Influence of the emulsification conditions on the microstructures and electrochemical characteristics of spinel lithium
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Lithium manganese oxide powders (LiMn2O4) with a spinel structure were synthesized via an optimized water-in-oil emulsion process. The influence of the emulsification conditions on the microstructures and physicochemical properties of LiMn2O4 powders was investigated. The phase purity of the synthesized powders significantly depends on the water-to-oil volume ratio in the emulsion. Increasing the water-to-oil ratio tends to decrease the stability of the emulsion that in turn leads to a segregation of water and oil phases. The unstable emulsion system results in the formation of an impure phase—Li2MnO3—that markedly decreases the charge and discharge capacities of the cathode materials. When water/oil volume ratio equals 1/5 or 1/10, monophasic spinel powders are formed at temperatures as low as 400 °C. In addition, decreasing the concentration of the aqueous phase substantially reduces the particle size of LiMn2O4 powders. Nanometered-LiMn2O4 powders with a particle size of 50 nm are obtained when the concentration of the aqueous phase is 1.0 M and the water-to-oil volume ratio is 1/5. Decreasing the particle size of LiMn2O4 powders was demonstrated to effectively increase the specific capacity and improve the cyclability of LiMn2O4 powders.
I. INTRODUCTION
Lithium-ion secondary batteries have attracted much interest globally because of their high energy density, high working voltage, low self-discharge rate, and long shelf life. Among all cathode materials currently being considered, lithium manganese oxides are more advantageous in terms of cost and toxicity than lithium cobalt and lithium nickel oxides.1 Thus, several forms of lithium manganese oxides have been investigated for electrochemical applications, notably the spinel typeLiMn2O4.2–8 In previous reports, solid-state reactions are commonly employed to synthesize LiMn2O4.9–14 Because of the low reactivity of reactants, this kind of reaction necessitates a prolonged reaction time (24–48 h) at an elevated temperature (600–1000 °C) for the completion of the reactions. The prolonged heating at high temperatures inevitably causes significant grain growth and consequently reduces the efficiency of lithium-ion utilization at high current rates. Moreover, the hightemperature heating will also result in the evaporation of
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J. Mater. Res., Vol. 18, No. 3, Mar 2003 Downloaded: 14 Mar 2015
lithium species, which not only affect the stoichiometry of the obtained LiMn2O4 powders, but also deteriorate their electrochemical properties. The chemical processing using a soft chemistry approach via a solution route is advantageous for preparing fine powders with a precise composition. It is because the solution process is controlled at the atomic or molecular level and the homogeneity of precursor powders can be effectively enhanced. The improved homogeneity can also facilitate the synthesis of powders in short reaction time and at moderate temperatures. For the purpose of practical applica
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