Ge Nanoparticles Encapsulated in Nitrogen-Doped Reduced

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Ge Nanoparticles Encapsulated in Nitrogen-Doped Reduced Graphene Oxide as an Advanced Anode Material for Lithium-Ion Batteries Yan Xu, Xiaoshu Zhu, Xiaosi Zhou, Xia Liu, Yunxia Liu, Zhihui Dai, and Jianchun Bao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509783h • Publication Date (Web): 21 Nov 2014 Downloaded from http://pubs.acs.org on November 25, 2014

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The Journal of Physical Chemistry

Ge Nanoparticles Encapsulated in Nitrogen-Doped Reduced Graphene Oxide as an Advanced Anode Material for Lithium-Ion Batteries Yan Xu,† Xiaoshu Zhu,‡ Xiaosi Zhou,*,† Xia Liu,† Yunxia Liu,† Zhihui Dai,*,† and Jianchun Bao† †

Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China ‡

Center for Analysis and Testing, Nanjing Normal University, Nanjing 210023, P. R. China

Abstract: Ge nanoparticles/C composites are desirable electrode materials for high energy and power density lithium-ion batteries. However, the production of well-dispersed Ge nanoparticles in a carbon network remains a challenge because of rapid grain growth during high-temperature thermal reduction. Herein, we report a PVP-assisted hydrolysis approach for fabricating Ge nanoparticles/reduced graphene oxide composite (denoted as Ge/RGO) made of ~5 nm Ge nanoparticles that are uniformly distributed within a nitrogen-doped RGO carbon matrix. The Ge/RGO composite exhibits an initial discharge capacity of 1475 mA h g−1 and a reversible capacity of 700 mA h g−1 after 200 cycles at a current density of 0.5 A g−1. Moreover, Ge/RGO shows a capacity of 210 mA h g−1 even at a high current density of 10 A g−1. The excellent performance of the Ge/RGO composite is attributed to its unique nanostructure, including Ge nanoparticles, homogeneous particle distribution, and highly conductive RGO carbon matrix. These properties alleviate the pulverization problem, prevent Ge particle aggregation, and facilitate electron and lithium ion transportation.

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Keywords: Germanium nanoparticle, nitrogen-doped carbon matrix, PVP-assisted hydrolysis, anode, Li-ion battery

1. Introduction Rechargeable lithium-ion batteries have attracted considerable attention in the area of portable electronics, electric vehicles, and the storage of renewable energy because of their high energy density and long cycling stability.1–5 However, current commercial graphite anodes have a limited theoretical capacity of 372 mA h g−1 and are thus far from meeting the increasing demand for high energy/power density.6–9 Therefore, it is critical to develop novel anode materials with high capacity and long cycle life as well as high rate capability.10–17 In this regard, germanium (Ge) has been considered as one of the most prospective anode materials for next generation lithium-ion batteries owing to its high theoretical capacity (1600 mA h g−1 for Li4.4Ge), fast lithium diffusivity, and high electrical conductivity.18–25 Nevertheless, the practical application of Ge is hindered by the pulverization problem caused by the large volume changes during Li insertion/extraction processes, thus leading to the loss of electrical conductivity and consequently a rapid capacity decline upon cycling.26–31 Previously studies have demonstrated that decreasing the Ge particle size into the nanometer range could alleviate the stress produced during Li uptake and release processes and suppress the tendency of the nanostructure to fracture.32–35 Moreover, the nanostructure can facilitate the diffusion of Li ion, resulting in high rate capability.36–39 However, the cycling performance remains unsatisfactory because the Ge nanoparticles aggregate during cycling.40 To simultaneously overcome the aforementioned issues, various Ge/C composites and Ge nanostructures have been synthesized, which exhibits enhanced electrochemical performance.41– 45

For example, Guo and co-workers reported a Ge@C/RGO composite that delivered a


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reversible capacity of 940 mA h g−1 after 50 cycles.41 The highly conductive carbon matrix is responsible for the enhanced electrode performance. Cho et al. synthesized a clusternanostructured Ge/C composite that exhibited a capacity of 896 mA h g−1 after 120 cycles at 1 C.42 The carbon shells can not only buffer the volume change but also reduce the particle agglomeration. More recently, high-density Ge nanowire arrays grew directly on a current collector and showed a high capacity of ~900 mA h g−1 after 1100 cycles at 0.2 C.45 This superior performance was attributed to the uniformly arrayed Ge nanowires with a mean diameter of 73 nm. Obviously, small Ge particle size, uniform particle distribution, and highly conductive carbon network are essential for attaining advanced lithium storage properties. However, preparation of Ge-based composites with these combined features remains challenging. In this work, we demonstrate a polyvinylpyrrolidone (PVP)-assisted hydrolysis method to embed Ge nanoparticles (with a mean size of ~5 nm) in N-doped reduced graphene oxide (denoted as Ge/RGO). This novel approach consists of a PVP-assisted hydrolysis of GeCl4 on the GO surface followed by thermal reduction, which is more affordable than previous approaches that involve hydrolysis of GeCl4 followed by carbon coating using acetylene gas. During the annealing process, GO and PVP are transformed into a highly conductive RGO network while the GeO2 is in situ reduced to Ge nanoparticles (~5 nm), which are uniformly distributed in the as-formed RGO carbon matrix, producing the ultimate Ge/RGO composite. Furthermore, some N species in the precursor GeO2-PVP-GO can be preserved in the composite, thereby generating N-doped RGO with a high conductivity.46,47 This unique structure endows the composite with excellent electrochemical performance when used as an anode material for lithium-ion batteries.



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2. Experimental Section 2.1. Synthesis of Ge/RGO composite Graphene oxide (GO) was first synthesized according to a modified Hummer’s method.48,49 In a typical synthesis of Ge/RGO, 5 mL of GO aqueous suspension (10 mg mL−1) was first washed with ethanol and then dispersed in 20 mL of ethanol dissolved with 500 mg of polyvinylpyrrolidone (PVP; molecular weight of ~55,000; Aldrich). Then, 0.5 mL of GeCl4 (99.9999%, Alfa Aesar) was added to the above suspension. After stirring for 2 h, 1.8 mL of deionized (DI) water was added to the mixture and followed by stirring for another 12 h. The precipitate GeO2-PVP-GO was collected by centrifugation, washed with ethanol, and dried under vacuum at 60 oC overnight. To obtain Ge/RGO composite, the GeO2-PVP-GO was annealed at 600 oC for 2 h under H2/Ar (5:95 v/v) flow. For comparison, 120-GeOx/RGO composite was synthesized following the same procedures as for Ge/RGO except that PVP was not added. RGO was also prepared following the same procedures as for Ge/RGO except that GeCl4 was not added. 2.2. Materials Characterization Scanning electron microscopy (SEM) measurements were performed on a JEOL JSM-7600F scanning electron microscope operated at 10 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations were conducted on a JEOL JEM-2100F transmission electron microscope operated at 200 kV. Energy dispersive X-ray spectrum (EDS) analysis and scanning transmission electron microscopy (STEM) meaurements as well as elemental mapping analyses were carried out on a Tecnai G2 F20 U-TWIN field emission transmission electron microscope equipped with an EDAX system. X-ray photoelectron spectroscopy (XPS) measurement was determined on an ESCALab250Xi electron spectrometer


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from VG Scientific using 300W Al Kα radiation. X-ray diffraction (XRD) pattern was recorded on a Rigaku D/max 2500/PC diffractometer using Cu Kα radiation. Elemental analysis was performed on a Vario EL III elemental analyzer. Raman measurements were taken on a Labram HR800 with a laser wavelength of 514.5 nm. Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449 F3 under air flow with a heating rate of 10 oC min−1 from room temperature to 800 oC. Assuming complete combustion of RGO and conversion from Ge to GeO2, the content of Ge in the Ge/RGO composite can be determined based on the following equation:

2.3. Electrochemical Measurements Electrochemical experiments were conducted using CR2032 coin cells. To fabricate working electrodes, Ge/RGO, Super-P carbon black, and poly(vinylidene fluoride) (PVDF) with a weight ratio of 80:10:10 were mixed into homogeneous slurry in N-methyl-2-pyrrolidone (NMP) using mortar and pestle. The resulting slurry was pasted onto pure Cu foil (99.9 %, Goodfellow) and then dried in a vacuum oven at 80 oC overnight. The mass loading of active material was 1.5–2.0 mg cm−2, that is, the mass loading of Ge in electrode was 0.29–0.39 mg cm−2. The electrolyte for all tests was 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 v/v). Glass fibers (GF/D) from Whatman were utilized as separators and pure lithium metal foil was used as the counter electrode. The coin cells were assembled in an argon-filled glove box (H2O, O2 < 0.1 ppm, MBraun). The charge and discharge measurements of the batteries were conducted on a Land CT2001A multi-channel battery testing system in the fixed voltage range of 0–1.5 V vs Li+/Li at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)



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were recorded on a PARSTAT 4000 electrochemical workstation. CV was measured at a scan rate of 0.1 mV s−1 while EIS was tested in the frequency range from 100 kHz to 100 mHz.

3. Results and Discussion The Ge/RGO composite was prepared by a simple thermal reduction of the GeO2-PVP-GO complex. The GeO2-PVP-GO precursor was synthesized via a PVP-assisted hydrolysis of GeCl4 on the surface of GO, as described in Experimental Section. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses (Supporting Information, Figure S1) of the as-prepared GeO2-PVP-GO shows that GeO2 grew on the GO surface because of the coordination effect between GeO2 and the functional groups of both PVP and GO, which formed a conformal GeO2 coating on GO. After annealing GeO2-PVP-GO at 600 oC under H2/Ar flow, the Ge/RGO composite was generated. Figure 1a shows the X-ray diffraction (XRD) pattern of the as-synthesized composite. All diffraction peaks could be well indexed to diamond cubic Ge (JCPDS card No. 04-0545), and the average size of Ge nanoparticles calculated from the FWHM of the (111) peak is around 5 nm. No obvious peak corresponding to carbon was detected, indicating the amorphous nature of carbon matrix.50 This amorphous nature was confirmed by the Raman spectrum of the Ge/RGO sample (Figure 1b), in which three characteristic peaks at 295, 1354 and 1589 cm−1 could be ascribed to the typical Raman band of Ge and the D and G bands of amorphous carbon, respectively.41,51 Generally, amorphous carbon possesses lots of vacancies and defects, which not only promote Li ion diffusion but offer numerous reversible sites for Li storage, thus boosting the total capacity of the composite.52


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Figure 1. (a) XRD pattern of Ge/RGO and standard XRD pattern of Ge (JCPDS card No. 040545). (b) Raman spectrum of Ge/RGO. The surface composition of the Ge/RGO composite was surveyed using X-ray photoelectron spectroscopy (XPS). The Ge 3d XPS spectrum (Figure 2a) exhibits two broad peaks at 32.7 eV (Ge4+ 3d) and 30.0 eV (Ge 3d), demonstrating that some Ge nanoparticles near the surface of the composite were oxidized to GeO2.53,54 The O 1s XPS spectrum (Figure 2b) of the composite indicates that the existence of oxygen can be assigned to RGO, GeO2, and O2, respectively.38,54 However, the formed GeO2 would be favorable to improve the total capacity of the composite because it has a high theoretical capacity of 2152 mA h g−1.53,55 Additionally, a small peak located at around 400 eV belonging to N 1s can be observed in the XPS survey scan of Ge/RGO (Figure 2c). High-resolution XPS spectrum of the N peak demonstrates pyridinic and pyrrolic nitrogen species in Ge/RGO (Figure 2d),56 implying the production of N-doped RGO. Elemental analysis further confirms that the as-formed Ge/RGO composite contains ~4.3 wt% N species. Usually, N-doping can improve the electrochemical performance of composites through enhancing the electric conductivity of the carbon matrix.47 The Ge content in the Ge/RGO composite determined by the thermogravimetric analysis (TGA, Supporting Information, Figure S2) was approximately 24.3 wt%. Besides, it is found that the usage of PVP in the hydrolysis



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process is crucial for the successful preparation of uniformly distributed Ge nanoparticles in Ndoped RGO matrix. In the absence of PVP, large and irregular GeO2 nanoparticles were generated on GO (Supporting Information, Figure S3). After the following thermal reduction, a GeOx/RGO composite with typical GeOx particle size of ~120 nm (denoted as 120-GeOx/RGO) was fabricated, as identified by XRD and TEM analyses (Supporting Information, Figure S4).

Figure 2. (a) High-resolution Ge 3d XPS spectrum of Ge/RGO. (b) High-resolution O 1s XPS spectrum of Ge/RGO. (c) The full XPS spectrum of Ge/RGO. (d) High-resolution XPS spectrum of the N 1s peak marked with a dashed rectangle in (c). The morphology of the as-prepared Ge/RGO composite was investigated using SEM and TEM. Figure 3a shows the SEM image of the composite, which presents as sheet-like shape. The high-magnification SEM image (Figure 3b) of Ge/RGO indicates that the obtained composite has a smooth surface. Only a few nanoparticles could be observed on the surface of the composite,


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demonstrating that most of Ge nanoparticles are encapsulated in the carbon network. As illustrated in Figure 3c, uniform Ge nanoparticles were homogeneously distributed in carbon matrix. The lighter framework represents the RGO network while the darker dots represent the Ge nanoparticles. The mean size of Ge particles is around 5 nm, which could be clearly observed from the high-resolution TEM (HRTEM) image (Figure 3d), agreeing well with the calculated result from the XRD analysis. The SAED pattern (inset of Figure 3d) demonstrates a polycrystalline structure and the diffraction rings could be perfectly indexed to crystal planes of cubic Ge phase, in accordance with the XRD results. The scanning transmission electron microscopy (STEM) image (Figure 3e) and energy dispersive X-ray spectrum (EDS) mappings (Figure 3f–h) of the as-formed sample shows that Ge nanopartciles are homogeneously embedded in the N-doped RGO network. Moreover, it should be noted that the distribution and signal intensity of carbon and germanium throughout the selected area are quite uniform, suggesting most of the Ge nanoparticles were homogeneously encapsulated in the RGO matrix. These results imply that the as-made Ge/RGO composite holds three key features: Ge nanoparticles, homogeneously distributed Ge nanoparticles, and highly conductive RGO-based carbon matrix. All of these features promise a desired performance for lithium storage.



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Figure 3. (a) SEM image, (b) high-magnification SEM image, (c) TEM image, (d) HRTEM image and SAED pattern (inset), (e) STEM image, (f, g, h) EDS elemental mapping images of Ge nanoparticles distributed in N-doped RGO network (Ge/RGO). The unique structure and morphology of the as-achieved Ge/RGO composite stimulate us to further test its electrochemical performance. Figure 4a displays the first five cyclic voltammetry (CV) curves of the Ge/RGO electrode at a scan rate of 0.1 mV s−1 between 0 and 1.5 V. During the first cathodic scan, the broad reduction peak at 0.66 V corresponds to the decomposition of


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the electrolyte to form solid electrolyte interphase (SEI) film, resulting in the capacity loss during the first cycle.21,57,58 The large peak between 0 and 0.51 V is assigned to the reduction of Ge. From the second cycle onward, three apparent peaks located at 0.01, 0.31, and 0.48 V can be attributed to the formation of LixGe alloy.21,57 In the anodic scan, oxidation peaks centered at 0.41, 0.53, and 0.92 V are ascribed to the dealloying reaction of LixGe. All peaks maintain stable after the first cycle, suggesting the reversibility of the electrochemical reactions of the Ge/RGO electrode. Furthermore, the absence of oxidative peak at 1.1 V indicates that Ge nanoparticles are well encapsulated in the RGO matrix.53

Figure 4. (a) CV curves of the first five cycles of Ge/RGO at a scanning rate of 0.1 mV s−1. (b) Galvanostatic charge-discharge profiles for different cycles of Ge/RGO at a current density of 0.5 A g−1. (c) Cycling performance and Coulombic efficiency of Ge/RGO under 0.5 A g−1. (d) Rate capability of Ge/RGO. Figure 4b shows the voltage profiles of the Ge/RGO electrode for different cycles between 0 and 1.5 V at a current density of 0.5 A g−1. The voltage profiles present typical characteristics of



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a Ge electrode and well correlate with the above CV curves.32 Note that all the specific capacity values are calculated based on the total mass of the Ge/RGO composite. The initial charge and discharge capacities of Ge/RGO are 903 and 1475 mA h g−1, respectively, corresponding to a Coulombic efficiency (CE) of 61.2%. The capacity loss is originated from the irreversible Li storage in the carbon matrix, as supported by the electrochemical performance of RGO (Supporting Information, Figure S5 and S6). The high capacity should benefit from the Ge particle size, uniform particle dispersion, and highly conductive carbon matrix, which enable the full utilization of Ge nanoparticles. More importantly, after 15 cycles the reversible capacity stabilizes at ~750 mA h g−1. The slow decrease in the charge/discharge capacities after the initial cycles suggests the as-obtained composite has high cycling stability. Figure 4c shows the cycling performance of the Ge/RGO electrode. Remarkably, the Ge/RGO electrode shows much better cycling stability than the 120-GeOx/RGO electrode (Supporting Information, Figure S7). After 200 cycles, the Ge/RGO electrode delivers a capacity as high as 700 mA h g−1, which is almost 2 times higher than that of commercial graphite electrode (372 mA h g−1). By subtracting the capacity contribution from RGO, a stable capacity of 1550 mA h g−1 can be reached for Ge on the basis of eq S1, Supporting Information. Furthermore, it should be pointed out that the CE of the second cycle exceeds 95% and then remains above 99% after 6 cycles, showing high reversibility of the Ge/RGO electrode. In contrast, only a low capacity of 143 mA h g−1 is retained for the 120-GeOx/RGO electrode after 100 cycles. The rapid capacity deterioration of the 120-GeOx/RGO electrode may be due to the particle pulverization or aggregation caused by the huge volume variation during charge/discharge processes, as verified by TEM characterization of the 120-GeOx/RGO electrode after 100 cycles (Supporting Information, Figure S8). Figure 4d shows the rate capability of the


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Ge/RGO electrode cycled at various current densities from 0.5 to 10 A g−1. The reversible capacity of the electrode declines moderately as the current density increases stepwise. When cycled at a low current density of 0.5 A g−1, its specific capacity can achieve 772 mA h g−1. Even at a high current density of 10 A g−1, it still delivers a reversible capacity of 210 mA h g−1. Similarly, the capacity contribution of Ge in the composite under 10 A g−1 is calculated to be 303 mA h g−1 based on the rate performance of RGO and eq S1 (Supporting Information, eq S1 and Figure S6c). In addition, the charge capacity can recover to 750 mA h g−1 when the current density finally returns to 0.5 A g−1, suggesting that the uniformly distributed Ge nanoparticles and highly conductive RGO matrix ensure the fast Li ion and electron transportation. To understand the excellent electrochemical performance of the Ge/RGO composite, TEM, STEM, and EDS elemental mapping analysis have been performed to investigate the morphology and structure change of the composite after 200 charge/discharge cycles under 0.5 A g−1. It is clearly observed from Figure 5a that the original appearance of the composite (as displayed in Figure 3c) has been well maintained even after long-term cycling. Integrated with the STEM image (Figure 5b), the elemental mapping images of carbon, germanium, and nitrogen reveal that all Ge nanoparticles are perfectly confined in the nitrogen-doped carbon matrix (Figure 5c–e), further confirming that the unique structure of the composite not only effectively prevents the nanoparticle aggregation but also successfully avoid the pulverization issue, thus guaranteeing the long cycle stability and high rate performance. Additionally, the improved electrical conductivity of the Ge/RGO composite was confirmed by electrochemical impendence spectroscopy (EIS, Supporting Information, Figure S9). Apparently, the Ge/RGO electrode presents a much lower charge-transfer resistance Rct than that of the 120-GeOx/RGO electrode (12.1 vs 35.7 Ω) based on the modified Randles equivalent circuit shown in the inset of Figure



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S9. The high electrical conductivity of Ge/RGO most likely arises from the N-doping of the RGO matrix.38,59

Figure 5. (a) TEM image and (b) STEM image of Ge/RGO after 200 cycles. (c, d, e) Corresponding C, Ge, and N elemental mapping images based on the area marked in (b). The superior electrochemical performance of the Ge/RGO composite can be attributed to the following aspects. First, the nanometer-sized Ge particles could significantly shorten the diffusion path of Li ion and considerably reduce the stress formed during Li insertion/extraction processes to alleviate the pulverization issue. Second, taking advantage of the uniform distribution of Ge nanoparticles, the produced strain during cycling would homogeneously spread in the composite, thus ensuring the integrity of the whole electrode. Finally, the highly conductive RGO network not only facilitates the electron transport but also allows for the large volume expansion/contraction of Ge nanoparticles upon prolonged cycling. These unique advantages make Ge/RGO very promising as an advanced anode material for lithium-ion batteries.


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4. Conclusions In summary, Ge/RGO composite with Ge nanoparticles (~5 nm) evenly encapsulated in Ndoped RGO matrix has been synthesized through a PVP-assisted hydrolysis technique. The asprepared Ge/RGO composite exhibits long cycle stability and high rate capability when evaluated as anode material for rechargeable lithium-ion batteries. The enhanced electrochemical performance is attributed to the nanometer-sized Ge particle size, uniform particle distribution, and highly conductive RGO network, which not only effectively alleviate the pulverization and agglomeration of Ge nanopartilces but also buffer the large volume expansion of Ge and facilitate electron and Li ion transport upon long-term cycling. This strategy may become a new promising avenue for developing high-capacity electrode materials for lithium and sodium storage.

Supporting Information SEM/TEM images of GeO2-PVP-GO, GeO2-GO, and 120-GeOx/RGO, TGA curve of Ge/RGO, XRD pattern of 120-GeOx/RGO, electrochemical performance of RGO and 120-GeOx/RGO, capacity contribution equation of Ge, and Nyquist plots of Ge/RGO and 120-GeOx/RGO. This information is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Authors *(X.Z.) E-mail: [email protected]. Telephone/Fax: +86-25-85891027 *(Z.D.) E-mail: [email protected]. Telephone/Fax: +86-25-85891051 Notes The authors declare no competing financial interest.



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Acknowledgements. This work was supported by the National Natural Science Foundation of China (Grant Nos. 21175069 and 21171096), the Natural Science Foundation of Jiangsu Province of China (BK20140915), the Scientific Research Foundation for Advanced Talents of Nanjing Normal University (2014103XGQ0073), the financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Program of Jiangsu Collaborative Innovation Center of Biomedical Functional Materials.

References (1) Armand, M.; Tarascon, J.-M. Building Better Batteries. Nature 2008, 451, 652–657. (2) Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem. Int. Ed. 2008, 47, 2930–2946. (3) Chen, J.; Cheng, F. Combination of Lightweight Elements and Nanostructured Materials for Batteries. Acc. Chem. Res. 2009, 42, 713–723. (4) Xin, S.; Guo, Y.-G.; Wan, L.-J. Nanocarbon Networks for Advanced Rechargeable Lithium Batteries. Acc. Chem. Res. 2012, 45, 1759–1769. (5) Jiang, J.; Li, Y.; Liu, J.; Huang, X.; Yuan, C.; Lou, X. W. Recent Advances in Metal Oxidebased Electrode Architecture Design for Electrochemical Energy Storage. Adv. Mater. 2012, 24, 5166–5180. (6) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666–686. (7) Park, M.-H.; Kim, K.; Kim, J.; Cho, J. Flexible Dimensional Control of High-Capacity LiIon-Battery Anodes: From 0D Hollow to 3D Porous Germanium Nanoparticle Assemblies. Adv. Mater. 2010, 22, 415–418.


Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Chan, C. K.; Zhang, X. F.; Cui, Y. High Capacity Li Ion Battery Anodes Using Ge Nanowires. Nano Lett. 2008, 8, 307–309. (9) Chen, X.; Li, X.; Ding, F.; Xu, W.; Xiao, J.; Cao, Y.; Meduri, P.; Liu, J.; Graff, G. L.; Zhang, J.-G. Conductive Rigid Skeleton Supported Silicon as High-Performance Li-Ion Battery Anodes. Nano Lett. 2012, 12, 4124–4130. (10) Ma, H.; Cheng, F.; Chen, J.; Zhao, J.; Li, C.; Tao, Z.; Liang, J. Nest-like Silicon Nanospheres for High-Capacity Lithium Storage. Adv. Mater. 2007, 19, 4067–4070. (11) Hu, Y.-S.; Demir-Cakan, R.; Titirici, M.-M.; Müller, J.-O.; Schlögl, R.; Antonietti, M.; Maier, J. Superior Storage Performance of a Si@SiOx/C Nanocomposite as Anode Material for Lithium-Ion Batteries. Angew. Chem. Int. Ed. 2008, 47, 1645–1649. (12) Zhou, X.; Yin, Y.-X.; Wan, L.-J.; Guo, Y.-G. Self-Assembled Nanocomposite of Silicon Nanoparticles Encapsulated in Graphene through Electrostatic Attraction for Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 1086–1090. (13) Luo, J.; Liu, J.; Zeng, Z.; Ng, C. F.; Ma, L.; Zhang, H.; Lin, J.; Shen, Z.; Fan, H. J. ThreeDimensional Graphene Foam Supported Fe3O4 Lithium Battery Anodes with Long Cycle Life and High Rate Capability. Nano Lett. 2013, 13, 6136–6143. (14) Zhou, X.; Dai, Z.; Liu, S.; Bao, J.; Guo, Y.-G. Ultra-Uniform SnOx/Carbon Nanohybrids toward Advanced Lithium-Ion Battery Anodes. Adv. Mater. 2014, 26, 3943–3949. (15) Wang, C.; Wu, H.; Chen, Z.; McDowell, M. T.; Cui, Y.; Bao, Z. Self-Healing Chemistry Enables the Stable Operation of Silicon Microparticle Anodes for High-Energy Lithium-Ion Batteries. Nat. Chem. 2014, 5, 1042–1048.



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(16) Zhang, G.; Wu, H. B.; Hoster, H. E.; Lou, X. W. Strongly Coupled Carbon Nanofiber– Metal Oxide Coaxial Nanocables with Enhanced Lithium Storage Properties. Energy Environ. Sci. 2014, 7, 302–305. (17) Wei, W.; Yang, S.; Zhou, H.; Lieberwirth, I.; Feng, X.; Müllen, K. 3D Graphene Foams Cross-linked with Pre-encapsulated Fe3O4 Nanospheres for Enhanced Lithium Storage. Adv. Mater. 2013, 25, 2909–2914. (18) Park, M.-H.; Cho, Y.; Kim, K.; Kim, J.; Liu, M.; Cho, J. Germanium Nanotubes Prepared by Using the Kirkendall Effect as Anodes for High-Rate Lithium Batteries. Angew. Chem. Int. Ed. 2011, 50, 9647–9650. (19) Yu, Y.; Yan, C.; Gu, L.; Lang, X.; Tang, K.; Zhang, L.; Hou, Y.; Wang, Z.; Chen, M. W.; Schmidt, O. G.; et al. Three-Dimensional (3D) Bicontinuous Au/Amorphous-Ge Thin Films as Fast and High-Capacity Anodes for Lithium-Ion Batteries. Adv. Energy Mater. 2013, 3, 281–285. (20) Xiao, Y.; Cao, M.; Ren, L.; Hu, C. Hierarchically Porous Germanium-Modified Carbon Materials with Enhanced Lithium Storage Performance. Nanoscale 2012, 4, 7469–7474. (21) Wang, Y.; Wang, G. Facile Synthesis of Ge@C Core–Shell Nanocomposites for HighPerformance Lithium Storage in Lithium-Ion Batteries. Chem. Asian J. 2013, 8, 3142–3146. (22) Ren, J.-G.; Wu, Q.-H.; Tang, H.; Hong, G.; Zhang, W.; Lee, S.-T. Germanium−Graphene Composite Anode for High-Energy Lithium Batteries with Long Cycle Life. J. Mater. Chem. A 2013, 1, 1821–1826. (23) Song, T.; Jeon, Y.; Samal, M.; Han, H.; Park, H.; Ha, J.; Yi, D. K.; Choi, J.-M.; Chang, H.; Choi, Y.-M.; et al. A Ge Inverse Opal with Porous Walls as an Anode for Lithium Ion Batteries. Energy Environ. Sci. 2012, 5, 9028–9033.


Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Qin, J.; Wang, X.; Cao, M.; Hu, C. Germanium Quantum Dots Embedded in N-Doping Graphene Matrix with Sponge-Like Architecture for Enhanced Performance in Lithium-Ion Batteries. Chem. Eur. J. 2014, 20, 9675–9682. (25) Cheng, J.; Du, J. Facile Synthesis of Germanium-Graphene Nanocomposites and Their Application as Anode Materials for Lithium Ion Batteries. CrystEngComm 2012, 14, 397–400. (26) Yan, C.; Xi, W.; Si, W.; Deng, J.; Schmidt, O. G. Highly Conductive and Strain-Released Hybrid Multilayer Ge/Ti Nanomembranes with Enhanced Lithium-Ion-Storage Capability. Adv. Mater. 2013, 25, 539–544. (27) Seo, M.-H.; Park, M.; Lee, K. T.; Kim, K.; Kim, J.; Cho, J. High Performance Ge Nanowire Anode Sheathed with Carbon for Lithium Rechargeable Batteries. Energy Environ. Sci. 2011, 4, 425–428. (28) Liu, X.; Zhao, J.; Hao, J.; Su, B.-L.; Li, Y. 3D Ordered Macroporous Germanium Fabricated by Electrodeposition from an Ionic Liquid and Its Lithium Storage Properties. J. Mater. Chem. A 2013, 1, 15076–15081. (29) Liu, J.; Song, K.; Zhu, C.; Chen, C.-C.; van Aken, P. A.; Maier, J.; Yu, Y. Ge/C Nanowires as High-Capacity and Long-Life Anode Materials for Li-Ion Batteries. ACS Nano 2014, 8, 7051– 7059. (30) Li, W.; Yang, Z.; Cheng, J.; Zhong, X.; Gu, L.; Yu, Y. Germanium Nanoparticles Encapsulated in Flexible Carbon Nanofibers as Self-Supported Electrodes for High Performance Lithium-Ion Batteries. Nanoscale 2014, 6, 4532–4537. (31) Zhong, X.; Wang, J.; Li, W.; Liu, X.; Yang, Z.; Gu, L.; Yu, Y. Facile Synthesis of Germanium-Reduced Graphene Oxide Composite as Anode for High Performance Lithium-Ion Batteries. RSC Adv. 2014, 4, 58184–58189.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

(32) Gu, J.; Collins, S. M.; Carim, A. I.; Hao, X.; Bartlett, B. M.; Maldonado, S. Template-Free Preparation of Crystalline Ge Nanowire Film Electrodes via an Electrochemical Liquid-LiquidSolid Process in Water at Ambient Pressure and Temperature for Energy Storage. Nano Lett. 2012, 12, 4617–4623. (33) Hwang, I.-S.; Kim, J.-C.; Seo, S.-D.; Lee, S.; Lee, J.-H.; Kim, D.-W. A Binder-Free GeNanoparticle Anode Assembled on Multiwalled Carbon Nanotube Networks for Li-Ion Batteries. Chem. Commun. 2012, 48, 7061–7063. (34) Ke, F.-S.; Mishra, K.; Jamison, L.; Peng, X.-X.; Ma, S.-G.; Huang, L.; Sun, S.-G.; Zhou, X.D. Tailoring Nanostructures in Micrometer Size Germanium Particles to Improve Their Performance as an Anode for Lithium Ion Batteries . Chem. Commun. 2014, 50, 3713–3715. (35) DiLeo, R. A.; Frisco, S.; Ganter, M. J.; Rogers, R. E.; Raffaelle, R. P.; Landi, B. J. Hybrid Germanium Nanoparticle Single-Wall Carbon Nanotube Free-Standing Anodes for Lithium Ion Batteries. J. Phys. Chem. C 2011, 115, 22609–22614. (36) Zhou, X.; Wan, L.-J.; Guo, Y.-G. Synthesis of MoS2 Nanosheet-Graphene Nanosheet Hybrid materials for Stable Lithium Storage. Chem. Commun. 2013, 49, 1838–1840. (37) Zhou, X.; Wan, L.-J.; Guo, Y.-G. Binding SnO2 Nanocrystals in Nitrogen-Doped Graphene Sheets as Anode Materials for Lithium-Ion Batteries. Adv. Mater. 2013, 25, 2152–2157. (38) Zhu, Z.; Wang, S.; Du, J.; Jin, Q.; Zhang, T.; Cheng, F.; Chen, J. Ultrasmall Sn Nanoparticles Embedded in Nitrogen-Doped Porous Carbon As High-Performance Anode for Lithium-Ion Batteries. Nano Lett. 2014, 14, 153–157. (39) Guan, C.; Wang, X.; Zhang, Q.; Fan, Z.; Zhang, H.; Fan, H. J. Highly Stable and Reversible Lithium Storage in SnO2 Nanowires Surface Coated with a Uniform Hollow Shell by Atomic Layer Deposition. Nano Lett. 2014, 14, 4852–4858.


Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Lee, H.; Kim, H.; Doo, S.-G.; Cho, J. Synthesis and Optimization of Nanoparticle Ge Confined in a Carbon Matrix for Lithium Battery Anode Material. J. Electrochem. Soc. 2007, 154, A343–A346. (41) Xue, D.-J.; Xin, S.; Yan, Y.; Jiang, K.-C.; Yin, Y.-X.; Guo, Y.-G.; Wan, L.-J. Improving the Electrode Performance of Ge through Ge@C Core-Shell Nanoparticles and Graphene Networks. J. Am. Chem. Soc. 2012, 134, 2512–2515. (42) Seng, K. H.; Park, M.-H.; Guo, Z. P.; Liu, H. K.; Cho, J. Self-Assembled Germanium/Carbon Nanostructures as High-Power Anode Material for the Lithium-Ion Battery. Angew. Chem. Int. Ed. 2012, 51, 5657–5661. (43) Zhong, C.; Wang, J.-Z.; Gao, X.-W.; Wexler, D.; Liu, H.-K. In Situ One-Step Synthesis of a 3D Nanostructured Germanium-Graphene Composite and Its Application in Lithium-Ion Batteries. J. Mater. Chem. A 2013, 1, 10798–10804. (44) Cho, Y. J.; Im, H. S.; Kim, H. S.; Myung, Y.; Back, S. H.; Lim, Y. R.; Jung, C. S.; Jang, D. M.; Park, J.; Cha, E. H.; et al. Atomic-Scale Observation of Lithiation Reaction Front in Nanoscale SnO2 Materials. ACS Nano 2013, 7, 9075–9084. (45) Kennedy, T.; Mullane, E.; Geaney, H.; Osiak, M.; O’Dwyer, C.; Ryan, K. M. HighPerformance Germanium Nanowire-Based Lithium-Ion Battery Anodes Extending over 1000 Cycles Through in Situ Formation of a Continuous Porous Network. Nano Lett. 2014, 14, 716– 723. (46) Wu, Z.-S.; Ren, W.; Xu, L.; Li, F.; Cheng, H.-M. Doped Graphene Sheets As Anode Materials with Superhigh Rate and Large Capacity for Lithium Ion Batteries. ACS Nano 2011, 5, 5463–5471.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

(47) Qie, L.; Chen, W.-M.; Wang, Z.-H.; Shao, Q.-G.; Li, X.; Yuan, L.-X.; Hu, X.-L.; Zhang, W.-X.; Huang, Y.-H. Nitrogen-Doped Porous Carbon Nanofiber Webs as Anodes for Lithium Ion Batteries with a Superhigh Capacity and Rate Capability. Adv. Mater. 2012, 24, 2047–2050. (48) Hummers, W. S., Jr.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339–1339. (49) Zhou, X.; Bao, J.; Dai, Z.; Guo, Y.-G. Tin Nanoparticles Impregnated in Nitrogen-Doped Graphene for Lithium-Ion Battery Anodes. J. Phys. Chem. C 2013, 117, 25367–25373. (50) Deng, D.; Lee, J. Y. Reversible Storage of Lithium in a Rambutan-Like Tin-Carbon Electrode. Angew. Chem., Int. Ed. 2009, 48, 1660–1663. (51) Hwang, J.; Woo, S. H.; Shim, J.; Jo, C.; Lee, K. T.; Lee, J. One-Pot Synthesis of TinEmbedded Carbon/Silica Nanocomposites for Anode Materials in Lithium-Ion Batteries. ACS Nano 2013, 7, 1036–1044. (52) Zheng, T.; Zhong, Q.; Dahn, J. R. High-Capacity Carbons Prepared from Phenolic Resin for Anodes of Lithium-Ion Batteries. J. Electrochem. Soc. 1995, 142, L211–L214. (53) Seng, K. H.; Park, M.-H.; Guo, Z. P.; Liu, H. K.; Cho, J. Catalytic Role of Ge in Highly Reversible GeO2/Ge/C Nanocomposite Anode Material for Lithium Batteries. Nano Lett. 2013, 13, 1230–1236. (54) Wang, D.; Chang, Y.-L.; Wang, Q.; Cao, J.; Farmer, D. B.; Gordon, R. G.; Dai, H. Surface Chemistry and Electrical Properties of Germanium Nanowires. J. Am. Chem. Soc. 2004, 126, 11602–11611. (55) Wang, X.-L.; Han, W.-Q.; Chen, H.; Bai, J.; Tyson, T. A.; Yu, X.-Q.; Wang, X.-J.; Yang, X.-Q. Amorphous Hierarchical Porous GeOx as High-Capacity Anodes for Li Ion Batteries with Very Long Cycling Life. J. Am. Chem. Soc. 2011, 133, 20692–20695.


Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(56) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Ggraphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780– 786. (57) Lv, D.; Gordin, M. L.; Yi, R.; Xu, T.; Song, J.; Jiang, Y.-B.; Choi, D.; Wang, D. GeOx/Reduced Graphene Oxide Composite as an Anode for Li-Ion Batteries: Enhanced Capacity via Reversible Utilization of Li2O along with Improved Rate Performance. Adv. Funct. Mater. 2014, 24, 1059–1066. (58) Wang, H.; Cui, L.-F.; Yang, Y.; Casalongue, H. S.; Robinson, J. T.; Liang, Y.; Cui, Y.; Dai, H. Mn3O4−Graphene Hybrid as a High-Capacity Anode Material for Lithium Ion Batteries. J. Am. Chem. Soc. 2010, 132, 13978–13980. (59) Ren, W.; Li, D.; Liu, H.; Mi, R.; Zhang, Y.; Dong, L. Lithium Storage Performance of Carbon Nanotubes with Different Nitrogen Contents as Anodes in Lithium Ions Batteries. Electrochim. Acta 2013, 105, 75−82.



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Ge Nanoparticles Encapsulated in Nitrogen-Doped Reduced

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