Preparation and characterization of Cr-doped LiMnO 2 cathode materials by Pechini’s method for lithium ion batteries W.K. Pang, J.Y. Lee, Y.S. Wei, S.H. Wu * Department of Materials Engineering, Tatung University, Taipei 104, Taiwan, ROC highlights < Stimulation effects of Cr-doping on the layered-spinel transition for LiMnO 2 -based cathodes upon cycling is proposed. < Effects of Cr-doping on the electrochemical properties of LiMnO 2 -based cathodes are demonstrated. < Crystallography of pristine and Cr-doped LiMnO 2 samples are studied using X-ray diffraction and Rietveld method. < Superiority of Pechini’s method on fine powder preparation is demonstrated. article info Article history: Received 29 June 2012 Received in revised form 5 December 2012 Accepted 13 January 2013 Keywords: Solegel growth Powder diffraction XAFS (EXAFS and XANES) Crystallography Electrochemical properties abstract o-LiMnO 2 and LiMn 1x Cr x O 2 (x ¼ 0.05 and 0.1) powders are synthesized via Pechini’s method. Crystalline structure, surface morphology, and electrochemical properties of the prepared powders are investigated by XRD, SEM, cyclic voltammetric and capacity retention studies. From results, it is found that a small amount of Cr-doping into LiMnO 2 cathode can not only lead to a topographic change from orthorhombic to monoclinic geometry and particle size reduction, but also improve cycling performance and reversible capacity. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Layered LiMnO 2 , both orthorhombic and monoclinic forms show similar topography as the most successful cathode material LiCoO 2 , are expected to have highly accessible Li þ ion diffusion pathways [1e 7]. Orthorhombic LiMnO 2 (o-LiMnO 2 ) crystallizes in space group of Pmnm having an ordered rocksalt structure in which edge-sharing LiO 6 and MnO 6 octahedra are arranged in alternate zig-zag (corrugated) layers along the (010) direction (see Fig. 1a) [5,8e10]. Monoclinic LiMnO 2 (m-LiMnO 2 ) crystallizing in phase group of C2/m shows a cationic ordering isotypic to a-NaFeO 2 structure that each cation occupies alternate octahedral site par- allel to the (111) plane of the cubic oxygen sub lattice (see Fig. 1b) [6,7,11]. Owing to the high-spin configuration of Mn 3þ that four unpaired electrons with parallel spin, in both polymorphs, the atomic arrangement is distorted by cooperative JahneTeller effect of Mn 3þ ions from ideal cubic-close-packing [6]. LiMnO 2 has a high theoretical capacity of 280 mAh g 1 . How- ever, the cycling stability of LiMnO 2 is not satisfied due to the induced capacity loss upon cycling by phase transformation to spinel-like phase, Mn dissolution, and JahneTeller distortion of Mn 3þ [12]. Certain methods have been developed, such as surface modification [12], stabilization with Li 2 MnO 3 component [13], and cationic substitution [6,14]. Cationic substitution is one of the most widely studied and useful approaches for improving the electro- chemical performance of LiMnO 2 . In literature, different cations have been employed and Cr ion is one of the most promising choices for this purpose [6,14e19]. Hwang et al. [17,18] revealed that Cr substitution for Mn site in LiMnO 2 prepared by ion- exchange method brings up the initial capacity from 120 to 190 mAh g 1 at 0.5 mA cm 2 . Wu et al. reported that a Cr-doped LiMnO 2 prepared by ion-exchange method shows high discharge capacity of 200þ mAh g 1 at 0.15 C [14]. Perhaps the difference in performance is coming from the different starting materials been used. Furthermore, Guo et al. claimed that Cr-doped LiMnO 2 prepared by a solution-assisted solid-state reaction method dem- onstrates much better cycling performance, which is attributed to a more regular local coordination symmetry around the * Corresponding author. Tel.: þ886 2 25922458; fax: þ886 2 25936897. E-mail address: shwu@ttu.edu.tw (S.H. Wu). Contents lists available at SciVerse ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.01.030 Materials Chemistry and Physics xxx (2013) 1e6 Please cite this article in press as: W.K. Pang, et al., Preparation and characterization of Cr-doped LiMnO 2 cathode materials by Pechini’s method for lithium ion batteries, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.01.030