Pliable Embedded-Type Paper Electrode of Hollow Metal Oxide

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Pliable Embedded-Type Paper Electrode of Hollow Metal Oxide@Porous Graphene with Abnormal but Superior Rate Capability for Li-Ion Storage Xiaoting Zhang, Jisheng Zhou, Xiaohong Chen, and Huaihe Song ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00087 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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ACS Applied Energy Materials

Pliable Embedded-Type Paper Electrode of Hollow Metal Oxide@Porous Graphene with Abnormal but Superior Rate Capability for Li-Ion Storage Xiaoting Zhang†, ‡, Jisheng Zhou*, †, Xiaohong Chen†, Huaihe Song*, † †

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of

Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P. R. China ‡

Graphene and Energy Storage R&D Centre, Beijing Wanyuan Industry Co., Ltd., Beijing 100176,

China AUTHOR INFORMATION Corresponding Author * J Zhou, Email: [email protected] ; *H Song, Email: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT: To improve the electrochemical performance of pliable graphene/metal oxide composite papers, a new kind of embedded-type PG@MO paper electrode including PG@Fe2O3 and PG@CuO were designed and prepared by a simple filtration and controllable oxidation. PG@MO paper electrodes have special structures in which hollow metal-oxide nanoparticles are embedded in the pores of porous graphene films and connected with the edges of the graphene to produce a strong interfacial interaction. Benefiting from this unique structure, the PG@MO paper electrodes for LIBs exhibit higher specific capacities, better cyclic stability and rate performance. In particular, the PG@MO electrodes show an interestingly abnormal rate performance in that the specific capacity increases with the current density from 50 to 500 mAg-1, which should be attributed to that Li-ion storage in graphene films is mainly controlled by diffusion, while electrochemical reactions of Fe2O3 are mainly controlled by Faradaic capacitance. At a higher current density, the Li-ion diffusion in the graphene layers is largely suppressed, so the graphene films can well play the role of a conductive network, and the activity of the Fe2O3 will be clearly enhanced. In addition, the PG@MO paper electrodes also show higher specific capacity and superior long-term cyclic ability at very high rates: for PG@Fe2O3, 741, 249, and 141 mAhg-1 at the rates of 10, 30, and 50 Ag-1 after 10,000 cycles, respectively; and for PG@CuO, 980, 314, and 168 mAhg-1 at the rates of 10, 50, and 100 Ag-1 after 10,000 cycles, respectively). The strategy for embedded-type PG@MO paper electrodes provides a direction for the design of flexible electrode materials with both high power density and energy density. KEYWORDS: porous graphene paper, hollow metal oxide, lithium-ion battery, anode, abnormal rate capability, superior rate capability

With the development of electric vehicles and portable electronic devices, electrochemical energystorage devices with high energy and power densities are urgently needed.1-5 As an important kind of electronic devices, flexible electronic devices receive extensive attention due to their usage of wearable devices and roll-up displays.6-8 For the flexible LIBs, one of the most important


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challenges is the design and fabrication of light-weight and high-performance pliable electrode materials.6-8 At present, pliable graphene/metal oxide (G/MO) composite papers, such as G/Fe3O4,9, 10

G/CuO,11 G/MnO2,12 G/SnO2,13-15 and G/TiO2,16,17 have attracted intense attention as anodes for

flexible LIBs due to the superior mechanical flexibility and charge transfer mobility of graphene as well as the eco-friendliness, low cost, and high theoretical specific capacities of metal oxides1820

. The G/MO paper electrodes9-17 usually not only efficiently alleviate the shortcomings of metal

oxides, such as low conductivity and pulverization, but they also simplify the assembly process for batteries due to their binder-free characteristics and favorable mechanical properties. However, the design of G/MO paper electrodes is still in its infancy stage.6-8 The electrochemical performance of G/MO paper electrodes including their rate capability and long-term cyclic performance still need to be improved to meet sustained demand for flexible electronics. To fully exhibit their merits, it is critical to optimize the microstructural design of graphene/metal oxide electrodes. On the one hand, the microstructure of a composite electrode should favor the facile diffusion of Li ion. In previous reports,9-17 G/MO paper electrodes are generally prepared by vacuum filtration, and MO nanoparticles are incorporated into two graphene nanosheets to form sandwich structures. These dense nanostructures lead to long diffusion pathways for Li ions. To improve the Li-ion diffusion, porous structural graphene-based hybrid electrodes are designed.21, 22 For example, Cheng et al. successfully fabricated a pliable hybrid electrode using 3D graphene foam synthesized via CVD, exhibiting outstanding rate performance.21 Recently, Niederberger et al. designed G/MO nanocomposite aerogels as anodes for LIBs, leading to high capacity and long-term cycling stability of the electrode.22 On the other hand, there should be a special focus on enhancing the interfacial interaction between MO and G to achieve a well-defined pliable G/MO paper electrode with high performance. This is because



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the strong interfacial interaction has been proven to be feasible in the promotion of the electrochemical performance of slurry-cast G/MO electrode materials by improving both electron transfer and the surface-confined effect.23-25 However, there is still a lack of deeper exploration for achieving the optimization of structural design by enhancing the interfacial interaction between MO and G in previous pliable paper electrodes.9-17 Generally, strong interfacial interaction can be generated on the edges and defects rather than on the basal planes of G due to the higher energies and dangling bonds on the sites of edges and defects.23, 26-28 It will be difficult to withstand higher rates during longer periods of discharge/charge cycling if there is only weak interfacial interaction between the MO and G basal planes resulting from van der Waals forces. Therefore, the edges and/or defects of G should be fully exposed to bind with MO for the design of advanced pliable G/MO paper electrodes. Hence, considering the Li-ion diffusion kinetics in the host materials, the design of pliable G/MO paper electrodes with strong interfacial interaction as well as a porous structure will produce an amazing effect on improving the rate performance and cyclic properties of the next generation of flexible LIBs. In this letter, following the basic rules above, we report the design of pliable embedded-type paper electrodes of porous graphene-coated metal oxide (PG@MO), in which hollow MO nanoparticles are embedded in the pores of the G films rather than simply dispersed on the basal planes of the G. In the PG@MO paper electrode, graphene formed a highly conductive network and the porous structure provided open channels for lithium-ion transport and storage. In addition, the bare edges of the pores on graphene films were bonded intensely with the metal oxides to form strong interfacial interaction, which not only enhanced the electron transfer but also buffered the volume expansion of the metal oxide. As expected, PG@MO electrodes exhibited both superior rate capacity and superior long-term cycling stability.


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Fig. 1. Schematic illustration of the synthesis procedure for PG@MO papers.

The preparation processes of pliable PG@MO electrodes are shown by Fig. 1. First, graphene oxide/metal salt (GO/MS) composite papers were prepared by filtration. Then, GO/MS composite paper was converted into porous-graphene-coated metal nanoparticle (PG@MNPs) composite paper under an Ar atmosphere. The formation process of PG@MNPs papers was very interesting. During annealing, the functional groups of GO were decomposed, and GO was converted to graphene. Simultaneously, pores were produced in the graphene because carbon atoms were etched by metal atoms via a carbothermal reduction reaction between G and MS,29, 30 and formed metal particles that will then be embedded into the pores to form PG@MNPs paper. This method is a general one by which various PG@MNPs papers, e.g., PG@Fe3C or Cu, can be prepared (Fig. S1S4). Finally, a series of PG@MO papers was prepared by oxidizing the PG@MNPs papers.



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Fig. 2. (a) Optical images of pliable PG@Fe2O3 paper, and microstructures: (b) the cross-section and (c) the top-view SEM images of PG@Fe2O3 paper, (d) I) A top-view SEM image of PG@Fe2O3 paper and corresponding element mapping images of II) C, III) O, and IV) Fe, (e and f) TEM images of hollow Fe2O3 NPs embedded in PG exfoliated from the paper and a typical hollow Fe2O3 embedded in the pore of graphene, respectively, (g) HRTEM image of Fe2O3 in the PG@Fe2O3 paper, and (h) SEM image of PG paper obtained after removing the Fe2O3 in the PG@Fe2O3 paper, (i) TEM image of the pores on the PG paper and (j) HRTEM image of an edge on the pores.

Typically, the morphology and structures of PG@Fe2O3 are investigated in detail (Fig. 2 and Fig. S2). The obtained PG@Fe2O3 paper retains its pliability very well (Fig. 2a). PG@Fe2O3 paper contains a packed layered structure and its thickness is approximately 10.8 μm according to the cross-section SEM image (Fig. 2b). In addition, the Fe2O3 nanoparticles are present not only on the surface of the paper but also in the cross section, indicating the nanoparticles are dispersed in the whole of the G films. Top-view SEM images and elemental mapping analysis clearly show


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that Fe2O3 nanoparticles are uniformly embedded in the pores of the G films (Fig. 2c and d). It is very interesting that there are spare holes beside the nanoparticles (Fig. 2b). More interestingly, the Fe2O3 nanoparticles possess hollow nanostructures (Fig. 2f), which should be attributed to the nanoscale Kirkendall effect.31-34 Firstly, oxygen atoms are adsorbed on the porous graphene paper and ionized by electrons coming from the iron, leading to an electrical field between the iron and the paper. Secondly, the iron carbide core diffuses outward and oxygen does inward due to the formed electrical field. In the process, heating accelerates the out-diffusion of core materials. Finally, hollow nanostructures are formed on account of faster outward diffusing of the iron carbide core than inward diffusing of oxygen. Oxygen is the driving force for the outward diffusion of core species.32 The average outer diameter of the hollow Fe2O3 nanoparticles is ca. 150 nm, and the average shell thickness is ca. 25 nm. Fig. 2f shows the TEM image of a typical hollow Fe2O3 nanoparticle embedded in the graphene pores. The nanoparticle has high crystallinity, which is consistent with the XRD results. The crystalline lattice of the shell is ca. 0.252 nm (Fig. 2g), corresponding to the (311) lattice plane of Fe2O3. The BET specific surface area of PG@Fe2O3 is measured to be 239 m2/g (Fig. S5). The content of Fe2O3 in the PG@Fe2O3 is ca. 44 wt% (Fig. S6). After removal of the Fe2O3 nanoparticles, the completed G films can be obtained, and the pores on the G films are fully exposed at the location of each nanoparticle (Fig. 2h), further indicating that Fe2O3 nanoparticles are embedded in the pores of the G films. Due to the absence of Fe2O3 nanoparticles, graphene stacked layer by layer. Therefore, the BET specific surface area of the PG paper decreases to 36 m2/g. Other PG@MO papers with similar structures, e.g., PG@CuO paper can also be prepared (Fig. S7-S10). Noticeably, the morphology and structures of PG@MO paper are rather different from those of G/MO composite paper,9-17, 21, 22 where typically the MO NPs are loaded onto the basal planes of graphene rather than being embedded in the graphene pores. High



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porosity, especially the spare holes near the nanoparticles, can provide the channels for Li-ion transfer. The high porosity also provides an abundance of exposed edges, which act as reactive sites to form heterojunctions with MO NPs.35 The MO NPs in contact with graphene edges are hard to remove fully by HCl (Fig. 2j). It can be found easily that some iron oxide nanoclusters are still trapped by the edges, indicating very strong covalent bonds between the MO NPs and the graphene edges according to a previous report. 36

C 1s

Fe2PG@Fe O3/PGO 2O3

(a)

(b)

Fe 2p

O 1s Fe 2p

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0

730

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(c)

C 1s

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O 1s Fe-O Fe-O-C

C-C/C=C

C-O C-O O-C=O 294

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Fig. 3. (a) Overall XPS spectrum of PG@Fe2O3 paper, and the (b) Fe 2p, (c) C 1s, and (d) O 1s spectra.

Moreover, X-ray photoelectron spectroscopy (XPS) measurements show that the strongly covalent bonds are C-O-metal bonds between the graphene edges and the MO nanoparticles. Fig. 3 shows an elemental chemical statement in the PG@Fe2O3 paper, which is composed of the elements C (285 eV, C1s), O (532 eV, O1s), and Fe (712 eV, Fe2p). The C1s spectra of the


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PG@Fe2O3 can be fitted to the mainly non-oxygenated C (C=C/C–C) in aromatic rings (284.9 eV), the C in C–O (286.1 eV), and the C in O–C=O (289.0 eV).25, 37-39 The O1s peak of PG@Fe2O3 is fitted to three peaks at 533.3, 531.7, and 530.3 eV. The peak at 533.3 eV can be attributed to the original and re-oxidized oxygen in the graphene oxide, while the peak at 530.3 eV should arise from Fe2O3. The middle peak at 531.7 eV should be caused by the bonds between Fe2O3 and graphene, indicating the formation of C-O-Fe bonds between the graphene and Fe2O3.25, 37, 40 XPS spectra also confirm that CuO nanoparticles are bonded with porous graphene paper by C-O-Cu bonds in the PG@CuO paper (Fig. S11). The transfer of electron in PG@MO paper should be affected by the transfer in the graphene basal plane, metal oxide, and the interface between metal oxide and graphene paper. As a bridge of two phases, the interface plays an important role on the properties of composites. Comparing with simple physical connection, the covalent interaction of C-O-metal bonds between MO and graphene are more tightly. Therefore, C-O-metal bonds will promote fast electron transfer between the MO NPs and graphene networks.37



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Fig. 4. (a) Cycle performance of PG@Fe2O3 at various current densities from 0.05 to 30 Ag-1; (b) the initial charge/discharge curves of PG@Fe2O3 at 50 and 500 mAg-1, respectively, and the differential capacity vs. voltage (dq/dv) plots of PG@Fe2O3 at (c) 50 and (d) 500 mAg-1 from the 2nd cycle to 10th cycle and (e) the comparison between dq/dv plots of the 10th cycle at 50 and 500 mAg-1, respectively; (f) CV curves of three cycles for PG@Fe2O3; (g) CV curves for the PG@Fe2O3 measured at various scan rates from 0.1 to 2 mV s-1 after 300 cycles at various current densities; (h) the plot of log(i) versus log(v), where the slope of the linear line corresponds to the b-value; (i) AC impedance spectra of PG@Fe2O3 after the initial three cycles at 50 and 500 mAg-1, respectively; and (j) schematic illustration of the formation of SEI films at the surface of PG@Fe2O3 at 50 and 500 mAg-1, respectively.


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Due to the novel nanostructures such as embedded-type nanostructures, strong interfacial interaction as well as the hollow interiors of the MO NPs, PG@MO paper electrodes are expected to result in superior electrochemical performance as anodes for LIBs. To demonstrate the benefits of the novel structures of paper electrodes, the electrochemical performances of PG@Fe2O3 and PG@CuO paper electrodes as examples are discussed in the details. PG@MO paper electrodes show an amazing rate performance. The specific capacity of PG@MO does not decrease but increases gradually when the current density was increased from 50 to 500 mAg-1. Fig. 4a shows the cyclic performance of PG@Fe2O3 at different current densities. The first discharge specific capacity and reversible capacity are 915 and 704 mAhg-1 at 50 mAg-1, respectively. After 10 cycles, the specific capacity remains at 546 mAhg-1. When the current density increases to 100 mAg-1, the specific capacity does not decrease but increases to 691 mAhg-1. At 500 mAg-1, the specific capacity further increases to 726 mAhg-1 and continuously increases to 860 mAhg-1 after 200 cycles. In previous reports, the phenomena of the increase of capacity with cycling can usually be observed in the graphene-based conversional electrode materials due to strong interfacial interactions.41, 42 To rule out the effect of the capacity increase on the rate performance, the current density returns to 50 mAg-1 after 200 cycles at 500 mA g-1 and interestingly, the specific capacity decreases intermediately to 631 mAhg-1, while when the current density increases to 500 mAg-1 again after 230 cycles, the specific capacity also increases to 962 mAhg-1. Likewise, PG@CuO also displays similar results. The first reversible capacity of PG@CuO is 353 mAhg-1 at 50 mAg1

, while it is 704 mAhg-1 at 500 mAg-1 (Fig. S12a). Generally, the specific capacity of electrode

materials decreases with an increase in current density. In contrast, the specific capacity of PG@MO electrodes increase with the current density from 50 to 500 mA g-1. This is an abnormal but a welcomed phenomenon, which will favor applications in high-power LIBs.



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What leads to this abnormal rate performance? The rate performance of LIBs is related to lithium ion diffusion and electron transfer during the lithiation/delithiation of the host materials. 43

Therefore, we tried to explain this abnormal phenomenon from the viewpoints of reactive

activity and kinetics by employing a PG@Fe2O3 paper electrode as an example (Fig. 4b-h). The Li-ion storage behaviors of PG@Fe2O3, observed from both the discharge/charge profiles and the cyclic voltammogram (CV) curves qualitatively resemble those in the previous reports (Fig. 4b and 4f).43 In the CV curves, there are two cathodic peaks at 1.35 and 0.93 V during the discharge process, which are attributed to the formation of cubic Li2Fe2O3 and the further reduction of Li2Fe2O3 to Fe0, respectively.44, 45 The peak at 0.93 V contributes to the main capacity. In the subsequent charging process, the two anodic peaks at 1.25 and 1.9 V correspond to the oxidative reactions of Fe0 to Fe2+ and Fe2+ to Fe3+, respectively. In addition, the peak at 0.01 V in the cathodic scanning should be attributed to the insertion of Li ions into the graphene layers.46, 47 First, the discharge/charge profiles and corresponding differential capacity versus voltage (dq/dV vs voltage) plots were analyzed in detail to investigate the Li-ion storage activity of the PG@Fe2O3 paper electrode at different current densities (Fig. 4b-e). In addition, the positions of the peaks in the dq/dV vs voltage profiles (Fig. 4c and 4d) correspond very well to the current peaks in the CV curves. Clearly, the intensity of the peaks in the dq/dV vs voltage profiles at 50 mAg-1, especially the main peak at 0.93 V, become weaker with the cycling (Fig. 4c), suggesting that the electrochemical reactivity gradually decreases. However, the peaks at 500 mAg-1 do not decrease but increase slowly with cycling. By comparing the dq/dV vs voltage profiles after 10 cycles at two current densities, it is interesting to find that the main differential peak intensity at 500 mAg1

are much higher than that at 50 mAg-1. Similar results are also observed in the PG@CuO


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electrode (Fig. S13), indicating that the Li-ion storage activity of MO in the PG@MO electrode is easily motivated at higher current density, which is in contrast to previous reports.48, 49 Moreover, to better understand the abnormal Li ion storage phenomenon of the PG@MO electrode, the charge-storage behaviors of every current peak in the CV curves of PG@Fe2O3 are evaluated by a sweep voltammetry method at various rates from 0.1 to 2 mV/s. The peak current intensity is clearly increased with the scan rate (Fig. 4g). The relationship between the peak current (i) and the scan rate (v) can be described by the equation: i=avb, where a and b are the adjustable parameters.50, 51 The b-value determines the type of Li ion storage, and can be obtained from the slope of the plot of log(i) versus log(v). When b=0.5, the electrochemical reaction process is controlled by intercalation/diffusion of the Li ion, while when b=1, the reaction mainly relies on the Faradaic capacitative effect. The b-value of the peak at 0.01 V is ca. 0.65, which is close to 0.5, indicating that Li-ion storage in graphene films is mainly controlled by diffusion. The b values of the four redox peaks attributed to Fe2O3 are 0.96, 0.88, 0.82 and 0.92, which is very close to 1, indicating that the electrochemical reactions of Fe2O3 are mainly controlled by the Faradaic capacitative effect. Similar results are also observed in the PG@CuO electrode (Fig. S14). Lower rate is beneficial for Li-ion diffusion process, while at higher rate Li-ion Faradic capacitive behavior will become more pronounced.50, 51 At lower current density of 50 mAg-1, Li-ion more easily diffuse into graphene layers, which is helpful to improve the Li-ion storage performance of graphene, but disadvantageous to enhance electron transfer capability of graphene films as conductive network. Therefore, activity of Fe2O3 is difficult to be stimulated fully. In contrast, at higher current density of 500 mAg-1, Li-ion diffusion in graphene layers is suppressed largely, so graphene films can play well a role of conductive network, and activity of Fe2O3 will be enhanced



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obviously. The Faradic capacitive feature of the electrochemical reaction process of Fe2O3 will lead to its very high rate of performance. At last, the electrochemical kinetics of PG@Fe2O3 and PG@CuO electrodes at 50 and 500 mAg1

were investigated by electrochemical impedance spectral (EIS) measurement (Fig. 4i and Fig.

S15-S17), which further support the conclusions from the dq/dV vs voltage profiles and CV curves. Both the film resistance (Rf) and charge-transfer resistance (Rct) of PG@Fe2O3 and PG@CuO at 50 mAg-1 increase with cycling, while those at 500 mAg-1 decrease with cycling. In addition, the Rf and Rct values of the PG@Fe2O3 and PG@CuO at 500 mAg-1 are obviously lower than those at 50 mAg-1. These results indicate that much thicker SEI films are formed on the surfaces of the graphene edges with more Fe2O3 nanoparticles at 50 mAg-1 than those at 500 mAg-1, similar to that exhibited by Fig. 4j, 52 which should be attributed more Li ions taking part in the intercalation reaction in graphene layers at lower current density. Although SEI films can enhance the cyclic performance of electrode materials to some extent, unduly thicker SEI films will seriously hinder the charge transfer, which leads to lower activity of PG@Fe2O3 and PG@CuO at 50 mAg-1 than that at 500 mAg-1.52, 53 In addition, the phase angle of the Warburg region of PG@Fe2O3 and PG@CuO at 50 mAg-1 decreases, whereas that at 500 mAg-1 becomes ever larger, suggesting that the kinetics of lithium ion transport become better and better at a high current density, and worse and worse at a low current density with cycling.9 More importantly, the PG@MO paper electrode exhibits superior cycling capacity at high current densities. The specific capacity of PG@Fe2O3 can retain 899 mAhg-1 at 5 Ag-1 after 4000 cycles and 741 mAhg-1 at 10 Ag-1 after 10000 cycles, respectively. Here, we find that the actual capacity is higher than the theoretical capacity of PG@Fe2O3 (857 mAhg-1, Fig. S6). The extra capacity observed here would possibly be attributed to the reversible formation and dissolution of


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SEI components, including various inorganic and organic compounds.54 Additionally, the metal nanoparticles that are formed during a cycle may perform as efficient catalysts to promote the reversible process.48 Further, we can find that within the first few cycles at 5 and 10 Ag-1, the specific capacity decreases gradually because of poor electrolyte wetting of the PG@Fe2O3 film electrode. 41, 49 However, the specific capacity gradually increases subsequently, and a bump is formed (Fig. 5a). The bump should be attributed to gradual electrolyte wetting, more defects and active sites generated from structural changes.41, 55 Then, the PG@Fe2O3 shows a stable cyclic performance until 10000 cycles. Then, we test the cyclic performance of PG@Fe2O3 at higher rate of 30 and 50 Ag-1 using the same battery. The specific capacities of PG@Fe2O3 can still retain 249 and 141 mAhg-1 at 30 Ag-1 (meaning a complete charge/discharge process in 30 s) and 50 Ag-1 (equal to finishing a charge or discharge process within 10 s) even after another 10000 cycles, respectively. In addition, the rate performances of PG@Fe2O3 electrode are not only higher than that of the compared GO/Fe2O3 electrode (Fig. S18) but also, to the best of our knowledge, superior to those of reported metal oxide/graphene anode materials (Table S1) and even comparable with other kinds of anode materials such as Si, porous carbon and doped graphene.54-61 For example, the Fe3O4 nanoparticles arrays on the surface of graphene foam prepared by Hu et al showed a reversible specific capacity of 300 mAhg-1 at 18.48 Ag-1 after 50 cycles.60 The free-standing nanoporous graphene/Fe3O4 electrode can only retain 282 mAhg-1 after 300 cycles at 10 Ag-1.10 CuO nanowires, by coating with graphene quantum dots, maintained 330 mAhg-1 after 70 cycles at 20 Ag-1.61 Likewise, Fig. S12b shows the PG@CuO can retain specific capacities of 980, 314, and 168 mAhg-1 at 10, 50, and 100 Ag-1 (finishing a charge or discharge process within 6 s) after 10000 cycles, respectively.



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Fig. 5. (a) Cycle performance of PG@Fe2O3 at 5, 10, 30, and 50 Ag-1 (Note: the cycle performances at 30 and 50 Ag-1 were tested using the same cell after 10000 cycles at 10 Ag-1), and morphology and structures of the PG@Fe2O3 paper electrode after 4000 cycles at 5 Ag-1 exhibited by (b) SEM and (c) TEM images.

It can reasonably be thought that the high capacity and rate capability of embedded-type PG@MO paper electrodes should be attributed to their unique architecture. First, graphene can form a complete conductive network due to the special structure that metal oxides are embedded into the pores of graphene films to form a good electrical contact with the graphene edges in nanopores,15 which will improve the charge transfer, especially at high current rate. Second, the open porous structure and the hollow nanoparticles can shorten the diffusion path of Li ions and provide favorable transport kinetics for Li ions. Finally, structural stability during cycles would also be one of the most important factors for superior electrochemical performance. After long-


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term cycling, both Fe2O3 (Fig. 5b and c) and CuO (Fig. S19) are still well embedded into the pores of graphene films, and their hollow structures are also preserved, demonstrating very good structural stability. Benefiting from the unique structure, the embedded-type PG@MO paper electrodes exhibit higher specific capacities, better cyclic stability, and rate performance when used as anodes for lithium ion batteries. In summary, a new type of embedded-type PG@MO paper electrodes were designed and prepared by a simple filtration and controllable oxidation. When used as anode materials for LIBs, a very interesting phenomenon was found, in that the PG@MO paper electrodes exhibit higher specific capacities at 500 mAg-1 than those at 50 mAg-1. This is because some lithium storage sites may be activated with cycling at a high current density. In addition, the PG@MO paper electrodes also show superior long-term cyclic ability and high rate performance. The special porous architecture can shorten the diffusion path of lithium ions and cause electrons to transfer faster on the surface of electrodes. These results would provide a new strategy and material for the nextgeneration LIBs.

ASSOCIATED CONTENT Supporting Information. The supporting information includes the details of the preparation process, characterization, and electrochemical measurements of PG@Fe2O3 and PG@CuO; the SEM and HRTEM images of PG@Fe3C, PG@Cu, and PG@CuO; the SEM images of PG; the XRD patterns of PG@Fe3C, PG@Fe2O3, PG@Cu, and PG@CuO; the nitrogen adsorption– desorption isotherms and TG curves of PG@Fe2O3 and PG@CuO; the XPS spectra of PG@CuO; the electrochemical performance of PG@CuO and GO/Fe2O3; and the AC impedance spectra and



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Randles equivalent circuit for PG@Fe2O3 and PG@CuO. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes “The authors declare no competing financial interest.” ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51572015 and 51272019), New Teachers' Fund for Doctor Stations, Ministry of Education of China (20120010120004), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001).

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TOC GRAPHICS


Pliable Embedded-Type Paper Electrode of Hollow Metal Oxide

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