High Performance Lithium-Ion Hybrid Capacitors Employing Fe3O4

suppress volume expansion of metal oxides to obtain a good cycling ..... observed the cycling life of Fe3O4-G//AC is better than most devices reported...
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High Performance Lithium-Ion Hybrid Capacitors Employing Fe3O4− Graphene Composite Anode and Activated Carbon Cathode Shijia Zhang,†,‡,§ Chen Li,†,‡,§ Xiong Zhang,*,†,‡ Xianzhong Sun,†,‡ Kai Wang,†,‡ and Yanwei Ma*,†,‡ †

Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China University of Chinese Academy of Sciences, Beijing 100049, P. R. China

‡

S Supporting Information *

ABSTRACT: Lithium-ion capacitors (LICs) are considered as promising energy storage devices to realize excellent electrochemical performance, with high energy−power output. In this work, we employed a simple method to synthesize a composite electrode material consisting of Fe3O4 nanocrystallites mechanically anchored among the layers of three-dimensional arrays of graphene (Fe3O4−G), which exhibits several advantages compared with other traditional electrode materials, such as high Li storage capacity (820 mAh g−1 at 0.1 A g−1), high electrical conductivity, and improved electrochemical stability. Furthermore, on the basis of the appropriated charge balance between cathode and anode, we successfully fabricated Fe3O4−G//activated carbon (AC) soft-packaging LICs with a high energy density of 120.0 Wh kg−1, an outstanding power density of 45.4 kW kg−1 (achieved at 60.5 Wh kg−1), and an excellent capacity retention of up to 94.1% after 1000 cycles and 81.4% after 10 000 cycles. The energy density of the Fe3O4−G//AC hybrid device is comparable with Ni-metal hydride batteries, and its capacitive power capability and cycle life is on par with supercapacitors (SCs). Therefore, this lithium-ion hybrid capacitor is expected to bridge the gap between Li-ion battery and SCs and gain bright prospects in next-generation energy storage fields. KEYWORDS: lithium-ion capacitor, electrode material, graphene, Fe3O4, prelithiation

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storage devices.9−11 High energy density (several times than SCs) can be readily realized because of the greater specific capacity of anode and the expanded working voltage window in the organic electrolyte, and the high power performance would also be maintained by the fast charge−discharge processes of cathode. Technically speaking, the physical and chemical properties of electrode materials, especially the nanostructure, are the key factors determining the electrochemical performance of LICs. To achieve high energy density, the electrodes should have (a) abundant micropores to provide sufficient sites for charge separation, (b) high electrical conductivity to offer efficient electron transfer,12−14 and (c) shorted Li-ion diffusion distances to promote ion transport. Well-developed mesopores and macropores are indispensable for high power density, which can serve as ion-diffusion channels to allow smooth ion transport.15−18 During the charge−discharge process of LICs, the intercalation/deintercalation at the LIB bulk anode will be accompanied by a different degree of volumetric expansion; however, the fast adsorption/desorption at cathode induces no volumetric change.19,20 This explains the much inferior cycling life of batteries than SCs, and how to minimize structural

INTRODUCTION The increasing demands of modern society for green energy, electric vehicles, and portable electronic devices necessitate the development of high-efficiency energy storage devices. Among them, lithium-ion batteries (LIBs) and supercapacitors (SCs) are considered as promising candidates due to their excellent electrochemical performances.1 LIBs have gained large-scale commercialization due to their high energy densities (150−200 Wh kg−1), but the limited power density (below 100 W kg−1) and inferior cycle life (less than 1000 cycles) restrict their further applications where quick energy release is needed.2,3 On the contrary, SCs can deliver high power density (10 kW kg−1) and outstanding cycling stability (up to a million cycles), but on the basis of their lower energy densities (5−10 Wh kg−1), it can be determined that they cannot be used as an independent energy storage device but in most cases need to be integrated with LIBs.4,5 The completely different energy storage mechanisms account for the tremendous performance discrepancy between the two devices, in which LIBs require redox reaction, whereas SCs depend on physical adsorption/ desorption at the electric double layer.6,7 Accordingly, it’s a challenging task to develop new energy storage devices to combine the advantages of LIBs and SCs.8 Lithium-ion capacitors (LICs), composed of a capacitor-type cathode and LIB-type anode, are expected to bridge the gap between LIBs and SCs by integrating the two different energy © 2017 American Chemical Society

Received: March 9, 2017 Accepted: May 5, 2017 Published: May 5, 2017 17136

DOI: 10.1021/acsami.7b03452 ACS Appl. Mater. Interfaces 2017, 9, 17136−17144

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of preparing the Fe3O4−G composites.

application and low cost.59 Finally, the soft-package LIC cells are assembled instead of coin cells to show practical performance in real applications. Furthermore, the mass matching of anode/cathode gets optimized from 1:1 to 1:5, and the optimal electrochemical performance is obtained with a mass ratio of 1:3. The energy density is up to 120.0 Wh kg−1, and the power density is up to 45.4 kW kg−1 (achieved at 60.5 Wh kg−1). The cycling life also gets greatly improved compared to that of other metal oxides: the capacity retention is up to 94.1% after 1000 cycles and 81.4% after 10 000 cycles.

distortion of anode on the nanoscale needs further investigation. In 2001, Amatucci et al. fabricated the first LIC hybrid system using commercially activated carbon (AC) as the cathode material and nanostructured Li4Ti5O12 (LTO) as the anode material.21 Since then, various hybrid systems have been proposed because many new materials have been applied to LICs and their merits and demerits are different. Currently, the cathode material is basically advanced nanoporous carbon. The most mature and widely applied cathode material is AC, which has a high surface area and lower cost.22 The anode materials are mainly insertion-type materials which involve carbonaceous materials, metal oxides,23−25 hydroxides,26 and some composites.27−29 Carbonaceous materials, including graphite30,31 and hard and soft carbon,32,33 have a low potential (0−0.25 V vs Li/ Li+ for graphite) after intercalation by Li+, leading to an extended operating voltage, whereas their low capacity (372 mAh g−1 for graphite)34−37 and inferior performance at high rate are undesirable. Metal oxides, such as Li4Ti5O12,38−41 TiO2,42 TiC,43 MoO2,24 Fe3O4,44,45 MnO2,46 MnO,43,47,48 Nb2O5,49 and NbN,50 are considered as suitable candidates for their high theoretical capacity (500−1000 mAh g−1) and relatively low-voltage plateau (around 0.8 V). Nevertheless, they will undergo a relatively violent volume distortion when lithium ions insert/extract into/from the electrode material, resulting in a quick capacity degradation.51−53 Many research attempts have been made recently for the purpose of enhancing the cycling stability of metal oxide−carbon composites. The coated carbon, like graphene, serves as an excellent conductive flexible network that could effectively suppress volume expansion of the metal oxides to obtain a good cycling stability.54,55 Significant progress has been made in recent years; the energy density of LICs is becoming increasingly higher (100− 150 Wh kg−1), but this improvement is usually accompanied by significant power losses or a cycling-life decline,27,37,48,50,56 which are the most important features of LICs compared with secondary batteries.57 However, the LICs (10−100 kW kg−1) cannot achieve both high energy density and power density at the same time.43,58 Hence, creating energy storage devices with comprehensive performances need further research. In this work, we pay more attention to the improvement of electrochemical performance in the light of practical applications. First, a composite anode material was synthesized by a simple and effective low-temperature thermal annealing method, which is composed of Fe3 O4 nanocrystallites mechanically anchored among the layers of three-dimensional (3D) arrays of graphene (Fe3O4−G). We carried out a series of physical and chemical characterization and electrochemical testing to prove its suitability as the electrode material for LIC. Then we chose AC as the cathode on account of its well-known

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EXPERIMENTAL SECTION

Synthesis of Fe3O4−G Composites. Graphite oxide (GO) was prepared by a modified Hummers’ method. The synthetic process is shown in Figure 1. GO was dispersed in deionized water by an ultrasonic dispersing method (2 mg mL−1). An aqueous solution of FeCl3·6H2O (128 mg mL−1) was blended with GO solution at a volume ratio of 1:4. Then, the mixture was stirred for 12 h, and vacuum filtration was applied for the solid product. The solid product was put in a crucible and heated at 350 °C under argon flow. Eventually, the composite was grinded and washed with deionized water and alcohol several times, and dried completely at room temperature. Material Characterization. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were conducted using a Zeiss SIGMA scanning electron microscope. Transmission electron microscopy (TEM) was carried out on a JEOL JSM-2010 electron microscope operated at 200 kV. X-ray diffraction (XRD) measurements were performed on a multifunctional Bruker D8 X-ray diffractometer with monochromatic Cu Kα radiation (λ = 1.54060 Å). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a PHI Quantear SXM (ULVAC-PHINC). Thermogravimetric analysis (TGA) was carried out on a Netzsch TG 209 F3 Tarsus under the heating rate of 10 °C/min in air atmosphere from 50 to 900 °C. Raman spectra were recorded with a LabRAM HR Raman spectrometer, with an excitation wavelength of 532 nm. The nitrogen adsorption−desorption measurements were carried out on a Micromeritics ASAP 2020 HD Analyzer at 77 K. The specific surface area based on the Brunauer−Emmett−Teller (BET) theory was calculated from the adsorbed amount of N2 at a relative pressure, P/P0, below 0.3. The pore size distribution was calculated according to the Barrett−Joyner−Halenda (BJH) model. Fabrication of Cells. For anode preparation, 80 wt % Fe3O4−G composite, 10 wt % Super C45 (SC), and 10 wt % poly(vinylidene difluoride) were mixed up in N-methyl-2-pyrrolidone to form slurry. Then, the slurry was coated on the 12 μm thick copper foil and dried for 12 h at 120 °C. The cathode was prepared comprising 85 wt % AC (YP50F), 10 wt % SC, 1 wt % carboxymethyl cellulose (CMC), and 4 wt % styrene−butadiene rubber. The materials were grinded in deionized water to form slurry, which was then coated on the 20 mm thick aluminum foil and dried for 12 h at 120 °C. The CR2032 type coin (13 mm diameter circular disks) was used for half-cells and 35 mm × 40 mm square electrodes for three-electrode full cells. The electrolyte is 1 M LiPF6 in the mixture solvent of dimethyl carbonate, 17137

DOI: 10.1021/acsami.7b03452 ACS Appl. Mater. Interfaces 2017, 9, 17136−17144

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ACS Applied Materials & Interfaces

Figure 2. (a) XRD pattern of Fe3O4−G and standard XRD pattern of Fe3O4. (b) Raman spectra of Fe3O4−G, commercial Fe3O4, and RGO. (c) TGA curves of Fe3O4−G and RGO. XPS spectrograms for Fe3O4−G, C 1s (d), Fe 2p (e), and O 1s (f).

Figure 3. (a, b) SEM images of the Fe3O4−G composite. (c) EDX mapping (C, O, Fe) of the Fe3O4−G composite. (d−f) TEM images of the Fe3O4−G composite.

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DOI: 10.1021/acsami.7b03452 ACS Appl. Mater. Interfaces 2017, 9, 17136−17144

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ACS Applied Materials & Interfaces

Figure 4. (a) N2 adsorption−desorption isotherm and BJH model pore size distribution of the Fe3O4−G composite. (b) First four charge−discharge curves of the Fe3O4−G composite at current density of 100 mA g−1. (c) Cyclic voltammograms of the Fe3O4−G composite at a scan rate of 0.1 mV s−1. (d) Rate performances of the Fe3O4−G composite and commercial Fe3O4 at different current densities.

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diethylcarbonate, and ethylene carbonate in a volume ratio of 1:1:1. The separator is Celgard 2400 membrane. The cells were fabricated in an argon-filled glovebox. Coin cells used lithium metal as the counter electrode. The schematic structure of the soft-package LIC cell is a sandwich structure (Figure S1): cathode− anode−Li reference electrode, divided by separator membranes in between. The active material mass ratio was set from 1:1 to 1:5 (anode/cathode). All cells were aged for 12 h at room temperature before electrochemical measurement. Prelithiation of the Fe3O4−G Composite. Prelithiation of the Fe3O4−G composite has an obviously different characteristic compared to that of LIBs and SCs. The purposes are to keep the anode stable at lower potential and match the capacity of the two electrodes. In this research, we improve the methods of prelithiation by introducing a Li reference electrode. First, the discharge−charge− discharge process (0.03−3.0 V) was performed between the anode and Li electrode several times at a current density of 50 mA g−1, which could eliminate the influence of irreversible capacity loss and lead to the infiltration of the anode by Li+ ions (Figure S3e,f). Second, the discharge−charge process (2.0−4.2 V) was conducted twice between the cathode and Li electrode, making the AC electrode get activated and filled with charge (Figure S3d). After these two processes, the electrodes were at fully charged state, respectively. Hence, discharging between positive and negative electrodes would achieve a complete matching of capacity between the two electrodes. Electrochemical Measurements. A Neware battery testing equipment (Neware Co., Shenzhen, China) was used to measure electrochemical performances of half-cells and full soft-packaging LIC cells, including prelithiation, galvanostatic charge−discharge progress, rate, and cycle-life testing. Cyclic voltammetry (CV) curves and electrochemical impedance spectroscopy plots were measured on a BioLogic VMP3 electrochemical station (Bio-Logic Co., France).

RESULTS AND DISCUSSION The XRD pattern of the Fe3O4−G composite is presented in Figure 2a. The diffraction peak located at 25.6° corresponds to the (200) plane of graphite, whereas the broadening of this peak suggests a random arrangement of the graphene layers in the composite. Other peaks can be perfectly assigned to the Fe3O4 crystals (PDF card no. 65-3107).56 Raman spectra of the Fe3O4−G composite, commercial Fe3O4, and reduced graphene oxide (RGO) are also displayed in Figure 2b. The characteristic D and G peaks of graphene are clearly observed at 1350 and 1580 cm−1 in the Fe3O4−G composite, and the Raman spectrum of Fe3O4−G also displays typical peaks corresponding to Fe3O4 below 800 cm−1.44 The ratio, I(D)/I(G), remains about 0.9 for both RGO and Fe3O4−G, which implies that the incorporation of Fe3O4 does not exert a significant change to the graphene structure. Figure 2d−f shows XPS spectrograms for the Fe3O4−G composite. The C 1s spectrum displays a sharp C−C peak at 284.8 eV and several weak peaks of oxygenfunctionalized groups similar to that of other RGOs upon chemical or thermal reduction. The Fe 2p peak (Figure 2e) shows that the two spin-orbit components of the Fe 2p1/2 and 2p3/2 peaks are centered at 711.4 and 724.9 eV, respectively. The satellite peak at 718.1 eV is the characteristic peak of Fe3+. In Figure 2f, the Lorentzian−Gaussian fitting of the O 1s spectrum shows two split peaks resulting from the different chemical bonding states of oxygen element in graphene and Fe3O4 crystalline phase. The above XRD, Raman spectroscopy, and XPS analyses validate a successful fabrication of the 17139

DOI: 10.1021/acsami.7b03452 ACS Appl. Mater. Interfaces 2017, 9, 17136−17144

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ACS Applied Materials & Interfaces

Figure 5. (a) CV curves of the Fe3O4−G//AC soft-packaging hybrid capacitors with a mass ratio of 1:3 (anode/cathode), tested at 5, 10, 20, and 50 mV s−1. (b) The galvanostatic charge−discharge curves of the Fe3O4−G//AC soft-packaging hybrid capacitors, with a mass ratio of 1:3, at different current densities in the voltage range of 1.0−4.0 V. (c) Ragone plots of the Fe3O4−G//AC soft-packaging hybrid capacitors with mass ratios from 1:1 to 1:5 and the picture of LIC lighting a red LED logo. (d) Ragone plots (mass ratio of 1:3) compared with the reported state-of-the-art literature on organic systems. (e) Cycling life, coulombic efficiency, and charge−discharge curves of the Fe3O4−G//AC soft-packaging hybrid capacitors, with a mass ratio of 1:3. (f) Cycling stability comparison with the literatures referred in graph (d).

Fe3O4−G composite simply through heat treatment. TGA curves of the Fe3O4−G composite and RGO are shown in Figure 2c. A sharp weight loss occurs between 350 and 500 °C, corresponding to the oxidation and decomposition of RGO in air. After about 500 °C, the TG traces are stable with no further weight loss, indicating the complete removal of RGO. The TGA curves reveal that graphene is the major component which takes up about 75 wt %, whereas Fe3O4 is about 25 wt %. SEM and TEM are shown in Figure 3 to check the overall morphology and microstructure of the Fe3O4−G composite. It can be observed that graphene appears wrinkled in the 3D structure, meaning that there is a fast, effective electron

transport, which is owing to thermodynamically stable bending, and Fe3O4 nanocrystallites are anchored between the arrays of graphene.43 The Fe3O4 nanoparticles have an extensive diameter distribution, from dozens to hundreds of nanometers, indirectly supported by the even distribution of C, Fe, and O in Figure 3c. The TEM images show a very high degree of coupling between the Fe3O4 nanocrystallites and graphene that is not only conducive to take full advantage of graphene’s function as a good conductive network but also prevents the agglomeration of Fe3O4 and exerts the large capacity of Fe3O4.54,55 The synergistic effect guarantees a good capacity and cycling performance of Fe3O4−G. The interplanar spacing 17140

DOI: 10.1021/acsami.7b03452 ACS Appl. Mater. Interfaces 2017, 9, 17136−17144

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it still surpasses 300 mAh g−1. In contrast, commercial Fe3O4 fades dramatically below 100 mAh g−1 at high rates. The good stability and rate performance of the Fe3O4−G composite is attributed to smaller volume variations, fast Li+ transport, and an effective electron transport during the charge−discharge progresses.23 On the basis of the above analysis, the Fe3O4−G composite has much potential as a good candidate for the electrode material. For asymmetric LICs, it is crucial that the capacities of cathode and anode should match with each other to fully utilize the electrochemical properties of both electrodes. Generally, the capacity of anode is much higher than that of cathode, which requires a higher mass loading of cathode according to the equation m+q+ = m−q− (q+ and q− stand for the specific capacity, and m+ and m− stand for mass of the active material in cathode and anode, respectively).56 To this end, we optimized the mass ratio of cathode and anode in the soft-package LICs to reach the best electrochemical performance. The half-cell analysis suggests that both the Fe3O4−G anode and AC cathode display a stable operation in the potential window of 0.03−1 and 2−4 V (vs Li/Li+), respectively. Therefore, we expect that the potential of 1−4 V should cover the whole working range of both electrodes. The CV curves and charge− discharge curves of the Fe3O4−G//AC hybrid capacitors with a mass ratio of 1:3 (anode/cathode) are displayed in Figure 5a,b. The CV curves, different from those of the conventional symmetric capacitors, gradually deviate from the ideal rectangular shape with increasing scan rates, which is due to the insertion-type energy storage mechanism of Fe3O4.56 However, the curve profiles reveal the rectangular part from 2.5−4 V, indicating the co-existence of the electrical double layer capacitor and battery storing charge features. The charge− discharge curves in Figure 5b exhibit a great linear relation especially at a large current density and only show a little bending at a low current density of 0.3 A g−1 between the potential from 2.5 to 3.0 V, corresponding to the redox peak in CV curves. A great linear relation means great power performance that can be demonstrated in the Ragone plot, as follows. Ragone plots of the Fe3O4−G//AC soft-packaging LIC cells, with different mass ratios from 1:1 to 1:5, are illustrated in Figure 5c. As we know that more AC results in higher energy, but volume expansion of the anode material also gets seriously, which leads to a quick attenuation of the rate performance and inferior cycling life.23 As shown in Figure 5c, optimal electrochemical performance happens in the LIC with a mass ratio of 1:3, which can deliver an energy densities of 120.0− 60.5 Wh kg−1 for power densities from 0.13 to 45.4 kW kg−1 (on the basis of active materials of the two electrodes). The LIC cell with a mass ratio of 1:2 also displays a great performance, which can achieve a high power density of 82.3 kW kg−1 maintaining an energy density of 34.3 Wh kg−1. The charge−discharge curves of the cell, cathode, and anode are shown in Figure S4; we can see that the potential of electrodes remain stable during different rates, which ensure stable electrochemical performances. Furthermore, there is a comparison between the Fe3O4−G//AC hybrid system and the recently reported excellent hybrid capacitors in Figure 5d, the concrete data lies in Table S1. We can see that the Fe3O4−G// AC hybrid system realizes a great energy−power combination compared to that for others, which can maintain higher energy density at high-power delivery.

of Fe3O4 in Figure 3f is 0.48 nm which comes from the (111) plane of Fe3O4. These results are consistent with the XRD characterization. The N2 adsorption−desorption isotherms and BJH pore size distribution curves of the Fe3O4−G composite and RGO are shown in Figures 4a and S2, respectively. The N2 adsorption−desorption isotherm of the Fe3O4−G composite shows type IV isotherms (IUPAC classification), with a distinct hysteresis loop at high partial pressures (P/P0 > 0.4), which indicates a mesoporous nature. The BET surface area of Fe3O4−G is about 73.59 m2 g−1, lower than that of RGO (about 204.28 m2 g−1), which is caused by the much heavier weight of Fe3O4 than that of graphene. The BJH pore size distribution also shows that mesopores and macropores are in majority. The relatively more large pores mean a smooth ion transport ensuring a good electrochemical performance at high rates.47,48 The BET surface area and pore size distribution of AC are shown in Figure S3a, indicating 1245 m2 g−1 surface area and typical micropore structure. The electrochemical performance of cathode half-cells is presented in Figure S3b,c. The galvanostatic charge−discharge curves have a significantly linear correlation at different current densities owing to their inherent nonfaradic capacitive properties. The rate performance of AC shows that the specific capacity of the AC half-cell is between 45 and 70 mAh g−1, within the voltage range of 2.0−4.2 V. Figure 4c shows the CV curves of the Fe3O4−G electrode in the first four intercalation/ deintercalation cycles at a scan rate of 0.1 mV s−1. Two obvious peaks are observed about 1 and 0.55 V during the cathodic process, which correspond to the formation of the solid− electrolyte interface (SEI) film and lithiation reaction shown in eqs 1 and 2. Fe3O4 + 2Li+ + 2e− → Li 2(Fe3O4 )

(1)

Li 2(Fe3O4 ) + 6Li+ + 6e− → 3Fe 0 + 4Li 2O

(2)

3Fe0 + 4Li 2O → Fe3O4 + 8Li+ + 8e−

(3)

The peak recorded at about 1.65 V in the anodic process should be attributed to the deintercalation reaction of eq 3.44 It should be noted that the cathodic and anodic peaks are polarized to 1.65 and 0.9 V during the cycles 2−4, respectively. We speculate that the formation of the SEI film in the first scan not only affected the rate of lithium-ion transport but also changed the charge distribution on the interface between the electrodes and electrolyte, thus leading to the increase of electrode polarization. Figure 4b shows the first four discharge−charge curves of the Fe3O4−G half-cell at a current density of 100 mA g−1, with a voltage window of 0.03−3.0 V. It can be seen that the first discharge capacity is about 1200 mAh g−1, reversible specific capacity is as high as 850 mAh g−1, and irreversible capacity loss is about 350 mAh g−1, which relates to formation of the SEI film.35 The first lithiation cycle contains a high percentage of irreversibility because extra charge is consumed in reducing oxides and building up the SEI as the potential goes down to 0.03 V (vs Li/Li+), whereas the following three cycles almost overlap with each other. As shown in Figure 4d, the Fe3O4−G composite exhibits a great rate performance, and commercial Fe3O4 is tested in comparison. It is obvious that the specific capacity of the Fe3O4−G composite decreases much slower than that of commercial Fe3O4, with increasing current densities. A specific charge capacity higher than 800 mAh g−1 can be obtained at 100 mA g−1, and even at a high current density of 3000 mA g−1 17141

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performance, which delivers an energy density of 120.0 Wh kg−1, a great power density of 45.4 kW kg−1 (achieved at 60.5 Wh kg−1), and reasonably good cycling stability with 94.1% capacity retention after 1000 cycles and 81.4% after 10 000 cycles. We believe that the successful fabrication of a low-cost LIC in this work benefits the development of next-generation advanced energy storage devices with high-power capabilities for electrical vehicles and portable electronics.

In the case of high-rate applications, like electrical vehicles and elevators,58 it is important that the energy storage devices should be able to endure repeated charge/discharge cycles without large capacity loss. For this purpose, we tested the cyclic performance of Fe3O4−G//AC LICs at a higher current density (10 A g−1) to demonstrate both the high-power capability and long cyclic life of our hybrid system (Figures 5e and S5). In Figure 5e, the capacity retention of Fe3O4−G//AC LIC is as high as 94.1% after 1000 cycles and 81.4% after 10 000 cycles, and the coulombic efficiency floats around 100% all along. Besides, it should be noted that the LIC can complete a charge−discharge cycle within 30 s (inset of Figure 5e), indicating its fast energy-harvesting capability comparable to that of SCs. The LIC cell of 1:2 also displays an excellent capacity retention of 94.6% after 1000 cycles and 83.7% after 10 000 cycles (Figure S5). The cycling capacity decay is caused mostly by the anode, which would undergo a large volume change during the charge−discharge process. The volume expansion would destroy the structure of the anode material resulting in the failure of the conductive link between the crystal grains and even the delamination of the anode material from current collectors. In Figure 5f, some devices with excellent Ragone performances in Figure 5d are chosen for cycling life comparison with the Fe3O4−G//AC hybrid capacitor (1:3), the detailed data also lies in Table S1. Most hybrid devices can hardly achieve 90% retention after 1000 cycles and 80% after 5000 cycles. It is observed that the cycling life of Fe3O4−G//AC is better than most devices reported, which indicates its promising application prospects. The high reversible capacity, excellent rate capability, and superior cycle life of the Fe3O4−G composite are attributed to the following features: (i) The agglomeration of the Fe3O4 nanoparticles is inhibited due to sufficient nucleation sites provided by the graphene layers, which favors the synergistic effect of the Fe3O4−G composite for a high reversible capacity.28 (ii) The 3D mesoporous networks provide efficient channels for fast Li+ diffusion and effective electron transport during faster charge/discharge rates.43,44 (iii) The flexible graphene sheets can effectively buffer the cracking or pulverization of the Fe3O4 nanoparticles during Li uptake/ release cycles.25,56 For a commercial packaged cell, the weight of the active electrode materials is usually 30−40% of the total cell weight;47,56 therefore, it is reasonable to expect that the LIC devices based on Fe3O4−G can deliver an energy density comparable to that of the Ni-metal hydride batteries, whereas the power performance is much higher than that of the lithiumion batteries and fuel cells (Figure S6). Such extraordinary properties of Fe3O4−G make it a promising candidate for hybrid capacitors with high energy density and power density; thus, they are capable of bridging the gap between SCs and lithium-ion batteries.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03452. LIC schematic structure; BET of RGO; electrochemical measurement of AC; prelithiation and charge−discharge curves; data table of relevant literature (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] (Y.M.). ORCID

Xiong Zhang: 0000-0001-9760-5206 Author Contributions §

S.Z. and C.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51472238 and 51677182), the Beijing Municipal Science and Technology Commission (Grant No. Z171100000917007), and the Beijing Nova Program (Grant No. Z171100001117073).

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REFERENCES

(1) Zhang, X.; Zhang, H.; Li, C.; Wang, K.; Sun, X.; Ma, Y. Recent Advances in Porous Graphene Materials for Supercapacitor Applications. RSC Adv. 2014, 4, 45862−45884. (2) Luo, J.; Liu, J.; Zeng, Z.; Ng, C. F.; Ma, L.; Zhang, H.; Lin, J.; Shen, Z.; Fan, H. J. Three-Dimensional Graphene Foam Supported Fe3O4 Lithium Battery Anodes with Long Cycle Life and High Rate Capability. Nano Lett. 2013, 13, 6136−6143. (3) Zhou, Y.; Rui, X.; Sun, W.; Xu, Z.; Zhou, Y.; Ng, W. J.; Yan, Q.; Fong, E. Biochemistry-Enabled 3D Foams for Ultrafast Battery Cathodes. ACS Nano 2015, 9, 4628−4635. (4) Niu, Z.; Chen, J.; Hng, H. H.; Ma, J.; Chen, X. A Leavening Strategy to Prepare Reduced Graphene Oxide Foams. Adv. Mater. 2012, 24, 4144−4150. (5) Wu, Z. S.; Sun, Y.; Tan, Y. Z.; Yang, S.; Feng, X.; Müllen, K. Three-Dimensional Graphene-Based Macro- and Mesoporous Frameworks for High-Performance Electrochemical Capacitive Energy Storage. J. Am. Chem. Soc. 2012, 134, 19532−19535. (6) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652−657. (7) Augustyn, V.; Simon, P.; Dunn, B. Pseudocapacitive Oxide Materials for High-Rate Electrochemical Energy Storage. Energy Environ. Sci. 2014, 7, 1597−1614. (8) Ma, Y.; Chang, H.; Zhang, M.; Chen, Y. Graphene-Based Materials for Lithium-Ion Hybrid Supercapacitors. Adv. Mater. 2015, 27, 5296−5308. (9) Jiang, H.; Hu, Y. J.; Guo, S. J.; Yan, C. Y.; Lee, P. S.; Li, C. Z. Rational Design of MnO/Carbon Nanopeapods with Internal Void

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CONCLUSIONS In summary, the Fe3O4−graphene composites are synthesized by a simple and effective low-temperature thermal annealing method. They have many advantages in their use as electrode materials, such as high Li storage capacity (820 mAh g−1 at 0.1 A g−1), high conductivity of the 3D nanoporous network structure, and better electrochemical stability. Moreover, the Fe3O4−G//AC soft-packaging LICs are successfully fabricated with different mass ratios from 1:1 to 1:5 (anode/cathode). The electrochemical testing results reveal that the LIC cell with a mass ratio of 1:3 displays optimal overall electrochemical 17142

DOI: 10.1021/acsami.7b03452 ACS Appl. Mater. Interfaces 2017, 9, 17136−17144

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DOI: 10.1021/acsami.7b03452 ACS Appl. Mater. Interfaces 2017, 9, 17136−17144