Graphene Mesoporous Composites for

Jun 2, 2014 - Critical Insight into the Relentless Progression Toward Graphene and Graphene-Containing Materials for Lithium-Ion Battery Anodes. Rinal...
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In Situ Self-Assembled FeWO4/Graphene Mesoporous Composites for Li-Ion and Na-Ion Batteries Wei Wang,† Liwen Hu,† Jianbang Ge,† Zongqian Hu,‡ Haobo Sun,† He Sun,† Haiqiang Zhang,† Hongmin Zhu,† and Shuqiang Jiao*,† †

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China ‡ Beijing Institute of Radiation Medicine, Beijing 100850, People’s Republic of China S Supporting Information *

ABSTRACT: With the growing demands for large-scale applications, rechargeable batteries with cost-effective and environmental-friendly characteristics have gained much attention in recent years. However, some practical challenges still exist in getting ideal electrode materials. In this work, three-dimensional FeWO4/graphene mesoporous composites with incredibly tiny nanospheres of 5−15 nm in diameter have been synthesized by an in situ self-assembled hydrothermal route. First-principles density functional theory has been used to theoretically investigate the crystal structure change and the insertion/ extraction mechanism of Li and Na ions. Unlike most graphene-coated materials, which suffer the restacking of graphene layers and experience significant irreversible capacity losses during charge and discharge process, the as-prepared composites have alleviated this issue by incorporating tiny solid nanospheres into the graphene layers to reduce the restacking degree. High capacity and excellent cyclic stability have been achieved for both Li-ion and Na-ion batteries. At the current density of 100 mA g−1, the discharge capacity for Li-ion batteries remains as high as 597 mAh g−1 after 100 cycles. The Na-ion batteries also exhibit good electrochemical performance with a capacity of 377 mAh g−1 at 20 mA g−1 over 50 cycles. The synthetic procedure is simple, cost-effective and scalable for mass production, representing a step further toward the realization of sustainable batteries for efficient stationary energy storage.

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INTRODUCTION Due to energy demand and the rapid growth in the price of fossil fuels as well as environmental problems, renewable energy sources will be of vital importance.1−3 The development of rechargeable batteries as energy storage devices has gained much attention in recent years.4−8 Among the various available energy storage technologies, lithium-ion batteries, with high energy and power densities, have been widely used in portable electric vehicles and electric power storage devices.9−12 As the smallest (effective radius) metal ion, lithium is an ideal ionic guest for transferring electronic charges into different insertion hosts. However, there is a serious concern about the availability of lithium for the limit of lithium reserves in the earth, especially in large-scale applications.13−15 Owing to the low cost and natural abundance of sodium and magnesium, recent research is focusing on rechargeable Na-based and Mg-based © 2014 American Chemical Society

batteries as potential alternatives to lithium-ion batteries for electric energy storage.16−18 Most recently, the eco-friendly Naion batteries operable at room temperature have received growing interest spurred by the rapid advances in rechargeable battery technology and fast increasing demand in the market.19−24 However, the radius of Na ions is 55% larger than Li ions, making it more difficult for them to be reversibly inserted into and extracted from host materials. In consideration of the aforementioned reason, among various electrode materials for Li-ion batteries, only a few are suitable host materials to accommodate Na ions and allow reversible insertion/extraction reactions. Received: March 30, 2014 Revised: May 30, 2014 Published: June 2, 2014 3721

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uniformly. 2.68 g of Na2C2O4 (20 mmol) was added into the above solution under stirring for 2 h. Then the above mixture was added into 40 mL of glycol (EG) under stirring for another 2 h. The above mixture was transferred to a sealed Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 180 °C for 8 h and was subsequently cooled to room temperature in an ice bath. The precipitate was collected with distilled water and alcohol by centrifuging. The above precipitate was dried in an oven at 120 °C for 12 h. The synthesis of FeWO4 nanosphere (NS) was the same as the above-mentioned, except the absence of GO. Characterizations. The structure and morphology of the asprepared powders were characterized by X-ray diffraction measurement (XRD, Rigaku, D/max-RB), field emission scanning electron microscopy (FESEM, JEOL, JSM-6701F), and transmission electron microscopy (TEM, JEOL, JEM-2010). Electrochemical Characterization. Samples of FeWO4 were ground and combined into a slurry that contained 75 wt % of the active material, 15 wt % of acetylene black conductive additive, and 10 wt % of Teflon (poly(tetrafluoroethylene), PTFE) binder. The mixtures were thoroughly mixed and laminated on a copper current collector and dried at 120 °C for 12 h to eliminate residual solvent. Then the tablet machine was used to make the electrode thin and smooth. The electrodes were cut into rounded pieces with a diameter of about 8 mm, and each piece was about 1 mg. Samples of FeWO4/ graphene were ground and combined into a slurry that contained 90 wt % of the active material (FWO4/graphene) and 10 wt % of Teflon (poly(tetrafluoroethylene), PTFE) binder. No conductive additive was added into the slurry. The following procedure was the same as that for samples of FeWO4. The electrochemical characterizations were measured by means of the coin-type cell CR2032. The active material electrode was used as the working electrode, and Li foil (or Na foil) was used as both the counter and reference electrodes. The electrolyte used for Li-ion batteries was 1.0 M LiPF6 in an electrolyte solution (EC/DMC/EMC = 1/1/1, V/V/V), with a porous polypropylene membrane (Celgard 2400) as the separator; the electrolyte used for Na-ion batteries was 1.0 M NaClO4 in a propylene carbonate (PC) electrolyte solution, with a glass fiber (GF/D) from Whatman as the separator. Coin cells were assembled in an argon filled glovebox. Galvanostatic charge and discharge cycles were conducted at room temperature under different current densities. The value of capacity is based on the total mass of FeWO4/graphene, whereas for FeWO4 and acetylene black composites, the value of capacity is based on the mass of FeWO4 only. The coin cell was also measured by electrochemical impedance spectroscopy (EIS, Solatron 1287/1255B) to evaluate the electrochemical performance, redox processes, and the ionic and electronic conductivity with a frequency range from 100 kHz to 0.1 Hz. Cyclic voltammetry (CV, CHI 1140A) measurements were performed with a scan rate of 0.01 mV s−1 between 0.01 and 3 V. Calculations. To illustrate the insertion−extraction mechanism of Li/Na ions in the FeWO4 electrode, the first-principles density functional theory (DFT) with the plane−wave pseudopotential method46 implemented in the CASTEP package47 was employed to calculate the formation energy and unit cell volume change for Li/Naion insertion. Generalized gradient approximation with normconserving pseudopotentials48 was used with the plane−wave cutoff of 400 eV. To study Li/Na insertion and extraction, we constructed a unit cell consisting of two FeWO4 molecules. Brillouine-zone integrations are made using (6 × 6 × 6) special k-point meshes according to the Monkhorst−Pack scheme.49 For each geometry optimization, both atomic positions and lattice parameters are fully relaxed using the quasi-Newton method.50 The convergence thresholds between optimization cycles for energy change, maximum force, maximum stress, and maximum displacement are set as 1 × 10−5 eV/ atom, 0.03 eV/Å, 0.05 GPa, and 0.001 Å, respectively. The optimization terminates when all of these criteria are satisfied. The choice of these computational parameters ensures good convergence in present studies. As Li/Na ions are successively incorporated into the FeWO4, referring to the following reaction:

Metal tungstates belong to an important family of inorganic functional materials with potential applications in various fields.25 Among them, FeWO4 is a very important electrode material for its good electron transport performance, which has been applied in catalysts, optical fibers, humidity sensors, laser hosts, scintillation detectors, phase-change optical recording devices, pigments, etc.26−31 Quite recently, as an anode material, FeWO4 has been used for Li-ion batteries.32,33 However, to the best of our knowledge, its application in Naion batteries has not been reported. In our work, a hydrothermal method was employed for the synthesis of FeWO4. However, due to poor electron and ion transport, the cyclic performance and capacity of this material we synthesized for rechargeable batteries are unsatisfactory. To improve the electrical conductivity, numerous studies have been carried out to confine the electrode material in a carbon matrix, such as hollow carbon spheres, microporous carbon spheres, hierarchical porous carbon, ordered mesoporous carbon, carbon nanotubes/nanofibers, and graphene. These carbonaceous materials have been investigated as the confining/conductive medium and have shown improved cyclic stability.34−40 Owing to its notably large surface area (ca. 2600 m2 g−1), excellent mobility of charge carriers (20 000 cm2 V−1 s−1), high chemical and thermal stability, high surface functionality, and superior mechanical flexibility, graphene has opened new possibilities and rapidly emerged as a twodimensional (2D) conductive support.41−43 Graphene is normally synthesized by the reduction of graphene oxide (GO). However, due to the strong π−π stacking and hydrophobic interactions, graphene sheets are prone to agglomerate during the reduction and drying process because of van der Waals forces, which may form graphite again, thus hindering Li (or Na) intercalation.44 Therefore, many unique properties of graphene are significantly compromised or even unavailable. On the basis of this understanding, self-assembly of graphene into macroscopic materials can translate the intriguing properties of graphene into the resulting macrostructures for practical applications. Due to the combination of a porous structure and excellent intrinsic properties of graphene, these well-defined interpenetrating structures provide macroscopic graphene materials with a large surface area, high mechanical strength, and fast mass and electron transport. So, in this work, hierarchical graphene was introduced to improve the electrical conductivity and cycling stability. The self-assembled three-dimensional (3D) architecture morphology FeWO4/graphene composites composed of numerous nanospheres have been synthesized and have greatly improved the electrochemical performance for both lithium- and sodiumion batteries.

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EXPERIMENTAL AND THEORETICAL METHODS

Preparation and Assembly of FeWO4 and FeWO4/Graphene. All materials and chemicals were purchased commercially and used as received. Graphene oxide (GO) was synthesized from natural graphite flakes by a modified Hummers method, the details of which are described elsewhere.45 Exfoliation was carried out by ultrasonicating the GO dispersion under ambient conditions. The FeWO4/graphene nanosphere (GNS) was synthesized by a hydrothermal method. In a typical procedure, 0.8109 g of FeCl3·6H2O (3 mmol) and 0.9896 g of Na2WO4·2H2O (3 mmol) were mixed together and dissolved in 20 mL of distilled water and the solution was kept stirring for 2 h. Fourteen milliliters of GO (2 mg mL−1) aqueous dispersion was dropped into the above solution under stirring and then the mixture was put into an ultrasonic vibration generator for 30 min to mix 3722

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FeWO4 + x M → Fe(1 − y)MxWO4 + y Fe

(1)

FeWO4 + 8M → 4M 2O + Fe + W

(2)

aggregation can be observed, demonstrating that the FeWO4 and graphene are uniform and well-attached with each other. Besides, some free graphene sheets serve as the highly conductive channels, which can decrease the inner resistance of electrodes and is favorable for stabilizing the electronic conductivity. The GNS appears very crumpled with a large amount of porosities, which can promote the diffusion of Li or Na ions in the electrode and enable the achievement of a highrate capability. To get a further understanding, the XRD spectroscopy images of NS and GNS, the FESEM image of FeWO4, and the crystal structure of iron tungstate are also displayed (see Figures S1 and S2 in the Supporting Information). Figure 1c is a typical TEM image of NS, which indicates that the as-prepared material has a nanosphere morphology with a diameter size in the range of 30−50 nm. Figure 1d shows the TEM image of GNS. Graphene serves as a three-dimensional (3D) conductive support with FeWO4 particles wrapped in graphene nanosheets. Interestingly, compared to the nanospheres shown in Figure 1c, the FeWO4 nanospheres in GNS are much smaller. To get a close view, HRTEM images are displayed in Figures 1e−h. The corresponding SAED patterns of FeWO4 (in NS and GNS) are also shown in Figures S3 and S4 (Supporting Information). It can be clearly seen that the FeWO4 particles have an incredibly tiny nanosphere morphology with a diameter size in the range of 5−15 nm, which are well-embedded in the graphene scaffold. Two reasons may account for the formation of these tiny nanospheres: (1) heterogeneous nucleation process and (2) kinetics inhibition of crystal growth. The detailed explanation is presented in Scheme 1. Meanwhile, the lattice fringes with interplanar spacings of

M = (Li, Na) the formation energy for reactions 1 and 2 is defined as

Ef (M) = Etot(Fe(1 − y)MxWO4 ) + yE tot (Fe) − xEtot(M) − Etot(FeWO4 )

(3)

and Ef (M) = 4Etot(M 2O) + Etot(Fe) + Etot(W) − 8Etot(M) − Etot(FeWO4 )

(4)

Here Etot(Fe(1−y)MxWO2) is the total energy of the FeWO4 containing x M atoms and replacing y Fe atoms, Etot(FeWO4) is the total energy of the FeWO4 without any sodium atom, and Etot(M), Etot(Fe), and Etot(W) are the total energy of a M/Fe/W atom in the reservoir (i.e., bulk Li, Na).

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RESULTS AND DISCUSSION The FESEM images of the as-prepared NS and GNS are shown in Figure 1a,b. The NS are composed of numerous ellipsoidal particles, and each microspheric particle is the aggregation of a large amount of small FeWO4 nanoparticles. The GNS exhibits ultrathin crumpled 3D nanosheets, and nearly no bulky FeWO4

Scheme 1. Schematic Illustration of the Formation Process of Mesoporous FeWO4/Graphene Composites

0.57 nm, 0.36 and 0.29 nm, corresponding to the (010), (110), and (111) planes of monoclinic FeWO4, are observed. The graphene layers and FeWO4 nanospheres interpenetrate into each other tightly, and this feature is favorable for good compatibility with organic electrolytes and offers easy access for Li ions or Na ions as well as facilitation of the fast electron transfer, promoting Li or Na storage.51−54 Scheme 1 shows a proposed route for the preparation of the self-assembled FeWO4/graphene nanosphere. In this work, graphene oxide (GO) is employed as a high surface template for the selective heterogeneous nucleation and growth of FeWO4 nanosphere. During the hydrothermal process, graphene oxide serves as the substrate (solid impurities) on the surface of which the heterogeneous nucleation and growth of FeWO4 take place. The graphene oxide is reduced to

Figure 1. FESEM images of (a) NS and (b) GNS and TEM images of (c) NS and (d) GNS. (e−h) HRTEM images of GNS. 3723

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Figure 2. Relationship between the formation energy and the number of Na (a and b) and Li (c and d) in FeWO4. The green, blue, red, and purple balls correspond to Fe, W, O, and Na (or Li) atoms, respectively.

stable are the Na ions located in the lattice. The total energy and electronic structure are calculated based on the optimized supercell model. From Figure 2a,b, the theoretical reaction calculated by first-principles is as the following describes. In the first stage, two Na atoms gradually replace the Fe sites, and the reaction is

graphene, forming a 3D architecture morphology composed of numerous FeWO4 nanospheres. Besides the growth on the surface of graphene, the mutual penetration of FeWO4 and graphene also happens. The 3D architecture is caused by the self-assembly of in situ reduced GO into a 3D architecture by the partial overlapping or coalescing of the flexible graphene during the hydrothermal process.55 Compared to NS, the GNS displays a smaller particle size (shown in Figure 1), which is attributed to the following reasons: (1) in the nucleation process, due to the existence of GO, which is employed as a high surface template, the reactants disperse uniformly on its surface. Instead of homogeneous nucleation, the heterogeneous nucleation occurs, which can inhibit the crystal growth. (2) In the process of hydrothermal reaction, the primarily formed crystalline grains distribute uniformly between the graphene layers. The hierarchical graphene can also inhibit the further growth of the crystals. Moreover, the particles are separated by the graphene layers and not easy to agglomerate. The 3D architecture morphology composed of numerous nanospheres can increase the contact area with the electrolyte and provide more and shorter Li-ion or Na-ion diffusion channels during the charge and discharge process. Besides, the overlapping or coalescing of the graphene can form an interconnected conducting network, which can accelerate the electronic transport. Owing to the superstrength of graphene, the cyclic stability of GNS is much improved.56 To better understand the insertion/extraction mechanism of Na (or Li) ions in the FeWO4 electrode, the plane−wave pseudopotential method implemented in the CASTEP package was employed to investigate the insertion of Na (or Li) into the FeWO4 structure. The structure of FeWO4 implies that the Li or Na cations would be likely to insert into the FeWO4 structure. The calculated formation energy with respect to the number of sodium ions in a FeWO4 unit cell is shown in Figure 2a,b. The more negative the formation energy, the more

FeWO4 + x Na → Fe(1 − y)NaxWO4 + y Fe

(5)

At the end of the first stage, the following reaction completely happens: FeWO4 + 2Na → Na 2WO4 + Fe

(6)

In the second stage, six Na atoms gradually replace the W sites, and the reaction is Na 2WO4 + x Na → Na 2 + xW(1 − y)O4 + y W

(7)

At the end of the second stage, the following reaction completely happens: Na 2WO4 + 6Na → Na8O4 + W

(8)

Consequently, eight Na atoms in total participate in the whole reaction and the overall reaction is FeWO4 + 8Na → 4Na 2O + Fe + W

(9)

Likewise, the reaction for Li atom insertion into the FeWO4 structure is as the following describes. The first stage: FeWO4 + x Li → Fe(1 − y)LixWO4 + y Fe

(10)

The second stage: Li 2WO4 + x Li → Li 2 + xW(1 − y)O4 + y W

(11)

The overall reaction: FeWO4 + 8Li → 4Li 2O + Fe + W 3724

(12)

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2.0 eV from experimental measurements,57 attributable to the well-known limitation of predicting accurate conduction band properties of the DFT method as implemented in the CASTEP code. The DOS of Fe0.5Li0.5WO4 in Figure 4b from 0 to 5 eV (valence band) moves to the left compared to the DOS of FeWO 4 , which means an enhanced conductivity of Fe0.5Li0.5WO4. This phenomenon also could be explained by the PDOS in Figure 4c,d. Figure 4c shows the orbital hybridization between the Fe 3d, W 3d, and O 2p states from 1 to 5 eV. It is evident from Figure 4c that there is an electron overlap between the W 3d state and O 2p state from 1 to 5 eV. It can be seen from the PDOS in Figure 4d that the PDOS of Li rarely affects the whole electronic structure but results in the offset of PDOS of the Fe 3d state, indicating the enhancement of conductivity and structural stability and improving the electrochemical performance of alkali metal ion batteries. As is shown in Figure 5a, the 1st, 2nd, 3rd, 50th, and 100th charge/discharge curves of GNS at 100 mA g−1 for Li-ion batteries are presented. The initial charge and discharge curves delivered a discharge capacity of 867 mAh g−1 and charge capacity of 725 mAh g−1 (the value of capacity is based on the total mass of FeWO4/graphene) with an initial Coulombic efficiency of 83.6%. The large capacity loss can be potentially attributed to the formation of a solid electrolyte interphase (SEI) layer on the electrode surface.58,59 In the initial cycle (the red curve), three voltage plateaus can be seen clearly. The first plateau occurs at 1.4 V, corresponding to the first stage (reaction 10) calculated from the first-principles approach. Likewise, the second plateau at around 0.7 V corresponds to the second stage (reaction 11). The appearance of the third plateau below 0.5 V is a result of the decomposition of the liquid electrolyte and the formation of an SEI layer, as was mentioned before. Despite the low Coulombic efficiency in the initial cycle, the Coulombic efficiency of the subsequent cycles is high (see Figure S5 in the Supporting Information), demonstrating a highly reversible charge and discharge process. Figure 5b shows the 1st, 2nd, 3rd, 10th, and 50th charge/ discharge curves of GNS at 20 mA g−1 for Na-ion batteries. It is found that the discharge capacity of the first cycle is particularly high, which is ascribed to some side reactions and the formation of SEI.60 The large capacity loss mainly occurs in the first few cycles, and the capacity loss between the 10th and 50th cycle is very little, which indicates a highly reversible performance. The corresponding Coulombic efficiency is shown in Figure S6 (Supporting Information). Two main voltage plateaus at 0.9 and 0.4 V can be seen, matching with the reaction in first and second stage (reactions 5 and 7), respectively. As an anode material, the voltage range of 3 V seems a bit wide; however, almost all discharge plateaus are below 1 V, which helps raise the operating voltage in full cells. The results of the rate performance for Li-ion batteries at the current densities up to 1 A g−1 are shown in Figure 5c. The discharge capacity remains 698 mAh g−1 for the current density of 100 mA g−1 after 10 cycles and decreases to 651, 490, 411, and 661 mAh g−1, following every 10 charge and discharge cycles with the current density of 200, 500, 1000, and 100 mA g−1, respectively. The capacity decreases slowly at the same current density. When the current density changes from 1 A g−1 to 100 mA g−1, its capacity nearly returns to the last value. Compared to the poor rate performance of NS (Figure S7, Supporting Information), the excellent rate performance of GNS is mainly attributed to the formation of highly efficient Li+

Thus, the theoretical capacity of FeWO4 is estimated to be 706 mAh g−1. The detailed first-principles geometry analysis (see Tables S1 and S2 in the Supporting Information) reveals that before the complete replacement of Fe or W by Na (or Li), the volume of the corresponding structure changes only a little, which can facilitate the intercalation of Na (or Li) ions. The electronic structure of FeWO4 has been studied in previous work; however, few study on the insertion/extraction mechanism of alkali metal atoms has been carried out. To get a further understanding of the electronic structure character and the insertion/extraction mechanism of Li (or Na) atoms in FeWO4, we have constructed the electron density difference contour maps associated with the DOS (density of states) and PDOS (partial density of states), as is shown in Figures 3 and 4.

Figure 3. Electron density difference contour maps of (a) FeWO4 and (b) Fe0.5Li0.5WO4.

The W atom is rarely replaced by Li (or Na) atoms before the formation of Li2O/Na2O. However, the mechanism of the replacement of Fe by Li (or Na) is complicated, which will be mainly discussed here. The intercalation and deintercalation of Li and Na atoms in the FeWO4 structure are similar to each other, so in this work, we only discuss the electron density difference contour maps for FeWO4 and Fe0.5Li0.5WO4. Figure 3 shows the plots of the electron density difference of FeWO4 and Fe0.5Li0.5WO4 in which Fe is replaced by Li. It can be seen from the comparison that the replacement of Fe by Li has little impact to the whole structure, which can enhance the cycle stability and rate performance. The same phenomenon also can be explained by the analysis in Tables S1 and S2 (Supporting Information). The corresponding DOS and PDOS of FeWO4 and Fe0.5Li0.5WO4 are shown in Figure 4. For the sake of clarity, only the DOS and PDOS around the Fermi energy are presented for the contribution from the lowest states (less than −10 eV). From Figure 4a, it can be seen that it is a semiconductor with a band gap of 1 eV, which is smaller than 3725

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Figure 4. Total and partial densities of states of FeWO4 (a and c) and Fe0.5Li0.5WO4 (b and d).

Figure 5. (a) 1st, 2nd, 3rd, 50th, and 100th charge/discharge curves of GNS at 100 mA g−1 for Li-ion batteries. (b) 1st, 2nd, 3rd, 10th, and 50th charge/discharge curves of GNS at 20 mA g−1 for Na-ion batteries. (c) Rate performance of GNS for Li-ion batteries at different current densities from 100 mA g−1 to 1 A g−1. (d) Rate performance of GNS for Na-ion batteries at different current densities from 20 to 500 mA g−1.

transportation pathways in the 3D conductive tiny mesoporous built by the mutual interpenetration of FeWO4 and graphene. The rate performance for Na-ion batteries at the current densities up to 500 mA g−1 is also shown and the results are similar to that for Li-ion batteries (Figure 5d and S8, Supporting Information). Admittedly, the electrochemical performance is no match for Li-ion batteries; however, as an anode material for Na-ion batteries, the capacity and cycle performance are superior in the field of Na-ion batteries.

Figure 6 shows the cycling performance of NS and GNS for Li-ion and Na-ion batteries. The cyclic stability of NS for both Li-ion and Na-ion batteries is very poor (Figure 6a,c). At the current density of 500 mA g−1, the discharge capacity nearly decays to 188 mAh g−1 for Li-ion batteries over 100 cycles and 46 mAh g−1 for Na-ion batteries over 50 cycles. On the contrary, the GNS exhibits high capacity and excellent cyclic stability. As for Li-ion batteries (Figure 6b), at the current density of 100 mA g−1, the discharge capacity remains 597 mAh 3726

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Figure 6. Cycling performances of (a) NS and (b) GNS for Li-ion batteries under different current densities over 100 cycles. Cycling performances of (c) NS and (d) GNS for Na-ion batteries under different current densities over 50 cycles.

Figure 7. Cyclic voltammetry curves of GNS for (a) Li-ion batteries and (b) Na-ion batteries at the scan rate of 0.1 mVs−1 in the voltage range of 0.01−3 V. AC impedance spectra of GNS for (c) Li-ion batteries and (d) Na-ion batteries.

g−1 after 100 cycles. When it came to a high current density of 500 mA g−1, the corresponding capacities could still retain 441 mAh g−1 over 100 cycles. The GNS for Na-ion batteries also shows an outstanding electrochemical performance, as shown in Figure 6d. When Na-ion batteries are discharging at 20 mA g−1, the specific discharge capacity can retain a reversible capacity of 377 mAh g−1 over 50 cycles. In fact, according to David et al.,61 graphene can also deliver a certain amount of capacity. However, due to the small content of graphene in FeWO4/graphene composites, the capacity contributed by graphene can be neglected. For both Li-ion and Na-ion

batteries, the capacity loss occurs mainly in the initial few cycles, and the following reasons are responsible for this phenomenon. The synthesized mesoporous GNS is composed of numerous tiny nanospheres, owning a large surface area. During the discharge process, some alkali metal ions are inserted in the surface of GNS, together with the formation of SEI, which can contribute to the discharge capacity as the irreversible part, especially in the initial cycle. Thus, the incorporation of graphene is responsible for the superior electrochemical performance: (1) the mutual penetration of FeWO4 and graphene makes the connection of the 3727

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NS electrode, leading to the improvement in rate performance. Likewise, as for the EIS of NS and GNS for Na-ion batteries (Figure 7d), the situation is exactly the same, and the GNS electrode has much lower resistance than the NS electrode. Meanwhile, to compare the electrical conductivity of NS and GNS, the four point conductivity is also tested. The electrical conductivity of NS is 7.2 × 10−6 S cm−1 whereas the electrical conductivity of GNS is improved by about 6 orders of magnitude, that is 13.2 S cm−1. The unique morphology and the mutual penetration of FeWO4 and graphene can accelerate the transmission of Li (or Na) ions and electrons, reduce ionic and electronic diffusion distance, and provide a thermodynamically stable system; all these will improve the electrochemical performance. Figure 8a,b shows the HRTEM images of GNS for Li-ion batteries and Na-ion batteries after a discharge process. The

two materials tight, forming a unique structure with high physical stability. Due to the heterogeneous nucleation during the formation of FeWO4, the 3D architecture morphology composed of tiny nanospheres (5−15 nm) is obtained; as for NS, however, the agglomeration reduces the contact area between the active material and the liquid electrolyte and prevents the ions and electrons from entering into the inner of the particles. Along with the high conductivity of graphene, these features can accelerate the transmission of Li (or Na) ions and electrons, reduce ionic and electronic diffusion distance, and provide a thermodynamically stable system and stable physical structure. (2) The whole process is a replacement reaction, and during the whole charging and discharging process, the structure changes a lot. However, during each step (reactions 5, 7, 10, and 11), the lattice parameters and unit cell volume change only a little, which can facilitate the intercalation of Li (or Na) ions. (3) It is worth mentioning that no additional conductive additive was added during the fabrication of the electrode. The combination of the in situ self-assembled composite GNS (in which graphene serves as the nominal conductive additive) is much tighter than the active material and conductive additive composites with physical combination, which can avoid the fracture and pulverization of the electrode and improve the cyclic stability. Besides, the amount of the graphene is very little (less than 3 wt % of the total electrode), indicating a higher mass efficiency. The cyclic voltammetry curves of Li-ion and Na-ion batteries for the initial three cycles are shown in Figure 7 and some pairs of redox peaks can be observed clearly. As can be seen in Figure 7a, three pairs of redox peaks can be clearly observed. The initial curve is a bit different from the other two curves; nevertheless, no significant changes are observed for the reduction peak between the second and subsequent scans, demonstrating that the electrochemical reaction is stable and highly reversible. The anodic peaks at the potentials of 1.3 and 0.7 V are attributed to the reaction in the first and second stage, respectively, which is also consistent with the charge and discharge curves in Figure 5a. The anodic peak in the initial curve below 0.5 V rapidly tends to be inconspicuous in the second and third scan, which is primarily due to the decomposition of the liquid electrolyte and formation of an SEI layer on the surface of the electrode, matching well with the large capacity loss of the initial cycle shown in Figure 5a. In Figure 7b, the initial curve is quite different from the other two curves, which may be attributed to the activation process of Na-ion batteries and the side reactions between the electrode and electrolyte. In the second and third curves, two anodic peaks were observed at 0.9 and 0.4 V, belonging to the reaction in the first and second stage, respectively, which is also in accordance with Figure 5b. The detailed reaction kinetics of the as-prepared NS and GNS electrode is investigated using EIS, and the corresponding Nyquist plots of NS and GNS for lithium-ion batteries are presented in Figure 7c. In the high frequency region, obvious depressed semicircles are found, which are attributed to charge/ discharge impedance. In the low frequency region, the slope line (about 45°) is due to Warburg impedance, attributable to the semi-infinite diffusion of Li ions into the electrode− electrolyte interface.62,63 Obviously, the transfer impedance of both materials is essentially low, representing a fast rate of intercalation and deintercalation of Li ions. Compared with NS, the GNS electrode displays a lower transfer impedance, which means that the GNS electrode has better conductivity than the

Figure 8. HRTEM images of GNS for (a) Li-ion batteries and (b) Naion batteries after a discharge process. Solution of GNS dispersed in anhydrous alcohol before and after a discharge process for (c) Li-ion batteries and (d) Na-ion batteries.

distance between the adjacent lattice fringes is 0.20 and 0.22 nm, which can be assigned to the interplanar distance of elementary Fe (110) and elementary W (110), respectively, confirming the occurrence of reactions 9 and 11. The GNS electrodes before and after a discharge process for Li-ion batteries and Na-ion batteries were dispersed in anhydrous alcohol, and after 20 min of ultrasonic shaking, we let the solution stand for 30 min. As is shown in Figure 8c,d, before a discharge process, the solution is transparent, which demonstrates the poor dispersibility of GNS in anhydrous alcohol. However, after a discharge process, the solution of the GNS electrode changes to a dark color. After a discharge process, this anode material is reduced to Fe, W, Na2O (or Li2O), and graphene, and these tiny particles and nanosheets can well disperse in anhydrous alcohol. We can also deduce that the mutual penetration and combination of FeWO4 and graphene are sufficient and close, which is favorable for the cyclic performance of Li-ion and Na-ion batteries.

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CONCLUSION Relying on an in situ self-assembled hydrothermal route, the 3D architecture morphology GNS composed of numerous nanospheres was synthesized. The mutual penetration of FeWO4 and graphene makes the connection of the two materials tight, forming a unique structure with large surface area and high 3728

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physical stability. Meanwhile, unlike the ellipsoidal agglomeration of FeWO4, the introduction of graphene results in a smaller particle size of FeWO4, which is due to the heterogeneous nucleation and the inhibition by hierarchical graphene. The rechargeable batteries based on Li-ion and Naion intercalation and deintercalation were proposed, and their electrochemical performance was demonstrated. First-principles calculations were employed to theoretically investigate the crystal structure change and the insertion/extraction mechanism of alkali metal ions in both Li-ion and Na-ion batteries. Excellent electrochemical performance of GNS was achieved, and the following reasons mainly account for its superiority: (1) the 3D architecture by the partial overlapping or coalescing of the flexible graphene and high surface-to-volume ratio with a more mesoporous structure; (2) the tiny nanospheres with a diameter size in the range of 5−15 nm; (3) the mutual penetration of FeWO4 and graphene; (4) the unique unit cell structure. Therefore, the present results suggest that this novel kind of FeWO4/graphene composite holds great potential as an anode material for both Li-ion and Na-ion batteries. This research may open a new avenue for preparation of other graphene-based composites with unique architecture and morphology.

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ASSOCIATED CONTENT

S Supporting Information *

XRD data, crystal structure, FESEM, SAED, Coulombic efficiency, rate performance and lattice parameters, unit cell volume, total energy, and formation energy calculated by firstprinciples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*S Jiao. E-mail: [email protected]. Fax: 0086-10-62334204. Tel: 0086-10-62334204. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. 51322402, 1301345), the Program for New Century Excellent Talents in University (NCET-2011-0577), Ministry of Education of China, the Fundamental Research Funds for the Central Universities (FRF-TP-12-002B, FRF-AS11-003A), and the National Basic Research Program of China (No. 2013CB632404).

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