X-ray Absorption Spectroscopy study of lithium insertion mechanism in Li 1.2 V 3 O 8
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X-ray Absorption Spectroscopy study of lithium insertion mechanism in Li1.2V3O8 N. Bourgeon, J. Gaubicher, D. Guyomard, G. Ouvrard Institut des Matériaux Jean Rouxel, 2 rue de la Houssinière, BP32229, 44322 Nantes, France
ABSTRACT X-ray absorption spectroscopy (XAS) measurements were performed to thoroughly understand lithium insertion mechanism in Li1.2V3O8. The evolution of the absorption pre-edge and edge corresponding to the local environment of the vanadium in the bulk has been examined by ex-situ XAS measurement at the vanadium K edge, during the first discharge-charge cycle. The results show a regular and reversible evolution of the pre-edge intensity, the edge position and the vanadium environment toward nearly perfect VO6 octahedra.
INTRODUCTION Lithium trivanadate, Li1.2V3O8, has been investigated as a positive electrode material for rechargeable lithium batteries during the past decade [1-6]. Much research work has been focused on the structural characterization and cyclability of this compound. The crystal structure of Li1.2V3O8 was first reported by Wadsley [7]. Thackeray et al. [8] confirmed that Li1.2V3O8 has a monoclinic structure with the space group P21/m and is composed of two basic structural units, VO6 distorted octahedra and VO5 distorted trigonal bipyramids. These two structural units share edges and corners to form layers. Lithium ions are inserted between layers in two different sites, octahedral and tetrahedral. The lithium insertion mechanism has been reported by Thackeray [8] and Kawakita [9]. When it is used as a positive in a lithium battery, the Li1.2V3O8 compound can accommodate up to 3.8 lithium ions in the host structure with an operating voltage between 3.7V and 2V [8]. For 0 ≤ x ≤ 1.7, lithium is inserted in a single-phase reaction process. The voltage plateau observed for 1.7 ≤ x ≤ 2.8 is attributed to the coexistence of two phases; the first is Li3V3O8, and the second is a defect rock salt structure with a nominal composition Li4V3O8. It was found that the electrochemical properties of Li1.2V3O8, such as discharge capacity and cyclability, depend on the synthesis method of the compound and the preparation conditions of the positive electrode [10-13]. In this paper, we focus on the characterization of the lithium insertion process during the first discharge-charge cycle. The local structure determination of the various phases and the modification generated by lithium insertion are investigated by XAS in order to get better understanding of the electrochemical behavior of Li1.2V3O8.
EXPERIMENTAL The lithium trivanadate is prepared by solid state reaction of Li2CO3 and V2O5 with a molar ratio of 1.2/3 at 580°C in air. The positive electrodes are prepared by mixing the active material, carbon black (Super P from Chemetal) and binder (polyvinylidene difluoride: PVDF) with the
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massic ratio (85:10:5), and coating the mixture onto an Al disk serving as the current collector, according to [14]. Such electrodes were vacuum-dried at 100°C for 1 h before entering a dr
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