© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 6250 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Hierarchical Hollow Spheres of Fe 2 O 3 @Polyaniline for Lithium Ion Battery Anodes Jae-Min Jeong, Bong Gill Choi,* Soon Chang Lee, Kyoung G. Lee, Sung-Jin Chang, Young-Kyu Han, Young Boo Lee, Hyun Uk Lee, Soonjo Kwon, Gaehang Lee, Chang-Soo Lee,* and Yun Suk Huh* Lithium ion batteries (LIBs) have mainly been employed as energy sources for portable electronics, and are now expanding their applications to a larger scale, particularly electric vehi- cles and grid storage. [1–3] Ever increasing demand in such applications has stimulated significant interest in the devel- opment of energy storage electrode materials. [4–9] In this con- text, α-Fe 2 O 3 is a non-toxic, low cost material and has a higher theoretical capacity value (1007 mAh g –1 ) than commercial graphite (372 mAh g –1 ). Thus, it has been considered as one of the most promising anode candidates. [10–12] Significant capacity fading, however, is still observed at both high rate and long-term charge/discharge cycling due to the large specific volume changes during battery cycling and kinetic limitations of its intrinsic nature. [13,14] The ease of fabrication of electrode materials that can be mass produced at low cost is also of great interest on an industrial scale. Hierarchical complexity, along with large surface area, a short diffusion length, and good mechanical integrity, has been appealing as an attractive structure for the design and fabrica- tion of electrode materials to overcome the current issues and challenges. [15–18] Recently, Lou et al. synthesized an urchin-like hollow structure of iron oxides and reported their improve- ment in the cycling life compared to a non-hollow struc- ture. [10,19,20] Despite this previous effort, the rate capability still remains insufficient to fully realize its potential in LIBs due to the intrinsically poor electronic conductivity of iron oxides. Construction of a core-shell structure, particularly carbon coating of the inner- or outer walls of active materials, could be a good approach to increasing the conductivity of the entire electrodes. [21,22] As a consequence, the hierarchical core-shell hollow structure of iron oxide@carbon can be expected to allow not only the efficient and rapid transfer of ion and electron, but also the accommodation of volume expansion during cycling. However, it is still a challenging task to develop a simple and reliable method for hierarchical core-shell hollow structures with a controlled size, morphology, and composition under mass manufacture. Most template-assisted methods for hollow structures often require a complicated experimental procedure of the preparation/removal of templates and suffer from partial structural collapse during the removal of templates. [20,23] The thermal decomposition of carbon precursors, which is used widely for carbon coatings, causes environmental problems because of the formation of volatile organic compounds, CO and CO 2 , and sometimes leads to the inferior reconstruction (i.e., structure, size, and phase) of materials. [24,25] Herein, we developed a simple and scalable approach for the fabrication of hierarchical hollow spheres of Fe 2 O 3 @poly- aniline (PANI) using a template-free method of iron oxides fol- lowed by a post PANI coating process. A conducting polymer coating allows us to an efficient approach to improving electron transport in electrodes because of its easy and environmental processability as well as the unique electrical properties of con- ducting polymers. [25,26] Of particular interest, we found that polymerization of aniline monomers using HCl solution leads to the simultaneous etching of iron oxides and PANI coating process, resulting in the successful formation of heterogeneous and hollow structures of Fe 2 O 3 @PANI. By using as-prepared composites, we were able to demonstrate their excellent electro- chemical characteristics as an anode for LIBs, such as improved large reversible capacity, rate capability, and long-term cycling stability, compared to the urchin-like Fe 2 O 3 . DOI: 10.1002/adma.201302710 [+] These authors contributed equally to this work. [†] Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States J.-M. Jeong, [+] Prof. C.-S. Lee Department of Chemical Engineering Chungnam National University Daejeon, 305–764, Republic of Korea E-mail: rhadum@cnu.ac.kr Dr. B. G. Choi, [+][†] Dr. S.-J. Chang, Dr. H. U. Lee, Dr. G. Lee Division of Materials Science, Korea Basic Science Institute Daejeon, 305–333, Republic of Korea E-mail: k1811@kaist.ac.kr S. C. Lee Department of Fine Chemical Engineering and Applied Chemistry Chungnam National University Daejeon, 305–764, Republic of Korea Dr. K. G. Lee Department of Chemical Engineering University of Michigan Ann Arbor, Michigan, 48109, United States Prof. Y.-K. Han Department of Energy and Materials Engineering Dongguk University-Seoul Seoul, 100–715, Republic of Korea Dr. Y. B. Lee Jeonju Center, Korea Basic Science Institute Jeonju, 561–180, Republic of Korea Prof. S. Kwon, Prof. Y. S. Huh Department of Biological Engineering Biohybrid Systems Research Center(BSRC) Inha University Incheon, 402–751, Republic of Korea E-mail: yunsuk.huh@inha.ac.kr Adv. Mater. 2013, 25, 6250–6255