© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 3052–3057 3052 www.advmat.de www.MaterialsViews.com COMMUNICATION By Khalil Amine,* Ilias Belharouak, Zonghai Chen, Taison Tran, Hiroyuki Yumoto, Naoki Ota, Seung-Taek Myung, and Yang-Kook Sun* Nanostructured Anode Material for High-Power Battery System in Electric Vehicles In recent years, gasoline prices have risen in real terms to levels not seen since the early 1980s. [1] As a result, demand has increased significantly for a high-performance battery to power hybrid elec- tric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and all-electric vehicles (EVs). At present, the total worldwide sales of HEVs based on the nickel/metal hydride battery system are only about 2–3 percent of all vehicle sales, even though this technology has been on the market since 1997, because of its high cost and poor performance. A necessary step for significant market pen- etration of battery-powered vehicles is the development of a cost- effective, long-lasting, abuse-tolerant battery system. Lithium-ion batteries, which exhibit the highest power and energy density of any existing battery systems, offer many advantages for applications in transportation. [2,3] Among the existing cathode chemistries used in lithium batteries, lithium manganese oxide spinel (LMO) offers outstanding power capa- bility, excellent safety characteristics, and low cost compared to Ni- and Co-based layered oxide materials. [4,5] However, when combined with the conventional carbon anode in a full cell configuration, the resulting cell shows poor cycle and calendar life, especially at elevated temperatures, due to the breakdown of the solid electrolyte interface (SEI) caused by Mn dissolution from the cathode. [4,6–8] In addition, carbon materials intercalate lithium at approximately 100 mV vs. Li/Li + ; and under high- power charge pulses, the carbon electrode can be polarized to such an extent that highly reactive lithium metal plates onto the surface of the negative electrode, [9] potentially causing thermal runaway because of internal shorts triggered by the formation of lithium dendrites. [10] To find an anode-cathode combination that will meet the demanding transportation requirements, we have investigated Li 4 Ti 5 O 12 (LTO) as the anode. Unlike the conventional carbon anode, which expands up to 16 vol% during charging, the LTO material can accommodate up to three lithium ions in the spinel structure with no volume change [11–15] ( Figure 1a). As a result, the integrity of the LTO electrode remains intact during extended cycling. Furthermore, the high operating voltage of LTO does not allow the growth of lithium dendrites; consequently, better safety is expected. Despite these advantages, cells with this anode material synthesized as micron-sized particles have failed to meet the transportation power requirement because of LTO’s poor electronic conductivity. [16–19] In an attempt to overcome this significant drawback, others have synthesized isolated nano-size LTO particles, which have shown improved rate capability in cell tests. [20,21] However, these nano-materials have very high surface area (over 200 m 2 /g) and low packing density, resulting in low volumetric energy density that falls short of meeting the energy requirement for transportation applications. Our approach to enable this anode material was to synthe- size a new LTO structure: micron-size ( ∼0.5–2 μm) secondary particles (Figure 1b, left) composed of nanometer-size ( <10 nm) primary particles (Figure 1b, right) which is similiar to our pre- viously reported nanoporous micro-LiFePO 4 materials; [22] this material is referred to as MSNP-LTO (MSNP stands for “micron secondary nano primary”). Unlike the conventional LTO with micron-size particles ( ∼5–10 μm), which exhibits low initial capacity and poor rate capability (Figure 1c), MSNP-LTO has a nano-porous structure that allows the electrolyte to penetrate inside the particles, resulting in high accessibility of the active material. In addition, the nano-primary particles of MSNP-LTO allow for fast lithium diffusion because of the short lithium pathway within the nanoparticle. As a result outstanding rate capability can be achieved with this material when tested in a half cell (Figure 1d) and in full cell (Figure 1Sa&b). In addition, the nanophase structure can allow for full lithium accessibility, resulting, thus, in high practical capacity (Figure 1d). To assess the potential of the MSNP-LTO anode for the HEV application, we conducted extensive testing using coin cells with LMO cathodes. For comparison, we also tested LTO (micron size)/LMO and carbon/LMO cells. Further details appear in the Methods section. Figure 2a compares the area specific impedance (ASI) for the MSNP-LTO/LMO, LTO/LMO, and carbon/LMO cells. Hybrid electric vehicles require very high charge and discharge pulse power for vehicle acceleration and regenerative braking. To meet the 25-kW pulse power requirement in HEVs, the ASI of electrodes in the cell must be less than 35 ohm cm. 2[23] The pulse power capability at each DOI: 10.1002/adma.201000441 [∗] Dr. K. Amine, I. Belharouak, Z. Chen Electrochemical Technology Program Chemical Sciences and Engineering Division Argonne National Laboratory 9700 South Cass Avenue, Argonne, Illinois 60439 (USA) E-mail: amine@anl.gov Dr. T. Tran, H. Yumoto, N. Ota Enerdel Lithium Power Systems 8740 Hague Road, Indianapolis, IN 46256 (USA) Prof. S.-T. Myung Department of Chemical Engineering Iwate University 4-3-5 Ueda, Morioka, Iwate 020-8551 (Japan) Prof. Y.-K. Sun Department of WCU Energy Engineering and Chemical Engineering Hanyang University Seoul 133-791 (Republic of Korea) E-mail: yksun@hanyang.ac.kr