© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1348 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Hee-Dae Lim, Kyu-Young Park, Hyelynn Song, Eui Yun Jang, Hyeokjo Gwon, Jinsoo Kim, Yong Hyup Kim, Márcio D. Lima, Raquel Ovalle Robles, Xavier Lepró, Ray H. Baughman, and Kisuk Kang* Enhanced Power and Rechargeability of a Li -O 2 Battery Based on a Hierarchical-Fibril CNT Electrode H.-D. Lim, K.-Y. Park, H. Gwon, J. Kim, Prof. K. Kang Department of Materials Science and Engineering Seoul National University 1 Gwanak-ro, Gwanak-gu Seoul 151-742, Republic of Korea E-mail: matlgen1@snu.ac.kr H. Song, E. Y. Jang, Prof. Y. H. Kim School of Mechanical and Aerospace Engineering Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea Prof. Y. H. Kim Institute of Advanced Aerospace Technology Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea M. D. Lima, R. O. Robles, X. Lepró, Prof. R. H. Baughman Alan G. MacDiarmid NanoTech Institute University of Texas at Dallas Richardson, TX 75083-0688, USA DOI: 10.1002/adma.201204018 With increasing demand for ultra-high-energy-density storage systems, considerable effort has been recently focused on Li -O 2 batteries. [1–4] The Li -O 2 battery can deliver substantially higher energy density ( ∼3500 Wh kg -1 ) than conventional Li -ion bat- teries. [4,5] However, such key limitations, including poor cycla- bility, low Coulombic efficiency, and relatively low capacity must be resolved for the Li -O 2 battery to be considered for applications. Recent studies have shown that identifying a stable electro- lyte is critically important for increasing the cycle life of Li -O 2 batteries. [6,7] Conventional carbonate-based electrolyte easily deteriorates upon attack of reactive oxygen radicals. [6,8] Even non-carbonate-based electrolytes, such as tetraethylene glycol dimethylether (TEGDME) and dimethoxyethane (DME), are not stable, and cannot sufficiently improve the cycle life. [7,9] Another important measure to improve rechargeability is to enhance reaction kinetics for the formation and decomposition of Li 2 O 2 by designing a nanostructured air electrode. Because the discharge product, Li 2 O 2 , continuously accumulates on the pores of an air electrode, it can potentially clog this electrode and become electrically disconnected, preventing further reac- tions. These isolated discharge products undergo minimal recharging. Therefore, the air electrode needs to be designed such that it minimizes the undesirable clogging and promotes the electrochemical reactivity. As the control of the morphology and porosity of the electrode greatly affects the capacity and rate capability, [10] various nanostructured air electrodes have been reported using carbon nanoparticles, graphene, graphene oxide, or carbon nanotubes (CNTs). [11–14] However, the poor cyclability and low rate capability remain as critical drawbacks of the Li -O 2 batteries, and the ideally designed electrode archi- tecture is still awaited. We here show that hierarchical porous electrodes comprising well-aligned CNTs fibrils can serve as an important model for controlled porosity, and demonstrate that they can significantly enhance the cycle stability and rate capability of the Li -O 2 batteries. The controlled porous framework of these woven CNT electrodes enables effective formation/decomposition of lithium peroxide by providing facile accessibility of oxygen to the inner side of the air electrode and preventing the clog- ging of pores by discharge product, even during the deep dis- charge. We found that the discharge products were uniformly deposited on the individual CNTs and CNT bundles, so pores are not clogged (see Figure 4 and Figure S7 of the Supporting Information (SI)). This unique feature led to the high cycle life and unprecedented high rate performance of the Li -O 2 cell. We believe that the facile controllability of porous morphology using well-aligned CNT fibrils can provide an important tool in identifying an ideally designed air electrode. The air electrode with controlled pore structure was fab- ricated by orthogonally plying individual sheets of aligned multiwalled nanotubes (MWNTs) without use of any binder or solvent. These MWNT sheets, which are drawn from specially prepared ∼400 μm high carbon nanotube forests, are initially an aerogel having about the density of air and a greater gravi- metric strength in the nanotube orientation direction than the strongest steel. [15] Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images indicate that the MWNTs have an outer diameter of ∼15 nm, contain ∼9 walls, form large bundles, and contain below 1 wt% catalyst. Figure 1 shows SEM images of the prepared air electrode. Well-aligned CNT fibril sheets were woven like a mesh struc- ture. One layer of the CNT fibril sheet was composed of a number of micro-sized strings. An individual string is a bundle of multiwalled CNTs, ∼15 nm in diameter (Figure 1b). Addi- tional SEM and TEM images with various magnifications, and atomic force microscopy (AFM) images are provided in the SI. The inset in Figure 1a shows that these well-ordered structures are maintained over a large area. Highly aligned, self-woven sheets were used as an electrode for the Li -O 2 cell. The air electrodes were prepared by orthogo- nally plying sheets of the CNT fibril on a Ni-mesh current col- lector. The electrolyte consisted of 0.21 mL of TEGDME with 1M LiPF 6 . Before the test, the air electrode and separator were soaked in the electrolyte to ensure wetting of the electrolyte. Adv. Mater. 2013, 25, 1348–1352