Journal of Power Sources 196 (2011) 10712–10716 Contents lists available at SciVerse ScienceDirect Journal of Power Sources jou rnal h omepa g e: www.elsevier.com/locate/jpowsour Short communication Facile synthesis and electrochemical performance of ordered LiNi 0.5 Mn 1.5 O 4 nanorods as a high power positive electrode for rechargeable Li-ion batteries Hyun-Wook Lee a , P. Muralidharan a , Claudio M. Mari b , Riccardo Ruffo b,∗ , Do Kyung Kim a,∗∗ a Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea b Dipartimento di Scienza dei Materiali, Università degli Studi di Milano-Bicocca, Milan 20125, Italy a r t i c l e i n f o Article history: Received 27 June 2011 Received in revised form 30 August 2011 Accepted 1 September 2011 Available online 7 September 2011 Keywords: Lithium ion battery High voltage cathode Lithium nickel manganese oxide Nanorod High rate capability a b s t r a c t One-dimensional ordered LiNi 0.5 Mn 1.5 O 4 nanorods have been fabricated and investigated for use as a high power cathode in rechargeable Li-ion batteries. These highly crystalline nanorods, with an ordered spinel structure and diameters and lengths around 130 nm and 1.2 m, respectively, were synthesized in two steps by using a hydrothermal reaction to produce ˇ-MnO 2 nanorods followed by solid-state lithiation. Electrochemical analysis showed the superior performance of nanorods as a cathode in Li-ion half cells. The specific charge and discharge capacities were found to be 120 and 116 mAh g -1 at a 0.5 C rate, and 114 and 111 mAh g -1 at a 1 C rate between 3.5 and 5.0 V vs. Li + /Li. Moreover, the nanorods exhibit high power capability, maintaining capacities of 103 and 95 mAh g -1 at specific currents of 732.5 and 1465 mA g -1 (5 and 10 C rates), respectively. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In the last few years, there has been an enormous increase in the use and demand for Li rechargeable batteries with higher energy densities and power capabilities for portable electronic devices and for hybrid and all-electric vehicles. For these appli- cations, batteries with increased energy are desired, and various alternate positive electrode materials have been widely investi- gated to obtain either high voltage or increased capacity [1,2]. The spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) is a promising material due to its high average discharge voltage (around 4.7 V vs. Li + /Li couple) [3–5]. In LNMO the manganese is present in its Mn(IV) valence state, and only the Ni 2+/4+ redox couple is responsible for the high voltage charge/discharge potential accompanying the insertion and extraction of Li + ions [6,7]. LNMO can exist in two different crys- tallographic structures: the cubic spinel with a P4 3 32 space group, which is called the “ordered” LiNi 0.5 Mn 1.5 O 4 , and the so called “dis- ordered” LiNi 0.5 Mn 1.5 O 4-ı with a Fd3m space group [8–10]. The disordered LiNi 0.5 Mn 1.5 O 4-ı has a special electrochemical behav- ior; the oxygen deficiency leads to the presence of manganese in the Mn(III) oxidation state, which can be oxidized to Mn(IV) at around 4 V [11], thus reducing the specific energy and inducing the ∗ Corresponding author. Tel.: +39 02 64485153; fax: +39 02 64485400. ∗∗ Corresponding author. Tel.: +82 42 350 4118; fax: +82 42 350 3310. E-mail addresses: riccardo.ruffo@unimib.it (R. Ruffo), dkkim@kaist.ac.kr (D.K. Kim). distortion of the spinel structure. Moreover, in disordered LNMO impurities such as NiO and Li x Ni y O are generally observed. On the other hand, disordered LNMO shows higher electronic conductiv- ity than the ordered stoichiometric phase due to the presence of the Mn 3+/4+ redox couple and Ni/Mn disordering [12,13]. Therefore, the preparation of the high energy ordered LNMO structure that also has high specific power (high rate capability) is a formidable challenge in this research area. To reach such a goal, two strategies are frequently employed: increasing the intrinsic electronic conductivity by the control of the electrode microstructure, and enhancing the Li-ion transport by reducing the bulk diffusion length in the active phase, which can be achieved by the use of nanostructured materials. Sev- eral researchers have already obtained high rate capability and cyclability through the use of nanosized materials [8,14]. In our previous work [15,16], we demonstrated that the one-dimensional (1-D) nanosized LiMn 2 O 4 spinel phase has a superior rate capa- bility due to a large area of contact with the electrolyte, shorter distances for Li-ion transport, and an enhancement of the elec- tronic pathway [17]. In addition, the 1-D nanosized structure may improve the mechanical stability of the electrode compared to zero-dimensional nanosized systems (nanopowders) which tend to undergo decrepitation during electrochemical reactions [18]. Since the ordered LNMO spinel has a cubic structure, its 1-D fabrication as nanowires or nanorods appears to be a difficult task, and it has not been demonstrated until now. In this study, we report on the preparation of ordered LNMO nanorods via a multistep synthesis: a hydrothermal reaction to 0378-7753/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2011.09.002