Synergistic effects of various morphologies and Al doping of spinel LiMn 2 O 4 nanostructures on the electrochemical performance of lithium-rechargeable batteries Won-Hee Ryu, a Ji-Yong Eom, b Ri-Zhu Yin, b Dong-Wook Han, a Won-Keun Kim a and Hyuk-Sang Kwon * a Received 11th January 2011, Accepted 2nd August 2011 DOI: 10.1039/c1jm10146c Nanostructured electrodes have recently received great attention as components in lithium rechargeable batteries, especially because of the high power produced by the fast kinetic properties of these unique structures. Here, we report the successful synthesis of various nanostructured morphologies of spinel lithium manganese oxide electrodes (nanorod, nanothorn sphere, and sphere) from a similarly shaped manganese dioxide precursor that was controlled with different aluminium contents by the hydrothermal method. Among these structures, nanothorn sphere structured LiAl 0.02 Mn 1.98 O 4 produces the highest discharge capacity of 129.8 mA h g 1 , excellent rate capability (94.6 mA h g 1 at 20 C, 72% of 0.2 C-rate discharge capacity) and stable cyclic retention for 50 cycles. The excellent kinetic properties of the nanothorn sphere structure are not only due to the nanothorn sphere electrode having high surface area but also because the critical amount of Al in the nanothorn sphere electrode was located at the Mn site (16d) instead of the Li site (8a). Introduction Recently, many studies of new cathode materials for lithium rechargeable batteries have focused their efforts on applications in next generation automobiles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). 1–4 To replace the conventional LiCoO 2 structure, new cathode materials should possess a distinguished set of properties comprising high power, excellent thermal and struc- tural stability, and fast charge/discharge. LiMn 2 O 4 , with a spinel structure, has been spotlighted as a prospective candidate for the cathode material for HEV applications, which is primarily due to its suitable Mn 4+ /Mn 3+ redox potential (4 V vs. Li + /Li), high power density, low material cost, and excellent structural stability in extreme environments. 5–7 Due to the excellent structural stability and hence the better safety, LiMn 2 O 4 with a three dimensional framework is preferred to the layered LiCoO 2 as a cathode material for use in large-scale lithium rechargeable batteries. LiMn 2 O 4 has better rate capability due to the 3-D lithium pathway in the spinel framework, as compared with olivine- structured LiFePO 4 , which only has a 1-D lithium pathway, and layered LiMO 2 (M ¼ Co, Ni, Mn), which only has a 2-D lithium pathway. 8–11 However, to maximize the kinetic properties to obtain high charge/discharge characteristics, nanostructured LiMn 2 O 4 is more suitable as an alternative cathode material. Nanostructured LiMn 2 O 4 has been shown to significantly increase the rate of Li + insertion/extraction due to the (1) large surface area providing innumerable reaction sites for Li and (2) short Li + diffusion length (t ¼ L 2 /D, t: reaction time, L: Li + diffusion length, D: diffusion coefficient) within the LiMn 2 O 4 particles. 12–14 In addition, it can effectively relieve the internal stress called the Jahn–Teller distortion, occurring during Li intercalation/deintercalation. Although 1-D nanostructured LiMn 2 O 4 is very effective in improving the rate capability during charge and discharge, it has a low tap-density or volumetric energy density for an electrode. Therefore, 3-dimensionally assembled structures such as randomly aligned nanorods on a microsphere are considered an ideal solution because the nanobranches around the core can not only increase the Li + sites on the nanoscaled large surface area to improve the kinetic properties but the microsphere core also increases the volumetric energy density and prevents a separator from tearing, causing electrical shorts and safety problems. 15,16 It is important to understand how to prepare the nano- structured MnO 2 precursor in order to synthesize nano- structured LiMn 2 O 4 because the morphology of LiMn 2 O 4 is generally influenced by the morphology of the MnO 2 precursor. 17 Various synthetic strategies have been developed for the prepa- ration of nanostructured MnO 2 , such as sol–gel processing, templating, and hydrothermal synthesis. 18–20 Among the many synthetic methods for producing nanostructured MnO 2 , hydro- thermal synthesis has recently received attention due to its a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Republic of Korea. E-mail: hskwon@kaist.ac.kr; Fax: +82-42-350-3310; Tel: +82- 42-350-3326 b Battery Development Team, Energy Business Division, Samsung SDI Co., Ltd., Cheonan, Chungcheongnam-do, 330-300, Republic of Korea This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21, 15337–15342 | 15337 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2011, 21, 15337 www.rsc.org/materials PAPER Downloaded by Korea Advanced Institute of Science & Technology / KAIST on 26 October 2011 Published on 30 August 2011 on http://pubs.rsc.org | doi:10.1039/C1JM10146C View Online / Journal Homepage / Table of Contents for this issue