Li 2 MTi 6 O 14 (M ¼ Sr, Ba): new anodes for lithium-ion batteries I. Belharouak * , K. Amine Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA Received 1 April 2003; received in revised form 7 April 2003; accepted 7 April 2003 Abstract Isostructural Li 2 MTi 6 O 14 (M ¼ Sr, Ba) materials, prepared by a solid state reaction method, have been investigated as insertion electrodes for lithium battery applications. These titanate compounds have a structure that consists of a three-dimensional network of corner- and edge-shared [TiO 6 ] octahedra, 11-coordinate polyhedra for the alkali-earth ions, and [LiO 4 ] tetrahedra in tunnels that also contain vacant tetrahedral and octahedral sites. Electrochemical data show that these compounds are capable of reversibly intercalating four lithium atoms in a three-stage process between 1.4 and 0.5 V vs. metallic lithium. The electrodes provide a practical capacity of approximately 140 mAh/g; they are, therefore, possible alternative anode materials to the lithium titanate spinel, Li 4 Ti 5 O 12 . The lithium intercalation mechanism and crystal structure of Li 2 MTi 6 O 14 (M ¼ Sr, Ba) electrodes are discussed and compared with the electrochemical and structural properties of Li 4 Ti 5 O 12 . The area-specific impedance (ASI) of Li=Li 2 SrTi 6 O 14 cells was found to be significantly lower than that of Li=Li 4 Ti 5 O 12 cells. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Lithium-ion batteries; Anode; Titanate; Structure 1. Introduction Rechargeable lithium-ion batteries with their light- weight and high energy density have become an important source for powering many applications, par- ticularly those in the electronics sector. These batteries are gaining more attention because of their possible application in high power devices such as hybrid electric vehicles [1]. Graphite/LiCoO 2 is the most commonly used electrochemical couple in lithium-ion batteries, in which LiCoO 2 plays the role of the positive electrode (cathode) and graphite acts as the negative electrode (anode). At full charge, the highly lithiated graphite (LiC 6 ) electrode is highly reactive because it operates close to the potential of metallic lithium. In order to address the safety limitations of lithium-ion cells, par- ticularly those containing nickel-based cathodes, alter- native anodes to graphite have been suggested, such as the spinel Li 4 Ti 5 O 12 that operates at approximately 1.5 V vs. Li 0 [2–5]. Although Li 4 Ti 5 O 12 is an insulator, doping the structure with small amounts of Mg 2þ , Al 3þ has been reported to improve the electronic conductivity of the spinel by many orders of magnitude [6]. In this paper, we introduce, as an alternative to Li 4 Ti 5 O 12 ,a new family of lithium-ion conducting titanate materials Li 2 MTi 6 O 14 (M ¼ Sr, Ba) that can act as insertion electrodes in lithium cells. 2. Experimental Li 2 MTi 6 O 14 (M ¼ Sr, Ba) compounds were prepared by heating stoichiometric quantities of Li 2 CO 3 , SrCO 3 or BaCO 3 , and TiO 2 . The reagents were carefully mixed and progressively heated up to 600 °C in air to decom- pose the carbonate salts. After grinding, the powders were sintered at 1000 °C for 24 h. Chemical analyses of the resulting products were performed by the inductively coupled plasma technique (ICP) to check their compo- sition. The analytical results were in excellent agreement with the expected theoretical values. For example, the (Sr + Ti)/Li ratio and the lithium content in the final Li 2 SrTi 6 O 14 product were determined to be 27.05 and 2.2 wt%, in contrast to the theoretically expected values of 27.01 and 2.26 wt%, respectively. Electrochemistry Communications 5 (2003) 435–438 www.elsevier.com/locate/elecom * Corresponding author. Tel.: 1-630-252-4450; fax: 1-630-252-4176. E-mail address: belharouak@cmt.anl.gov (I. Belharouak). 1388-2481/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1388-2481(03)00090-0