Structural, Magnetic and Electrochemical Properties of the Spinel LiMn 2-y Co y O 4 Nanosized Powders
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Structural, Magnetic and Electrochemical Properties of the Spinel LiMn2-yCoyO4 Nanosized Powders Nourredine Amdouni1, François Gendron2, Alain Mauger3, and Christian M. Julien2 1 PCMS, Tunis University, Campus Universitaire, El Manar, Tunis, 2092, Tunisia 2 INSP, University Paris 6, 140 rue de Lourmel, Paris, 75015, France 3 MPPU, CNRS, 140 rue de Lourmel, Paris, 75015, France ABSTRACT We present the synthesis, structure, magnetic properties and electrode behavior of LiMn2-yCoyO4 (0≤y≤0) spinel oxides prepared by the wet-chemistry via the carboxylic acid route. LiMn2-yCoyO4 samples crystallize with the cubic spinel-like structure (Fd3m S.G.). Optical spectra indicate that the vibrational mode frequencies and relative intensities of the bands are sensitive to the covalency of the (Co,Mn)-O bonds. Magnetic susceptibility and electron spin resonance measurements show the compositional dependence of the magnetic parameters, i.e. Curie temperature, Curie-Weiss constant and Néel temperature, when Mn is substituted by Co. The overall electrochemical capacity of LiMn2-yCoyO4 oxides have been reduced due to the 3d6 metal substitution, however, a more stable charge-discharge cycling performances have been observed when electrodes are charged up to 4.3 V as compared to the performance of the native oxide. INTRODUCTION Lithium-ion batteries using transition-metal oxides as positive electrodes are currently dominating the portable power sources industry for laptop computers and mobile telephones. The great success of Li-ion type cells has stimulated searches on high-performance for alternative positive electrodes such as LiMn2O4, but the electrochemical properties of spinel LiMn2O4 are highly dependent on the synthesis route, composition, and thermal history of the samples [1]. The partial substitution of metal cations for the Mn forming the spinel derivatives LiMn2-yMyO4 (M=Ni, Co, Al, Cr, Fe, etc.) solid solutions is a strategy to improve significantly the electrochemical cycling of LiMn2O4 materials, but at the expense of a decrease in initial capacity within the useful voltage window, i.e. below 4.4 V [2-7].. Such a successful result is due to replacement of the Mn3+ Jahn-Teller (JT) ions, which provoke a tetragonal phase transition in the 3-V region [8-9]. Despite the decreasing first discharge capacity upon substitution, cobalt-doped spinels seem to be the most promising substituted materials investigated so far [9-12]. Two main effects appear with the presence of Co in LiMn2O4: (1) an enhancement of the exchange current density and (2) an easy charge-transfer reaction of the active material [9]. In this work, we present the characterization and electrode behavior of LiMn2-yCoyO4 (0≤y≤1) prepared by wet-chemistry via the citrate precursor routes. The phase evolution is studied as a function of the cobalt substitution and the modification on the intercalation and de-intercalation of Li ions. EXPERIMENT Undoped LiMn2O4 and LiMn2-yCoyO4 (0≤y≤1) powders were prepared with the same technique, i.e. the carboxylic a
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