Article pubs.acs.org/cm
Reversible Storage of Lithium in Three-Dimensional Macroporous Germanium Haiping Jia, Richard Kloepsch, Xin He, Juan Pablo Badillo, Pengfei Gao, Olga Fromm, Tobias Placke,* and Martin Winter* MEET Battery Research Center, Institute of Physical Chemistry, University of Münster, Corrensstr. 46, 48149 Münster, Germany S Supporting Information *
ABSTRACT: In this work, the preparation of novel macroporous germanium (p-Ge) and its electrochemical characterization as anode material for lithium-ion batteries is presented. Three-dimensional (3D) macroporous germanium particles with a hexagonal-like morphology were successfully prepared using a magnesiothermic reduction method, in which GeO2 serves not only as the template, but also as the germanium source. The obtained material demonstrates uniform pores within the particles, which serve as buffer zone to effectively accommodate the big volume changes of germanium during electrochemical lithiation and gives rise to an improved electrochemical performance. The p-Ge anode delivers not only a high reversible capacity of 1131 mAh g−1 at a rate of 1 C after 200 cycles, but also a high rate capability with a capacity of 717 mAh g−1 at 5 C. The capacity retention for charge/discharge cycling of more than 96% after 200 cycles is also remarkably improved, compared to nonporous Ge materials. commercialization of this anode material.8 Furthermore, as with silicon, tin, and their derivatives15,16 severe particle pulverization can be triggered by a large volume change during the lithium alloying (to form LixGe) and dealloying processes (to reform Ge), which results in electronically disconnected germanium particles as well as a continuous reactivity of the anode with the electrolyte. As a result, the electrode suffers from a rapid capacity fading upon cycling. Many approaches have been pursued to accommodate the volume changes of germanium, such as preparation of Ge/ carbon composites to improve the electronic conductivity and prevent the breakdown of the particles.9,17 However, these methods are at the expense of the specific capacity, which is less than 1200 mAh g−1, to improve the cycling performance. Furthermore, control of the volume change was attempted, by designing a special morphology of germanium (such as, e.g., nanoparticles,12 nanowires,18 or nanotubes).19 In addition, the fabrication of Ge-based alloy composites, like Ge/Sn nanocomposites, or composite heterosturctures such as Sn78Ge22/ carbon structures,13,20 is a promising approach for improving the cycling performance. In recent years, more attention has been paid to these anode materials to avoid the common problems of ultrafine powders (especially those with a particle size of less than 100 nm), such as a strong aggregation tendency, which results in increased lithium ion diffusion pathways, their difficulty to handle and an enhanced binder requirement leading to a lower energy density. The basic strategy is that a porous structure can offer sufficient
1. INTRODUCTION Currently, green energy has drawn the worldwide attention again as the nonrenewable energy sourcesmainly fossil fuels, such as petroleumare depleting. The further development of the lithium-ion battery as one of the clean and convenient sources of energy is now of great demand.1,2 Rechargeable lithium-ion batteries have revolutionized the small format battery market for portable electronic devices, such as smart phones or laptop computers, in the 1990s. However, in the case of emerging electric transportation systems, including hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and zero emission electric vehicles (EVs), state-ofthe-art lithium-ion batteries still exhibit an insufficient low specific energy, which severely limits the range of electric vehicles.1,3 In order to further increase the specific energy of lithium-ion batteries, the development of high capacity electrode materials, in particular anode materials, is of great interest. The current carbonaceous anode materials cannot satisfy the demand of a high specific capacity, since, e.g., commercial graphite only displays a theoretical capacity of 372 mAh g−1.4 In particular, germanium (Ge) has received much attention when compared to other alloy anode materials such as silicon5,6 or tin,7,8 because of its still sufficiently high theoretical capacity of 1600 mAh g−1 (4.4 Li+ ions per Ge atom), good lithium ion diffusivity (more than 400 times higher than in silicon at room temperature: 1.41 × 10−14 cm2 s−1 for Si and 6.51 × 10−12 cm2 s−1 for Ge), high electronic conductivity (104 times higher than silicon)9−11 and a lower specific volume change during the lithium insertion/extraction process.12−14 Nevertheless, compared to silicon, tin, or other lithium storage metals, the high price of germanium is the major drawback for the © XXXX American Chemical Society
Received: July 10, 2014 Revised: September 10, 2014
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dx.doi.org/10.1021/cm5025124 | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
compared to the self-prepared porous germanium in the electrochemical investigations. In addition, with the view of comprehensive comparison, we also reduced GeO2 to obtain germanium directly by Ar/H2 (95/5 vol %) atmosphere at 650 °C for 10 h (hereafter abbreviated as “Ge (Ar/H2)”, BET surface area = 4.35 m2 g−1). Furthermore, commercial GeO2 (Sigma−Aldrich, BET specific surface area = 0.14 m2 g−1; average particle size = 1 μm) was also introduced to get germanium (hereafter abbreviated as “Ge (comm-GeO2)”, BET surface area = 4.78 m2 g−1) via a magnesiothermic reduction method. All characterizations and electrochemical studies of these last two materials will be demonstrated in the Supporting Information. 2.3. Structure and Morphology Characterization. X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Avance X-ray diffractometer (Bruker AXS GmbH) equipped with a copper target X-ray tube (radiation wavelength: λ = 0.154 nm). The morphologies of the samples were observed by a field-emission scanning electron microscope (FESEM, JEOL JSM-7401F). Transmission electron microscopy (TEM, JEOL JEM-100CX) was used to investigate the microstructures of GeO2 and Ge. The BET specific surface area has been determined by nitrogen adsorption measurements using an ASAP 2020 (Accelerated Surface Area and Porosimetry Analyzer, Micromeritics GmbH). Before the measurement, the samples were degassed at 120 °C until a static pressure of
