Structural and electrochemical evaluation of (1  x)Li 2 TiO 3 (x)LiMn 0.5 Ni 0.5 O 2 electrodes for lithium batteries Christopher S. Johnson * , Jeom-Soo Kim, A. Jeremy Kropf, Arthur J. Kahaian, John T. Vaughey, Michael M. Thackeray Electrochemical Technology and Basic Sciences Program, Chemical Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA Abstract X-ray diffraction (XRD), in situ X-ray absorption spectroscopy (XAS), and chemical lithiation experiments were used to evaluate the phases associated with the electrochemistry of the mixed-metal layered LiMn 0.5 Ni 0.5 O 2 oxide electrode. These results, along with coin-cell cycling data from the substituted layered (1  x)Li 2 TiO 3 (x)LiMn 0.5 Ni 0.5 O 2 composite oxide electrode are reported. The cycling behavior of Li/0.05Li 2 TiO 3 0.95LiMn 0.5 Ni 0.5 O 2 (x ¼ 0:95) cells over an extended voltage window (4.3 or 4.6–1.25 V) under moderate current rate have yielded rechargeable capacities above 250 mAh/g. These large capacities and structural data suggest that both the composite (1  x)Li 2 TiO 3 (x)LiMn 0.5 Ni 0.5 O 2 and LiMn 0.5 Ni 0.5 O 2 (standard) layered electrodes operate predominantly off two-electron redox couples, Ni 4þ /Ni 2þ and Mn 4þ /Mn 2þ , approximately between 4.6 and 2.0V, and between 2.0 and 1.0 V versus metallic Li, respectively. The LiMn 0.5 Ni 0.5 O 2 layered oxide is shown to reversibly react chemically or electrochemically with Li to form a stable, but air-sensitive dilithium compound, Li 2 Mn 0.5 Ni 0.5 O 2 (Li 2 MO 2 ;M ¼ metal ion) that can be indexed to the space group P-3m1. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Lithium batteries; Layered electrodes; Lithium–nickel–manganese oxides 1. Introduction The synthesis, electrochemical, and structural properties of layered (1  x)Li 2 M 0 O 3 (x)LiMO 2 composite cathodes where M 0 ¼ Mn, Ti, Zr, Ru, and M ¼ Mn, Ni, Co, are under investigation. This approach to stabilize layered LiMO 2 electrode structures emanated from earlier studies of the manganese system (1  x)Li 2 MnO 3 (x)LiMnO 2 that can be derived from the rock-salt phase Li 2 MnO 3 by (1) acid- treatment to remove Li 2 O and (2) relithiation of the (1  x)Li 2 MnO 3 (x)MnO 2 product by either chemical or electrochemical methods [1,2]. This research has demon- strated that a layered Li 2 MnO 3 component (or an isostruc- tural component, Li 2 TiO 3 or Li 2 ZrO 3 ) contributes to the stabilization of the layered LiMnO 2 electrode [3]. This approach has recently led to attempts to stabilize a layered LiMn 0.5 Ni 0.5 O 2 electrode [4,5] by incorporating a small amount of Li 2 TiO 3 in a composite structure for 0  x < 0:1 [6]. Although 0.05Li 2 TiO 3 0.95LiMn 0.5 Ni 0.5 O 2 electrodes did not improve the initial capacity loss effects that are observed when standard Li/LiMn 0.5 Ni 0.5 O 2 cells are cycled between 4.3 and 3 V (i.e. during break-in cycles), they displayed a higher coulombic efficiency on extended cycling between 4.6 and 2.5 V [6]. This has been attributed to a decrease in the oxygen activity at the surface of delithiated 0.05Li 2 TiO 3 0.95Li 1z Mn 0.5 Ni 0.5 O 2 (0 < z  1) electrode particles at high states of charge. In this paper, results of continued electrochemical and structural characterization of (1  x)Li 2 TiO 3 (x)LiMn 0.5 - O 2 are reported for the composite electrode at x ¼ 0:95. Of particular significance has been the observation that Li/0.05Li 2 TiO 3 0.95LiMn 0.5 Ni 0.5 O 2 and standard Li/ LiMn 0.5 Ni 0.5 O 2 cells can be cycled over a large voltage window between 4.6 and 1.25 V with surprisingly good reversibility, leading to capacities in excess of 300 mAh/g [7,8]. To address the underlying factors associated with the large capacities observed, additional synthesis, XRD and XAS studies of the parent compound, LiMn 0.5 Ni 0.5 O 2 , and the substituted composite 0.05Li 2 TiO 3 0.95LiMn 0.5 Ni 0.5 O 2 electrode were conducted. Furthermore, we have obtained crystallographic data for the dilithium, layered compound, Li 2 Mn 0.5 Ni 0.5 O 2 , with hexagonal symmetry P-3m1 and report the results herein. The retention of a layered Journal of Power Sources 119–121 (2003) 139–144 * Corresponding author. Tel.: þ1-630-252-4787; fax: þ1-630-252-4176. E-mail address: johnsoncs@cmt.anl.gov (C.S. Johnson). 0378-7753/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-7753(03)00143-5