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reduced graphene oxide nanocomposite as anode materials for Li-ion batteries
Author’s Accepted Manuscript Facile and green synthesis of MnFe2O4/reduced graphene oxide nanocomposite as anode materials for Li-ion batteries Kaipeng Wu, Guorong Hu, Yanbing Cao, Zhongdong Peng, Ke Du www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30455-9 http://dx.doi.org/10.1016/j.matlet.2015.08.100 MLBLUE19461
To appear in: Materials Letters Received date: 9 June 2015 Revised date: 14 August 2015 Accepted date: 20 August 2015 Cite this article as: Kaipeng Wu, Guorong Hu, Yanbing Cao, Zhongdong Peng and Ke Du, Facile and green synthesis of MnFe2O4/reduced graphene oxide nanocomposite as anode materials for Li-ion batteries, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.100 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 galley proof before it is published in its final citable 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.
Facile and green synthesis of MnFe2O4/reduced graphene oxide nanocomposite as anode materials for Li-ion batteries Kaipeng Wu, Guorong Hu, Yanbing Cao, Zhongdong Peng, Ke Du* School of Metallurgy and Environment, Central South University, Changsha 410083, China *Corresponding author, E-mail: [email protected], TEL: +8615084864373 Abstract: MnFe2O4/reduced graphene oxide nanocomposite (MnFe2O4/rGO) was synthesized by a facile and green strategy, which involves in situ reduction of GO in presence of Fe2+ as well as co-precipitation of Fe3+ and Mn2+ onto the surface of rGO. The composite consists of MnFe2O4 with its primary particles (~10nm) decorated by rGO sheets, which could prevent the aggregation of the particles and ensure a relatively large specific surface area. It exhibits a specific capacity of 813.4 mAh·g-1 at the current density of 0.2 A·g-1 and a capacity retention of 80.3% after 100 cycles. Moreover, the proposed synthesis strategy can also be easily extended to prepare other MxFe1-xO4/rGO (M=Ni, Co, Zn, Mg) composite materials. Key Words: Energy storage and conversion; Nano MnFe2O4; Reduced graphene oxide; Nanocomposites; Anode materials. 1. Introduction As an important spinel ferrite material, MnFe2O4 has been widely applied in electronics, microwave devices, magnetic and energy storage [1-2]. It has recently been proposed as a promising anode for Li-ion batteries (LIBs) due to its low intercalation
1
potential for lithium ions and high theoretical capacity of 930 mAh·g-1 [3]. However, the MnFe2O4 has poor electrical conductivity and suffers from severe volume expansion during the charge-discharge process, which may lead to a large irreversible capacity, poor cycle and rate performance [4]. To overcome these problems, fabrication of a composite with carbonaceous materials is regarded as an effective strategy. Recently, rGO was employed as an ideal matrix to synthesis anode composites with enhanced electrochemical performance for LIBs [5]. And various wet chemical methods have been adopted for the preparation of MnFe2O4/rGO, such as solvothermal [3], ultrasonic [4], hydrothermal [6], and coprecipitation method [7]. In spite of these efforts, the traditional methods to prepare MnFe2O4/rGO were reported to be time consuming and demand strict reaction conditions, especially high temperature and pressure. Moreover, toxic reductant, such as hydrazine hydrate [7], was used for the reduction of GO to graphene. Therefore, a facile and green strategy is still highly desirable for preparation of MnFe2O4/rGO anode materials for LIBs. In this paper, MnFe2O4/rGO was synthesized by a facile and green strategy, which involves in situ reduction of GO in presence of Fe2+ as well as coprecipitation of Fe and Mn ions onto the surface of rGO. Compared with other methods, the proposed strategy has some advantages, such as simplicity and environmentally friendly due to its noninvolvement of any toxic reductant. The synthesized MnFe2O4/rGO exhibits excellent electrochemical performance as anode materials for LIBs.
2
2. Experimental MnFe2O4/rGO was synthesized by an in situ reduction-coprecipitation method. Typically, GO (3.8 g) was first dispersed into 500 mL distilled water by ultrasonication. Then, MnSO4 (0.05 mol) and FeSO4 (0.1 mol) were added into the GO solution and stirred at 95
℃ under the protection of Ar. After 2h of continuous stirring, the protection
of Ar was removed and the solution was left exposed in air and stirred for another 1h. After that, 2M NaOH solution was added to adjust the solution to pH=11 and further stirred for 2h. Final products were obtained by centrifugation, washed by distilled water and dried in a vacuum oven. MnFe2O4/rGO was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS K-Alpha 1063), TG analysis (NETZSCH STA-449C), Raman spectroscopy (LabRAM HR 800 UV) and high-resolution transmission electron microscope (HRTEM JEM 2010FEF). The working electrode was built by mixing the MnFe2O4/rGO, black carbon, and PVDF binder (weight ratio 80:10:10). These were then coated onto a Cu foil and dried in vacuum for 10 h at 120
℃. The area density of active material on each piece is about
1.78 mg·cm-2. The lithium foil was used as the negative electrode, Celgard 2400 as the separator, and 1M LiPF6 dissolved in EC+DMC (1:1 in volume) as the electrolyte. CR2025 coin-type cells were assembled in an Ar-filled glove box. The galvanostatic charge-discharge was performed on LAND battery test system. Cyclic voltammetry was performed by Model 2273A system at 0.1 mV·s-1 scan rate between 0.01-3.0 V.
3
3. Results and discussion Scheme 1 shows the formation mechanism of the MnFe2O4/rGO. After adding MnSO4·H2O and FeSO4·7H2O into the GO suspension, the positively charged Mn2+ and Fe2+ would be absorbed on the negatively charged surface of GO by electrostatic interactions [8]. And these Fe2+ ions could be then oxidized into Fe3+ ions effectively by the oxygen containing functional groups on the GO surface at the high temperature condition (T=95
℃), accompanied by an in situ reduction of GO to rGO. Meanwhile, the
pH value of the solution is less than 4 due to the hydrolyzation of Fe3+, which could inhibit the oxidization of Mn2+ when the solution was subsequently stirred in air. Finally, the oxidized Fe3+ and Mn2+ were homogenously co-precipitated on the surface of rGO sheet by using NaOH as the precipitant to form MnFe2O4/rGO.
Scheme 1 Schematic illustration of the formation mechanism of the MnFe2O4/rGO.
Fig.1. XRD pattern (a), TEM (b) and HRTEM (c) images (Inset is the HRTEM image of the selected region.) of MnFe2O4/rGO; Raman spectra (d) of GO and MnFe2O4/rGO; The XRD pattern of the MnFe2O4/rGO was shown in Fig. 1a. All diffraction peaks match well with standard MnFe2O4. In addition, no rGO peak is observed, which can attributed to its weak diffraction in comparison with MnFe2O4. The rGO content of 4
MnFe2O4/rGO was about 14.46%, determined by TG analysis (Inset in Fig. 1a). From Fig. 1b-c, we can see that MnFe2O4 particles with a size of about 10nm are uniformly anchored on rGO sheets. This specific structure is expected to prevent the aggregation of the nanoparticles, and ensure a large specific surface area as well as alleviate the volume change during the charge-discharge process [9]. The HRTEM image (Inset in Fig. 1c) further shows the clear lattice fringes with the interplanar distance of 0.24 nm, corresponding to the (222) plane of cubic MnFe2O4. In the Raman spectra (Fig. 1d), it is noted that the MnFe2O4/rGO exhibits a higher ID/IG value than that of GO, which can be ascribed to the restoration of numerous graphitic domains from amorphous regions of GO [10]. Moreover, the G band of MnFe2O4/rGO shifts from 1598 cm-1 to 1583 cm-1, indicates the restoration of conjugated π systems [11].
Fig.2. XPS spectra of C1s for GO (a), C1s(b), Mn2p (c) and Fe2p(d) for MnFe2O4/rGO. XPS measurements were carried out to further confirm the element composition of the GO and MnFe2O4/rGO. As shown in Fig.2b, after in-situ reduced by Fe2+, the peaks of oxygen containing functional groups, such as -C-O at 286.8 eV, -C=O at 287.1 eV and -COO at 288.9 eV, decrease significantly compared to those of GO (Fig.2a), demonstrating that the majority of the conjugated rGO networks are restored, which also agrees with the Raman results. Moreover, from the Mn2p XPS spectra (Fig.2c), the two peaks centered at 642.5 and 654.1eV, corresponding to the Mn 2p3/2 and Mn 2p1/2, 5
indicates the appearance of Mn2+ [12]. The Fe 2p3/2 and Fe 2p1/2 peaks (Fig.2d) at 711.4 and 725.0eV, respectively, confirms the Fe3+ oxidation state in the sample [13]. Thus, the XPS results further demonstrate the formation of MnFe2O4/rGO.
Fig.3.
Electrochemical performance
of
MnFe2O4/rGO: (a) CV curves;
(b)
Charge-discharge profiles; (c) rate performance (Inset is the cycling performance). The electrochemical performance of the synthesized MnFe2O4/rGO was studied. Shown in Fig. 3a are the first three consecutive CV curves of MnFe2O4/rGO electrode. In the first cathodic process, the peak at 0.52 V may correspond to the reduction reaction of MnFe2O4 to metallic Mn and Fe as well as the formation of Li2O [4]. Moreover, this peak slightly shifted to higher potentials in the subsequent cycles due to the structure rearrangement of the MnFe2O4. The weak cathodic peak, appeared at 1.65 V in the first cycle and disappeared in the subsequent cycles, can be assigned to the lithium interactions with the residual oxygen-containing functional groups within rGO [7]. The broad anodic peak at around 1.8 V in the first cycle can be assigned to the oxidation of metallic Mn and Fe to MnO and Fe3O4, respectively, and which positively shifted to 2.0 V in subsequent cycles due to the polarization of electrode materials [14]. Furthermore, after the first cycle, the CV curves of the MnFe2O4/rGO almost overlapped during the subsequent two cycles, illustrating the good reversibility. Fig. 3b shows the charge-discharge profiles of the MnFe2O4/rGO for the first three 6
cycles at the current density of 0.2 A·g-1. The MnFe2O4/rGO exhibits the discharge capacity of 1293.6 mAh·g-1and a charge capacity of 813.4 mAh·g-1 in the first cycle. The irreversible capacity loss is mainly attributed to the incomplete conversion reaction and the formation of a solid electrolyte interface (SEI) film during the first cycle [15]. The rate capability of the MnFe2O4/rGO is shown in Fig. 3c. Specific capacities of 731.2, 624.5, 525.1 and 432.8 mAh·g-1 are obtained at the current density of 0.4, 0.8, 1.6 and 3.2 A·g-1, respectively. Moreover, the MnFe2O4/rGO electrode retains a reversible capacity of 653.2 mAh·g-1 after 100 cycles at the current density of 0.2 A·g-1, and shows about 80.3% capacity of the second cycle. The synthesized MnFe2O4/rGO exhibits quite good electrochemical performance, which can be ascribed to three main factors. Firstly, the synthesis strategy proposed in this paper ensures the in situ formed MnFe2O4 densely disperses on the rGO sheets to form a specific structure, which could prevent the aggregation of the particles as well as increase the conductivity of electrodes. Secondly, the nano-sized MnFe2O4 particles could bring a rapid Li-ion diffusion rate during the electrochemical reaction. Finally, it is expected that the rGO in MnFe2O4/rGO, which has a distinctive feature of high surface area and good mechanical flexibility, can facilitate the penetration of the electrolyte to the surface of active particles as well as relieve the volumetric change during the charge-discharge process that has a beneficial effect on cycling performance [3].
7
4. Conclusions We have developed a facile and green strategy to synthesize MnFe2O4/rGO as anode materials for LIBs. The composite consists of nano-MnFe2O4 with its primary particles (~10nm) decorated by rGO sheets, which could prevent the aggregation of the particles and alleviate the volume change as well as accelerate the lithium-ion transfer more effectively during the charge-discharge process. The MnFe2O4/rGO exhibits quite good electrochemical performance with high reversible capacity, excellent rate capability and cycling stability. Moreover, the proposed synthesis strategy can be easily extended to prepare other MxFe1-xO4/rGO (M=Ni, Co, Zn, Mg) composite materials. References [1] Topkaya R, Kurtan U, Baykal A, Toprak MS. Ceram Int. 2013;39:5651-8. [2] Wu KP, Hu GR, Du K, Peng ZD, Cao YB. Mater Lett. 2015;152:217-9. [3] Li SM, Wang B, Li B, Liu JH, Yu M, Wu XY. Mater Res Bull. 2015;61:369-74. [4] Xiao YL, Zai JT, Tao LQ, Li B, Han QY. Phys Chem Chem Phys. 2013;15:3939-45. [5] Feng JK, Wang CS, Qian YT. Mater Lett. 2014;122:327-30. [6] Sankar KV, Selvan RK. J Power Sources. 2015;275:399-407. [7] Tang H, Gao PB, Xing A, Tian S, Bao ZH. Rsc Adv. 2014;4:28421-5. [8] Cong HP, Ren XC, Wang P, Yu SH. Acs Nano. 2012;6:2693-703. [9] Liu HF, Ji SF, Zheng YY, Li M, Yang H. Powder Technol. 2013;246:520-9. [10] Ferrari AC, Basko DM. Nat Nanotechnol. 2013;8:235-46.
8
[11] Mei XG, Ouyang JY. Carbon. 2011;49:5389-97. [12] Yao YJ, Cai YM, Lu F, Wei FY, Wang SB. J Hazard Mater. 2014;270:61-70. [13] Li BJ, Cao HQ, Shao J, Qu MZ. Chem commun. 2011;47:10374-6. [14] Wang NN, Ma XJ, Wang YP, Yang J, Qian YT. J Mater Chem A. 2015;3:9550-5. [15] Liu SY, Xie J, Su QM, Du GH, Zhang SC, Cao GS. Nano Energy. 2014;8:84-94.
Highlights MnFe2O4/rGO was synthesized by an in situ reduction-coprecipitation method. MnFe2O4 particles with a size of about 10nm are uniformly anchored on rGO sheets. MnFe2O4/rGO exhibits an excellent electrochemical performance.
9
PII: DOI: Reference:
S0167-577X(15)30455-9 http://dx.doi.org/10.1016/j.matlet.2015.08.100 MLBLUE19461
To appear in: Materials Letters Received date: 9 June 2015 Revised date: 14 August 2015 Accepted date: 20 August 2015 Cite this article as: Kaipeng Wu, Guorong Hu, Yanbing Cao, Zhongdong Peng and Ke Du, Facile and green synthesis of MnFe2O4/reduced graphene oxide nanocomposite as anode materials for Li-ion batteries, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.100 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 galley proof before it is published in its final citable 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.
Facile and green synthesis of MnFe2O4/reduced graphene oxide nanocomposite as anode materials for Li-ion batteries Kaipeng Wu, Guorong Hu, Yanbing Cao, Zhongdong Peng, Ke Du* School of Metallurgy and Environment, Central South University, Changsha 410083, China *Corresponding author, E-mail: [email protected], TEL: +8615084864373 Abstract: MnFe2O4/reduced graphene oxide nanocomposite (MnFe2O4/rGO) was synthesized by a facile and green strategy, which involves in situ reduction of GO in presence of Fe2+ as well as co-precipitation of Fe3+ and Mn2+ onto the surface of rGO. The composite consists of MnFe2O4 with its primary particles (~10nm) decorated by rGO sheets, which could prevent the aggregation of the particles and ensure a relatively large specific surface area. It exhibits a specific capacity of 813.4 mAh·g-1 at the current density of 0.2 A·g-1 and a capacity retention of 80.3% after 100 cycles. Moreover, the proposed synthesis strategy can also be easily extended to prepare other MxFe1-xO4/rGO (M=Ni, Co, Zn, Mg) composite materials. Key Words: Energy storage and conversion; Nano MnFe2O4; Reduced graphene oxide; Nanocomposites; Anode materials. 1. Introduction As an important spinel ferrite material, MnFe2O4 has been widely applied in electronics, microwave devices, magnetic and energy storage [1-2]. It has recently been proposed as a promising anode for Li-ion batteries (LIBs) due to its low intercalation
1
potential for lithium ions and high theoretical capacity of 930 mAh·g-1 [3]. However, the MnFe2O4 has poor electrical conductivity and suffers from severe volume expansion during the charge-discharge process, which may lead to a large irreversible capacity, poor cycle and rate performance [4]. To overcome these problems, fabrication of a composite with carbonaceous materials is regarded as an effective strategy. Recently, rGO was employed as an ideal matrix to synthesis anode composites with enhanced electrochemical performance for LIBs [5]. And various wet chemical methods have been adopted for the preparation of MnFe2O4/rGO, such as solvothermal [3], ultrasonic [4], hydrothermal [6], and coprecipitation method [7]. In spite of these efforts, the traditional methods to prepare MnFe2O4/rGO were reported to be time consuming and demand strict reaction conditions, especially high temperature and pressure. Moreover, toxic reductant, such as hydrazine hydrate [7], was used for the reduction of GO to graphene. Therefore, a facile and green strategy is still highly desirable for preparation of MnFe2O4/rGO anode materials for LIBs. In this paper, MnFe2O4/rGO was synthesized by a facile and green strategy, which involves in situ reduction of GO in presence of Fe2+ as well as coprecipitation of Fe and Mn ions onto the surface of rGO. Compared with other methods, the proposed strategy has some advantages, such as simplicity and environmentally friendly due to its noninvolvement of any toxic reductant. The synthesized MnFe2O4/rGO exhibits excellent electrochemical performance as anode materials for LIBs.
2
2. Experimental MnFe2O4/rGO was synthesized by an in situ reduction-coprecipitation method. Typically, GO (3.8 g) was first dispersed into 500 mL distilled water by ultrasonication. Then, MnSO4 (0.05 mol) and FeSO4 (0.1 mol) were added into the GO solution and stirred at 95
℃ under the protection of Ar. After 2h of continuous stirring, the protection
of Ar was removed and the solution was left exposed in air and stirred for another 1h. After that, 2M NaOH solution was added to adjust the solution to pH=11 and further stirred for 2h. Final products were obtained by centrifugation, washed by distilled water and dried in a vacuum oven. MnFe2O4/rGO was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS K-Alpha 1063), TG analysis (NETZSCH STA-449C), Raman spectroscopy (LabRAM HR 800 UV) and high-resolution transmission electron microscope (HRTEM JEM 2010FEF). The working electrode was built by mixing the MnFe2O4/rGO, black carbon, and PVDF binder (weight ratio 80:10:10). These were then coated onto a Cu foil and dried in vacuum for 10 h at 120
℃. The area density of active material on each piece is about
1.78 mg·cm-2. The lithium foil was used as the negative electrode, Celgard 2400 as the separator, and 1M LiPF6 dissolved in EC+DMC (1:1 in volume) as the electrolyte. CR2025 coin-type cells were assembled in an Ar-filled glove box. The galvanostatic charge-discharge was performed on LAND battery test system. Cyclic voltammetry was performed by Model 2273A system at 0.1 mV·s-1 scan rate between 0.01-3.0 V.
3
3. Results and discussion Scheme 1 shows the formation mechanism of the MnFe2O4/rGO. After adding MnSO4·H2O and FeSO4·7H2O into the GO suspension, the positively charged Mn2+ and Fe2+ would be absorbed on the negatively charged surface of GO by electrostatic interactions [8]. And these Fe2+ ions could be then oxidized into Fe3+ ions effectively by the oxygen containing functional groups on the GO surface at the high temperature condition (T=95
℃), accompanied by an in situ reduction of GO to rGO. Meanwhile, the
pH value of the solution is less than 4 due to the hydrolyzation of Fe3+, which could inhibit the oxidization of Mn2+ when the solution was subsequently stirred in air. Finally, the oxidized Fe3+ and Mn2+ were homogenously co-precipitated on the surface of rGO sheet by using NaOH as the precipitant to form MnFe2O4/rGO.
Scheme 1 Schematic illustration of the formation mechanism of the MnFe2O4/rGO.
Fig.1. XRD pattern (a), TEM (b) and HRTEM (c) images (Inset is the HRTEM image of the selected region.) of MnFe2O4/rGO; Raman spectra (d) of GO and MnFe2O4/rGO; The XRD pattern of the MnFe2O4/rGO was shown in Fig. 1a. All diffraction peaks match well with standard MnFe2O4. In addition, no rGO peak is observed, which can attributed to its weak diffraction in comparison with MnFe2O4. The rGO content of 4
MnFe2O4/rGO was about 14.46%, determined by TG analysis (Inset in Fig. 1a). From Fig. 1b-c, we can see that MnFe2O4 particles with a size of about 10nm are uniformly anchored on rGO sheets. This specific structure is expected to prevent the aggregation of the nanoparticles, and ensure a large specific surface area as well as alleviate the volume change during the charge-discharge process [9]. The HRTEM image (Inset in Fig. 1c) further shows the clear lattice fringes with the interplanar distance of 0.24 nm, corresponding to the (222) plane of cubic MnFe2O4. In the Raman spectra (Fig. 1d), it is noted that the MnFe2O4/rGO exhibits a higher ID/IG value than that of GO, which can be ascribed to the restoration of numerous graphitic domains from amorphous regions of GO [10]. Moreover, the G band of MnFe2O4/rGO shifts from 1598 cm-1 to 1583 cm-1, indicates the restoration of conjugated π systems [11].
Fig.2. XPS spectra of C1s for GO (a), C1s(b), Mn2p (c) and Fe2p(d) for MnFe2O4/rGO. XPS measurements were carried out to further confirm the element composition of the GO and MnFe2O4/rGO. As shown in Fig.2b, after in-situ reduced by Fe2+, the peaks of oxygen containing functional groups, such as -C-O at 286.8 eV, -C=O at 287.1 eV and -COO at 288.9 eV, decrease significantly compared to those of GO (Fig.2a), demonstrating that the majority of the conjugated rGO networks are restored, which also agrees with the Raman results. Moreover, from the Mn2p XPS spectra (Fig.2c), the two peaks centered at 642.5 and 654.1eV, corresponding to the Mn 2p3/2 and Mn 2p1/2, 5
indicates the appearance of Mn2+ [12]. The Fe 2p3/2 and Fe 2p1/2 peaks (Fig.2d) at 711.4 and 725.0eV, respectively, confirms the Fe3+ oxidation state in the sample [13]. Thus, the XPS results further demonstrate the formation of MnFe2O4/rGO.
Fig.3.
Electrochemical performance
of
MnFe2O4/rGO: (a) CV curves;
(b)
Charge-discharge profiles; (c) rate performance (Inset is the cycling performance). The electrochemical performance of the synthesized MnFe2O4/rGO was studied. Shown in Fig. 3a are the first three consecutive CV curves of MnFe2O4/rGO electrode. In the first cathodic process, the peak at 0.52 V may correspond to the reduction reaction of MnFe2O4 to metallic Mn and Fe as well as the formation of Li2O [4]. Moreover, this peak slightly shifted to higher potentials in the subsequent cycles due to the structure rearrangement of the MnFe2O4. The weak cathodic peak, appeared at 1.65 V in the first cycle and disappeared in the subsequent cycles, can be assigned to the lithium interactions with the residual oxygen-containing functional groups within rGO [7]. The broad anodic peak at around 1.8 V in the first cycle can be assigned to the oxidation of metallic Mn and Fe to MnO and Fe3O4, respectively, and which positively shifted to 2.0 V in subsequent cycles due to the polarization of electrode materials [14]. Furthermore, after the first cycle, the CV curves of the MnFe2O4/rGO almost overlapped during the subsequent two cycles, illustrating the good reversibility. Fig. 3b shows the charge-discharge profiles of the MnFe2O4/rGO for the first three 6
cycles at the current density of 0.2 A·g-1. The MnFe2O4/rGO exhibits the discharge capacity of 1293.6 mAh·g-1and a charge capacity of 813.4 mAh·g-1 in the first cycle. The irreversible capacity loss is mainly attributed to the incomplete conversion reaction and the formation of a solid electrolyte interface (SEI) film during the first cycle [15]. The rate capability of the MnFe2O4/rGO is shown in Fig. 3c. Specific capacities of 731.2, 624.5, 525.1 and 432.8 mAh·g-1 are obtained at the current density of 0.4, 0.8, 1.6 and 3.2 A·g-1, respectively. Moreover, the MnFe2O4/rGO electrode retains a reversible capacity of 653.2 mAh·g-1 after 100 cycles at the current density of 0.2 A·g-1, and shows about 80.3% capacity of the second cycle. The synthesized MnFe2O4/rGO exhibits quite good electrochemical performance, which can be ascribed to three main factors. Firstly, the synthesis strategy proposed in this paper ensures the in situ formed MnFe2O4 densely disperses on the rGO sheets to form a specific structure, which could prevent the aggregation of the particles as well as increase the conductivity of electrodes. Secondly, the nano-sized MnFe2O4 particles could bring a rapid Li-ion diffusion rate during the electrochemical reaction. Finally, it is expected that the rGO in MnFe2O4/rGO, which has a distinctive feature of high surface area and good mechanical flexibility, can facilitate the penetration of the electrolyte to the surface of active particles as well as relieve the volumetric change during the charge-discharge process that has a beneficial effect on cycling performance [3].
7
4. Conclusions We have developed a facile and green strategy to synthesize MnFe2O4/rGO as anode materials for LIBs. The composite consists of nano-MnFe2O4 with its primary particles (~10nm) decorated by rGO sheets, which could prevent the aggregation of the particles and alleviate the volume change as well as accelerate the lithium-ion transfer more effectively during the charge-discharge process. The MnFe2O4/rGO exhibits quite good electrochemical performance with high reversible capacity, excellent rate capability and cycling stability. Moreover, the proposed synthesis strategy can be easily extended to prepare other MxFe1-xO4/rGO (M=Ni, Co, Zn, Mg) composite materials. References [1] Topkaya R, Kurtan U, Baykal A, Toprak MS. Ceram Int. 2013;39:5651-8. [2] Wu KP, Hu GR, Du K, Peng ZD, Cao YB. Mater Lett. 2015;152:217-9. [3] Li SM, Wang B, Li B, Liu JH, Yu M, Wu XY. Mater Res Bull. 2015;61:369-74. [4] Xiao YL, Zai JT, Tao LQ, Li B, Han QY. Phys Chem Chem Phys. 2013;15:3939-45. [5] Feng JK, Wang CS, Qian YT. Mater Lett. 2014;122:327-30. [6] Sankar KV, Selvan RK. J Power Sources. 2015;275:399-407. [7] Tang H, Gao PB, Xing A, Tian S, Bao ZH. Rsc Adv. 2014;4:28421-5. [8] Cong HP, Ren XC, Wang P, Yu SH. Acs Nano. 2012;6:2693-703. [9] Liu HF, Ji SF, Zheng YY, Li M, Yang H. Powder Technol. 2013;246:520-9. [10] Ferrari AC, Basko DM. Nat Nanotechnol. 2013;8:235-46.
8
[11] Mei XG, Ouyang JY. Carbon. 2011;49:5389-97. [12] Yao YJ, Cai YM, Lu F, Wei FY, Wang SB. J Hazard Mater. 2014;270:61-70. [13] Li BJ, Cao HQ, Shao J, Qu MZ. Chem commun. 2011;47:10374-6. [14] Wang NN, Ma XJ, Wang YP, Yang J, Qian YT. J Mater Chem A. 2015;3:9550-5. [15] Liu SY, Xie J, Su QM, Du GH, Zhang SC, Cao GS. Nano Energy. 2014;8:84-94.
Highlights MnFe2O4/rGO was synthesized by an in situ reduction-coprecipitation method. MnFe2O4 particles with a size of about 10nm are uniformly anchored on rGO sheets. MnFe2O4/rGO exhibits an excellent electrochemical performance.
9