Vibrational Spectroscopic Studies of the Local Environment in 4-Volt Cathode Materials
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ABSTRACT We report the vibrational spectra of numerous 4-volt cathode materials, the transition metal oxides which are potential materials for advanced Li-ion batteries. They provide high specific energy density, high voltage, and remarkable reversibility for lithium intercalation-deintercalation process. Studied were carried out by Raman and FTIR spectroscopies. Oxides such as LiMn 20 4 , LiNiVO 4 , LiCoVO 4 spinels, LiMeO 2 (Me=Co, Ni, Cr) layered compounds and their mixed compounds have been investigated. The local environment of cations against oxygen neighboring atoms has been determined by considering tetrahedral and octahedral units building the lattice. Structural modifications induced by the intercalation-deintercalation process, by the cation substitution, or by the low-temperature preparation route are also examined. The results are compared with those of end members. INTRODUC[ION The recent interest in developing advanced rechargeable lithium batteries such as lithium-ion type cells, has stimulated investigation on high-performance positive electrodes, the so-called four-volt cathode materials [1]. Transition metal oxides are potential cathode materials providing high specific energy density, high voltage, and remarkable reversibility for lithium intercalationdeintercalation process. According to their crystallographic structure, one can distinguish two classes of materials: those with a spinel-type structure such as LiMn 20 4 , LiNiVO 4 , LiCoVO 4 , and
those with a layered-type structure such as LiMeO 2 (Me=Co, Ni, Cr) compounds and their mixed compounds. Since the electrochemical characteristics of the 4-volt materials depend on their crystal nature, size and shape, various kinds of preparation methods have been developed to improve the characteristics of these cathodes [2]. Also, it has been pointed out that the rechargeability can be markedly improved and the fading loss upon cycling can be minimized for mixed or for substituted transition metal oxides [3-4]. The current debate consists in the investigation of the local structure of these compounds because of the difficulty encountered in using powder x-ray and neutron diffraction for an unambiguous structural determination [5]. Vibrational spectroscopies, i.e., Raman scattering (RS) and Fourier transform infrared (FTIR), are sensitive to the short-range environment of oxygen coordination around the cations in oxide lattices. Since RS and FTlR techniques can solve the problem of phase determination when various environments are present. Spectra consist of a superposition of the components of all local entities. The frequencies and relative intensities of the bands are sensitive to coordination geometry and oxidation states. Thus, spectra are less affected by the grain size or the degree of long-range order of the lattice. We present in this paper the vibrational spectra of numerous 4-volt cathode materials with spinel-type and layered-type structure. Oxides such as LiMn 2 0 4 , LiMeVO 4 (Me=Ni, Co) spinels, LiMeO 2 (Me--Co, Ni, Cr) layered compoun
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