Graphene 3D Network Anode

Research Article pubs.acs.org/journal/ascecg

Synthesis of Vesicle-Like MgFe2O4/Graphene 3D Network Anode Material with Enhanced Lithium Storage Performance Yanhong Yin,*,†,‡,§ Wenfeng Liu,†,‡,§ Ningning Huo,†,‡,§ and Shuting Yang*,†,‡,§ †

School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang Henan 453007, China National and Local Joint Engineering Laboratory of Motive Power and Key Materials, Xinxiang, Henan 453007, China § Collaborative Innovation Center of Henan Province for Green Motive Power and Key Materials, Henan Normal University, Xinxiang, Henan 453007, China Downloaded via DURHAM UNIV on June 28, 2018 at 19:40:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Vesicle-like MgFe2O4/graphene 3D network anode material was synthesized via a simple one-step in situ growth of solvothermal technique. The as-obtained unique 3D nanostructure consists of MgFe2O4 particles randomly anchored on mutual cross-linking graphene sheets functioning as a mechanical support and an efficient electron conducting pathway. The as-synthesized anode material shows an excellent rate capability and outstanding cycling stability. A specific capacity of 1300 mAh g−1 can be maintained at 1000 mA g−1 in a prolonged charge/discharge process (200 cycles). When cycling at a high current density of 10,000 mA g−1, a specific capacity of 597 mAh g−1 can still be achieved. The superior battery performance can be attributed to the unique 3D network structure, which provides an efficiently conductive network, buffers the volume expansion, and improves the structure integrity of the electrode. KEYWORDS: MgFe2O4, Graphene, Anode, Lithium ion batteries, Specific capacity

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discharge capacity rapidly decreased to 300 mAh g−1 after only 10 cycles. Then, Pan et al.13 synthesized MgFe2O4 by the sol−gel method, and the capacity maintained only 493 mAh g−1 after 50 cycles. A modification technique was used to improve the battery performance. Qiao et al.14 reported that MgFe2O4 nanofibers were synthesized by an electrospinning technique and annealed at 800 °C. The obtained 1D MgFe2O4 nanofibers showed an improved electrochemical performance with a capacity of 714 mAh g−1 after 100 cycles. Gong et al.15 prepared irregular MgFe2O4 nanoparticles (60 nm) via a coprecipitation and calcining process, and then, MgFe2O4 and pyrrole were added into autoclaves and heated at 550 °C for 5 h to obtain a carbon-coated MgFe2O4. A specific capacity of 600 mAh g−1 after 50 cycles was obtained. Alok et al.16 prepared an MgFe2O4/graphene nanocomposite through a urea-assisted autocombustion method. The MgFe2O4 particles presented serious aggregation on the graphene, whose capacity maintained 764.4 mAh g−1 at 0.04 C after 60 cycles. To date, there still exists a big gap between the experimental and theoretical specific capacities of MgFe2O4. Meanwhile, it is still very urgent to improve their cycling and rate performances. As the previous reports have shown, the poor performance of MgFe2O4 is mainly caused by insufficient structural stability and conductivity, which can be overcome through appropriate

INTRODUCTION Lithium ion batteries (LIBs) are one of the most promising power supplies for electric vehicles (EVs) and hybrid electronic vehicles (HEVs).1,2 In order to meet the increasing demands of high energy density and long cycling lifetime, it is essential to develop electrode materials with high charge/discharge capacity and high-rate performance. Due to the low cost, high theoretical specific capacity, and environmentally benign nature, transition metal oxides (TMOs) are becoming promising alternative anode materials for LIBs.3−6 Among them, bicomponent transition metal oxides (AB2O4, A and B denote divalent and trivalent metal cations, respectively) have attracted increasing interests due to the relatively low activation energy of electron transfer and rich redox reaction. Cations with different valences accommodate in octahedral or tetrahedral sites of the close-packed oxygen atoms, which can synergistically enhance the intrinsic properties of each component,7−11 such as electrical conductivity, electrochemical reactivity, and mechanical stability. MgFe2O4, consisting of high electrochemical active Fe2O3 and inactive MgO, can not only provide high specific capacity but also relieve the volume expansion to a certain extent. However, there are only few reports on the electrochemical behavior of nanocrystalline MgFe2O4 as anode materials for lithium ion batteries. Sivakumar et al.12 employed a high-speed milling technique to prepare MgFe2O4 and first researched it as anode material for lithium ion batteries, which achieved a high discharge capacity of 1400 mAh g−1. Nevertheless, the © 2016 American Chemical Society

Received: August 15, 2016 Revised: November 19, 2016 Published: December 5, 2016 563

DOI: 10.1021/acssuschemeng.6b01949 ACS Sustainable Chem. Eng. 2017, 5, 563−570

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration for the synthetic procedure of MFO/G.

morphology control and structure design.4,9,17−21 However, the influences of morphology and structure on the electrochemical performance are seldom investigated. As is well known, the solvothermal technique is an effective method to prepare materials via one-step in situ growth and to control the morphology and structure by adjusting the reaction conditions. Herein, vesicle-like MgFe2O4/graphene 3D network anode material was designed and synthesized via a simple one-step in situ growth of solvothermal techniques. In the target product, the graphene layers cross-link with each other to form the 3D conductive network, and the MgFe2O4 nanoparticles are randomly anchored on both sides of the self-packaged graphene sheets through electrostatic interaction. The 3D conductive network can improve the conductivity, buffer the volume expansion, and act as a barrier to suppress the aggregation of active particles during the lithiation/delithiation process.18,22−25 As a result, the hybrid MgFe2O4/graphene electrode exhibits high lithium-storage capacity, superior cycling stability, and impressive rate performance. When cycling at 1000 mA g−1, a specific capacity of 1300 mAh g−1 can be maintained after 200 cycles. When cycling at 10000 mA g−1, a specific capacity of 597 mAh g−1 still can be achieved.

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laser line at room temperature. BET surface area and pore diameter distribution were determined by Tristar 3020 analyzer at 77 K. Electrochemical Measurements. The working electrodes were prepared by mixing the active material (70 wt %), conducting agent (Super P 20 wt %), and polyvinylidene fluoride (PVDF 10 wt %) well in N-methyl-2-pyrrolidinone (NMP) solvent to form slurry, and then, the slurry was brushed onto a Cu-current collector and dried at 60 °C in vacuum for 24 h. The electrode was cut into discs 12 mm in diameter and pressed at 6 MPa. The active material of each elctrode was about 1.4 mg cm−2. CR2032 type coin cells were assembled in an Ar-filled glovebox using Celgard 2400 as separator, Li foil as the counter and reference electrode, and 1.0 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume) as the electrolyte. Galvanostatic charge/discharge profiles between 0.01 and 3.0 V (vs Li+/Li) and rate capability at various current densities were recorded on a LAND Cell test system (CT2001A, Wuhan, China), and specific capacity of the composite was calculated by the total mass of MgFe2O4 and graphene. Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) of the test cells were carried out on an electrochemical workstation (CHI760E, Shanghai, China). CV tests were performed between 0.01 and 3.0 V (vs Li+/Li) at a scan rate of 0.2 mV s−1. The data of EIS were collected with a frequency range from 0.1 Hz to 100 kHz and a signal amplitude of 5 mV.

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EXPERIMENTAL SECTION

RESULT AND DISCUSSION

Figure 1 illustrates our simple yet effective synthesis process of MFO/G. Graphene was dispersed in ethylene glycol under ultrasonication for 2 h, and then, metal salts and urea were dissolved uniformly in the solution. During the solvothermal reaction, urea decomposed into CO2 and NH3, which interacted with Fe3+ and Mg2+ to form tiny spindle M(OH)CO3 crystal nucleii uniformly depositing on the graphene sheets via electrostatic self-assembly. With reaction time increasing, the tiny crystal nucleii coarsened to form irregular spheres along with the pyrolysis of subcarbonate. Consequently, under the effect of the Ostwald ripening, vesicle-like MgFe2O4 nanoparticles were formed on the graphene sheets. XRD patterns of as-synthesized MFO and MFO/G are shown in Figure 2. All of the reflection peaks in the patterns can be well indexed to the spinel-structured MgFe2O4 (JCPDS 881939, Fd3̅m (227)). The diffraction peaks of 2θ at 30.0°, 35.4°, 37.1°, 43.1°, 47.2°, 53.4°, 57.0°, and 62.5° are ascribed to the (220), (311), (222), (400), (331), (422), (511), and (440) planes of MgFe2O4. Compared with bare MFO, the characteristic diffraction peak at 26.3° in the MFO/G composite confirms the existence of graphene layers, and all the characteristic peaks in MFO/G become broad and weak, implying a smaller grain size of MFO/G. The average grain sizes of MFO and MFO/G calculated by the Scherrer equation

Materials Synthesis. Graphene oxide (GO) was synthesized based on a modified Hummer’s method and then reduced as previously reported.26,27 An exfoliated graphene suspension (100 mL, approximately 150 mg reduced graphene oxide) was obtained by dispersing the reduced graphene oxide (rGO) in ethylene glycol (EG) under ultrasonication for 2 h. 5 mmol of MgCl2·6H2O, 10 mmol of FeCl3·9H2O, and 9 mmol of urea were added to the above suspension under stirring. The mixture was transferred into a Teflon-lined autoclave (80% degree of filling), subsequently heated at 200 °C for 24 h. The as-obtained MgFe2O4/grapheme (MFO/G) was washed with deionized water and ethanol several times and dried in vacuum at 60 °C for 24 h. For comparison, bare MgFe2O4 (MFO) was also prepared under the same conditions without reduced graphene oxide. Materials Characterization. Powder X-ray diffraction (XRD) patterns in the 2θ range of 20°−80° were obtained by Bruker AXS D8 using Cu Kα radiation (λ = 0.1541 nm). The morphology features of the samples were characterized by the field emission scanning electron microscope (FESEM, JSM-6700F). Transmission electron microscopic (TEM) images were collected by a JEOL JEM-2100 microscope with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos Axis Ultra spectrometer with a monochromatic Al Kα radiation (hυ = 1486.6 eV). Thermal gravimetric analysis (TGA) was used to measure the content of graphene on a NETZSCH (STA 449 F3) thermal analyzer with a heating rate of 10 °C min−1 in flowing air atmosphere. Raman spectra were recorded with a Renishaw Invia spectrometer using a 532 nm 564


Research Article


structures with many internal pores. The HRTEM image of MFO (Figure 4b) demonstrates the interplanar spacing is 0.26 nm, consistent with the lattice fringe distance of the (311) lattice plane of spinel MgFe2O4. Figure 4d displays a lattice resolved HRTEM image of MFO/G. The lattice fringe spacings of 0.27 and 0.31 nm are in accordance with the spacing of the (311) and (220) crystal planes of MgFe2O4, respectively. The winding lattice fringe spacing of 0.35 nm at the edge of the particle corresponds to (002) planes of the graphene sheets. Raman spectra and TG were used to obtain further information on structure and composition of the samples. Figure 5a is the Raman spectra of as-synthesized MFO and MFO/G. For MFO, peaks at 475 and 684 cm−1 can be assigned to the characteristic peaks of MgFe2O4,28−30 and the broad and weak peaks at approximately 1330 and 1580 cm−1 can be ascribed to the typical D and G bands of carbon materials derived from the EG solvent of solvothermal. For MFO/G, the characteristic Raman spectrum of MgFe2O4 becomes inconspicuous. While the characteristic peaks at ∼1350 cm−1 (Dband), ∼1580 cm−1 (G-band), and ∼2730 cm−1 (2D-band) become distinguishable, distinctly indicating that the MFO particles are coated by graphene sheets. In addition, the weight fraction of the rGO shown by thermogravimetric analysis (TGA) (Figure 5b) is approximately 10.5 wt % in the final composite. The weight of MFO increased slightly about 0.5 wt %. It may be ascribed to a trace of Fe2+, which is too little to be detected by XRD. XPS analysis was performed to investigate the detailed information about surface composition and electronic structure of the samples. As indicated in the XPS survey spectrum (Figure 6a), Mg, Fe, O, and C characteristic peaks, energy loss peaks, and Auger electron peaks at different electron orbits or transition states are observed for both MFO and MFO/G. The deconvoluted C 1s peak (Figure 6b) was fitted to four contributions (C1, C2, C3, and C4) for MFO/G. The C1 peak at 284.7 eV and C2 peak at 285.6 eV can be assigned to the sp2

Figure 2. XRD patterns of the synthesized MFO and MFO/G.

are 56 and 32.2 nm, respectively. The result can be due to the existence of graphene sheets limiting the growth of MFO particles during the synthesis process. Morphologies and sizes of the MFO and MFO/G were characterized by FESEM in Figure 3. Figure 3a and b show the FESEM images of the MFO particles. The narrowly distributed sphere-like particles aggregate together with some pinholes on their surface, whose average particle size is about 160 nm. Morphology of MFO in MFO/G (Figure 3c,d) is very similar to that of the bare MFO, but the smaller average particle (about 100 nm) and better dispersity imply that the existence of graphene can limit the growth and aggregation of MFO particles. The MFO particles are randomly anchored on both sides of the self-packaged graphene sheets, and the graphene layers cross-link with each other to form the 3D network structure. Figure 4a and c show the TEM images of MFO and MFO/G. In accordance with the FESEM results, MFO particles are randomly anchored on both sides of the graphene sheets. Interestingly, some transparent area can be observed on the MFO particles, indicating that MFO particles are vesicle-like

Figure 3. FESEM images of MFO (a, b) and MFO/G (c, d) at different magnification. 565


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Figure 4. TEM and HRTEM images of MFO (a, b) and MFO/G (c, d).

Figure 5. Raman spectra (a) and TG profiles (b) patterns of the synthesized MFO and MFO/G.

(C−C) and sp3 (C−O) hybridized graphite carbon, respectively. The C3 peak at 287.3 eV might correspond to the carbonyl and carboxylic (O−CO) groups. The C4 peak at 291.0 eV corresponds to either the carbonyl group or a π−π* shake up satellite structure which is the characteristic of conjugated systems.31−33 As shown in Figure 6c, the Fe 2p peak can be resolved into two components (Fe 2p 1/2 and Fe 2p 3/2), which can be deconvoluted into four peaks (marked with 1, 2, 3, and 4) using Gaussian functions, corresponding to a tiny Fe2+ (B-site) subpeak at 708.6 eV, Fe3+ subpeak at the A-site (710.8 eV), Fe3+ subpeak at the B-site (712.6 eV), and the shakeup satellite peak, respectively.34,35 From Figure 6d, it is obvious that the Fe 2p peak for MFO shifts to higher bonding energy after adding graphene, which may be caused by the interaction between the metal cation and the surface oxygenic functional groups of RGO. The porous feature of MFO and MFO/G was characterized by the N2 adsorption−desorption measurement. In Figure 7, both obtained nitrogen isotherm patterns present the typical structural signature of type IV isotherm. For MFO/G, due to the existence of a slit pore caused by the layer structure of

graphene, the H3 type of hysteresis loop is formed within a relative pressure P/P0 range of 0.4−1 between the desorption and absorption isotherm, and its Brunauer−Emmett−Teller (BET) specific surface area is calculated to be about 47.15 m2 g−1, which is higher than that of bare MFO (10.14 m2 g−1). On the basis of the Barrett−Joyner−Halenda model, the pore size of MFO/G distributes between 2 and 300 nm. The high surface area of MFO/G with the meso- and macro-porous features is benefit for electrolyte accessibility and rapid Li+ diffusion. Cyclic voltammograms (CVs) of the MFO and MFO/G electrodes are shown in Figure 8a−c. The CV curves of the two electrodes are similar, indicating that a similar electrochemical reaction occurs during the cycling process. As previously reported,13,14,16 in the first cathodic scan, two reduction peaks are observed at 0.6 and 0.1 V, corresponding to the reduction of MgFe2O4 to Fe, MgO, and the formation of a solid electrolyte interface (SEI) film layer on the electrode material surface. In the first anodic scan, a broad peak is recorded between 1.5 and 2.2 V, corresponding to a reversible oxidation reaction. In the subsequent cycles, the two reduction peaks slightly shift to ∼0.76 and 1.2 V, which can be attributed to the structure transformation after the first cycle. Figure 8c shows 566


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Figure 6. XPS survey spectra of the synthesized MFO and MFO/G (a), the deconvoluted C 1s spectra (b) and Fe 2p spectra (c) of MFO/G, and the comparison of Fe 2p spectra between MFO and MFO/G (d).

down to 0.01 V can be attributed to the formation of solid electrolyte interface (SEI) film. After the first cycle, the voltage plateau becomes less apparent. Instead, two sloping regions of 1.5−1.0 V and 1.0−0.75 V appear, in accordance with the CV profiles, indicating that the structure transformation occurs after the first complete cycle. With the cycling number increasing, the MFO electrode (Figure 8d) shows an obvious deterioration trend. The capacity decreases progressively (622, 388, and 331 mAh g−1 for the 30th, 50th, and 100th cycles, respectively), and the polarization is increasingly severe. However, the MFO/G electrode (Figure 8e) exhibits a slowly enchanced reversible capacity (997, 1096, and 1341 mAh g−1 for the 30th, 50th, and 100th cycles, respectively). It is worth noticing that, in comparison with the first and 100th discharge/ charge curves of the two electrodes (Figure 8f), when introducing graphene, the MFO/G electrode appears to have higher specific capacity and lower polarization. This may be because graphene with large specific surface area increases the contact area between the active material and electrolyte and facilitates Li+ diffusion, and consequently, the electrodic polarization decreases. Moreover, the better conductivity of graphene and the smaller particle size of MFO in the composite can also reduce the polarization. Figure 9a−c show the cycle and rate performances of the MFO and MFO/G at different current densities between 0.01 and 3 V. When cycling at a current density of 500 mA g−1 (Figure 9a), the initial discharge and charge capacities of the MFO/G electrode are 1246 and 896 mAh g−1, respectively. The corresponding Coulombic efficiency is 71.9%, and the capacity loss can be attributed to the formation of SEI and the irreverse insertion. Subsquently, the specific capacity shows a continuous increase to 1341 mAh g−1 at the 100th cycle, with a stable Coulombic efficiency higher than 98%. Notably, the specific capacity of 1341 mAh g−1 is larger than the theoretical value of MgFe2O4. The increased capacity is mainly due to the

Figure 7. N2 adsorption−desorption isotherms and pore diameter distribution of the as-synthesized MFO and MFO/G.

the comparison of the first two cycles of MFO and MFO/G. Obviously, for the MFO/G electrode, the electrical potential difference between the cathodic and anodic peaks is much smaller than bare MFO, implying that reversibility of the MFO/G electrodes is relatively better. This may be because that adding high conductivity graphene can reduce the electrodic polarization and improve the reversibility of electrode process. Figure 8d−f show the charge and dischage curves of the two electrodes at a current density of 500 mA g−1 between 0.01 and 3.0 V (vs Li+/Li). It is found that the first discharge/charge voltage profiles for the two electrodes are very similar, which are consistent with their corresponding CV plots. In the first discharge step, a rapid potential drop to 0.75 V is indicative of partial Li+ inserting into the lattice to form LixMgFe2O4. The voltage plateau at about 0.75 V corresponds to the conversion reaction of Fe3+ to Fe2+ and Fe0. Subsequently, a sloping curve 567


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Figure 8. CV (a−c) and discharge/charge curves (d−f) of the synthesized MFO and MFO/G.

Figure 9. Cycling (a, b), rate performance (c), and EIS (d) of the synthesized MFO and MFO/G.

capacitance contribution from interfacial lithium storage and/or reversible formation of polymeric/gel-like films.36−38 The initial discharge and charge capacities of the bare MFO electrode are 1029 and 790 mAh g−1 with a relatively higher initial Coulombic efficiency of 76.8%. However, its specific capacity quickly fades. After 100 cycles, only 334 mAh g−1 (41.9%) is retained. Importantly, the MFO/G electrode still exhibits a superior cycling performance at the current density of 1000 mA g−1 (Figure 9b). Specific capacity of the MFO/G electrode appears to continually increase to about 1300 mAh g−1 at the 160th cycle and remains stable until 200 cycles, while capacity of the bare MFO rapidly decays to 340 mAh g−1. Rate performances of the two elecrodes are shown in Figure 9c with current densities increased stepwise from 100 to 10000 mA g−1. Obviously, the MFO/G electrode presents excellent rate performance. At a current density of 100 mA g−1, the average specific capacity of the MFO/G electrode is about 1080 mAh g−1. Even cycling at 10,000 mA g−1, a capacity of 597 mAh g−1

and a capacity retention of 55.3% can still be retained, which is more remarkable than the previous reports.13,15,16,39,40 Moreover, when the current density returns to 100 mA g−1, a capacity of 1307 mAh g−1 can be attained again, which is even higher than that of the first 10 cycles. Figure 9d shows EIS measurements of the two electrodes after 50 cycles at 1000 mA g−1. Both Nyquist plots of the two electrodes exhibit two semicircles in the high-medium frequency range and an inclined line in the low frequency range, corresponding to the SEI film and contact resistance (Rs), charge transfer resistance (Rct) between the electrolyte and active material, and the Warburg impedance related to the solid-state diffusion of Li+ into the bulk of the active materials.41,42 Apparently, the MFO/G electrode shows a smaller semicircle diameter and a larger inclined line, implying a relatively lower charge transfer resistance and faster Li+ diffusion. This can be attributed to the large specific surface area, high conductivity of the graphene substrate, and smaller particle size of MFO in the composite, 568


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which not only increases the contact area between active material and electrolyte and facilitates the Li+ and electron transport but also shortens the pathway of Li+, leading to the improved rate capability of MFO/G. In summary, the 3D construction of vesicle-like MFO on the graphene nanosheets contributes to a superior electrochemical performance. First, introducing graphene can limit the growth of MFO particles to form the smaller nanoparticles with a more active site and shorten the pathway of Li+ diffusion. Second, MFO/G composites with a higher specific surface area provide more interfaces and better penetrability of the electrolyte. Third, graphene can act as a 3D conductive network to improve the electron transmission of the active material. Meanwhile, graphene as a flexible matrix can buffer the volume expansion, prevent the grain aggregate, and keep the electrode structure integrity.

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CONCLUSION A spherical vesicle-like MFO/G hybrid system was synthesized successfully via a simple one-step in situ growth of solvothermal techniques. The graphene layers cross-link with each other to form the 3D conductive network, and the MFO nanoparticles are randomly anchored on both sides of the self-packaged graphene sheets through electrostatic interaction. This unique 3D nanostructure can not only enhance the electrical conductivity but can also buffer the volume expansion and improve the structural integrity of the electrode. The MFO/G electrode exhibits high lithium-storage capacity, superior cycling stability, and impressive rate capability. This result shows that the vesicle-like MgFe2O4/graphene 3D network hybrid system is a promising next generation anode material for LIBs with high energy and power density.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: (+86)-373-3326439. Tel: (+86)-373-3326439 (Yanhong Yin). *E-mail: [email protected]. Fax: (+86)-373-3326439. Tel: (+86)-373-3326439 (Shuting Yang). ORCID

Shuting Yang: 0000-0001-7841-0016 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Nature Science Foundation of China under award (No. 21471049). REFERENCES

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Graphene 3D Network Anode

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