Facile synthesis of germanium–reduced graphene oxide composite as anode for high performance lithium-ion batteries

Abstract

Germanium is a promising anode material for lithium ion batteries (LIBs) due to its high specific capacity, but it still suffers from poor cyclability. A simple method was developed to synthesize Ge–reduced graphene oxide nanocomposites using organic germanium as a precursor. The nanocomposites exhibit improved electrochemical performance with a reversible specific capacity of 814 mA h g⁻¹ after 50 cycles at a current density of 0.1 A g⁻¹. When cycled at a high current density of 2 A g⁻¹, they still deliver a reversible specific capacity of 690 mA h g⁻¹ after 150 cycles. The improved electrochemical performance is attributed to the unique nanostructure (0D electroactive particles in 2D mixed conducting matrix), which conferred a variety of advantages: high flexibility of the graphene sheets for accommodating the volume change, good electrochemical coupling and short transport length for ions and electrons, enabling low contact resistances.

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Rechargeable lithium ion batteries (LIBs) have been widely used in portable electronic devices, electric vehicles, and implantable medical devices.^1–3 To meet the increasing demand for advanced energy storage techniques for application in electric vehicles (EV), hybrid electric vehicles (HEV) and smart grids, extensive attention has been paid to developing LIBs with both high energy density and high power density. Currently, the most dominant anode material of LIBs is traditional graphite. However, graphite does not still satisfy the requirements for practical application in the next generation of large scale LIBs, because of its lower theoretical capacity (372 mA h g⁻¹).^2,4–19 Therefore, extensive attention has been paid to seeking alternatives with high capacity. Much effort has been devoted to the use of Li alloys of group IV materials, such as Li_xSi, and Li_xGe, because of their higher theoretically reversible specific capacities (4200 mA h g⁻¹ for Si, 1600 mA h g⁻¹ for Ge).²⁰ Compared with silicon, germanium (Ge) has emerged as a promising anode material due to its excellent lithium-ion diffusion coefficient (400 times higher than that of Si) and high electrical conductivity (∼10⁴ times higher than that of Si).^20–27 However, practical implementation of Ge in LIBs is greatly hampered by poor cyclability and poor rate performance. The main reason for poor cyclability is the huge volume change (∼300%) caused by Li alloying/de-alloying reactions, resulting in both mechanical failure and loss of electrical contact at the electrodes. The other reason is the aggregation of Ge nanoparticles during charge–discharge process.^28,29 Recently, various Ge-based nanostructures (nanoparticles,^30–32 nanowires,^33–35 nanotubes,³⁶ 3D porous Au,⁸ and nanocomposites^29,37) have been designed to overcome these problems. Among these various approaches, dispersion of Ge nanoparticles in a carbon-based matrix (such as carbon nanotubes,³⁷ carbon nanofibers,³⁸ and graphene matrix³⁹) has been demonstrated as the most attractive way to improve the cycling performance and rate capability.

Graphene is a two-dimensional carbon honeycomb-like lattice with excellent thermal, mechanical and electronic properties, which makes graphene ideal to work as a conductive matrix to suppress large volume expansions resulting from the charge/discharge process of Ge nanoparticles.⁴⁰ Recently, various reduced graphene oxide (RGO)/Ge hybrid nanostructures have been synthesized as anodes for LIBs with improved electrochemical performance.^31,41 Compared to other carbon matrixes (graphite, carbon nanotubes, and carbon nanofibers), RGO matrix is a very good mixed conductor (conductive for e⁻ and Li⁺) as well as helpful in alleviating the huge volume expansion during the Li uptake and release process.⁴¹ To date, most Ge–RGO composites reported in literature were prepared by mixing RGO and Ge-base nanoparticles in solution phase, leading to a low-quality loading amount of Ge. In addition, the weak adhesion between the RGO and active materials (Ge nanoparticles) could easily result in poor capacity retention and short cycle life.

Herein, we report a facile, straightforward one-pot synthesis of Ge–RGO nanocomposites using organic germanium (OGe) as a precursor. As an anode material, the in situ synthesized Ge–RGO nanocomposites display improved cyclability (∼814 mA h g⁻¹ after 50 cycles at 0.1 A g⁻¹) and good rate capability (690 mA h g⁻¹ after 150 cycles at 2 A g⁻¹). The improved electrochemical performance of Ge–RGO nanocomposites can be attributed to the unique nanostructure (combination of 0D electroactive particles and 2D mixed conducting matrix) that offers low contact resistances, good electrochemical coupling via short transport length for ions and electrons, and excellent percolation mechanical de-coupling of particles, which prevents cracking as well as affords morphological stability.

Fig. 1 schematically illustrates the experimental procedures. In a typical synthesis, OGe–RGO nanocomposites were first synthesized using solvothermal reactions by mixing tetramethoxygermane (TMOG) precursor and graphene oxide (GO) with different mass ratios: 5 : 1 (OGe–RGO-5) and 10 : 1 (OGe–RGO-10) at 200 °C. Then, TMOG decomposed and chemical bonding occurred on both sides of the GO sheets in the presence of polyvinyl pyrrolidone (PVP). Moreover, the GO was synchronously reduced to RGO during the solvothermal reactions. Finally, Ge–RGO nanocomposites were obtained with a carbonization process conducted 650 °C in a 5% H₂/95% Ar atmosphere for 3 hours.⁴²

Fig. 1 Schematic illustration of the process for preparing the Ge–RGO nanocomposites. (1) Solvothermal reactions at a temperature of 200 °C and (2) carbonization of the OGe–RGO nanocomposites.

Fig. 2a and b show transmission electron microcopy (TEM) images of the OGe–RGO-5 nanocomposites and Ge–RGO-5 nanocomposites, respectively. In Fig. 2a, the OGe precursors densely and uniformly cover the surface of RGO sheets. After heating in an Ar/H₂ atmosphere, the OGe was transformed to Ge nanoparticles with a size of 30–90 nm (Fig. 2b), while the organic composition decomposed. A high-resolution transmission electron microscopy (HRTEM) micrograph (Fig. 2c) further reveals that the Ge nanoparticles are in close contact with RGO sheets. Some multi-layered RGO sheets (about 2–5 layers) were observed resulting from stacking of single sheets of RGO (Fig. 2c). Moreover, scanning transmission electron microscope-energy-dispersive X-ray spectroscopy (STEM-EDX) mapping (Fig. 2d) was performed to show that Ge precursors were fully transformed to Ge nanoparticles and uniformly distributed on the RGO sheets.

Fig. 2 TEM micrographs of the OGe–RGO-5 nanocomposites (a) and the Ge–RGO-5 nanocomposites (b). (c) HRTEM micrographs of the Ge–RGO-5 nanocomposites. (d) STEM-EDS mapping profiles of C (red) and Ge (yellow and orange) in the nanocomposites.

Fourier transform infrared (FTIR) spectroscopy was further used to characterize the OGe–RGO-5 nanocomposites and the Ge–RGO-5 nanocomposites (Fig. S1†). Along with the existence of two bands of C–O and C=C vibrations for the RGO and Ge precursors, the OGe–Ge nanocomposites show two typical bands of GeO₂ at 882 cm⁻¹ and 3340 cm⁻¹, which is in accordance with the results for commercial GeO₂.^38,42 After the carbonization process, the absence of GeO₂ peaks indicated that the Ge precursors were fully reduced to Ge nanoparticles.²⁹

X-ray diffraction (XRD) (Fig. S2†) characterization demonstrates that the obtained Ge–RGO-5 and Ge–RGO-10 products are mixtures of Ge and graphene. No GeO₂ phase was detected, which is in good agreement with the FTIR results. The peaks at 27.38°, 45.35°, and 53.78° can be indexed to (111), (220), and (311) reflections of the standard cubic phase Ge (JCPDS card no. 4-545), respectively. There are no obvious peaks corresponding to carbon and graphene in the XRD pattern, which indicates that the carbon in the sample is not well crystallized. Thermo gravimetric analysis (TGA) curves for the Ge–RGO-5 and Ge–RGO-10 nanocomposites show that the carbon weight contents are approximately 60.7% and 39.5%, respectively (see Fig. S3†).

The electrochemical performance of Ge–RGO-5 nanocomposites was evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge cycling between 0.005 and 1.2 V using a CR2032 coin cell. Fig. 3a shows the CV curves of the Ge–RGO-5 electrode at a scanning rate of 0.2 mV s⁻¹. During the first lithiation step, two peaks at 0.01 V and 0.62 V were observed, which are attributed to lithium alloying with Ge to form Li_xGe and the decomposition of electrolyte to form the solid-electrolyte-interphase (SEI), respectively.⁴³ One anodic peak at ∼0.5 V in the first charge step can be ascribed to the phase transition of Li_xGe to Ge.³⁴ After the first cycle, the presence of another three cathodic peaks at 0.5 V, 0.3 V and 0.01 V suggested that a number of different Li–Ge phases were formed during electrochemical lithiation.⁴³ Moreover, the difference between the first and the latter cycles can be ascribed to SEI formation and the local structure rearrangement in the electrode during alloying/de-alloying.⁴⁴

Fig. 3 (a) Cyclic voltammograms of the Ge–RGO-5 nanocomposites at a scan rate of 0.2 mV s⁻¹ (voltage range: 0.005 V to 1.2 V). (b) Capacity-cycle number curves of the Ge–RGO-5 and Ge–RGO-10 nanocomposite electrodes cycled between 0.005 V and 1.2 V vs. Li⁺/Li at a current density of 0.1 A g⁻¹. (c and d) Electrochemical performance of the Ge–RGO-5 nanocomposite electrodes cycled between 0.005 V and 1.2 V vs. Li⁺/Li. (c) Voltage profiles of the Ge–RGO-5 nanocomposites at a current density of 0.1 A g⁻¹; and (d) discharge capacity of the Ge–RGO-5 nanocomposites as a function of discharge current density (0.1 to 10 A g⁻¹).

We tested the cycling performance of Ge–RGO with different mass ratios of Ge and RGO. As shown in Fig. 3b, the Ge–RGO-5 nanocomposites exhibit a higher specific capacity and better stability than the Ge–RGO-10 nanocomposites. The Ge–RGO-5 nanocomposites show a higher reversible specific capacity of over 950 mA h g⁻¹ in the first 10 cycles and maintains a reversible capacity of ∼810 mA h g⁻¹ after 50 cycles at a current density of 0.1 A g⁻¹. In the case of the electrode made of the Ge–RGO-10 nanocomposites, it displays a discharge capacity (∼1000 mA h g⁻¹) for the first several cycles, and then the capacity fades rapidly. After 50 cycles, the Ge–RGO-10 nanocomposites maintain a reversible capacity of ∼400 mA h g⁻¹, resulting from the larger size of Ge nanoparticles (see Fig. S4†). In addition, the RGO delivers a reversible capacity of 280 mA h g⁻¹(see Fig. S5†). Fig. 3c displays the voltage profiles of the Ge–RGO-5 electrode under a current density of 0.1 A g⁻¹. The voltage profiles of the Ge–RGO-5 composite electrode exhibit the typical characteristics of the Ge anode.⁴⁵ The initial discharge and charge capacities of the Ge–RGO-5 electrode are 1731 and 1068 mA h g⁻¹, respectively. The large initial discharge capacity of the nanocomposite can be attributed to the formation of SEI films on the surface of the electrode and the high surface contact area between the graphene and electrolyte.^45–48 After the initial capacity loss, the reversibility of the capacity was significantly improved, with the coulombic efficiency reaching ∼97% from the second cycle. The rate capability, an important, challenging and key aspect in Ge anode operations, was also tested under the galvanostatic mode at various discharge–charge rates. As shown in Fig. 3d, the Ge–RGO-5 nanocomposites can deliver a reversible capacity of 950, 906, 851, 794, 633, and 335 mA h g⁻¹ at current densities of 0.2, 0.5, 1, 2, 5, and 10 A g⁻¹, respectively. When the current density recovers to 0.1 A g⁻¹, the specific capacity comes back to 963 mA h g⁻¹, indicating the good reversibility and stability of the Ge–RGO-5 nanocomposites.

To further examine the long-term stability of the Ge–RGO-5 nanocomposites, a cycling test for 150 cycles was carried out. Fig. 4a and b exhibit the excellent cycling stability of the Ge–RGO-5 at high current densities. To stabilize the electrodes, we first cycled the batteries for five cycles at a current density of 0.1 A g⁻¹ followed by a current density of 1 A g⁻¹ and 2 A g⁻¹. The capacity is 710 mA h g⁻¹ after 150 cycles at a current density of 1 A g⁻¹ and still 690 mA h g⁻¹ even after 150 cycles at a current density of 2 A g⁻¹. The improved rate capability of the Ge–RGO-5 nanocomposites could be attributed to the combination of 0D electroactive particles and 2D mixed conducting matrix, characterized by embedding nanosized electroactive nanoparticles in 2D carbon nanosheets of mixed conducting carbon.⁴¹ We also compared the current work with other graphene-Ge nanocomposite electrodes reported in the recent literature,^31,32,49,50 and these results are shown in ESI Table S1.† By comparison, the Ge–RGO-5 nanocomposites show superior lithium storage properties.

Fig. 4 (a and b) Cycle performance of Ge–RGO-5 electrodes at (a) 1 A g⁻¹ (b) 2 A g⁻¹ for 150 cycles. (The batteries were activated first at a current density of 0.1 A g⁻¹ for five cycles); (c) TEM micrograph of the Ge–RGO-5 electrode after 50 cycles between 0.005 V and 1.2 V vs. Li⁺/Li at a current density of 0.1 A g⁻¹.

To confirm the structure stability of the Ge–RGO-5 nanocomposites, we investigated the morphology of the Ge–RGO-5 electrodes after 50 cycles at a current density of 0.1 A g⁻¹. The nanostructure of Ge–RGO-5 nanocomposites remained almost unchanged (Fig. 4c). Notably, the surface of the Ge nanoparticles on the RGO sheets after cycling displayed a rough texture that could have resulted from the formation of a SEI layer.

In conclusion, we have successfully developed a facile method for preparing Ge-graphene nanocomposites as an anode for Li-ion batteries. When used as an anode material, the Ge–RGO-5 nanocomposites deliver a reversible capacity of ∼814 mA h g⁻¹ after 50 cycles at a current density of 0.1 A g⁻¹. Even after 150 cycles at high current densities of 1 and 2 A g⁻¹, they still retain reversible specific capacities of 710 and 690 mA h g⁻¹, respectively. The excellent cycling performance and rate capability could be attributed to the unique nanostructure (0D electroactive particles in 2D mixed conducting matrix), which conferred a variety of advantages: high flexibility of the graphene sheets for accommodating the volume change, good electrochemical coupling and a short transport length for ions and electrons, enabling low contact resistances. This facile method could provide an effective approach to improve the cyclability and rate capability of Ge-based anode electrode materials.

Experimental

Preparation of Ge–RGO nanocomposites

In a typical synthesis, 0.1 g graphene oxide (GO) (prepared by a modified Hummers method) and 0.1 g polyvinyl pyrrolidone (PVP) were transferred to a Teflon-lined stainless-steel autoclave filled with 30 ml N,N-dimethylformamide (DMF). Subsequently, the mixture was sonicated until GO was homogeneously dispersed in the solution. Then, different weights of tetramethoxygermane (TMOG) were added to the mixture to control the Ge loading (TMOG : GO = 5 : 1 and 10 : 1). After solvothermal treatment at 200 °C for 36 hours, the products (OGe–RGO nanocomposites) were collected after centrifugation, then thoroughly washed three times with ethanol, and finally dried at 75 °C. The Ge–RGO nanocomposite was obtained via calcination of the OGe–RGO nanocomposite at 650 °C for 3 h under a 5% H₂/95% Ar atmosphere. The different weights of tetramethoxygermane (TMOG) were added to the mixture to control the Ge loading (TMOG : GO = 5 : 1 and 10 : 1). The yield of Ge–RGO nanocomposites obtained via this method is about 60%.

Characterization

The structure of the Ge–RGO nanocomposite was characterized by X-ray diffraction (XRD) (TTR-III, Rigaku, Japan) using Cu Kα radiation. Fourier transform infrared (FTIR) spectra were recorded using a Thermo Nicolet 8700 FTIR spectrophotometer. A JEOL 4000EX transmission electron microscope (HRTEM) (JEOL, Tokyo, Japan) was used to study the morphology and microstructure of the OGe–RGO and Ge–RGO nanocomposites.

Electrochemical characterization

The Ge–RGO nanocomposite electrodes were prepared by mixing the active materials, carbon black and poly vinylidene fluoride (PVDF) at a weight ratio of 80 : 10 : 10 and spread onto copper foil. The loading mass of the working electrodes was about 0.8 mg cm⁻². The coin type half-cells (CR2032) were prepared in an argon-filled glovebox. The electrolyte was 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) (1 : 1 = w/w), and the Celgard 2400 membrane was used as a separator. Cycle voltammetry measurements were conducted at a scan rate of 0.2 mV s⁻¹ on a CHI 660D electrochemical workstation (Chenhua Instrument Company, Shanghai, China).

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21171015 and no. 21373195), the Recruitment Program of Global Experts, the program for New Century Excellent Talents in the University (NCET-12-0515), the Fundamental Research Funds for the Central Universities (WK2060140014, WK2060140016), the Collaborative Innovation Center of Suzhou Nano Science and Technology and the Sofja Kovalevskaja Award from the Alexander von Humboldt Foundation.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08797f

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