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In-situ single-step chemical synthesis of graphene-decorated CoFe2O4 composite with enhanced Li ion storage behaviors
Accepted Manuscript In-situ single step chemical synthesis of graphene-decorated CoFe2O4 composite with enhanced Li ion storage behaviors Kaipeng Wu, Diwei Liu, Yun Tang PII:
S0013-4686(18)30074-4
DOI:
10.1016/j.electacta.2018.01.047
Reference:
EA 31029
To appear in:
Electrochimica Acta
Received Date: 28 October 2017 Revised Date:
11 December 2017
Accepted Date: 8 January 2018
Please cite this article as: K. Wu, D. Liu, Y. Tang, In-situ single step chemical synthesis of graphenedecorated CoFe2O4 composite with enhanced Li ion storage behaviors, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.01.047. 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.
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Graphical Abstract
ACCEPTED MANUSCRIPT In-situ single step chemical synthesis of graphene-decorated CoFe2O4 composite with enhanced Li ion storage behaviors Kaipeng Wu*, Diwei Liu, Yun Tang
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State Key Laboratory Cultivation Base for Nonmetal Composites and Functional
Materials, Southwest University of Science and Technology, Mianyang 621010, China
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*Corresponding author, E-mail: [email protected]. Abstract
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Herein, we introduce a facile strategy for constructing graphene-decorated CoFe2O4 nanoparticles (CoFe2O4/rGO) as anode material for Li-ion batteries. The CoFe2O4/rGO composites are synthesized via single step chemical synthesis method, which involves an in situ oxidation-reduction process between GO and Fe2+ and the
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subsequent nucleation and growth of CoFe2O4 onto the surface of rGO. Well-designed experiments are conducted to investigate the detailed formation mechanism of
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this kind of CoFe2O4/rGO hybrid. The key feature of this work is that by using the oxidizing characteristic of GO and reducing characteristic of Fe2+, toxic reducing agents,
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such as hydrazine and hydroquinone, can be avoid skillfully, which make the procedure more straightforward and economical. Compared with the pure CoFe2O4, the as-prepared CoFe2O4/rGO with a low rGO content of 16.7 wt% shows enhanced Li ion storage behaviors. It exhibits a high initial reversible capacity of 901.2 mAh g-1 and a capacity retention of 80.7% after 100 cycles when operated at the current density of 200 mA g-1.
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ACCEPTED MANUSCRIPT Key Words: In situ oxidation-reduction; CoFe2O4/rGO; Nanocomposite; Formation mechanism; Anode materials. 1. Introduction
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Electrochemical energy storage and conversion devices such as fuel cells[1-3], supercapacitors [4, 5], Li-O2 [6] and Li/Na-ion batteries are propelled by their
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potentiality to construct zero emission energy technologies [7, 8]. Among them, Li-ion batteries have been used extensively in portable electronic apparatus, electric vehicles,
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and energy storage due to their high energy and power density, long-cycle life and high specific capacity [9-11]. The electrochemical properties of Li-ion batteries mainly depended on the physical and chemical properties of both cathode and anode materials [12, 13]. The current commercially used anode material is graphite, which could avoid
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the formation of Li dendrite, guarantee good cyclability and safety [14]. However, the relatively low theoretical specific capacity and poor rate capability cannot satisfy the
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demand of high-energy battery systems [15]. Therefore, developing advanced anode materials with superior electrochemical performance is of vital significance [9, 16].
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Among the available anode materials, CoFe2O4, as an important kind of spinel
ferrite, has recently been proposed as a promising candidate because of its relatively high theoretical capacity, excellent chemical stability and environmentally benign [17-21]. The electrochemical reaction of CoFe2O4 in Li-ion battery can be described as: CoFe2O4 + 8Li+ + 8e ↔2Fe + Co + 4Li2O [22]. According the equation, each formula unit of CoFe2O4 can store eight Li ions via reversibly conversion reactions and exhibits
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ACCEPTED MANUSCRIPT a theoretical capacity up to 916 mAh g-1 [23], which is about 2.5 times greater than that of commercial graphite anodes. However, CoFe2O4 electrodes often suffer from poor electric conductivity, severe volume expansion and pulverization during the
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charge-discharge process, which leads to poor rate capability and rapid capacity fading [24-26]. To address these problems, combination of designed CoFe2O4 nanostructures
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and carbonaceous materials to form hybrid composites to shorten the diffusion distance of Li ions and improve the electrical conductivity of the anode composites is regarded
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as the most applicable strategy [27-29].
Graphene, as a promising electron conducting additive, has been extensively used to fabricate hybrid composites as both cathode and anode materials for Li-ion batteries due to its chemical stability, large specific area, superior
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mechanical strength, and high carrier (ion and electron) mobility [30]. Generally, the enhanced electrochemical properties of the hybrids can be attributed to the
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synergetic interaction between graphene and active materials, in which the graphene sheets can serve not only as a “space block” to prevent the aggregation of the active
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particles and increase the conductivity of electrodes but also as a structural buffer to effectively relieve the volumetric change of embedded particles during the Li+ insertion/extraction process [16, 31-34]. Currently, great efforts have been made to fabricate nano-structured CoFe2O4 and graphene hybrid composites with enhanced rate and
cycling
performance.
For
instance,
Jia
and
coworkers
prepared
carbon-encapsulated CoFe2O4/graphene by a hydrothermal strategy and obtained a high
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ACCEPTED MANUSCRIPT capacity of 925.6 mAh g-1 at the current density of 100 mA g-1 after 100 cycles [29]. Zhao's group synthesized self-assembled CoFe2O4/graphene sandwich via a in situ solvothermal route, which exhibits obvious improvement in cycling stability [35]. Li et
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al. farbricated hierarchical CoFe2O4@graphene hybrid films via electrophoretic deposition combine thermal annealing process, which delivers a specific capacity
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up to 866.67 mAh g-1 at 1000 mA g-1, and the reversible capacity can retain to 860 mAh g-1 after 200 cycles [36]. Despite the above progress to date, the
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exploration of CoFe2O4/graphene composite with enhanced electrochemical performance is far from enough and the current reported synthesis process usually involve tedious procedures, various toxic reducing agents, and rigorous reaction conditions (involved high heat and pressure), which greatly qualified
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their industrial mass production. Thus, a straightforward and cost-effective method to prepare CoFe2O4/graphene with improved electrochemical properties is eagerly sought.
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In this study, we present a single step chemical synthesis approach for the preparation of CoFe2O4/rGO composite, which was first proposed and used to obtain a
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composite of MnFe2O4 and rGO in our previous work [37]. It involves in situ oxidation-reduction process between GO and Fe2+ in aqueous solutions and the subsequent co-precipitation to form CoFe2O4 onto the surface of rGO. In the process, GO can be used as a structural platform firstly to uniformly incorporate metal ions through mutual electrostatic interactions, and then wrap the CoFe2O4 nanoparticles to form well confined composites. Combining the advantages of the size effects of
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ACCEPTED MANUSCRIPT CoFe2O4 nanoparticles and the superior physicochemical properties of rGO, the as-synthesized CoFe2O4/rGO composite exhibits significantly improved rate and
application value as advanced anodes for Li-ion batteries. 2. Experimental
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2.1 Synthesis of the CoFe2O4/rGO
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cycling performance compared to pure CoFe2O4, thus demonstrating great potential
Graphite oxide (GO) were obtained from flake graphite powder via a modified
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Hummer's method [38]. CoFe2O4/rGO was prepared using a solution-based in situ oxidation-reduction-precipitation method. Typically, 4.3g GO was dispersed in 500 ml ultra-pure water by ultrasonication to form a homogeneous dispersion. Then, stoichiometric ratios of CoSO4·H2O (0.05 mol) and FeSO4·7H2O (0.1 mol) were added
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to the dispersion under stirring and the protection of argon at 95 . After 60 min, argon was removed and the solution was kept stirring in air at 95
for another 60 min.
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Subsequently, 3.0 M sodium hydroxide solution was added dropwise into the mixture to maintain the pH value to 12.5. The solution mixture was continuously stirred at a
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constant speed for 120 min. After cooled naturally, the precipitates were collected, washed with ultra-pure water repeatedly, and dried overnight to obtain CoFe2O4/rGO. In comparison, pure CoFe2O4 was synthesized according to the same procedure in the absence of GO. 2.2 Characterization The crystallographic structure of the samples was performed on a Rigaku X-ray
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ACCEPTED MANUSCRIPT diffraction (XRD) which was conducted by Cu Kα irradiation. Raman spectra were investigated on a Raman spectroscopy with an excitation of 633 nm laser light. The morphological characteristics of the obtained particles were determined using TEM (,
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JEOL, JEM-2010FEF) and high-resolution TEM. The chemical environments of the samples were recorded by X-ray photoelectron spectroscopy (XPS K-Alpha 1063). The
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graphene content of the CoFe2O4/rGO was determined by infrared C-S analysis equipment (HW 2000).
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2.3 Electrochemical measurements
Electrochemical performance of the samples was evaluated using CR2025 coin-type half-cells. The active materials, conductive carbon black and PVDF were mixed with a ratio of 8:1:1 in N-methyl-2-pyrrolidone and then cast-dried on a copper
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foil to fabricate the working electrode. The coin-type cells were assembled in an Ar filled glove box using metal Li foil as counter electrode, glass fiber membrane as
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separator, and 1 M LiPF6 with 1:1 ( in volume) mixture of DMC and EC as electrolyte. The charge-discharge tests were carried out in the voltage range of 0.01-3.0 V at
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specific currents densities on a Land CT 2001A battery test system (Wuhan, China). The cyclic voltammetry (CV) tests were carried out using a Model 2273A Electrochemical Instruments (PerkinElmer Co., USA). Electrochemical impedance spectroscopy (EIS) analysis was determined by CHI660D potentiostat in the frequency range from 0.01 Hz to 100 kHz. 3. Results and discussion
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ACCEPTED MANUSCRIPT The formation process of CoFe2O4/rGO is schematically illustrated in Fig. 1. In the process, in order to obtain the uniform distribution of CoFe2O4 nanoparticles, the positively charged metal ions (Fe2+ and Co2+) were first anchored on the GO surface by
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electrostatic interactions. At this stage, Fe2+ ions could be oxidized into Fe3+ effectively by the oxygen-based functional groups on the GO nanosheets, accompanied by an in
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situ reduction of GO to rGO. After addition of NaOH as the precipitant, CoFe2O4 nanocrystals can be nucleated under high temperature, which in situ grew on the surface
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of rGO nanosheets to form CoFe2O4/rGO composite. In order to further demonstrate the rationality of the proposed formation mechanism of CoFe2O4/rGO in this work, we performed additional well-designed experiments and the results were illustrated in Supplementary Information (Fig. S1 and S2). It can be deduced from the results that
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Fe2+ can be oxidized by GO to form FeOOH intermediate during the in situ reaction process, which then transformed into CoFe2O4 in the presence of Co2+ under high pH
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value condition as well as in situ simultaneously deposited on the surface of rGO nanosheets. Moreover, it seems that NaOH in the second step can act not only as a
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precipitator to enable the co-precipitation and transformation of the metal ions to form CoFe2O4 nanoparticles but also as a reductant to further remove the epoxide and hydroxyl functional groups of rGO. XRD patterns recorded for GO, CoFe2O4/rGO and pure CoFe2O4 samples are illustrated in Fig.2a, where the diffraction peaks of both CoFe2O4/rGO and pure CoFe2O4 can be indexed to standard reference card (JCPDS No. 22-1086) [39]. It is a
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ACCEPTED MANUSCRIPT cubic inverse spinel structure with lattice parameter of ac= 8.342Å and ac= 8.369Å for CoFe2O4/rGO and pure CoFe2O4 respectively. The strong characteristic peaks demonstrate the high crystallinity nature of the products. The rGO content of the
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CoFe2O4/rGO was about 16.7 wt%, estimated by infrared C-S analysis. However, at such a high content, no obvious diffraction peaks corresponding to rGO is observed,
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which can be attributed to its weak diffraction in comparison with CoFe2O4. The structural changes of the carbon network during the in situ reduction process from GO
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to rGO was monitored by Raman spectrum analyses. As shown in Fig.2b, both samples exhibit two main peaks, which correspond to the D and G bands of carbon-based materials, respectively. By comparison with GO, the G band of CoFe2O4/rGO shifts from 1593 to 1578 cm-1, indicating the successful reduction of GO [40]. Moreover, the
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CoFe2O4/rGO exhibits a higher ID/IG ratios (~1.06) than GO (~0.91), which can be attributed to the decrease in the average size of sp2 domains and an increased number of
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these domains [41].
The size and shape features of the prepared pure CoFe2O4 and CoFe2O4/rGO
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nanoparticles were characterized by TEM and HRTEM. As shown in Fig.3a, the obtained
pure
CoFe2O4
particles
occurred in forms
of
both irregularly shaped particles and cubic crystal aggregations with a size of about 50 nm. In comparison, the CoFe2O4 particles in CoFe2O4/rGO (Fig.3b) are spheroidal, with diameters ranging from 10 to 20 nm, which are densely deposited on the rGO surface. The rGO sheets, dispersed between the CoFe2O4 nanoparticles, could not only connect
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ACCEPTED MANUSCRIPT the nanoparticles with each other to improve the electronic conductivity of the composites but also immobilize the CoFe2O4 nanoparticles and suppress the particles aggregation degree. Fig.3c shows the enlarged image of the CoFe2O4/rGO, which also
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support the morphology of the CoFe2O4/rGO nanocomposites, and the HRTEM image of the selected area from CoFe2O4 nanoparticles is shown in Fig.3d, the clear lattice
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fringes with spacing of 0.30 nm are correspond to the (220) planes of CoFe2O4 phase, implying the well-defined crystalline nature of CoFe2O4.
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XPS analyses were carried out to investigate the elemental components of the samples. As shown in Fig.4a, four different peaks at 284.9, 287.0, 287.8 and 288.9 eV are observed in the XPS spectra of C 1s in GO, corresponding to C-C, -C-O, -C=O and –COO groups, respectively. However, after in situ oxidation-reduction process, the
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intensities of the -C-O, -C=O and -COO peaks for rGO in CoFe2O4/rGO decrease obviously (shown in Fig.4b), revealing that most of the oxygenic functional groups
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were removed. The XPS spectra of Co 2p is shown in Fig.4c. The peaks at 781.8 eV with a satellite peak at 786.1 eV is from Co 2p3/2 (781.8 eV, Co2+ in octahedron site;
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786.1 eV, Co2+ in tetrahedron site), while the peak at 797.8 eV can be ascribed to the Co 2p1/2 level, with a satellite peak at 803.5 eV (797.8 eV, Co2+ in octahedron site; 803.5 eV, Co2+ in tetrahedron site) [42, 43]. And the binding energy difference between the Co 2p3/2 and Co 2p1/2 peaks is 15.0 eV. These results confirm that Co mainly exists in the Co2+ state in CoFe2O4. The Fe 2p level with binding energies of 711.2 and 725.4 eV (shown in Fig.4d) is assigned to Fe 2p3/2 and Fe 2p1/2, respectively. In addition, binding
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ACCEPTED MANUSCRIPT energies at 718.8 eV is in agreement with the shake-up satellite structure, and the separation of the 2p doublet is ~14.2 eV. All these features are typical of Fe3+ in the composites [44].
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The lithium storage behavior of the CoFe2O4/rGO and pure CoFe2O4 electrodes were investigated by CV tests at the scanning rate of 0.1 mV s-1 and the representative
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curves for the first three cycles are shown in Fig. 5. In the first cathodic process, a strong and broad peak located at ~0.5 V is attributed to the reduction reaction of
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CoFe2O4 to metallic Co and Fe, along with the formation of Li2O and irreversible reaction with the electrolyte to form solid-electrolyte interface (SEI) [45, 46]. The cathodic
process
can
be
understand
as
followings:
CoFe2O4 + 8Li+ + 8e → 2Fe + Co + 4Li2O [47]. Interestingly, this cathodic peak shifts
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to higher potentials of ~0.7 V in the 2nd and 3rd cycles and the corresponding peak intensity decrease significantly, suggesting the presence of an irreversible redox reaction
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and complete structural recovery to CoFe2O4 cannot occur. As for the anodic scan, a broad peak at about 1.85 V in the initial cycle correspond to the oxidation of metallic Fe
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and Co [48]: 4Li2O + 2Fe + Co→Fe2O3 + CoO + 8Li+ + 8e, and this anodic peak positively shifted to high voltage in the subsequent cycles because of the polarization of electrode materials. From the above analysis, we can see that CoFe2O4 electrode is transformed into a mixture of CoO and Fe2O3 after the first lithiation/delithiation process and the whole electrochemical process after the first cycle can be described as: CoO+Fe2O3 + 8Li+ + 8e ↔ 2Fe + Co + 4Li2O [49]. Evidently, by comparison with pure
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The galvanostatic charge-discharge profiles of the CoFe2O4/rGO and pure CoFe2O4 at 200 mA g-1 are shown in Fig. 6a-b. Obviously, both samples exhibit the plateau
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region in the first discharge curves, which correspond to the reduction reactions of metal ions and formation of Li2O. Afterwards, these discharge curves fade rapidly and no
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longer show any plateau regions due to the inadequate interfacial stability [50]. The initial discharge and charge capacities of CoFe2O4/rGO are 1346.8 and 881.5 mAh g-1, respectively, with an irreversible capacity of 465.3 mAh g-1, which is higher than that of pure CoFe2O4 (~411.5 mAh g-1). It is because that the irreversible capacity loss is
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mainly attributed to the the decomposition of electrolyte and the formation of solid-electrolyte interface (SEI) film during the first cycle. By comparison with the pure
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CoFe2O4, CoFe2O4/rGO shows higher specific surface area and smaller particle size distribution, which will consume more Li ions during the formation process of SEI film,
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leading to the larger irreversible capacity loss. The coulombic efficiency of CoFe2O4/rGO for the initial cycle is about 65.5%. However, it increased rapidly to 97.2% in the 3rd cycle, demonstrating an excellent reversible Li+ intercalation/extraction performance. In contrast, the pure CoFe2O4 delivers an initial discharge capacity of 1201.8 mAh g-1, while at the 3rd cycle it is 561.4 mAh g-1, exhibiting a low capacity retention of about 46.7%. The enhancement in higher capacity of CoFe2O4/rGO can be
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ACCEPTED MANUSCRIPT explained by two main factors. On one hand, the incorporation of rGO can effectively decrease the particle size of CoFe2O4 as described previously, which increased the contact area between CoFe2O4 and the electrolyte. In this case, polymeric film can be
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easily formed, which would deliver extra capacity due to the pseudocapacitive character [51]. On the other hand, the exposed portion of rGO nanosheets in CoFe2O4/rGO,
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especially for the edges, can also offer additional Li-storage sites [52]. In addition, the almost overlapped charge and discharge curves of CoFe2O4/rGO at the 2nd and 3rd cycle
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also evidenced its better reversibility than pure CoFe2O4.
The rate performance of the CoFe2O4/rGO and pure CoFe2O4 are compared in Fig. 6c. Apparently, the CoFe2O4/rGO electrode exhibits better discharge capacity. The average reversible capacities of 731.2, 624.5, 525.1 and 432.8 mAh g-1 are obtained at
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400, 800, 1600 and 3200 mA g-1, respectively, which are much higher than those of pure CoFe2O4. Moreover, the average specific capacity still turns back to 790 mAh g-1 when
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the current density returns to 200 mA g-1 after 50 cycles. This means that the structure of CoFe2O4/rGO can be well maintained even after undergoing the high-rate cycles. The
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cycling performance of CoFe2O4/rGO and pure CoFe2O4 electrodes at 200 mA g-1 is shown in Fig. 6d. Obviously, the CoFe2O4/rGO exhibits higher reversible capacity and better cycling stability than the pure CoFe2O4. The CoFe2O4/rGO electrode can maintain a charge capacity of 727.2 mA g-1 from its initial value of 901.2 mA g-1 after 100 cycles, while for pure CoFe2O4, its discharge capacity decreases quickly from 783.8 to 129.6 mAh g-1 after the same cycle number with the corresponding capacity retention of
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EIS analysis was carried out to investigate the different electrochemical performance of the materials. As shown in Fig. 7, both the Nyquist plots consist of a
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depressed semicircle in high-frequency region and a slanted line in the low frequency region [53-55]. The EIS data can be fitted into an equivalent circuit (the inset in Fig. 7),
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in which contain ohmic resistance of the electrolyte resistance (Re), SEI layer resistance (Rf), charge-transfer resistance (Rct), and Warburg diffusion impedance (Zw). CPEf and CPEct are the dielectric relaxation capacitance and double layer capacitance, respectively. It is obvious that the equivalent series resistance (Re+Rf+Rct) for
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CoFe2O4/rGO is smaller than that of pure CoFe2O4 (Fig. 7), indicating that the CoFe2O4/rGO electrode has a faster charge-transfer process than pure CoFe2O4
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electrode, which might account for the better electrochemical performance, and can be ascribed to the incorporation of rGO with superior conductivity.
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To further understand the outstanding rate and cycling performance of the
CoFe2O4/rGO nanocomposite, the morphologies and structures change of both samples before and after 100 cycles at a current density of 200 mA g-1 were characterized by SEM (Fig. 8). As shown in Fig.8c, it is clear that the CoFe2O4/rGO particles are still uniformly distributed and the original structure of spheroidal CoFe2O4 particles are basically maintained after 100 cycles when compared with the the SEM image before
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insertion/extraction process. The results demonstrate that rGO sheets can effectively suppress the particle aggregation as well as accommodate the strain of volume change
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of CoFe2O4 during the cycling process.
On the basis of the aforesaid discussion, the electrochemical performance
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enhancement of the CoFe2O4/rGO can be ascribed to the following main reasons. Firstly, the synthesis method proposed in this work makes the in situ formed CoFe2O4 can combine with rGO to form a specific structure, which could not only restrain the growth of CoFe2O4 crystals, but also suppresses its aggregation as well as prevent the restacking
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of rGO nanosheets during the preparation process. Secondly, the nano-sized CoFe2O4 particles, highly conductive rGO, can ensure a rapid and sustained transportation of
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both Li-ion and electrons through electrodes during electrochemical reaction. Finally, due to its large surface area and excellent mechanical features, rGO in, can also increase
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the BET surface areas of the composites and provide extra space for accommodating the volumetric expansion, which can facilitate the penetration of the electrolyte to the surface of active particles, and maintain the structural integrity of the electrodes during the continuous Li+ insertion/extraction process. In order to gain a more intuitive comparison for the advantages of the strategy used in this work, we compared this strategy with the traditional approach used during the
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ACCEPTED MANUSCRIPT preparation of CoFe2O4/graphene composites. As shown in Table 1, by comparison with the traditional methods, the proposed strategy shown great advantages: (1) the fabrication process is very simple, whereas the traditional methods usually require
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tedious multistep procedures. (2) the strategy introduced herein is especially efficiency, which takes only 3 hours. (3) no addictives, especially toxic reducing agents, is required
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during the synthesis process. In addition, it is important to note that, despite with a low rGO content of about 16.7%, it possesses a comparable, or preferable electrochemical
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performance than the materials obtained by traditional methods. 4. Conclusions
In summary, we have developed a straightforward and cost-effective strategy for the fabrication of CoFe2O4/rGO nanocomposites. During the preparation process, a in
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situ oxidation-reduction occur between GO and Fe2+ at first, which results in the formation of a kind of FeOOH intermediate, and then was co-precipitated with Co2+
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onto the surface of rGO to form CoFe2O4/rGO composite. It can be deduced that NaOH in the approach can act not only as a precipitator to enable the co-precipitation and
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transformation of the metal ions to form CoFe2O4 nanoparticles but also as a reductant to further remove the epoxide and hydroxyl functional groups of rGO. The in situ formed CoFe2O4 particles could combine with rGO to build a specific structure, which would prevent the aggregation of the composite, accommodate the volumetric expansion of the active materials during the charge-discharge process. The prepared CoFe2O4/rGO demonstrated a high initial reversible capacity of 901.2 mAh g-1 at the
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ACCEPTED MANUSCRIPT current density of 200 mA g-1 and a capacity retention of 80.2% after 100 cycles. Acknowledgements
of Science and Technology (Grant No.17zx7126 ). References
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This work was supported by the Scientific Research Fund of Southwest University
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(2015) 5535-5546.
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hybrids for lightweight and highly efficient microwave absorber, J Mater Chem A, 3
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ACCEPTED MANUSCRIPT Figures List Fig. 1. Schematic illustration of the formation of CoFe2O4/rGO. Fig.2. XRD patterns (a) of as-prepared GO, CoFe2O4/rGO and pure CoFe2O4; Raman
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spectra (b) of GO and CoFe2O4/rGO.
Fig.3. TEM image (a) of pure CoFe2O4; TEM (b) and HRTEM images (c-d) of
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CoFe2O4/rGO.
Fig.4. XPS spectra of C1s for GO (a), C1s (b), Co2p (c) and Fe2p (d) for CoFe2O4/rGO.
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Fig.5. CV curves of CoFe2O4/rGO (a) and pure CoFe2O4 (b).
Fig.6. Charge-discharge profiles of CoFe2O4/rGO (a) and pure CoFe2O4 (b); rate (c) and cycling (d) performance of CoFe2O4/rGO and pure CoFe2O4. Fig. 7. Nyquist plots of pure CoFe2O4 and CoFe2O4/rGO.
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Fig. 8. SEM images of CoFe2O4/rGO (a) and pure CoFe2O4 (b) fresh anodes without cycling; SEM images of CoFe2O4/rGO (c) and pure CoFe2O4 (d) anodes after 100 cycles
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at a current density of 200 mA g-1.
Table 1 Comparisons of the proposed strategy and the traditional approach for the
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preparation of CoFe2O4/graphene composites.
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Fig. 1
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Fig.2.
Fig.3.
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Fig.4.
Fig.5.
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Fig. 7
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Synthesis time
Graphene content
Addictive
3h
16.7%
—
Hydrothermal
21 h
20%
Hydrothermal+ pyrolysis
13 h
33.2%
Ultrasonic + anneal
15.5 h
15%
4h
21%
Solvothermal
12.5 h
20%
Hydrothermal+calcine
12 h
In situ oxidation-reduction-
Co-precipitation Reflux
5h 23 h
Current density
Remaining capacity
Ref.
200 mA g-1
790 mAh g-1
This work
—
50
100 mA g-1
910 mAh g-1
[23]
Glucose
50
100 mA g-1
571.5 mAh g-1
[29]
Hydrazine hydrate
50
100 mA g-1
839 mAh g-1
[33]
Citric acid
10
200 mA g-1
906 mAh g-1
[49]
PEG
30
91.4 mA g-1
1040 mAh g-1
[50]
Oxalate, acetate
475
100 mA g-1
907.3 mAh g-1
[56]
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Hydrothermal+ anneal
100
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co-precipitation
Cycle number
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Method
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Table 1 Comparisons of the proposed strategy and the traditional approach for the preparation of CoFe2O4/graphene composites.
—
Hydrazine hydrate
As an adsorbent
[57]
—
TEG, acetate
As a microwave-absorbing material
[58]
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ACCEPTED MANUSCRIPT Highlights ►A straightforward approach for constructing graphene-decorated CoFe2O4 was introduced. ►CoFe2O4/rGO was synthesized by an in situ oxidation-reduction-coprecipitation
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method. ►FeOOH intermediate may be formed during the in situ reaction process.
►CoFe2O4/rGO shows enhanced electrochemical properties as anode material.
►The strategy can be extended to fabricate other MFe2O4 (M=Cu, Zn, Ni etc)/rGO.
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►Detailed formation mechanism of this kind of CoFe2O4/rGO hybrid was
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investigated.
S0013-4686(18)30074-4
DOI:
10.1016/j.electacta.2018.01.047
Reference:
EA 31029
To appear in:
Electrochimica Acta
Received Date: 28 October 2017 Revised Date:
11 December 2017
Accepted Date: 8 January 2018
Please cite this article as: K. Wu, D. Liu, Y. Tang, In-situ single step chemical synthesis of graphenedecorated CoFe2O4 composite with enhanced Li ion storage behaviors, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.01.047. 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.
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Graphical Abstract
ACCEPTED MANUSCRIPT In-situ single step chemical synthesis of graphene-decorated CoFe2O4 composite with enhanced Li ion storage behaviors Kaipeng Wu*, Diwei Liu, Yun Tang
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State Key Laboratory Cultivation Base for Nonmetal Composites and Functional
Materials, Southwest University of Science and Technology, Mianyang 621010, China
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*Corresponding author, E-mail: [email protected]. Abstract
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Herein, we introduce a facile strategy for constructing graphene-decorated CoFe2O4 nanoparticles (CoFe2O4/rGO) as anode material for Li-ion batteries. The CoFe2O4/rGO composites are synthesized via single step chemical synthesis method, which involves an in situ oxidation-reduction process between GO and Fe2+ and the
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subsequent nucleation and growth of CoFe2O4 onto the surface of rGO. Well-designed experiments are conducted to investigate the detailed formation mechanism of
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this kind of CoFe2O4/rGO hybrid. The key feature of this work is that by using the oxidizing characteristic of GO and reducing characteristic of Fe2+, toxic reducing agents,
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such as hydrazine and hydroquinone, can be avoid skillfully, which make the procedure more straightforward and economical. Compared with the pure CoFe2O4, the as-prepared CoFe2O4/rGO with a low rGO content of 16.7 wt% shows enhanced Li ion storage behaviors. It exhibits a high initial reversible capacity of 901.2 mAh g-1 and a capacity retention of 80.7% after 100 cycles when operated at the current density of 200 mA g-1.
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ACCEPTED MANUSCRIPT Key Words: In situ oxidation-reduction; CoFe2O4/rGO; Nanocomposite; Formation mechanism; Anode materials. 1. Introduction
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Electrochemical energy storage and conversion devices such as fuel cells[1-3], supercapacitors [4, 5], Li-O2 [6] and Li/Na-ion batteries are propelled by their
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potentiality to construct zero emission energy technologies [7, 8]. Among them, Li-ion batteries have been used extensively in portable electronic apparatus, electric vehicles,
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and energy storage due to their high energy and power density, long-cycle life and high specific capacity [9-11]. The electrochemical properties of Li-ion batteries mainly depended on the physical and chemical properties of both cathode and anode materials [12, 13]. The current commercially used anode material is graphite, which could avoid
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the formation of Li dendrite, guarantee good cyclability and safety [14]. However, the relatively low theoretical specific capacity and poor rate capability cannot satisfy the
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demand of high-energy battery systems [15]. Therefore, developing advanced anode materials with superior electrochemical performance is of vital significance [9, 16].
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Among the available anode materials, CoFe2O4, as an important kind of spinel
ferrite, has recently been proposed as a promising candidate because of its relatively high theoretical capacity, excellent chemical stability and environmentally benign [17-21]. The electrochemical reaction of CoFe2O4 in Li-ion battery can be described as: CoFe2O4 + 8Li+ + 8e ↔2Fe + Co + 4Li2O [22]. According the equation, each formula unit of CoFe2O4 can store eight Li ions via reversibly conversion reactions and exhibits
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ACCEPTED MANUSCRIPT a theoretical capacity up to 916 mAh g-1 [23], which is about 2.5 times greater than that of commercial graphite anodes. However, CoFe2O4 electrodes often suffer from poor electric conductivity, severe volume expansion and pulverization during the
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charge-discharge process, which leads to poor rate capability and rapid capacity fading [24-26]. To address these problems, combination of designed CoFe2O4 nanostructures
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and carbonaceous materials to form hybrid composites to shorten the diffusion distance of Li ions and improve the electrical conductivity of the anode composites is regarded
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as the most applicable strategy [27-29].
Graphene, as a promising electron conducting additive, has been extensively used to fabricate hybrid composites as both cathode and anode materials for Li-ion batteries due to its chemical stability, large specific area, superior
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mechanical strength, and high carrier (ion and electron) mobility [30]. Generally, the enhanced electrochemical properties of the hybrids can be attributed to the
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synergetic interaction between graphene and active materials, in which the graphene sheets can serve not only as a “space block” to prevent the aggregation of the active
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particles and increase the conductivity of electrodes but also as a structural buffer to effectively relieve the volumetric change of embedded particles during the Li+ insertion/extraction process [16, 31-34]. Currently, great efforts have been made to fabricate nano-structured CoFe2O4 and graphene hybrid composites with enhanced rate and
cycling
performance.
For
instance,
Jia
and
coworkers
prepared
carbon-encapsulated CoFe2O4/graphene by a hydrothermal strategy and obtained a high
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ACCEPTED MANUSCRIPT capacity of 925.6 mAh g-1 at the current density of 100 mA g-1 after 100 cycles [29]. Zhao's group synthesized self-assembled CoFe2O4/graphene sandwich via a in situ solvothermal route, which exhibits obvious improvement in cycling stability [35]. Li et
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al. farbricated hierarchical CoFe2O4@graphene hybrid films via electrophoretic deposition combine thermal annealing process, which delivers a specific capacity
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up to 866.67 mAh g-1 at 1000 mA g-1, and the reversible capacity can retain to 860 mAh g-1 after 200 cycles [36]. Despite the above progress to date, the
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exploration of CoFe2O4/graphene composite with enhanced electrochemical performance is far from enough and the current reported synthesis process usually involve tedious procedures, various toxic reducing agents, and rigorous reaction conditions (involved high heat and pressure), which greatly qualified
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their industrial mass production. Thus, a straightforward and cost-effective method to prepare CoFe2O4/graphene with improved electrochemical properties is eagerly sought.
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In this study, we present a single step chemical synthesis approach for the preparation of CoFe2O4/rGO composite, which was first proposed and used to obtain a
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composite of MnFe2O4 and rGO in our previous work [37]. It involves in situ oxidation-reduction process between GO and Fe2+ in aqueous solutions and the subsequent co-precipitation to form CoFe2O4 onto the surface of rGO. In the process, GO can be used as a structural platform firstly to uniformly incorporate metal ions through mutual electrostatic interactions, and then wrap the CoFe2O4 nanoparticles to form well confined composites. Combining the advantages of the size effects of
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ACCEPTED MANUSCRIPT CoFe2O4 nanoparticles and the superior physicochemical properties of rGO, the as-synthesized CoFe2O4/rGO composite exhibits significantly improved rate and
application value as advanced anodes for Li-ion batteries. 2. Experimental
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2.1 Synthesis of the CoFe2O4/rGO
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cycling performance compared to pure CoFe2O4, thus demonstrating great potential
Graphite oxide (GO) were obtained from flake graphite powder via a modified
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Hummer's method [38]. CoFe2O4/rGO was prepared using a solution-based in situ oxidation-reduction-precipitation method. Typically, 4.3g GO was dispersed in 500 ml ultra-pure water by ultrasonication to form a homogeneous dispersion. Then, stoichiometric ratios of CoSO4·H2O (0.05 mol) and FeSO4·7H2O (0.1 mol) were added
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to the dispersion under stirring and the protection of argon at 95 . After 60 min, argon was removed and the solution was kept stirring in air at 95
for another 60 min.
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Subsequently, 3.0 M sodium hydroxide solution was added dropwise into the mixture to maintain the pH value to 12.5. The solution mixture was continuously stirred at a
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constant speed for 120 min. After cooled naturally, the precipitates were collected, washed with ultra-pure water repeatedly, and dried overnight to obtain CoFe2O4/rGO. In comparison, pure CoFe2O4 was synthesized according to the same procedure in the absence of GO. 2.2 Characterization The crystallographic structure of the samples was performed on a Rigaku X-ray
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JEOL, JEM-2010FEF) and high-resolution TEM. The chemical environments of the samples were recorded by X-ray photoelectron spectroscopy (XPS K-Alpha 1063). The
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graphene content of the CoFe2O4/rGO was determined by infrared C-S analysis equipment (HW 2000).
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2.3 Electrochemical measurements
Electrochemical performance of the samples was evaluated using CR2025 coin-type half-cells. The active materials, conductive carbon black and PVDF were mixed with a ratio of 8:1:1 in N-methyl-2-pyrrolidone and then cast-dried on a copper
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foil to fabricate the working electrode. The coin-type cells were assembled in an Ar filled glove box using metal Li foil as counter electrode, glass fiber membrane as
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separator, and 1 M LiPF6 with 1:1 ( in volume) mixture of DMC and EC as electrolyte. The charge-discharge tests were carried out in the voltage range of 0.01-3.0 V at
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specific currents densities on a Land CT 2001A battery test system (Wuhan, China). The cyclic voltammetry (CV) tests were carried out using a Model 2273A Electrochemical Instruments (PerkinElmer Co., USA). Electrochemical impedance spectroscopy (EIS) analysis was determined by CHI660D potentiostat in the frequency range from 0.01 Hz to 100 kHz. 3. Results and discussion
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ACCEPTED MANUSCRIPT The formation process of CoFe2O4/rGO is schematically illustrated in Fig. 1. In the process, in order to obtain the uniform distribution of CoFe2O4 nanoparticles, the positively charged metal ions (Fe2+ and Co2+) were first anchored on the GO surface by
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electrostatic interactions. At this stage, Fe2+ ions could be oxidized into Fe3+ effectively by the oxygen-based functional groups on the GO nanosheets, accompanied by an in
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situ reduction of GO to rGO. After addition of NaOH as the precipitant, CoFe2O4 nanocrystals can be nucleated under high temperature, which in situ grew on the surface
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of rGO nanosheets to form CoFe2O4/rGO composite. In order to further demonstrate the rationality of the proposed formation mechanism of CoFe2O4/rGO in this work, we performed additional well-designed experiments and the results were illustrated in Supplementary Information (Fig. S1 and S2). It can be deduced from the results that
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Fe2+ can be oxidized by GO to form FeOOH intermediate during the in situ reaction process, which then transformed into CoFe2O4 in the presence of Co2+ under high pH
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value condition as well as in situ simultaneously deposited on the surface of rGO nanosheets. Moreover, it seems that NaOH in the second step can act not only as a
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precipitator to enable the co-precipitation and transformation of the metal ions to form CoFe2O4 nanoparticles but also as a reductant to further remove the epoxide and hydroxyl functional groups of rGO. XRD patterns recorded for GO, CoFe2O4/rGO and pure CoFe2O4 samples are illustrated in Fig.2a, where the diffraction peaks of both CoFe2O4/rGO and pure CoFe2O4 can be indexed to standard reference card (JCPDS No. 22-1086) [39]. It is a
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ACCEPTED MANUSCRIPT cubic inverse spinel structure with lattice parameter of ac= 8.342Å and ac= 8.369Å for CoFe2O4/rGO and pure CoFe2O4 respectively. The strong characteristic peaks demonstrate the high crystallinity nature of the products. The rGO content of the
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CoFe2O4/rGO was about 16.7 wt%, estimated by infrared C-S analysis. However, at such a high content, no obvious diffraction peaks corresponding to rGO is observed,
SC
which can be attributed to its weak diffraction in comparison with CoFe2O4. The structural changes of the carbon network during the in situ reduction process from GO
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to rGO was monitored by Raman spectrum analyses. As shown in Fig.2b, both samples exhibit two main peaks, which correspond to the D and G bands of carbon-based materials, respectively. By comparison with GO, the G band of CoFe2O4/rGO shifts from 1593 to 1578 cm-1, indicating the successful reduction of GO [40]. Moreover, the
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CoFe2O4/rGO exhibits a higher ID/IG ratios (~1.06) than GO (~0.91), which can be attributed to the decrease in the average size of sp2 domains and an increased number of
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these domains [41].
The size and shape features of the prepared pure CoFe2O4 and CoFe2O4/rGO
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nanoparticles were characterized by TEM and HRTEM. As shown in Fig.3a, the obtained
pure
CoFe2O4
particles
occurred in forms
of
both irregularly shaped particles and cubic crystal aggregations with a size of about 50 nm. In comparison, the CoFe2O4 particles in CoFe2O4/rGO (Fig.3b) are spheroidal, with diameters ranging from 10 to 20 nm, which are densely deposited on the rGO surface. The rGO sheets, dispersed between the CoFe2O4 nanoparticles, could not only connect
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ACCEPTED MANUSCRIPT the nanoparticles with each other to improve the electronic conductivity of the composites but also immobilize the CoFe2O4 nanoparticles and suppress the particles aggregation degree. Fig.3c shows the enlarged image of the CoFe2O4/rGO, which also
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support the morphology of the CoFe2O4/rGO nanocomposites, and the HRTEM image of the selected area from CoFe2O4 nanoparticles is shown in Fig.3d, the clear lattice
SC
fringes with spacing of 0.30 nm are correspond to the (220) planes of CoFe2O4 phase, implying the well-defined crystalline nature of CoFe2O4.
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XPS analyses were carried out to investigate the elemental components of the samples. As shown in Fig.4a, four different peaks at 284.9, 287.0, 287.8 and 288.9 eV are observed in the XPS spectra of C 1s in GO, corresponding to C-C, -C-O, -C=O and –COO groups, respectively. However, after in situ oxidation-reduction process, the
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intensities of the -C-O, -C=O and -COO peaks for rGO in CoFe2O4/rGO decrease obviously (shown in Fig.4b), revealing that most of the oxygenic functional groups
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were removed. The XPS spectra of Co 2p is shown in Fig.4c. The peaks at 781.8 eV with a satellite peak at 786.1 eV is from Co 2p3/2 (781.8 eV, Co2+ in octahedron site;
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786.1 eV, Co2+ in tetrahedron site), while the peak at 797.8 eV can be ascribed to the Co 2p1/2 level, with a satellite peak at 803.5 eV (797.8 eV, Co2+ in octahedron site; 803.5 eV, Co2+ in tetrahedron site) [42, 43]. And the binding energy difference between the Co 2p3/2 and Co 2p1/2 peaks is 15.0 eV. These results confirm that Co mainly exists in the Co2+ state in CoFe2O4. The Fe 2p level with binding energies of 711.2 and 725.4 eV (shown in Fig.4d) is assigned to Fe 2p3/2 and Fe 2p1/2, respectively. In addition, binding
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ACCEPTED MANUSCRIPT energies at 718.8 eV is in agreement with the shake-up satellite structure, and the separation of the 2p doublet is ~14.2 eV. All these features are typical of Fe3+ in the composites [44].
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The lithium storage behavior of the CoFe2O4/rGO and pure CoFe2O4 electrodes were investigated by CV tests at the scanning rate of 0.1 mV s-1 and the representative
SC
curves for the first three cycles are shown in Fig. 5. In the first cathodic process, a strong and broad peak located at ~0.5 V is attributed to the reduction reaction of
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CoFe2O4 to metallic Co and Fe, along with the formation of Li2O and irreversible reaction with the electrolyte to form solid-electrolyte interface (SEI) [45, 46]. The cathodic
process
can
be
understand
as
followings:
CoFe2O4 + 8Li+ + 8e → 2Fe + Co + 4Li2O [47]. Interestingly, this cathodic peak shifts
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to higher potentials of ~0.7 V in the 2nd and 3rd cycles and the corresponding peak intensity decrease significantly, suggesting the presence of an irreversible redox reaction
EP
and complete structural recovery to CoFe2O4 cannot occur. As for the anodic scan, a broad peak at about 1.85 V in the initial cycle correspond to the oxidation of metallic Fe
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and Co [48]: 4Li2O + 2Fe + Co→Fe2O3 + CoO + 8Li+ + 8e, and this anodic peak positively shifted to high voltage in the subsequent cycles because of the polarization of electrode materials. From the above analysis, we can see that CoFe2O4 electrode is transformed into a mixture of CoO and Fe2O3 after the first lithiation/delithiation process and the whole electrochemical process after the first cycle can be described as: CoO+Fe2O3 + 8Li+ + 8e ↔ 2Fe + Co + 4Li2O [49]. Evidently, by comparison with pure
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ACCEPTED MANUSCRIPT CoFe2O4 (Fig. 5b), the CV curves of the CoFe2O4/rGO (Fig. 5a) almost overlapped during the 2nd and 3rd cycles, indicating that the CoFe2O4/rGO exhibits better electrochemical reversibility after introduction of rGO.
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The galvanostatic charge-discharge profiles of the CoFe2O4/rGO and pure CoFe2O4 at 200 mA g-1 are shown in Fig. 6a-b. Obviously, both samples exhibit the plateau
SC
region in the first discharge curves, which correspond to the reduction reactions of metal ions and formation of Li2O. Afterwards, these discharge curves fade rapidly and no
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longer show any plateau regions due to the inadequate interfacial stability [50]. The initial discharge and charge capacities of CoFe2O4/rGO are 1346.8 and 881.5 mAh g-1, respectively, with an irreversible capacity of 465.3 mAh g-1, which is higher than that of pure CoFe2O4 (~411.5 mAh g-1). It is because that the irreversible capacity loss is
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mainly attributed to the the decomposition of electrolyte and the formation of solid-electrolyte interface (SEI) film during the first cycle. By comparison with the pure
EP
CoFe2O4, CoFe2O4/rGO shows higher specific surface area and smaller particle size distribution, which will consume more Li ions during the formation process of SEI film,
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leading to the larger irreversible capacity loss. The coulombic efficiency of CoFe2O4/rGO for the initial cycle is about 65.5%. However, it increased rapidly to 97.2% in the 3rd cycle, demonstrating an excellent reversible Li+ intercalation/extraction performance. In contrast, the pure CoFe2O4 delivers an initial discharge capacity of 1201.8 mAh g-1, while at the 3rd cycle it is 561.4 mAh g-1, exhibiting a low capacity retention of about 46.7%. The enhancement in higher capacity of CoFe2O4/rGO can be
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ACCEPTED MANUSCRIPT explained by two main factors. On one hand, the incorporation of rGO can effectively decrease the particle size of CoFe2O4 as described previously, which increased the contact area between CoFe2O4 and the electrolyte. In this case, polymeric film can be
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easily formed, which would deliver extra capacity due to the pseudocapacitive character [51]. On the other hand, the exposed portion of rGO nanosheets in CoFe2O4/rGO,
SC
especially for the edges, can also offer additional Li-storage sites [52]. In addition, the almost overlapped charge and discharge curves of CoFe2O4/rGO at the 2nd and 3rd cycle
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also evidenced its better reversibility than pure CoFe2O4.
The rate performance of the CoFe2O4/rGO and pure CoFe2O4 are compared in Fig. 6c. Apparently, the CoFe2O4/rGO electrode exhibits better discharge capacity. The average reversible capacities of 731.2, 624.5, 525.1 and 432.8 mAh g-1 are obtained at
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400, 800, 1600 and 3200 mA g-1, respectively, which are much higher than those of pure CoFe2O4. Moreover, the average specific capacity still turns back to 790 mAh g-1 when
EP
the current density returns to 200 mA g-1 after 50 cycles. This means that the structure of CoFe2O4/rGO can be well maintained even after undergoing the high-rate cycles. The
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cycling performance of CoFe2O4/rGO and pure CoFe2O4 electrodes at 200 mA g-1 is shown in Fig. 6d. Obviously, the CoFe2O4/rGO exhibits higher reversible capacity and better cycling stability than the pure CoFe2O4. The CoFe2O4/rGO electrode can maintain a charge capacity of 727.2 mA g-1 from its initial value of 901.2 mA g-1 after 100 cycles, while for pure CoFe2O4, its discharge capacity decreases quickly from 783.8 to 129.6 mAh g-1 after the same cycle number with the corresponding capacity retention of
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ACCEPTED MANUSCRIPT 16.5%. Therefore, it is believed that modification with rGO is an effective way to prevent rapid capacity fading and improve the electrochemical performance of CoFe2O4 anode material.
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EIS analysis was carried out to investigate the different electrochemical performance of the materials. As shown in Fig. 7, both the Nyquist plots consist of a
SC
depressed semicircle in high-frequency region and a slanted line in the low frequency region [53-55]. The EIS data can be fitted into an equivalent circuit (the inset in Fig. 7),
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in which contain ohmic resistance of the electrolyte resistance (Re), SEI layer resistance (Rf), charge-transfer resistance (Rct), and Warburg diffusion impedance (Zw). CPEf and CPEct are the dielectric relaxation capacitance and double layer capacitance, respectively. It is obvious that the equivalent series resistance (Re+Rf+Rct) for
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CoFe2O4/rGO is smaller than that of pure CoFe2O4 (Fig. 7), indicating that the CoFe2O4/rGO electrode has a faster charge-transfer process than pure CoFe2O4
EP
electrode, which might account for the better electrochemical performance, and can be ascribed to the incorporation of rGO with superior conductivity.
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To further understand the outstanding rate and cycling performance of the
CoFe2O4/rGO nanocomposite, the morphologies and structures change of both samples before and after 100 cycles at a current density of 200 mA g-1 were characterized by SEM (Fig. 8). As shown in Fig.8c, it is clear that the CoFe2O4/rGO particles are still uniformly distributed and the original structure of spheroidal CoFe2O4 particles are basically maintained after 100 cycles when compared with the the SEM image before
13
ACCEPTED MANUSCRIPT cycling (Fig.8a). In contrast, the electrode for pure CoFe2O4 (Fig. 8d) after 100 cycles is mostly destroyed without clear boundaries between the particles, which indicates that the pure CoFe2O4 electrode may suffer from severe structure damage during Li+
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insertion/extraction process. The results demonstrate that rGO sheets can effectively suppress the particle aggregation as well as accommodate the strain of volume change
SC
of CoFe2O4 during the cycling process.
On the basis of the aforesaid discussion, the electrochemical performance
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enhancement of the CoFe2O4/rGO can be ascribed to the following main reasons. Firstly, the synthesis method proposed in this work makes the in situ formed CoFe2O4 can combine with rGO to form a specific structure, which could not only restrain the growth of CoFe2O4 crystals, but also suppresses its aggregation as well as prevent the restacking
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of rGO nanosheets during the preparation process. Secondly, the nano-sized CoFe2O4 particles, highly conductive rGO, can ensure a rapid and sustained transportation of
EP
both Li-ion and electrons through electrodes during electrochemical reaction. Finally, due to its large surface area and excellent mechanical features, rGO in, can also increase
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the BET surface areas of the composites and provide extra space for accommodating the volumetric expansion, which can facilitate the penetration of the electrolyte to the surface of active particles, and maintain the structural integrity of the electrodes during the continuous Li+ insertion/extraction process. In order to gain a more intuitive comparison for the advantages of the strategy used in this work, we compared this strategy with the traditional approach used during the
14
ACCEPTED MANUSCRIPT preparation of CoFe2O4/graphene composites. As shown in Table 1, by comparison with the traditional methods, the proposed strategy shown great advantages: (1) the fabrication process is very simple, whereas the traditional methods usually require
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tedious multistep procedures. (2) the strategy introduced herein is especially efficiency, which takes only 3 hours. (3) no addictives, especially toxic reducing agents, is required
SC
during the synthesis process. In addition, it is important to note that, despite with a low rGO content of about 16.7%, it possesses a comparable, or preferable electrochemical
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performance than the materials obtained by traditional methods. 4. Conclusions
In summary, we have developed a straightforward and cost-effective strategy for the fabrication of CoFe2O4/rGO nanocomposites. During the preparation process, a in
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situ oxidation-reduction occur between GO and Fe2+ at first, which results in the formation of a kind of FeOOH intermediate, and then was co-precipitated with Co2+
EP
onto the surface of rGO to form CoFe2O4/rGO composite. It can be deduced that NaOH in the approach can act not only as a precipitator to enable the co-precipitation and
AC C
transformation of the metal ions to form CoFe2O4 nanoparticles but also as a reductant to further remove the epoxide and hydroxyl functional groups of rGO. The in situ formed CoFe2O4 particles could combine with rGO to build a specific structure, which would prevent the aggregation of the composite, accommodate the volumetric expansion of the active materials during the charge-discharge process. The prepared CoFe2O4/rGO demonstrated a high initial reversible capacity of 901.2 mAh g-1 at the
15
ACCEPTED MANUSCRIPT current density of 200 mA g-1 and a capacity retention of 80.2% after 100 cycles. Acknowledgements
of Science and Technology (Grant No.17zx7126 ). References
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This work was supported by the Scientific Research Fund of Southwest University
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[52] D. Pan, S. Wang, B. Zhao, M. Wu, H. Zhang, Y. Wang, Z. Jiao, Li storage
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properties of disordered graphene nanosheets, Chem Mater, 21 (2009) 3136-3142. [53] J. Xie, W.T. Song, G.S. Cao, T.J. Zhu, X.B. Zhao, S.C. Zhang, One-pot synthesis of
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ultrafine ZnFe2O4 nanocrystals anchored on graphene for high-performance Li and Li-ion batteries, Rsc Adv, 4 (2014) 7703-7709. [54] S.M. Li, B. Wang, B. Li, J.H. Liu, M. Yu, X.Y. Wu, Self-assembly of 2D sandwich-structured MnFe2O4/graphene composites for high-performance lithium storage, Mater Res Bull, 61 (2015) 369-374. [55] Q.Q. Xiong, J.P. Tu, S.J. Shi, X.Y. Liu, X.L. Wang, C.D. Gu, Ascorbic acid-assisted
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ACCEPTED MANUSCRIPT synthesis of cobalt ferrite (CoFe2O4) hierarchical flower-like microspheres with enhanced lithium storage properties, J Power Sources, 256 (2014) 153-159. [56] M. Zhang, M.Q. Jia, Y.H. Jin, Q.Q. Wen, C. Chen, Reduced graphene
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oxide/CoFe2O4-Co nanocomposite as high performance anode for lithium ion batteries, J Alloy Compd, 566 (2013) 131-136.
Adsorption
of
Pb(II)
and
Hg(II)
from
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[57] Y.K. Zhang, L.G. Yan, W.Y. Xu, X.Y. Guo, L.M. Cui, L. Gao, Q. Wei, B. Du, aqueous
solution
using
magnetic
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CoFe2O4-reduced graphene oxide, J Mol Liq, 191 (2014) 177-182.
[58] X.H. Li, J. Feng, Y.P. Du, J.T. Bai, H.M. Fan, H.L. Zhang, Y. Peng, F.S. Li, One-pot synthesis of CoFe2O4/graphene oxide hybrids and their conversion into FeCo/graphene
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(2015) 5535-5546.
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hybrids for lightweight and highly efficient microwave absorber, J Mater Chem A, 3
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ACCEPTED MANUSCRIPT Figures List Fig. 1. Schematic illustration of the formation of CoFe2O4/rGO. Fig.2. XRD patterns (a) of as-prepared GO, CoFe2O4/rGO and pure CoFe2O4; Raman
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spectra (b) of GO and CoFe2O4/rGO.
Fig.3. TEM image (a) of pure CoFe2O4; TEM (b) and HRTEM images (c-d) of
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CoFe2O4/rGO.
Fig.4. XPS spectra of C1s for GO (a), C1s (b), Co2p (c) and Fe2p (d) for CoFe2O4/rGO.
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Fig.5. CV curves of CoFe2O4/rGO (a) and pure CoFe2O4 (b).
Fig.6. Charge-discharge profiles of CoFe2O4/rGO (a) and pure CoFe2O4 (b); rate (c) and cycling (d) performance of CoFe2O4/rGO and pure CoFe2O4. Fig. 7. Nyquist plots of pure CoFe2O4 and CoFe2O4/rGO.
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Fig. 8. SEM images of CoFe2O4/rGO (a) and pure CoFe2O4 (b) fresh anodes without cycling; SEM images of CoFe2O4/rGO (c) and pure CoFe2O4 (d) anodes after 100 cycles
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at a current density of 200 mA g-1.
Table 1 Comparisons of the proposed strategy and the traditional approach for the
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preparation of CoFe2O4/graphene composites.
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Fig. 1
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Fig.2.
Fig.3.
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Fig.4.
Fig.5.
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Fig.6.
Fig. 7
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Synthesis time
Graphene content
Addictive
3h
16.7%
—
Hydrothermal
21 h
20%
Hydrothermal+ pyrolysis
13 h
33.2%
Ultrasonic + anneal
15.5 h
15%
4h
21%
Solvothermal
12.5 h
20%
Hydrothermal+calcine
12 h
In situ oxidation-reduction-
Co-precipitation Reflux
5h 23 h
Current density
Remaining capacity
Ref.
200 mA g-1
790 mAh g-1
This work
—
50
100 mA g-1
910 mAh g-1
[23]
Glucose
50
100 mA g-1
571.5 mAh g-1
[29]
Hydrazine hydrate
50
100 mA g-1
839 mAh g-1
[33]
Citric acid
10
200 mA g-1
906 mAh g-1
[49]
PEG
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91.4 mA g-1
1040 mAh g-1
[50]
Oxalate, acetate
475
100 mA g-1
907.3 mAh g-1
[56]
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Hydrothermal+ anneal
100
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Cycle number
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Method
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Table 1 Comparisons of the proposed strategy and the traditional approach for the preparation of CoFe2O4/graphene composites.
—
Hydrazine hydrate
As an adsorbent
[57]
—
TEG, acetate
As a microwave-absorbing material
[58]
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ACCEPTED MANUSCRIPT Highlights ►A straightforward approach for constructing graphene-decorated CoFe2O4 was introduced. ►CoFe2O4/rGO was synthesized by an in situ oxidation-reduction-coprecipitation
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method. ►FeOOH intermediate may be formed during the in situ reaction process.
►CoFe2O4/rGO shows enhanced electrochemical properties as anode material.
►The strategy can be extended to fabricate other MFe2O4 (M=Cu, Zn, Ni etc)/rGO.
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►Detailed formation mechanism of this kind of CoFe2O4/rGO hybrid was
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investigated.