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Electrochimica Acta
jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta
e3O4 nanoparticles embedded in carbon-framework as anode material for higherformance lithium-ion batteries
ang Yua, Yongchun Zhua,∗, Huaxu Gonga, Yanmei Maa, Xing Zhanga, Na Lia, Yitai Qiana,b,∗
Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR ChinaSchool of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, PR China
r t i c l e i n f o
rticle history:eceived 9 April 2012eceived in revised form 2 August 2012ccepted 2 August 2012
a b s t r a c t
Fe3O4/C composites have been prepared by sucrose calcining with Fe3O4 particles obtained from ferrousoxalate decomposition. The scanning electron microscopy (SEM) images show that Fe3O4 nanoparticles(Fe3O4 NPS) with average size of 200 nm are embedded in the three-dimensional (3D) carbon-framework.As an anode material for rechargeable lithium-ion batteries, the Fe3O4/C composite delivers a reversible
vailable online 10 August 2012
eywords:agnetite
arbon-framework
capacity of 773 mAh g−1 at a current density of 924 mA g−1 after 200 cycles, higher than that of the bareFe3O4 NPS which only retain a capacity of 350 mAh g−1. When the current density rises to 1848 mA g−1,Fe3O4/C material still remains 670 mAh g−1 even after 400 cycles. The enhanced high-rate performancecan be attributed to the 3D carbon-framework, which improves the electric conductivity, relaxes the
s the
ithium-ion batteryigh-rate performance
strain stress and prevent
. Introduction
In the last few decades, graphite is mostly used as a commercialnode material in the lithium-ion batteries (LIBs) with a theoreti-al capacity of 372 mAh g−1 [1,2]. With the development of the higherformance LIBs, transition metal oxides (MO, where M is Fe, Co,i or Cu, etc.) have been studied as a new series of anode materialsue to their higher specific capacity compared with that of graphite3–11]. Among these available alternative anode materials, mag-etite (Fe3O4) has always been regarded as an appealing candidateue to its high theoretical specific capacity (∼924 mAh g−1), as wells nontoxicity, high corrosion resistance and low processing cost12]. As reported in the literatures [13–15], the Fe3O4 materials
ain follow the conversion reaction mechanism and are reduced tomall metal clusters accompanying with the Li+ uptake and release.he electrochemical reactions can be described as follows:
e3O4 + 8Li+ + 8edisch arge−→ 3Fe + 4Li2O (1)
Li+ + 8echarge−→ 8Li (2)
e3O4 + 8Li ↔ 3Fe + 4Li2O (3)
Fe3O4 based anodes undergo a significant volume change,esulting in large potential hysteresis, capacity fading and poor
∗ Corresponding authors at: Hefei National Laboratory for Physical Science aticroscale and Department of Chemistry, University of Science and Technology of
cycling performance [16]. In nowadays, carbon coating is knownas one of simplest and the most effective strategies in improv-ing the electric conductivity and restrain volume change duringthe charge/discharge process. For instance, a dispersed Fe3O4nanospindle coated with carbon can remain 530 mAh g−1 after80 cycles at a current density of 460 mA g−1 [17]. The Fe3O4/Cnanofibers exhibit a reversible capacity of 1000 mAh g−1 after80 cycles at 200 mA g−1 [18]. Fe3O4/C core–shell nanospherespresent a capacity of 636 mAh g−1 over 50 cycles at 1000 mA g−1
[19]. Graphene sheets modified Fe3O4 NPS deliver a capacity of550 mAh g−1 even after 300 cycles at 1000 mA g−1 [20].
Herein, we prepared Fe3O4/C composites in which Fe3O4 NPSwere embedded in the 3D carbon-framework. The composites candeliver a high reversible capacity of 773 mAh g−1 at 924 mA g−1
after 200 cycles, together with a capacity of 670 mAh g−1 at ahigher current density of 1848 mA g−1 until the 400th cycle. While,without carbon coating, the bare Fe3O4 NPS can only deliver350 mAh g−1 up to 200 cycles at a current density of 924 mA g−1.The remarkable high-rate performance of the composites indi-cates its promising application as anode material for lithium-ionbatteries.
2. Experimental
2.1. Preparation of Fe3O4 NPS
In a typical synthesis, a homogeneous solution containingFeSO4·7H2O 8 mmol (2.224 g) and 5.043 g citric acid was first pre-pared in 30 mL distilled water, meanwhile, another solution wasprepared by dissolving H2C2O4·H2O 10 mmol (1.08 g) in 10 mL
istilled water, then, the oxalic acid solution was added to the for-er solution under continuous stirring. Subsequently, the yellow
errous oxalate precipitate was filtered and washed. Ferrous oxalaterecipitate and 5 mL ethanol were sealed in a 20 mL stainless steelutoclave and calcined at 550 ◦C for 5 h. After the autoclave cooledown, the black powders were washed with absolute ethanol andistilled water for several times.
.2. Preparation of Fe3O4/C composites
The obtained Fe3O4 powders were milled with sucroseFe3O4/sucrose = 3:1, w/w) in a ball mill (400 r/s, 6 h). Then, the
ixture was annealed in a quartz tube with a slow ramping rate of◦C/min to 600 ◦C for 5 h in Ar atmosphere. The Fe3O4/C compositesere obtained.
.3. Characterization
X-ray powder diffraction (XRD) patterns of the products wereecorded on a Philips X’pert X-ray diffractometer with Cu Kaadiation (� = 1.54178 A). X-ray photoelectron spectra (XPS) wereested on a VGESCA-LAB MKII X-ray photoelectron spectrometer,sing non-monochromated Mg K� X-ray radiation as the excita-ion source. Raman spectrum was carried out on a JYLABRAM-HRonfocal Laser Micro-Raman spectrometer with 514.5 nm from anrgon laser at room temperature. The scanning electron microscopySEM) images were taken by using a JEOL-JSM-6700F field-emittingFE) scanning electron microscope. The high-resolution transmis-ion electron microscope (HRTEM) images were taken on a JEOL010 HRTEM at an acceleration voltage of 200 kV. EIS were per-ormed by a Zahner Elektrik IM6 (Germany) impedance instrumentver the frequency range from 100 kHz to 0.01 Hz.
The active materials, acetylene black and poly (vinylideneifluoride) (PVDF) with a weight ratio of 60:30:10 were mixedomogeneously with N-methyl-2-pyrrolidone (NMP), the obtainedlurry was coated on a copper foil and dried at 100 ◦C for 12 h inacuum. The coin cells (size: 2016) were assembled in an argon-lled glove box with lithium foil as the anode, celgard 2400 ashe separator, and a solution of 1.0 M LiPF6 in ethylene carbon-te (EC)/diethyl carbonate (DEC) (1:1 by volume) as the electrolyte.he total loading of active material in the electrode is 1.77 mg cm−2.he galvanostatic charge and discharge tests were carried out on aAND-CT2001A instrument in the potential range of 0.01–3.00 Vversus Li/Li+). The cells were charged/discharged at 25 ◦C in aH3600 electro-thermal constant-temperature incubator (Taisite,hina).
. Results and discussion
The crystallographic structures of the samples are identified byRD. Fig. 1 shows the typical XRD patterns of the as-obtained prod-cts. The diffraction peaks of Fe3O4 NPS (Fig. 1a) and the Fe3O4/Composite (Fig. 1b) can be readily indexed as face-centered cubictructure Fe3O4 with a space group of Fd3m (JCPDS Card No: 75-033). From Fig. 1b, no diffraction peak for carbon can be observed.n addition, in our XRD patterns, there are no typical �-Fe2O3 (JCPDSard No: 39-1346) peaks such as (1 1 0), (2 1 0), (2 1 1) (the inten-ities of these peaks are even more than (1 1 1) existing in thebtained XRD patterns), indicating the absence of �-Fe2O3.
The representative Raman spectrum of the as-obtained Fe3O4/Composite is used to investigate the presence of carbon. As shownn Fig. 2a, the two peaks at 1350 and 1587 cm−1 are the charac-
eristic peaks corresponding to the D-band and G-bond of graphite21]. D-band is associated with the vibration of carbon atoms withangling bonds in-plane terminations of disordered graphite, and-bond is related to the vibration of sp2-bonded carbon atoms in a
Fig. 1. Typical XRD patterns of the as-obtained products (a) Fe3O4 nanoparticles (b)the Fe3O4/C composite.
2D hexagonal lattice. Here, the ratio of ID/IG is 0.757, implying poorcrystallinity of carbon [22].
The composition of the as-obtained composite is further identi-fied by XPS. Fig. 2b is the wide scan of product, and the sharp peaksof the C1s, O1s and Fe2p indicate the existence of carbon, oxygenand iron elements in the composite. Fig. 2c is the binding ener-gies of Fe2p, the peaks located at 711 and 724.5 eV correspondingto Fe2p3/2 and Fe2p1/2, respectively, which are in good agreementwith the reported values of Fe3O4 in the literatures [23,24].
The morphologies and structures of the as-obtained productsare characterized by SEM and TEM. The typical SEM image of Fe3O4NPS is shown in Fig. 3a, which shows the particles with an averagesize of 200 nm. Fig. 3b is a representative SEM image at a low magni-fication of the composite. It indicates that the main products consistof some particles embedded in framework. Fig. 3c is the TEM imageof the composite, which shows strong contrast between the lightand dark parts in the enlarged area (inset in Fig. 3c). The HRTEMimage (Fig. 3d) of the dark part in Fig. 3c shows clear crystal lat-tices with d-spacing of 0.294 nm, corresponding to the (2 2 0) planeof the face-centered cubic Fe3O4 crystals. The light part is consid-ered as carbon. To further investigate the morphology of carbon,we remove Fe3O4 NPS by stirring the Fe3O4/C composites in theconcentrated hydrochloric acid for 12 h. The left black powders arecarbon. Fig. 3e is the SEM image of the as-remained carbon. It isfound that the square area is the position of Fe3O4 NPS and carbonlayers interconnect to each other to form a 3D framework structure.Fig. 3f is the TEM image of the carbon-framework. The carbon edgeand the void area indicate that the Fe3O4 NPS are actually embed-ded uniformly in the interconnected carbon-framework. Sucrosecan mix with Fe3O4 particles uniformly after a long-milling. Whenthe temperature is elevated, sucrose is gradually carbonized in situ.The Fe3O4 particles disperse in a carbon matrix after the annealingprocess to form the 3D Fe3O4/C composite. The similar process hasbeen reported in other researches [25].
The as-prepared Fe3O4 NPS and Fe3O4/C composite materialsare assembled into coin cells to investigate their electrochemicalbehaviors. As shown in Fig. 4a, the charge/discharge profiles of theFe3O4/C electrodes are tested at a current density of 92 mA g−1. Thefirst discharge curve is with a long voltage plateau at about 0.75 Vversus Li/Li+, which is close to that reported in the literature forFe3O4/C anodes [26] and could be attributed to the reduction ofFe2+/Fe3+ to Fe0 [27], and the discharge specific capacity is as highas ∼1360 mAh g−1. The over discharge capacity is approximately
consistent with the sloping voltage below 0.37 V. It is attributed tothe formation of the SEI film and further lithium consumption viainterfacial reactions [28]. The voltage plateau of the first chargecurve is present at about 1.8 V, corresponding to the reversible
Y. Yu et al. / Electrochimica Acta 83 (2012) 53– 58 55
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Fig. 2. (a) Raman spectrum of the Fe3O4/C composite; XPS su
xidation of Fe0 to Fe2+/Fe3+. The charge/discharge curves of theollowing cycles (2nd–5th) appear to overlap and the obtainedpecific capacity remains unchanged and stables at 924 mAh g−1,hich is almost consistent with the theoretical value. Similarly,
he wide voltage plateau of these charge curves centered at 1.8 Vs observed, while the plateau of the discharge curves is present at.0 V.
Fig. 4b is the cycle performances of the bare Fe3O4 NPS ande3O4/C composites. The electrodes are firstly tested at a cur-ent density of 92 mA g−1 for five cycles, then at 924 mA g−1 untilhe 200th cycle. Finally, the current density returns to 92 mA g−1.lthough the initial discharge capacity of the Fe3O4 NPS electrode
s as high as 1432 mAh g−1, it decreases to 660 mAh g−1 at the 5thycle and fades to 200 mAh g−1 immediately as the density riseso 924 mA g−1, which is only about 71.4% and 21.6% of the theo-etical specific capacity, respectively. It is note that the capacityan stabilize at ∼350 mAh g−1 (37.9% of the theoretical specificapacity) after the density returns to 92 mA g−1, which may bettributed to its high crystallinity [29]. By comparison, the firstve cycles of the Fe3O4/C composites is the same as Fig. 4a. It stillan deliver a capacity of ∼924 mAh g−1 at the 5th cycle. As theensity rises to 924 mA g−1, the cell experiences a gradual capac-
ty fading process. After the 40th cycle, the capacity rises steadilynd reaches 773 mAh g−1 at the 200th cycle which is ∼83.7% ofhe theoretical capacity of Fe3O4 and more than double theoret-cal capacity of graphite (372 mAh g−1). The stage of the capacityecrease at the initial 40 cycles may be explained by the struc-ure re-organization of the carbon coatings [30]. The phenomenonf the gradual increased capacity is attributed to the reversiblerowth of a polymeric gel-like film resulting from kinetically acti-
ated electrolyte degradation, which is well-documented in theiteratures [31–33] and similar results have been reported for
any transition metal oxides [34,35]. In addition, the reversibleapacity of the Fe3O4/C composites reaches about 1100 mAh g−1
pectra of the Fe3O4/C composite: (b) wide scan and (c) Fe2p.
after 200 cycles, which indicates the good cyclic stability of thecomposites.
Rate-performance of the Fe3O4/C composite is further investi-gated. As shown in Fig. 5a, after 5 cycles, the current density isgradually increased with a few cycles at current densities of 924,1848 and 2310 mA g−1. The reversible capacities are 700, 540 and500 mAh g−1, which are 75.8%, 58.4% and 54.1% of the theoreticalspecific capacity, respectively. More importantly, when the currentdensity is reduced to 924 and then 92 mA g−1, the capacities swiftlyapproach to the same values which obtained at the same density inthe previous cycles. Note that even at the highest current density of2310 mA g−1, the specific capacity is still higher than the theoreticalcapacity of graphite.
We further test the cyclic stability of the Fe3O4/C compositesat a higher density. A new cell is cycled at 92 mA g−1 for 3 cycles,followed by cycling at 1848 mA g−1 for 400 cycles (Fig. 5b). Thereversible specific capacity is around 670 mAh g−1 (72.5% of thetheoretical specific capacity) with virtually no capacity loss for thecycles, except the stage of the capacity decrease at the former 40cycles which is the same as the phenomenon in Fig. 5b. The coulom-bic efficiency (the black dots in Fig. 5b) maintains consistently at∼98% throughout the cycling.
To gain further understanding of the 3D carbon-framework, theelectrochemistry impedance spectra (EIS) of the Fe3O4 particlesand Fe3O4/C electrodes are investigated. Fig. 6 shows the typicalNyquist plots of the cells after 10 cycles at 924 mA g−1. The inter-cept at the real (Z′) axis in high frequency range corresponds tothe ohmic resistance (Re). The semicircle in the middle frequencyindicates the charge transfer resistance (Rct). The inclined straightline relates to the Warburg impedance (Zw). It is reported that
the cell impedance is mainly determined by Rct [36]. As can beseen in Fig. 6, the semicircle diameter of the Fe3O4/C compositeis much smaller than that of Fe3O4 particles. The result of theEIS analysis indicates that carbon-framework has an important
56 Y. Yu et al. / Electrochimica Acta 83 (2012) 53– 58
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ig. 3. (a) SEM image of Fe3O4 particles; (b) SEM image, (c) TEM image and (d) HRramework; the inset in (c) is the enlarged TEM image of the selected area.
ole in reducing the Rct of cells during charging/discharging.
he good electrochemical performance of the Fe3O4/C compos-te is believed to be attributed to the unique carbon-frameworktructure. Firstly, the 3D carbon-framework not only remarkablynhances the electric conductivity, but also provides continuous
ig. 4. (a) Charge/discharge profiles of the Fe3O4/C composite at a current density of 92 m current density of 924 mA g−1 for 200 cycles.
image of the Fe3O4/C composite; (e) SEM image and (f) TEM image of the carbon-
paths between Fe3O4 NPS, thus ensures the fast and continuous
transportation of electrons between Fe3O4 NPS and carbon, whichis favorable for electrons moving unimpeded over particles to attaina high rate capability [25]. Secondly, the interconnected carbon lay-ers and Fe3O4 NPS build a special micro-nanostructure, which can
A g−1. (b) Cycle performance of Fe3O4/C composite and the bare Fe3O4 particles at
Y. Yu et al. / Electrochimica Acta 83 (2012) 53– 58 57
Fig. 5. (a) Rate performance of the Fe3O4/C composite at various current densities. (b)
density of 1848 mA g−1 (the front three cycles at 92 mA g−1).
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Fig. 6. Impedance spectra of the anodes obtained after 10 cycles.
table the composite, prevent the active materials aggregation orulverization and effectively improves the cyclic performance ofaterials [37]. Thirdly, the uniformity of carbon distribution based
n the framework can relax the strain stress, buffer the volumexpansion and hence improve the cyclic stability of the materials.he above three important roles favor the high reversible capacitynd superior cyclic performance of the Fe3O4/C anode material inithium-ion batteries.
. Conclusions
In summary, we have prepared Fe3O4/C composites in whiche3O4 NPS embedded in the 3D carbon-framework. As an anodeaterial for rechargeable lithium-ion batteries, the Fe3O4/C
omposites deliver a reversible capacity of 773 mAh g−1 at a cur-ent density of 924 mA g−1, while the Fe3O4 NPS only deliver50 mAh g−1 after 200 cycles. More importantly, the Fe3O4/Composites deliver a high reversible capacity (670 mAh g−1)t a higher current density (1848 mA g−1) until the 400thycle. The high-rate capability and good cyclic performanceay be attributed to the 3D conductive carbon-framework,hich can favor fast electrons transportation and high struc-
ural stability during the reversible charge/discharge process.his study suggests that the optimized Fe3O4/C composite is aromising anode material for high-power rechargeable lithium-
on batteries. Meanwhile, it provides an approach to preparehe other carbon-coated materials. The further work is inrogress.
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Cycle performance and Coulomb efficiency of the Fe3O4/C composite at a current
Acknowledgments
This work was financially supported by the National Natural Sci-ence Fund of China (No. 91022033), the 973 Project of China (No.2011CB935901), the Fundamental Research Funds for the CentralUniversities (No. WK 2340000027) and Anhui Provincial NaturalScience Foundation (1208085QE101).
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