Article pubs.acs.org/JPCC
Enhancement of the Cyclability of a Si Anode through Co3O4 Coating by the Sol−Gel Method Yoon Hwa, Won-Sik Kim, Byeong-Chul Yu, Seong-Hyeon Hong, and Hun-Joon Sohn* Department of Materials Science and Engineering, Seoul National University, Seoul, 151-744, Korea ABSTRACT: A nano-Si/Co3O4 core−shell nanostructured material has been successfully prepared using the sol−gel method. The reaction mechanism between Li and the nano-Si/ Co3O4 core−shell nanostructured electrode was investigated by ex situ analyses. Metallic Co, Li21Si8, and amorphous Li2O phases were formed when fully lithiated, and an amorphous Si, Co3O4, and Co phases were identified after charge, indicating that the Co3O4 was partially reversible. The nano-Si/Co3O4 core−shell nanostructured electrode showed excellent cyclability, with a reversible capacity of ca. 850 mAh/g over 100 cycles.
1. INTRODUCTION In recent years, Si has become a powerful candidate as an anode material for Li-ion batteries to replace the conventional graphite anode, due to the demand for high power and high energy density.1−12 As is generally known, a Si electrode can store a large amount of Li (Li15Si4: 3579 mAh/g at room temperature) and exhibits low operating voltage (0.4−0.5 V vs Li/Li+).1 In addition, Si has many advantages for an electrode material, such as its abundant, cheap, and environmentally benign nature. However, a large volume change of up to 300% during cycling leads to pulverization of Si, which is a critical reason for poor cycle performance.1 To solve the problem, various unique nanostructures have been synthesized for Sibased electrodes, such as nanowires,3,7 core−shell structures,8−10 and hollow spheres.11,12 These nanostructured Si electrodes have been shown to successfully alleviate the stress caused by volume change during cycling, and show relatively excellent electrochemical properties. Among nanostructures, the core−shell structure is one of the promising nanostructures for electrode materials.8−10 The core−shell structure consists of an inner core material surrounded by other material as the shell, which has been a traditional concept of the semiconductor field.13 In general, the core material is the main component of the electrode, while the shell material acts as a protection layer, to keep the electrochemical performance of the core material, or to provide a new property. Shell materials also prevent active cores from contacting the electrolyte directly, to avoid or restrict unnecessary side reactions. Poor electrical conductivity, or large volume change during cycling of the core material, could be overcome by this structure.13 Since Poizot et al.14 reported a nanosized transition-metal oxide as an anode material for Li-ion batteries, many studies for applying transition-metal oxide to anode core material have been conducted, such as iron oxide,15−17 cobalt oxide,18−29 nickel oxide,30−32 and copper oxide.33−35 Among these © 2013 American Chemical Society
transition-metal oxides, Co3O4 is a representative material as an anode material for Li-ion batteries, due to its high theoretical capacity (890 mAh/g), and stable capacity retention.14−29 The theoretical capacity of a Co3O4 electrode is based on the conversion reaction, and its reaction chemistry is21 Co3O4 + 8Li+ + 8e− → 4Li 2O + 3Co
(1)
In this study, facile preparation of the nano-Si/Co3O4 core− shell nanostructured material has been successfully conducted using the sol−gel method, from which the coating of a structurally stable Co3O4 shell on the Si nanoparticle can help enhance cycle retention of the nano-Si electrode. Recently, a similar approach employing Co3O4 nanoparticles grafted on a Si nanowire electrode for Li-ion batteries was reported by Sun et al.36 However, the cutoff voltage during discharge was set at 0.4 V to study the electrochemical behaviors of Co3O4, and Si did not contribute the capacity. The reaction mechanism for the nano-Si/Co3O4 core−shell nanostructured electrode was investigated by ex-situ X-ray diffraction (XRD), and highresolution transmission electron microscopy (HRTEM), with selected area electron diffraction (SAED).
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The nano-Si/Co3O4 core−shell nanostructured material was prepared using the following procedure. Commercially available nano-Si particles (0.075 g, Aldrich,
