Lithium oxide solution in chloride melts as a medium to prepare LiCoO 2 nanoparticles
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Lithium oxide solution in chloride melts as a medium to prepare LiCoO2 nanoparticles Vladimir Khokhlov, Dmitriy Modenov, Vasiliy Dokutovich, Viktor Kochedykov, Irina Zakir’yanova, and Emma Vovkotrub, Institute of High-Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences, 620990 Ekaterinburg, Russia Igor’ Beketov, Institute of Electrophysics, Ural Branch, Russian Academy of Sciences, 620016 Ekaterinburg, Russia Address all correspondence to Vladimir Khokhlov at [email protected] (Received 24 July 2013; accepted 19 December 2013)
Abstract The paper describes a new technique of molten salt synthesis (MSS) that is based on the direct oxidation of halide ions with molecular oxygen in thermally stable halide melts to prepare nanoparticles of complex oxides. Lithium cobaltate (LiCoO2) was chosen as a model compound for testing this method. Synthesis was achieved in LiCl–CoCl2 melts at 600 and 700 °C, respectively, under a dry-air atmosphere. Fourier transform infrared (FTIR) and Raman spectroscopies, x-ray diffraction (XRD), and transmission electron microscopy (TEM) were used to study the products obtained. The route suggested results in the formation of stoichiometric high-temperature (HT) LiCoO2 powders.
Introduction Lithium cobaltate (LiCoO2) is a common cathode material used for making rechargeable lithium-ion batteries (LIBs). It is thermally stable and ensures large specific capacity and high operating voltage of the LIBs.[1,2] A number of studies[3–5] have shown that the use of the submicron LiCoO2 particles in the manufacture of cathodes raises the energy density and the quantity of the recharging cycles of the batteries. At present, much attention is being given to a search for suitable ways of reproducibly synthesizing LiCoO2 nanoparticles. Molten salt synthesis (MSS) is considered to be among the most promising techniques to prepare ultrafine powders.[6–10] The MSS of LiCoO2 is generally conducted by using nitrates, oxalates, and carbonates as precursors that undergo thermal decomposition to form Li2O and Co3O4. The subsequent step of annealing at temperatures of about 900 °C and above is required to complete the synthesis, but this process results in a hard-to-control aggregation of the particles (up to micron sizes) that reduces the LIBs discharge capacity and cycle life considerably.[5] The direct oxidation of halide ions with molecular oxygen in halide melts can be used for the synthesis of oxide compounds as well. Alkali, alkaline earth, rare earth, and transition metal halides are thermally stable over a wide temperature range. After being melted, they enter into reactions with oxygen giving metal oxides that dissolve in halide melts to a variable degree.[11–15] For molten salt mixtures like LiCl–MCl2 (M = Mn, Co, and Ni), lithium and transition metal oxides formed in situ interact with one another yielding insoluble mixed oxide compounds at much lower temperatures than in the event of the conventional solid state or molten salt methods.
Considerable aggregation of
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