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Mar 18, 2014 - [15] M. H. Park, Y. Cho, K. Kim, J. Kim, M. Liu, J. Cho, Angew. Chem. ... [32] F. F. Cao, J. W. Deng, S. Xin, H. X. Ji, O. G. Schmidt, L. J. Wan, Y. G..

DOI: 10.1002/ente.201300167

Synthesis of Ultrathin GeO2–Reduced Graphene Oxide (RGO) Sheets for a High-Capacity Lithium-Ion Battery Anode De-Long Ma,[a] Shuang Yuan,[a] Xiao-Lei Huang,[b] and Zhan-Yi Cao*[a] GeO2 is a promising high-performance anode material for Li-ion batteries (LIBs) because of its high theoretical reversible capacity, low operation voltage, and high thermal stability. Herein, a simple, cheap, and easily scaled-up synthetic procedure for preparing ultrathin GeO2–reduced graphene oxide (RGO) sheets is proposed and realized through a freeze-drying method. The as-prepared GeO2–RGO show non-aggregated graphene sheets and homogeneously dispersed GeO2 nanoparticles. As a high-performance anode material for LIBs, the composite shows a high specific capacity (1200 mAh g1 at current density of 100 mA g1), good cycling performance, and rate capability.

There is great interest in developing novel anode materials for high-performance Li-ion batteries (LIBs), which are essential for portable electronics, electric vehicles, and the storage of renewable energy.[1–10] Compared with the commercial graphite anode (the theoretical specific capacity is 372 mAh g1), the group IV elements have attracted increasing attention as high-capacity anode materials, such as Si, Sn, SnO2, Ge, and GeO2. Especially, due to the intrinsic kinetic superiority (the better electrical conductivity and the higher Li diffusion coefficient), there is increased interest in Ge and GeO2 as potential anode materials for high-capacity and high-power LIBs.[11–19] GeO2 has great potential for application as a high-performance anode material for LIBs, because of its high theoretical reversible capacity (1125 mAh g1), low operating voltage (below 1.5 V), and higher thermal stability compared with Ge. However, like other high-capacity anode materials, its practical implementation is still hindered by poor battery performance, due to pulverization and exfoliation of the active materials from the current collector, caused by the drastic volume change of the active materials during the lith[a] D.-L. Ma, S. Yuan, Prof. Z.-Y. Cao Key Laboratory of Automobile Materials, Ministry of Education School of Materials Science and Engineering Jilin University Changchun 130012 (P R China) E-mail: [email protected] [b] Dr. X.-L. Huang State Key Laboratory of Rare Earth Resource Utilization Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130012 (P R China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ente.201300167. Part of a Special Issue on “Energy Storage Materials“. To view the complete issue, visit: http://onlinelibrary.wiley.com/doi/10.1002/ente.v2.4/issuetoc

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ium uptake and release processes. To overcome this problem, tremendous efforts have been made to design materials that can buffer the volume change.[20–22] Among the various approaches, combining nanoparticles (NPs) with conductive buffer materials is an attractive one to solve this problem. On one hand, decreasing the particle size could shorten both the electronic and ionic pathways during Li + reaction with active materials. On the other hand, the combination with conductive buffer materials could provide a fast electronic path through the electrode and buffer the volume change of the active materials during battery cycling. Among the various buffer materials, graphene is a good candidate to host active NPs because of its high specific surface area, excellent conductivity, and good mechanical flexibility.[23–26] It could afford good dispersion of the NPs, buffer the volume change (due to its good mechanical flexibility), and provide a two-dimensional conductive network, which is helpful for achieving high capacity and rates. Although various hybrid nanostructures of GeO2 containing graphene have been designed as electrode materials, these methods still suffer from severe drawbacks such as the requirement of a special solvent, a complicated preparation process, high energy and materials costs, and/or difficulty in scaling up the processes. It is still a challenge to enhance both the cycleability and rate capability of GeO2 anode materials. In this work, we present a simple, inexpensive, and easily scaled-up synthetic procedure for the preparation of ultrathin GeO2–reduced graphene oxide sheets (GeO2–RGO) using a freeze-drying method with graphene oxide (GO) and a (NH4)2GeO3 solution according to the reaction mechanism: Freeze drying (NH4)2GeO3 ƒƒƒƒƒƒ!GeO2 + NH4OH, inspired by a natural principle in sea ice.[27,2 8] The as-prepared GeO2–RGO show non-aggregated graphene sheets and homogeneously dispersed GeO2 NPs. Such a hybrid structure is an ideal electrode material for LIBs. The GeO2–RGO sheets show much improved specific capacity (1200 mAh g1 at a current density of 100 mA g1), good cycling performance, and high rate capability, for use as an anode material for LIBs. The strategy is simple yet very effective and also because of its versatility, it may be easily extended to other next-generation electrode materials. A schematic illustration of the synthesis route for GeO2– RGO is shown in Scheme 1. Briefly, the synthesis began with the formation of the (NH4)2GeO3 solution by the reaction of GeO2 with NH4OH. Then the (NH4)2GeO3 solution/GO mixture was frozen quickly in liquid nitrogen (N2). In this process, the (NH4)2GeO3 decomposed to GeO2 and NH3·H2O. The generated GeO2 was expelled from the forming ice and nucleated and to grow along the surface of the GO, wherein the GO was entrapped within the channels between the ice

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Scheme 1. Schematic representation of the synthesis of the GeO2–RGO composites.

crystals to avoid agglomeration during vacuum drying. After vacuum drying, the NH3·H2O and ice were sublimated and the GeO2–GO composite was obtained. Finally, the GeO2– RGO sheets were obtained after heat treating the precursor GeO2–GO under an Ar atmosphere, wherein the GO was reduced through thermal decomposition. X-ray diffraction (XRD) analysis was performed to investigate the crystal phase of the synthesized samples. All the diffraction peaks of the fabricated samples after freeze drying could be indexed to the trigonal phase GeO2 (JCPDS card No. 36-1463), as shown in Figure 1 a. It was found that the GeO2 was unchanged whereas the GO was reduced to RGO after heat treatment (400 8C for 1 h) under an Ar atmosphere, which was reflected by Raman spectroscopy (Figure 1 b). There is an increased D/G intensity ratio in GeO2– RGO (from 0.95 to 1.01) compared with GeO2–GO, indicating that the GO was reduced and subsequently more defects were formed during the heat-treatment process.[7, 29, 30] Note that the absence of peaks for GO or RGO in the XRD patterns (Figure 1 a) could be attributed to its covering by GeO2, leading to no significant restacking of the GO or RGO, which is consistent with the morphology results (see below). Thermogravimetric (TG) analysis was then used to determine the amount of GeO2 in the obtained GeO2–RGO (Figure S1). The weight loss of 12 % from 25 to 200 8C could be attributed to the loss of absorbed water. Almost no additional weight loss was observed, which implies that the composites are stable between 200 and 400 8C. The rapid mass loss between 400 and 620 8C is caused by the oxidation of RGO. At 750 8C, the total mass loss was 30 %. According to the results of the TG analysis, the mass percentages of RGO and GeO2 in the composites were calculated to be 18 % and 82 %, respectively. The morphology and structure of the asprepared GeO2–GO and GeO2–RGO samples were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Interestingly, it was found that both of them inherit the large-area sheet structure of GO and there was no large-particle aggregation of the GeO2 on the surface of the GO (Figure S2 a and b) and RGO (Figure 1 c and d), which was further confirmed by the TEM images. The GeO2 was found to be homogeneously distributed on the GO (Figure S2 c and d) and RGO (Figure 1 e, f and Energy Technol. 2014, 2, 342 – 347

Figure S3) sheets with a sheet-like structure (  20–50 nm in diameter). However, HRTEM images could not be obtained due to the deformation of the particles when focused under the electron beam.[14] Further structural details are also evident in the atomic force microscopy image (AFM) shown in Figure 2 a. A typical AFM image supports the morphological conclusions drawn from the TEM observations and the linear scan confirms that the thickness of GeO2–RGO is approximately 3.8 nm (the thickness of a GO sheet is  1 nm, as shown in Figure S4). There has been no report about such ultrathin GeO2–RGO sheet composites until now, which is different from previously published work.[11] One can expect that these ultrathin sheets will be particularly favorable in accomodating the volume change of GeO2 during cycling. To further confirm the distribution of GeO2 on RGO, dispersive spectroscopic (EDS) mapping was employed (Figure 2 c and d). The white square in Figure 2 b shows the mapping area of Figure 2 c and d. It was observed that the Ge and C elements distribute in the same region, which further confirms the uniform distribution of GeO2 on RGO. This structure could be helpful for preventing aggregation or restacking of RGO sheets and GeO2 upon cycling. Moreover, as an ideal electron conductor, RGO would play an important role as a conductive network within the electrode, which would benefit the electrochemical performance (see below). Coin cells with metallic Li counter electrodes were assembled and the galvanostatic charge/discharge technique was utilized to evaluate the electrochemical performance at room temperature. Note that the capacity is calculated based on the weight of the composite materials. Figure 3 a shows the discharge–charge voltage profiles cycled under a current density of 100 mA g1 over the voltage range 0–1.5 V vs. Li + /Li. The initial specific capacities for discharging and charging were 2700 and 1250 mAh g1, respectively. The large initial discharge capacity of the composite could be attributed to the formation of solid–electrolyte interphase (SEI) films on the surface of the electrode due to the decomposition of the electrolyte and the irreversible reaction of GeO2, similar to that previously reported for nanosized lithium-alloy systems.[31,32] In addition, the RGO networks with high surface area may also contribute to this issue. Though the coulombic efficiency is low in the first cycle, the reversibility of the capacity is significantly improved from the second cycle. As shown in Figure 3 a (inset), the GeO2–RGO composite still retains a reversible capacity of 1100 mAh g1 after 55 cycles. The lithium storage mechanisms of as-prepared samples were investigated by using cyclic voltammetry (CV) and the profiles were plotted in Figure 3 b. In the first reduction scan of the GeO2–RGO sample (inset, Figure 3 b), a peak at 0.7 V is most likely due to the formation of the SEI layer, as it is not reversible. In the voltage region between 0.5 and 0.01 V, the peaks could be related to the lithium reactions with GeO2. In the corresponding oxidation scan, the broad peak at 0.4 V could be related to the dealloying reaction, by which lithium was removed from the Li–Ge alloy. The broad humps at approximately 1.2 V could be related to the reoxidation of Ge to GeO2. In the second reduction scan, the

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Figure 1. a) XRD patterns and b) Raman spectra of the as-prepared sample before and after heat treatment (400 8C for 1 h); c, d) SEM and e, f) TEM images of the GeO2–RGO.

peaks at 0.45 and 0.1 could be related to the conversion reaction of GeO2 with lithium and the alloying reaction of Ge with lithium (corresponding oxidation peaks at 0.35 and 1.1 V). The results of the CV are in accordance with charge–

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discharge profiles. As shown in Figure 3 a, there are also two charge–discharge plateaus at the similar voltages in the CV curves. It should be noted that the reduction peaks shifted to high voltage in the subsequent cycles, which is due to the re-

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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formance. Moreover, the battery performance could be improved in future work by using a more-suitable binder in future work, such as carboxymethylated cellulose (CMC). In addition, the homogeneously distributed ultrathin GeO2– RGO nanosheets, like other ultrathin structure materials (such as graphene), could be used in many other fields. For example, they could be easily prepared into flexible electrodes through a simple filtration and press method (as shown in Figure S5), which is widely used in flexible Li-ion batteries. They could also be fabricated into an electron device used for semiconductor materials.

Conclusions

Figure 2. a, b) AFM and TEM images of GeO2–RGO; c, d) EDS characterization of GeO2–RGO; c) germanium; d) carbon mapping of the boxed region shown in b.

duction of the size of GeO2 during cycling. To demonstrate the improved cycling performance of the GeO2–RGO, a commercial GeO2 powder was also tested for comparison. Figure 3 c compares the cycling performance of the GeO2–RGO and the commercial GeO2 powders at 500 mA g1. The GeO2–RGO exhibits a higher specific capacity and better cycling stability than the commercial GeO2. After 100 cycles, GeO2–RGO still retains a reversible capacity of 750 mAh g1, whereas the commercial GeO2 only shows a specific capacity of 50 mAh g1. Figure 3 d shows the rate capability of the GeO2–RGO composite as the current density was increased from 0.1 to 2 A g1. It was found that the GeO2–RGO could deliver a capacity of 900 mAh g1 even at a current density of 0.2 A g1. Even under a high current density of 2 A g1, it could also deliver a capacity of 250 mAh g1. However, the commercial GeO2 shows very poor rate capability. All these results show that the GeO2–RGO composite holds superior rate capability. It should be noted that the capacity can be recovered if the rate is reduced to 0.1 A g1, indicating the good stability of the GeO2–RGO composite during the highrate discharge/charge cycles, which was also supported by the cycling test. The superior electrochemical performance of the GeO2– RGO can be attributed to the following factors: 1) The ultrathin structure sheets could shorten the Li + and electron diffusion pathway and effectively accommodate the huge volume change caused by the Li + insertion/extraction, which accounts for the high capacity and cycling performance; 2) The uniformly dispersed GeO2 on the conductive RGO could endow each GeO2 with good conductivity which would significantly improve the rate capability. Therefore, the GeO2–RGO exhibited excellent electrochemical perEnergy Technol. 2014, 2, 342 – 347

We have developed a simple, inexpensive, and easily scaledup solution method to chemically synthesize ultrathin GeO2– reduced graphene oxide (RGO) sheets. The synthesis endowed the GeO2–RGO composite with a morphology of uniformly dispersed GeO2 and non-aggregated RGO. The electrochemical performance shows the remarkably high reversible capacity, good cycling stability, and high rate capability of the GeO2–RGO composites. The simple and effective synthesis strategy, coupled with the excellent electrochemical performance, makes GeO2–RGO a promising anode material for LIBs.

Experimental Section Graphene oxide (GO) was prepared by a method previously reported in the literature[33] and was dispersed in deionized water up to the concentration of 1 mg g1. In a typical synthesis, commercial GeO2 (80 mg) was dissolved in H2O (5 mL) containing a small amount of concentrated NH3·H2O (100 mL). Then the solution was mixed with the as-prepared GO suspension (20 mg). Then, the mixed suspension was frozen in liquid N2 and the precursor GeO2–GO sheets were obtained by freeze drying. Finally, the ultrathin GeO2–RGO sheets were obtained by heat treatment at 400 8C for 1 h under an Ar atmosphere. The electrodes were prepared by mixing the active materials (80 wt %), acetylene black (10 wt %), and polyvinylidene fluoride (PVDF, 10 wt %) in N-methyl-2-pyrrolidone (NMP). After the above slurries were uniformly spread onto a Cu foil, the electrodes were dried at 80 8C in vacuum for 12 h. Then the electrodes were pressed and cut into disks before transferring into an argon-filled glove box. Coin cells (CR2025) were laboratory-assembled by using Li metal as the counter electrode, Celgard 2400 membrane as the separator, and 1 m LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (EC/DMC, 1:1 wt %) as the electrolyte. The galvanostatic charge–discharge tests were performed by using a Land Battery Measurement System (Land, China). Cyclic voltammetry (CV) was performed using a VMP3 Electrochemical Workstation (Bio-logic Inc.)

Acknowledgements This work was financially supported by the 100 Talents Program of The Chinese Academy of Sciences, National Program on Key Basic Research Project of China (973 Program, Grant No. 2012CB215500), the Foundation for Innovative Research

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Figure 3. Electrochemical activation of the GeO2–RGO. a) Charge–discharge curves at a current density 100 mA g1 (inset is the cycle performance); b) Cyclic voltammetry profiles at the scan rate 0.5 mVs1; c) Cycling stability (current density: 500 mA g1); and d) Rate performances of the GeO2–RGO composite and commercial GeO2.

Groups of the National Natural Science Foundation of China (Grant No. 20921002), the National Natural Science Foundation of China (Grant No. 21101147), and the Jilin Province Science and Technology Development Program (Grant No. 20100102 and 20116008).

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