Journal of Power Sources 160 (2006) 558–562 Synthesis and characterization of spherical morphology [Ni 0.4 Co 0.2 Mn 0.4 ] 3 O 4 materials for lithium secondary batteries Sung Woo Oh a , Sang-Ho Park a,b , K. Amine b , Yang-Kook Sun a,∗ a Center for Information and Communication Materials, Department of Chemical Engineering, Hanyang University, Seungdong-Gu, Seoul 133-791, Republic of Korea b Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA Received 8 November 2005; received in revised form 3 January 2006; accepted 5 January 2006 Available online 17 February 2006 Abstract Spherical morphology [Ni 0.4 Co 0.2 Mn 0.4 ] 3 O 4 materials have been synthesized by ultrasonic spray pyrolysis. The Li[Ni 0.4 Co 0.2 Mn 0.4 ]O 2 powders were prepared at various pyrolysis temperatures between 500 and 900 ◦ C. The Li[Ni 0.4 Co 0.2 Mn 0.4 ]O 2 material prepared at a pyrolysis temperature of 600 ◦ C samples are exhibited excellent electrochemical cycling performance and delivered the highest discharge capacity at over 180 mAh g -1 between 2.8 and 4.4V. The structural, electrochemical, morphological property and thermal stability of the powders were characterized by X-ray diffraction (XRD), galvanostatic charge/discharge testing, scanning electron microscopy (SEM), and differential scanning calorimeter (DSC), respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Lithium secondary batteries; Spray pyrolysis; Positive materials; Layered materials; Electrochemical properties 1. Introduction Extensive research for alternatives to LiCoO 2 as the positive material in rechargeable batteries has been conducted over the past few years. Alternative cathode materials are being sought because LiCoO 2 is expensive and toxic. The promising alterna- tives include hexagonal -NaFeO 2 structured-layered lithium transition metal oxides LiMO 2 (M = Co, Ni, Mn), particularly LiNiO 2 and LiMnO 2 . Although there has been much progress in optimizing these two materials, there are still problems that need to be overcome. For example, LiNiO 2 cannot be used in its current form because stoichiometric LiNiO 2 is known to be difficult to synthesize and delithiated Li x NiO 2 decomposes exothermally at around 200 ◦ C [1–3]. LiMnO 2 , on the other hand, is thermodynamically unstable as a layered structure, but thermodynamically stable as orthorhombic phase o-LiMnO 2 [4]. The Mn 3+ (d 4 ) ions cause a cooperative distortion of the MnO 6 octahedral due to Jahn–Teller stabilization leading to a mono- clinic unit cell. When Li is deintercalated from the LiMnO 2 , ∗ Corresponding author. Tel.: +82 2 2220 0524; fax: +82 2 2282 7329. E-mail address: yksun@hanyang.ac.kr (Y.-K. Sun). both the m- and o-LiMnO 2 have been observed to undergo a detrimental phase transformation to a spinel-like phase through minor atomic rearrangements leading to eventual degradation of electrode performance [5,6]. Several recent attempts have been made to enhance the elec- trochemical performance of LiNiO 2 –LiCoO 2 –LiMnO 2 solid solution [7,8]. Among them, Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 has been suggested as the most promising alternative to LiCoO 2 since Ni, Co, and Mn can substitute each other to form a solid solution of any percentage without disturbing the layer structure. Recently, we reported on the structural and electrochemical properties of Li[Ni 0.5 Mn 0.5 ] 1-x Co x O 2 (x in 0–0.33) materials [9]. In this study, we synthesized [Ni 0.4 Co 0.2 Mn 0.4 ]O y precur- sor at various pyrolysis temperatures in order to obtain opti- mized Li[Ni 0.4 Co 0.2 Mn 0.4 ]O 2 cathode materials. The structural and electrochemical properties of these cathode materials were investigated by X-ray diffraction and electrochemical measure- ments. 2. Experimental Li[Ni 0.4 Co 0.2 Mn 0.4 ]O 2 powder was synthesized by a spray pyrolysis method. At first, stoichiometric amounts of nickel 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.01.023