Materials Chemistry and Physics xxx (2015) 1e4
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Facile synthesis of Ge/C nanocomposite as superior battery anode material Wei Guo a, *, Lin Mei b, **, Qingqin Feng a, Jianmin Ma b, c a
College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455002, China Key Laboratory for Micro-/Nano-Optoelectronic Devices of the Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China c Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522, Australia b
h i g h l i g h t s Ge/C nanocomposite was synthesized through annealing the precursors GeO2 and polyvinylpyrrolidone (PVP) in Ar/H2 atmospheres. Ge/C nanocomposite delivered a reversible specific capacity of 530 mAh/g after 100 cycles at a current density of 200 mA/g. The excellent electrochemical performance of Ge/C nanocomposite was attributed to the carbon layers structure.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2015 Received in revised form 26 October 2015 Accepted 9 November 2015 Available online xxx
Germanium (Ge) is a high capacity anode material for lithium-ion batteries (LIBs), but still suffers from poor cyclability and stability due to the huge volume change during lithium-ion intake/remove process. In this work, we present a synthetic method for preparing Ge/C nanocomposite through annealing the precursors GeO2 and polyvinylpyrrolidone (PVP) in Ar/H2 atmospheres. The as-synthesized Ge/C nanocomposite delivers a reversible specific capacity of 530 mAh/g after 100 cycles at a current density of 200 mA/g, and a high rate capability due to its composite structure. The excellent electrochemical performance can be attributed to the carbon layers structure, which not only provides the room for accommodation of volume expansion, but also enhances the electronic conductivity of the electrode. Such Ge/C nanocomposite are promising as a potential anode material for LIBs. © 2015 Elsevier B.V. All rights reserved.
Keywords: Composite materials Heat treatment Electron microscopy Thermogravimetric analysis Microstructure
1. Introduction In the past decade, high-capacity anode materials have been intensively studied for constructing high-energy-density LIBs, in portable electronics [1]. Great progress has been made in the design, synthesis and construction of anode materials to replace commercial graphite anode with a theoretical capacity of 372 mAh/ g [2e9]. A varies of high-capacity anode materials have been well developed, such as silicon (Si, 4200 mAh/g), tin (Sn, 993 mAh/g) and Ge (1623 mAh/g) to replace the graphite anode [10e15]. Among them, Ge has attracted intensive attention due to its high
* Corresponding author. ** Corresponding author. E-mail addresses:
[email protected] (W. Guo),
[email protected] (L. Mei).
capacity, higher conductivity, and the lithium-ion diffusivity relative to silicon [16]. In addition, Ge also exhibits a lower specific volume change during the Li insertion/extraction process than Si, which leads to better cycling performance [17]. Therefore, it is meaningful to study Ge-based anodes for high-capacity LIBs. Pure Ge suffers from the poor capacity retention and rate capability, which hinders the applications of the Ge anode materials in LIBs. The main reason for the poor electrochemical performance is attributed to the significant volume change during charge/ discharge processes, resulting in both mechanical failure and loss of electrical contact at the current collector [18]. To address the question, a varies of strategies have been used for improving their cycling and rate capabilities, such as coating layer [19,20], composition with other materials [21e23], the design of electrode structures, and shape control of nanomaterials [24e26]. Among them, as a buffer of Ge particle volume change, carbonaceous materials could minimize the mechanical stress to overcome the
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pulverization and provide both Liþ and electron transport paths [17,27,28]. Thus, it is of interest to synthesize Ge-based material/ carbon composites as advanced anode materials. Recently, we synthesized porous amorphous Ge/C with excellent electrochemical performances using oleic acid as the carbon precursor [28]. In this work, we have facilely synthesized Ge/C nanocomposite using a simple method with GeO2 and PVP as precursors via an annealing process. The as-synthesized Ge/C nanocomposite has exhibited a high capacity of 530 mAh/g after 100 cycles at a current density of 200 mA/g. In addition, they also showed excellent rate capabilities, delivering an average discharge capacity of 550 mAh/g at a current density of 400 mA/g, 455 mAh/g at 800 mA/g, and even 287 mAh/g at 1600 mA/g. The good electrochemical performance is attributed to the high conductivity transport of electron along the carbon layers. Meanwhile, the carbon layers can effectively buffer the huge volume expansion during cycling. 2. Material and methods All the chemicals were of analytical grade and used as received without further purification. In a typical synthetic process, 1 g PVP10, 2 g GeO2 and 2 ml ethylenediamine (en) were mixed together for half an hour. Then, the mixture was heated in Ar at 650 C for 4hrs with a heating rate of 5 C/min. The as-obtained sample was directly characterized and used as anode materials without any post-treatment. 2.1. Characterization The microstructure of the as-prepared samples was characterized by X-ray diffraction (XRD; GBC MMA diffractometer) with Cu Ka radiation at a scanning rate of 2 /min. Thermogravimetric analysis (TGA) of the Ge/C composite was carried out with a TGA/ DSC1 type instrument (METTLER TOLEDO, Switzerland) at a heating rate of 5 C/min from 25 to 600 C in air. The morphology of the Ge/C composite was evaluated using a JEOL JEM-ARM200F transmission electron microscope (STEM, JEOL, Tokyo, Japan). Energy dispersive X-ray spectroscopy (EDX, JEOL 7500FA) was used to confirm the C, Ge and N contents. 2.2. Electrochemical measurements The electrode was fabricated, and composed of 80 wt% Ge/C nanocomposite, 10 wt% carbon black and 10 wt% carboxymethylcellulose (CMC) binder. Prior to the assembly, the electrode was dried in a vacuum oven at 80 C over 12 h. Electrochemical cells (CR2032 coin type) using the active materials as working electrode, Li foil as the counter electrode and reference electrode, a microporous polypropylene film as the separator, and 1 M LiPF6 in a 1:1 (v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte were assembled in an Ar-filled glove box (H2O, O2 < 0.1 ppm, Mbraun, Unilab, USA). The cells were galvanostatically charged and discharged over a voltage range of 0.02e3 V versus Li/Liþ at different constant current densities based on the weight of the samples on a Land CT2001A cycler. Cyclic voltammetry (CV) was performed on an Ametek PARSTAT®2273 electrochemistry workstation. 3. Results The preparation process of Ge/C nanocomposite could be easily realized. GeO2 and PVP were firstly mixed manually for half an hour, and then annealledat 650 C for two hours to obtain the Ge/C nanocomposite. For the first step, GeO2 could be dispersed around
PVP molecular. During the annealing step, PVP could be carbonized into carbon, with the reduction of GeO2 into Ge in Ar/H2 at high temperature. This method could afford the complete reduction of GeO2 to Ge, confirmed by the XRD pattern of the as-synthesized nanocomposite (Fig. 1a, JCPDS Card no. 04-0545). It should be noted that there is no carbon peaks in the pattern, indicating that carbon is amorphous [29]. Thermogravimetric analysis (TGA) in Fig. 1b results indicated the Ge amount in the Ge/C composite is 68.3%, calculated from the weight loss upon carbon combustion and that Ge is fully oxidized to GeO2 in air [30]. The morphology and structure of the as-synthesized Ge/C nanocomposite were further characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) attaching to TEM. Fig. 2a shows a typical TEM image of the Ge/C nanocomposite, indicating Ge nanoparticles are well immersed in carbon. HR-TEM image in Fig. 2b further shows the amorphous carbon layer has a thickness of about 3 nm. Fig. 2c shows a high-angle annular dark field scanning TEM (HAADFSTEM) image of the as-synthesized Ge/C nanocomposite. The sample was further mapped through TEM and energy-dispersive Xray spectroscopy (EDS) by displaying the integrated intensity of C, Ge and N signals as a function of the beam position when operating the TEM in scanning mode (STEM). The element mapping results (Fig. 2def) reveal that the C, Ge and N are homogeneously distributed in Ge/C composite. The ratio of C: Ge: N in weight is about 30.55: 68: 1.45. The facile synthetic method is easily used to scale up the preparation of the Ge/C nanocomposite. Combined with the structural characteristics of the Ge/C nanocomposite, the assynthesized Ge/C nanocomposite is promisingly applied as nextgeneration anode material for advanced LIBs. Thus, it is necessary to study the electrochemical performance of the as-synthesized Ge/ C nanocomposite for LIBs. Fig. 3a shows the charge/discharge curves of the as-synthesized Ge/C nanocomposite electrode at 200 mA/g in the voltage range of 0.01e3.0 V vs. Li/Liþ. The shape of the discharge/charge profiles for the 1st, 2nd, 10th, 30th, 50th and 100th cycles is similar to those of Ge/C composites in reported literatures [17,27,28]. The initial discharge and charge capacities of the Ge/C nanocomposite electrode are 1793 and 871 mAh/g, respectively. The initial discharge capacity is even higher than the theoretical value of Ge (about 1600 mAh/g). Additionally, the discharge capacity decreases rapidly from the 1st to the 10th cycle. The high irreversible capacity in first ten cycles should be attributed to the decomposition of the electrolyte and the formation of the SEI film on the carbon matrix [25e27]. After 10 cycles, the Ge/C nanocomposite electrode keeps relatively stable, as shown in Fig. 3a. Fig. 3b shows the cycling performance of the Ge/C nanocomposite electrode. The data in Fig. 3b further indicates that the charge/discharge capacities after ten cycles keep stable, and no significant decrease is observed. Fig. 3c shows the rate capability of the Ge/C nanocomposite electrode, which presents a capacity-decreasing trend with increasing current rates. When the current density gradually increases from 200 mA/g to 1600 mA/g, the capacity of the Ge/C nanocomposite electrode drops from 638 mAh/g to 283 mAh/g. When the current density returns to 200 mA/g, the specific storage capacity of the Ge/C nanocomposite electrode can reach 589 mAh/ g. Fig. 3d further shows the average discharge capacities at various current densities. As shown in Fig. 3d, the average discharge capacities are 650, 550, 455 and 283 mAh/g at current densities of 200, 400, 800, 1600 mA/g, respectively. These results indicate that the Ge/C nanocomposite synthesized by our method is superior. The excellent electrochemical performance can be attributed to the carbon layers structure, which provides the room for the accommodation of volume expansion. Meanwhile, the carbon layers on
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(a)
(b) 100
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Fig. 1. (a) XRD pattern and (b) TGA curve of the as-synthesized Ge/C nanocomposite.
Fig. 2. (a) TEM image; (b) HR-TEM image (c) HAADF-STEM image of the Ge/C composite; (def) Corresponding C, Ge and N elemental maps.
Fig. 3. (a) Charge/discharge curves of the Ge/C nanocomposite electrode at a current density of 200 mA/g for various cycles; (b) Cycling performance of the Ge/C nanocomposite electrode at a current density of 200 mA/g; (c) Rate performance of the Ge/C nanocomposite electrode at various current densities; (d) Corresponding discharge capacity at various current densities.
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the surfaces of the Ge nanoparticles enhances the electronic conductivity of the electrode. 4. Conclusion In summary, we have facilely prepared the Ge/C nanocomposite via an annealing process, using GeO2 and PVP as precursors in Ar/ H2 atmospheres. The electrochemical results showed that the assynthesized Ge/C nanocomposite could maintain a discharge capacity of 530 mAh/g after 100 cycles at a current density of 200 mA/ g, and have excellent rate capability with a discharge capacity 550 mAh/g at a current density of 400 mA/g, 455 mAh/g at 800 mA/ g, and even 287 mAh/g at 1600 mA/g. The as-synthesized Ge/C composite could be considered as a potential anode material for LIBs. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51302079). References [1] N. Nitta, G. Yushin, Part. Part. Syst. Charact. 31 (2014) 317e336. [2] S. Goriparti, E. Miele, F.D. Angelis, E.D. Fabrizio, R.P. Zaccaria, C. Capigli, J. Power Sources 257 (2014) 421e443. [3] L.W. Ji, Z. Lin, M. Alcoutlabi, X.W. Zhang, Energy Environ. Sci. 4 (2011) 2682e2699. [4] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Chem. Rev. 113 (2013) 5364e5457. [5] X. Li, J.T. Xu, Z.J. Zhang, L. Mei, C.Y. Cui, H.K. Liu, J.M. Ma, S.X. Dou, J. Mater. Chem. A 3 (2015) 3257e3260. [6] Y. Cai, Xiu Li, L. Wang, H.Y. Gao, Y.N. Zhao, J.M. Ma, J. Mater. Chem. A 3 (2015) 1396e1399. [7] T. Lv, X. Li, J.M. Ma, RSC Adv. 4 (2014) 49942e49945. [8] G.D. Du, Z.P. Guo, S.Q. Wang, R. Zeng, Z.X. Chen, H.K. Liu, Chem. Commun. 46
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Please cite this article in press as: W. Guo, et al., Facile synthesis of Ge/C nanocomposite as superior battery anode material, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.11.016