin Xiao-Hui Zhang Qi-Chang Pan Zhi-Xiong Yan Hong-Qiang Wang Jian-Jun Chen Qing-Yu Li PII: DOI: Reference:
S0013-4686(15)30304-2 http://dx.doi.org/doi:10.1016/j.electacta.2015.08.054 EA 25522
To appear in:
Received date: Revised date: Accepted date:
1-7-2015 10-8-2015 11-8-2015
Please cite this article as: You-Guo Huang, Xi-Le Lin, Xiao-Hui Zhang, QiChang Pan, Zhi-Xiong Yan, Hong-Qiang Wang, Jian-Jun Chen, Qing-Yu Li, [email protected]
nanocapsules/expanded graphite as anode materials for lithium ion batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.08.054 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.
nanocapsules/expanded graphite as anode materials for lithium ion batteries You-Guo Huanga, Xi-Le Lina, Xiao-Hui Zhanga, Qi-Chang Pana, Zhi-Xiong Yana, Hong-Qiang Wanga,c* [email protected]
, Jian-Jun Chend, Qing-Yu Lib* [email protected]
School of Chemical and pharmaceutical Sciences, Guangxi Normal University, Guilin, 541004,
Guangxi Key Laboratory of Low Carbon Energy Materials, Guilin, 541004, China
Hubei Key Laboratory for Processing and Application of Catalytic Materials, College of
Chemical Engineering, Huanggang Normal University, Huanggang, 438000, China d
Research Institute of Tsinghua University in Shenzhen, Shenzhen, 518057, China
ABSTRACT [email protected]
nanocapsules/expanded graphite composite was successfully prepared by a new and facile method, including mix of starting materials and heat treatment of the precursor. It is featured by unique 3-D structure, where expanded graphite acts as scaffold to ensure a continuous entity, and Fe3C particles coated by carbon nanocapsules are embedded intimately. The Fe3C nanoparticles encased in carbon nanocapsules act as catalyst in the modification of SEI film during the cycles. The interesting 3-D architecture which aligns the conductivity paths in the planar direction with expanded graphite and in the axial direction with carbon nanocapsules minimizes the resistance and enhances the reversible capacity. The prepared composite exhibits a high reversible capacity and excellent rate performance as an anode material for lithium ion batteries. The composite maintains a reversible capacity of 1226.2 mAh/g after 75 cycles at 66 mA/g. When the current density increases to 200 mA/g, the reversible capacity maintains 451.5 mAh/g. The facile synthesis method and excellent electrochemical performances make the composite expected to be one of the most potential anode material for lithium ion batteries.
Keywords: Lithium ion batteries; Anode materials; Iron carbide; Expanded graphite.
1. Introduction With the fast development of mobile electric application, lithium ion batteries with high energy/power density and better cycling performance are in ever-increasing need. As commercial anode currently, graphite owes high electrical conductivity, better recyclability. However, the low theoretical specific capacity (372 mAh/g) and poor rate capability restrict its application on high performance batteries. Therefore, the development of new carbon-based anode materials with enhanced reversible capacity and fast charge/discharge ability is of utmost importance. Expanded graphite (EG), the most important graphite derivative, has aroused considerable interest[2-6]. Compared with graphite, EG exhibits much higher capacity owing to its larger pores, larger surface area, high conductivity, structural defects, chemical stability and extra space for lithium-ion storage. Bai et al applied modified Hummers’ method to prepare EG which maintained 412 mAh/g after 30 cycles at the current density of 0.2 mA/cm2. Mildly EG was synthesized by using perchloric acid as both intercalating agent and oxidizing agent, displaying a rate capacity as high as 397 mAh/g at 0.2C and 250 mAh/g at 1.6C. From the previous work, EG shows a higher reversible capacity than traditional carbon material, but it is still very low. The loss of capacity partly attributes to the irreversible formation of solid electrolyte interface (SEI) film. Recently, Zhou's group proposed that the newly-generated transition metal nanoparticles could activate the reversible transformation of some SEI components 3
and further benefited reversible capacity[9, 10]. Especially, some authors pointed out that iron carbide (Fe3C) could improve the electrochemical performance of the carbonaceous anode under similar mechanism[1, 11-13]. In 2012, an [email protected]
/C composite was first reported as an anode material for lithium ion batteries, which gave a capacity of 500 mAh/g after 30 cycles due to the modification of SEI film by Fe3C . The obvious extra capacity can be attributed to the reversible formation and dissolution of polymeric gel-like SEI film along with the discharge and charge process. Indeed, this reversible gel-like film is part of SEI components. Fe3C shells can reduce some SEI components. Considering the catalysis function of Fe3C, many researches have adopted various methods to prepare Fe3C/C composite with interesting morphologies and excellent electrochemical performances (shown in Table 1 and Table 2), including in-situ electrospinning, polymerization-pyrolysis[1, 12], sol-gel process. However, these traditional synthesis of Fe3C/C composite are often difficult because of high reaction temperature, using hazardous and expensive chemical precursors, advanced equipment and tedious steps, and the as-obtained composite demonstrates the highest capacity reported so far 1098 mAh/g. In this work, we attempt to design [email protected]
nanocapsules (CNCs)/EG composite with cheap ferroence and EG through a facile process including reflux and heat treatment under low temperature (below 600
C). The as-obtained
/EG exhibits excellent electrochemical performances, including cycle ability and rate performance.
2. Experimental 4
2.1. Materials Synthesis [email protected]
/EG was fabricated by an in-situ synthesis method as following: The starting materials were ferrocene (Xi Long Chemical Share Co. Ltd), hydrogen peroxide (Guang Dong Guang Hua Sci-Tech Co. Ltd), EG (Qing Dao Graphite Material Co. Ltd), acetone (Xi Long Chemical Share Co. Ltd). In a typical synthesis, ferrocene (12.00 g) was dissolved in acetone to form a clear solution. EG (1.00 g) was added into above solution with vigorous stirring for 8 h. When the temperature cooled down, H2O2 (11.00 g) was added into the above solution. Then the solution was maintained boiling for 12 h and allowed to cool down to room temperature naturally. The precipitate was washed by ethanol for several times and dried at 50 oC for 12h. Then [email protected]
/EG was obtained through calcination under nitrogen atmosphere at 550 oC for 0.5 h. 2.2. Materials characterization The morphologies of the as-prepared [email protected]
/EG composite material and EG were characterized by scanning electron microscope (SEM, Phillips, FEI Quanta 200FEG) with EDS analyzer and transmission electron microscope (TEM, JEM-2100F). Crystal structure of the [email protected]
/EG composite material was characterized by X-ray diffraction (XRD, Rigaku D/max 2500) with Cu Ka radiation (λ=1.5405 Å). The graphitization degree was evaluated by Raman spectroscopy (Ren-ishaw 1000) with laser excitation at 514 nm. X-ray photoelectron spectroscopy (XPS) measurement was carried out on PHI X-Tool. TGA-DSC data of the
/EG composite material was obtained by thermal analyzer (Labsys TG-DTG/DSC, France) with a heating rate of 10 oC /min in flowing air atmosphere. 2.3. Electrochemical Measurements The electrochemical measurements of [email protected]
/EG were examined using CR2025 cells with lithium metal as the counter electrode. The working electrode were prepared by mixing the [email protected]
/EG (50 wt%), super p (30 wt%) and polyvinylidene fluoride (20 wt%) in N-methyl-2-pyrrolidide and pasted onto the copper foils. The average weight of the working electrodes was approximately 0.8 mg. The electrodes were dried at 80 oC for 12h. A thin porous polypropylene film was used as the separator (Celgard 2000, 16 μm). 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by weight) was used as electrolyte. The cells were assembled in an argon-filled glove box with the concentrations of moisture and oxygen below 0.1 ppm. Galvanostatic electrochemical experiments were carried out with BT2013A Battery Test System. Electrochemical impedance spectroscopy (EIS) were recorded between 200 kHz and 5 mHz with an amplitude of 5 mV on IM6 (Germany). Cyclic voltammetry (CV) measurements were carried out on the same IM6 over the potential range 0-3.5 V at a scan rate of 0.2 mV/s.
3. Results and discussion As shown in Fig. 1a, the crystal structure of pristine EG was studied by X-ray diffraction. In the pattern of EG diffraction peaks are observed at 2θ= 26.5°, 42.2°, 6
44.4°, 54.5° and 77.2°: these peaks can be indexed to the Graphite-2H (002), (100), (101), (004) and (110) faces, respectively (PDF No. 41-1487), which is representative of EG in the crystalline phase. The XRD analyses illustrated in Fig. 1b indicates a similar curve with EG except some peaks located at the range from 35° to 50°, which implies that the addition of ferrocene and hydrogen peroxide does not change the structure of EG. A close look at the enlarged image shown in Fig. 1b can attribute these peaks to orthorhombic phase of Fe3C (PDF No. 65-2413). These indicate that the material consists of multiple phases including graphitic carbon and Fe3C. However, peaks of Fe3C are not very sharp and narrow, which implies the unique structure. The Fe3C phase is encased by CNCs, which will be further discussed by high resolution TEM and EDS. The surface composition is of particular interest and the surface content of Fe was examined by high-resolution X-ray photoelectron spectroscopy (XPS). As shown in Figure. 1c, XPS signals are noisy and try to increase the number of scans, which however does not lead to any improvement unless for the sample with higher elemental contents. A trivial Fe-2p signal（Fe 2p1/2 at 724 eV and Fe 2p3/2 at 711 eV of Fe3+）is observed for [email protected]
/EG[13, 15-17], signifying a negligible share of iron atoms (1.6 at%) on the surface. The low iron surface contents imply that the Fe3C nanoparticles are exclusively encased by carbon layers, which can be further verified by other tests like high resolution TEM and EDS. Fig. 2 revealed the TGA/DSC results. The initial weight loss of 10 wt % from 100 o
C to 350 oC corresponds to the evaporation of the weakly adsorbed water and the
oxidation of some remained organic groups[13, 16, 23]. With increasing temperature, 7
an obvious weight loss occurs near 350 oC extending to 580 oC, accompanying with an exothermic peak at 550 oC in the DSC curve. This is due to the oxidation of Fe3C. A striking weight loss happens between 580 oC to 740 oC with a strong and sharp exothermic peak at 700 oC, which is related to the oxidation of graphitic carbon. That is deduced by other homemade composites based the same EG which also show the similar exothermic peak around 700 oC. According to the chemical equations: C + O2 = CO2, 2Fe3C + 13/2O2 = 3Fe2O3 + 2CO2, the mass contents of Fe3C and graphitic carbon in the sample are calculated about 18.75 wt% and 81.25 wt%, respectively. The Raman spectra of [email protected]
/EG and EG are shown in Fig. 3, both displaying a characteristic graphite G-band at 1580 cm-1, the D-band at 1350 cm-1 and a 2D-band at 2710 cm-1. Additionally, it is found that ID/IG of [email protected]
/EG is 0.725, lower than that of EG (0.777), which supplies the evidence of the higher graphitic degree of [email protected]
/EG. The results originating from the unique 3-D structure and the catalysis role of iron element enhances electric conductivity and thus promotes charge transfer. The morphology and microstructure of as-obtained sample were investigated by SEM and TEM. Fig. 4a shows the rippled and fluffy nature of thermally-exfoliated graphite sheets, which is used as the starting material of [email protected]
/EG. Some scrolling of the EG sheets are caused by thermal exfoliation. EG shows abundant pores of different sizes ranging from scores of nanometers to a few micrometers. A careful look at the raw EG reveals the surface is quite smooth. When compared to the raw material EG, the sheets of [email protected]
/EG composite material become rough 8
with uniform bright dots distributed on, as shown in Fig. 4b. Fig. 4c provides a different magnification of [email protected]
/EG composite. Viewed from the edge, the submicrometer-particles sandwiched by EG are observed as one tectonic unit, forming an alternating architecture as a whole. Some bright and white dots with an average diameter of 60 nm adhering to the surface of the EG sheets or between pores of EG are clearly observed. The white and bright dots are in-situ created during the oxidation and subsequent heat treatment process. Fig. 4d provides a more close connection between EG and dots. In order to determine the internal structure and component of [email protected]
/EG composite, elemental mapping by EDS spectrum was used to analyze elements distribution. The peaks of C and Fe are obviously identified in Fig. 5. A higher Fe content can be found in the bright and white dot region (Fig. 5b) than that in other region (Fig. 5a). It can be verified that Fe3C nanoparticles should be located mainly in the bright and white dot region. The inset in Fig. 4d presents the dots in detail. Around 60 nm Fe3C nanoparticles are well encapsulated in CNCs. The layer of CNCs is about 4 nm. Based on the analyses of crystal structure and morphology, a schematic representation of the formation process of the self-assembled 3-D [email protected]
/EG is proposed in Fig. 6. During the reflux process, driven by the π-π conjugate interaction, ferrocene is enriched up between EG sheets and dispersed on its surface using acetone as the solvent. When the H2O2 is added, the iron atom in ferroncene is oxidized. When the mixture is subjected to calcination, forrocene thermally decomposes into iron nanoparticles and carbonaceous gas[18, 19]. Carbon invades the Fe catalyst seeds 9
very rapidly, as carbon diffusion is very fast, thus reaching almost instantaneously the carbon solubility limit. The carbon then has to precipitate, either directly in the form of graphite, or in the form of Fe3C. However, Fe3C, not graphite should precipitate first from an iron solution because of the insufficient temperature (below 600 oC) and then forms graphitic carbon-coated Fe3C nanoparticles. Fe3C would release carbon atoms to the graphitic carbon, leading to the growth of CNTs as well as creating carbon vacancies in the Fe3C lattice that migrates to the nanoparticle’s surface where the gas is decomposed. CNTs growth follows the tip-growth mechanism, maintaining catalyst at its top. But as insufficient temperature and short calcination time is operated, CNCs, a special type of CNTs are obtained[20-22]. Fig. 7a presents the discharge-charge profiles of [email protected]
/EG in selected cycles. The composite shows a discharge capacity of 2021.9 mAh/g and charge capacity of 896.8 mAh/g in the first cycle, showing a low columbic efficiency of 44.4% due to the formation SEI film. Interestingly, the charge capacity delivers 1017.4 mAh/g at the tenth cycle with a columbic efficiency of 96.0%. The subsequent cycles deliver a stable charge capacity around 1200 mAh/g and nearly 100% columbic efficiency, which will be given in Fig. 8c and Fig. 8d. The capacity change with the increase of cycling numbers agrees with the previous reports. It is due to the modification of SEI film by Fe3C nanoparticles. To better explain the catalysis mechanism, CV was measured for [email protected]
/EG. Fig. 7b shows the CV curve of [email protected]
/EG. The material shows cathodic peaks at 1.3 V, 0.7 V and 0.1 V in the first cycle. Enlightened by the previous work, the peak at 1.3 V can be attributed to 10
the preliminary decomposition of electrolytes and the formation of SEI film, whereas the low-potential peak at 0.7 V is assigned to the further electrochemical reduction of some SEI components. The peak at 0.1 V is related to the lithium ion intercalation into the graphitic carbon. From the second cycle, the peak at 0.7 V increases to 0.8 V under the catalysis of Fe3C nanoparticles, but the peak at 1.3 V disappears. There are two anodic peaks at 0.2 V and 1.1 V, representing the removal of lithium and the lithium extraction from SEI film respectively, repeats in the subsequent cycles[10, 12, 13]. That can suggest the cathodic peak at 0.8 V and the anodic peak at 1.1 V are associated with the formation and decomposition of partially reversible SEI film. Fig. 7c displays the CV curve after rate performance, delivering almost overlapped curves. It implies the extremely good cyclic stability of [email protected]
/EG and its well-designed 3-D architecture. Fig. 7d supplies the first and second discharge-charge curves of [email protected]
/EG and EG at 66 mA/g. The first and second discharge-charge curves of [email protected]
/EG conform to its CV curves shown in Fig. 7b. However, the first and second discharge-charge curves of EG show a big difference compared with that of [email protected]
/EG. It only appears a small discharge plateau at 0.7 V in the first one and disappears in the second one. Moreover, there is no charge plateau around 1.1 V. The difference between [email protected]
/EG and EG results from the catalysis the Fe3C nanoparticles which improves the reversible capacity and benefits the cycle ability. In order to well understand the specific capacity, cyclic stability and rate performance of the [email protected]
/EG composite materials in lithium ion battery, the 11
electrochemical properties with respect to Li insertion/extraction are investigated. Fig. 8a shows the capacities of [email protected]
/EG electrode at various charge/discharge rates. The cell is first cycled at 100 mA/g for 20cycles, followed by cycling with a stepwise increase to 500 mA/g, and finally back to 100 mA/g. Meanwhile, reversible capacity of more than 350 mAh/g at 500 mA/g is achieved. By returning to the initial 100 mA/g, the electrode rebounds to the original one, demonstrating excellent rate performance and stability of [email protected]
/EG electrode. The results are superior to pure EG electrode (Fig. 8b). The cycling performance of [email protected]
/EG and EG under long-term cycling are examined at 66 mA/g in the range of 0.01-3.5 V. As shown in Fig. 8c, both electrodes indicate good cyclic performance and reversibility. After 75 cycles, the [email protected]
/EG electrode still holds about a specific reversible capacity of 1226.2 mAh/g and the columbic efficiency stays at around 100% except the initial few cycles. By contrast, the EG electrode has a reversible capacity of only around 355.2 mAh/g under the same test condition. The [email protected]
/EG composite still delivers 451.5 mAh/g at 200 mA/g, as shown in Fig. 8d. All demonstrates that [email protected]
/EG has extremely excellent electrochemical stability. EIS measurements are further conducted to elucidate the electrode reaction features. Fig. 9 shows the Nyquist plots of [email protected]
/EG and EG. Both profiles consist of a semicircle in the high frequency region associated with the charge transfer resistances, and a long low-frequency line corresponding to the lithium-diffusion process within electrodes. Obviously, the intercept of the semicircle with real axis for the electrode [email protected]
/EG is much smaller than that of EG electrode, indicating the smaller 12
charge transfer resistance and improved electrical conductivity of [email protected]
/EG electrode. This improved electrical conductivity is beneficial to lithium ions or electrons transfer, leading to high rate capacity, and reduced polarization for [email protected]
/EG electrode, which matches well with Raman results and electrochemical performances. The highly reversible capacity, good cyclic stability and rate performance can be ascribed to the unique 3-D architecture and in-situ generated Fe3C nanoparticles. Firstly, the 3-D sandwich-like structure where EG acts as scaffold to ensure a continuous entity and CNCs embedded intimately aligns the conductivity paths in the planar direction with EG sheets and in the axial direction with CNCs, leading to minimal resistance[9, 19]. CNCs directly bonding to the EG sheets enables the contact resistance at junction to minimize, consequently facilitating the current approach to the anode as well as enhancing the diffusion of lithium ion toward the electrode. Secondly, although Fe3C is only 26 mAh/g, negligible to the whole [email protected]
/EG electrode, the Fe3C nanoparticles also paly an incomparable role to acquire additional capacity from SEI film. SEI films have various organic compounds with carboxyl or carbonyl bonds, which have the capability to react with Li+ and present considerable capacity[9, 10]. And how much is the additional capacity? The contribution from graphitic carbon in [email protected]
/EG composite and super p should be deducted. The graphitic carbon is assumed to be theoretical capacity of 372 mAh/g and the average capacity of super p is tested to be 199.5 mAh/g at 66 mA/g (shown in Fig. 10). As revealed by TG/DSC curve in Fig. 2, the mass contents of Fe3C and graphitic carbon 13
in the sample are calculated about 18.75 wt% and 81.25 wt%, respectively. A minimum reversible capacity of 3863.7 mAh/g can be attributed to the extra capacity from SEI film, indicating the good catalysis effect from Fe3C nanoparticles. For approximate processing, the calculated capacity of 3863.7 mAh/g is based on the following
0.8125)]/0.1875=[1226.2-199.5-(372 * 0.8125)]/0.1875=3863.7 mAh/g.
4. Conclusion In summary, a novel [email protected]
/EG composite with in-situ generated Fe3C nanoparticles and interesting 3-D structure was synthesized by a new and facile method, which includes mix of starting materials and heat treatment under low temperature. It is featured by unique 3-D structure, where EG acts as scaffold to ensure a continuous entity, and Fe3C coated by CNCs are embedded intimately on the EG matrix. The Fe3C nanoparticles encased in CNCs act as catalyst in the modification of SEI film during the cycles, combining the interesting 3-D structure, the composite exhibits a high reversible capacity, stable recyclability, rate performance, and reduced resistance. It delivers a high reversible capacity of 1226.2 mAh/g after 75 cycles at 66 mA/g. The excellent electrochemical performance as well as the facile synthesis route makes [email protected]
/EG composite promising for application in high-performance lithium ion batteries.
Acknowledgements This work is supported by the National Natural Science Foundation of China (U1401246
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Table 1 Synthesis methods and electrochemical performances of Fe3C/C composites reported Fe3C/C Synthesis Tested Synthesis Current Cycling N-do composite method capacity temperature density number ped (mAh/g) (mA/g) o Fe/Fe3C-CNFS electrospinning 500 550 C 200 70 No o C/Fe3C polymerization- 750 700 C 100 120 Yes pyrolysis Graphene/Fe- pyrolysis 1098 800 oC 100 48 Yes Fe3C [email protected]
/C hydrothermal 500 600 oC 50 30 No method and calcination
Table 2 Structures of Fe3C/C composites reported Fe3C/C Structure composite Fe/Fe3C-CNFS randomly oriented, overlapped, continuous and interconnected nanofibers. C/Fe3C some small white spheres with a diameter of about 300 nm are embedded in the bulk materials Graphene/Fe-Fe3 Fe and Fe3C particles are loaded homogeneously on the C graphene sheets. [email protected]
/C wire and film-like carbon coexisted between [email protected]
/C core-shell nanoparticles
Figures Fig. 1 (a) XRD patterns of EG and (b) [email protected]
/EG. The inset in (b) is an enlarged
/EG Fig. 2 TGA/DSC curve of the [email protected]
/EG Fig. 3 Raman spectra of [email protected]
/EG and EG Fig. 4 (a) SEM image of EG; (b，c) [email protected]
/EG with different magnification; (d) TEM image of [email protected]
/EG. The inset in (d) shows the TEM image of [email protected]
in detail Fig. 5 EDS spectra of [email protected]
/EG Fig. 6 Self-assembly of 3-D [email protected]
/EG Fig. 7 (a) Rate capability of [email protected]
/EG and (b) EG; (c) Cycle ability of [email protected]
/EG and EG at the current density of 66 mA/g; (d) Cycle ability of [email protected]
/EG at different current densities Fig. 8 (a) Discharge-charge curves of [email protected]
/EG at selected cycles; (b) CV curve of fresh [email protected]
/EG electrode and (c) after rate performance test at the scan rate of 0.2 mV/s; (d) The first and second discharge-charge curves of 21
/EG and EG Fig. 9 Impedance plots for fresh [email protected]
/EG and EG electrodes Fig. 10 Cycle ability of super P at the current density of 66 mA/g
Fig. 8 25