Article pubs.acs.org/Langmuir
Amorphous Ge/C Composite Sponges: Synthesis and Application in a High-Rate Anode for Lithium Ion Batteries Qiuyang Ma, Wanwan Wang, Peiyuan Zeng, and Zhen Fang* Key Laboratory of Functional Molecular Solids, Ministry of Education, Center for Nano Science and Technology, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, P. R. China S Supporting Information *
ABSTRACT: A Ge/C spongelike composite is prepared by the facile and scalable single-step pyrolysis of the GeOx/ ethylenediamine gel process, which has a feature with three-dimensional interconnected pore structures and is hybridized with nitrogen-doped carbon. A detailed investigation shows that the pore in the sponge is formed for the departure of the gaseous products at the evaluated temperature. As an anode for lithium ion batteries, the obtained composite exhibits superior specific capacity in excess of 1016 mA h g−1 at 100 mA g−1 after 100 cycles. Moreover, the amorphous Ge/C sponge electrode also has a good rate capacity and stable cycling performance. The obtained amorphous Ge/C sponges are a good candidate anode for nextgeneration lithium ion batteries.
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INTRODUCTION The technological advancement of lithium ion batteries (LIBs) has governed the energy storage devices for portable electronics and electric vehicles.1−5 Currently, the available commercial anodes in LIBs are graphite. However, graphite has a specific capacity of 372 mA h g−1, which significantly limits the desire for second batteries with higher energy densities and higher power densities.6−9 In recent years, germanium (Ge) has attracted intensive interest as a promising potential alternative anode material for LIBs because of its high gravimetric theoretical capacity, good lithium diffusivity, and low charge/ discharge potential.10−14 Nevertheless, the bulk Ge anode has an essential attribute of considerable volume change (about 370%) during Li alloying/dealloying reactions, which leads to severe pulverization of the electrode and the loss of the electrical interphase contact, thereby resulting in poor cyclability and rapidly declining capacity upon prolonged cycling.15−19 To buffer such drastic volume changes in Ge anodes, numerous effects have been achieved. Yu and coworkers have reported the Ge/C nanowires that can effectively relax the volume strain and provide channels for electron transport.20 Kim et al. synthesized amorphous Ge/GeO2/ carbon that exhibited a high reversible capacity of 1631 mA h g−1 with superior capacity retention after 90 cycles at 0.5 A g−1.21 The above-mentioned reports suggest that various types of nanostructures (e.g., nanoparticles, nanowires, thin films, nanotubes, and porous architectures) and the use of a conductive matrix such as carbonaceous materials can effectively reduce the degree of volume change and improve the cycling stability.4,13,21−30 However, the established synthesis © XXXX American Chemical Society
process for Ge is usually rather complicated, tedious, and costly. For instance, Ge embedded in a conductive matrix such as carbon nanotubes or graphene is too costly to generate for large-scale practical applications. Given this, we design a cost-effective approach to preparing amorphous Ge/C spongelike materials by the controlled pyrolysis of GeOx/ethylenediamine (GeOx/EDA) gel. It should be noted that it is the first spongelike Ge/C structure as far as we know. The obtained structure has several advantages for achieving excellent electrochemical performance. First, spongelike porous materials possess favorable diffusion pathways, which lead to the convenient transportation of charge and electrolyte31 and result in much-improved rate performance. Second, the spongelike structure with uniform distributed pores balances the mechanical stress caused by the huge volumetric fluctuation during the lithiation/delithiation process, which improved the cyclic performance. The amorphous Ge/C sponge electrode possesses a superior reversible specific capacity (1016 mA h g−1 at 100 mA g−1 after 100 cycles), exceptional rate capability (733 mA h g−1 at 10 A), and stable cycling performance (824 mA h g−1 for up to 200 cycles at 200 mA g−1). Synthesis of Amorphous Ge/C Sponges. Commercial GeO2 (0.5 g) was dispersed in distilled water, and then 0.25 mL of ethylenediamine was introduced. A colorless and transparent solution was obtained. GeOx/EDA gel was obtained after aging Received: December 12, 2016 Revised: February 3, 2017 Published: February 14, 2017 A
DOI: 10.1021/acs.langmuir.6b04444 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir Scheme 1. Schematic Illustration of the Fabrication Process of Amorphous Ge/C Sponges
Figure 1. (a) TG curve of the dried GeOx/EDA gel under an N2 atmosphere. (b) XRD patterns of Ge/C sponges collected at different temperatures: Ge/C-500, Ge/C-600, and Ge/C-700. Panoramic SEM image of (c) a dried GeOx/EDA gel and (d) Ge/C-600. (e, f) Highmagnification surface image marked by rectangles in (c) and (d).
reduction was used to prepare Ge/C sponges. Scheme 1 shows a brief schematic illustration of the synthesis procedure for the amorphous Ge/C sponges. The ethylenediamine used in the first step can serve not only as the chelating agent but also as the carbon source for the carbothermic reduction. The resulting solid, called a GeOx/EDA gel, was used as a precursor that was subjected to thermal decomposition and deoxidization at high temperature. The homogenous porous structure obtained here is attributable to the effective release of gas (H2O, NH3, and alkane) during the pyrolysis of the precursor, which could be detected easily by ordinary litmus paper and KMnO4 aqueous solution in the tail gas.34 Figure 1a gives a TG curve of the dried GeOx/EDA gel. The weight loss of 18.87% under 200 °C could be assigned the dehydration of the water existing in the gel. The decomposition of the gel mainly takes place at 200−500 °C with a weight loss of 11.75%. The mass tends toward a constant value, and a
the solution overnight at room temperature. The gel was dried in an electric oven at 60 °C for 5 h to obtain a white dry gel. For the carbothermic reduction of GeOx/EDA and to form amorphous Ge/C sponges, the dried gel was pyrolyzed at 600 °C for 1 h in N2 at a heating rate of 2 °C min−1, and the obtained sample was denoted as Ge/C-600. For comparison, the dried gel was also pyrolyzed at 500 and 700 °C with other conditions keep constant. The obtained samples are denoted as Ge/C-500 and Ge/C-700, respectively. The material characterization and electrochemical measurement sections are described in the Supporting Information.
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RESULTS AND DISCUSSION Previous reports suggest that Ge nanoparticles were prepared by the reaction between GeBr2 and oleylamine32 or the solvothermal reduction by NaBH4.33 In this paper, the organic−inorganic chelation reaction and subsequent thermal B
DOI: 10.1021/acs.langmuir.6b04444 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir residue of 67.50% is obtained at 700 °C. The XRD profiles of the sample obtained at different temperatures are shown in Figure 1b. The obtained Ge/C product exhibited a broad diffraction peak at lower pyrolysis temperature (
