Lithium intercalation into single-wall carbon nanotube bundles Solange B. Fagan a , S. Guerini b , J. Mendes Filho b , V. Lemos b, * a Centro de Cie ˆncias Naturais e Tecnolo ´gicas, Centro Universita ´rio Franciscano, UNIFRA, Santa Maria, RS 97010-032, Brazil b Departamento de Fı ´sica, Universidade Federal do Ceara ´, Centro de Cie ˆncias, Caixa Postal 6030, Campus do Pici, 60455-970 Fortaleza, Ceara ´, Brazil Available online 18 April 2005 Abstract The insertion of lithium atoms in the channels of the single-wall carbon nanotube (SWNT) bundles is investigated using an ab initio calculation. The relaxed structure as well as the electronic band structure were obtained. Results reveals that Li insertion modifies the band structure by shifting the Fermi level to a higher density of states region, and this shift scales with the rate of insertion. The Li/SWNT band structure allows to predict strongly modified electronic properties. q 2005 Elsevier Ltd. All rights reserved. Keywords: Lithium intercalation; Single-wall carbon nanotube bundles; Electronic properties; Ab initio calculation 1. Introduction In lithium batteries the Li-ions intercalate into the carbon host material during the charge process and reverse back to the cathode during the discharge. Several carbon forms served as anode in the construction of Li-rechargeable batteries, including graphite and disordered carbon com- pounds. The limit for lithium intercalation into graphite is one lithium atom per six carbon atoms, LiC 6 , the battery attaining, at this limit, the specific capacity of 372 mA h/g [1]. In the recent literature the replacement of such compounds by single-wall carbon nanotubes (SWNTs) was predicted to result in a substantial increase of the alkali metal storage capacity [2]. In fact, SWNTs were found to have higher Li-storage capacity than graphite or multi-wall carbon nanotubes, [3,4] with storage rate up to LiC 3 when the SWNTs are etched to short segments, allowing for insertion into the tubes [5]. The insertion of lithium in a bundle may occur inside or outside the tubes. Insertion through the walls or closed ends of the tubes is forbidden due to the height of the potential barrier [6]. For opened ends tubes the outside position was found to be the most stable, energetically [7]. Therefore, the ions in the channels between the tubes may be the easiest process for reversible intercalation thus requiring further investigation of this process to improve the Li-battery research. Here, calculation of the minimal energy configuration and electronic band structure was performed for lithium insertion into SWNT bundles using first principle methods [8]. Several densities of intercalation in the intersticial sites of the bundles were considered. The Fermi level was observed to shift with lithium insertion, the shifting scaling with the amount of lithium atoms inserted. As the Fermi level fall in a region highly crowded with electronic states for Li-intercalation, the electronic properties of the material changes considerably. 2. Results and discussion This study is based on first-principles density functional theory. The SIESTA code, [9] was employed to perform fully self-consistent calculation solving the Kohn–Sham equations. The Kohn–Sham orbitals are expanded using linear combinations of pseudoatomic orbitals as proposed by Sankey and Niklewski [10]. For the exchange and correlation term, the generalized gradient approximation— GGA is used as described by Perdew et al. [11]. Core electrons are replaced by nonlocal, norm conserving Troullier and Martins pseudopotential [12]. Valence electrons are described within a linear combination of atomic orbitals (LCAO) and a double-zeta basis set with a polarization function. A cut-off of 150 Ry for the grid integration was utilized to represent the charge density. Microelectronics Journal 36 (2005) 499–501 www.elsevier.com/locate/mejo 0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.02.059 * Corresponding author. E-mail address: volia@fisica.ufc.br (V. Lemos).