Optimization of the Synthesis Conditions of Nanostructure 3DOM Li4Ti5O12 spinel with High Rate Capability

Li₄Ti₅O₁₂ spinel (LTO) is a potential candidate for anode materials in power LIBs due to the high Li-ion mobility and cyclability. At 1.56V Li₄Ti₅O₁₂ inserts 3 lithium ions with a theoretical capacity of 175 mAh/g. Li₄Ti₅O₁₂ spinel consists of lithium and titanium atoms randomly distributed on one-half of the octahedral sites and lithium atoms filling one-eighth of the tetrahedral sites within the oxygen close packed lattice.  As current is applied and lithium is passed, Ti⁴⁺ is reduced to Ti³⁺ within the octahedrally coordinated framework, allowing a topotactic transition between  Li₄Ti₅O₁₂ →current →Li₇Ti₅O₁₂. The inserted lithium ions and the tetrahedrally coordinated lithium move to occupy adjacent octahedral sites.

A variety of preparation methods, including sol-gel, hydrothermal and template synthesis, have been developed in order to obtain structures with large surface area and open pores as the three dimensionally ordered macroporous (3DOM) spinels. The synthesis of pure phases of 3DOM-LTO structures presents difficulties; some samples results in different amounts of Li₂TiO₃, rutile, and anatase as side products and the high temperatures destruct the porous structure. As calcination time increases, grains of lithium titanate grow larger and, consequently, the 3DOM structure breaks down. 3DOM architecture, however, comes at the expense of phase purity. In spite of the interest for this material, to the best of our knowledge reports on its preparation as 3DOM material are scarce. Since we found the result critically depended on the salt used and the thermal treatment, particular emphasis was given to the understanding of the formation process.

This study aims to achieve the best synthesis conditions to prepare nanostructured 3DOM-LTO. Samples have been synthesized by a colloidal templating process with and without hydrothermal conditions. It is also studied the effect of a pre-forming step and subsequent incorporation of lithium or directly addressing the chemical processes in one step. We have found that a well-defined sequence of steps during synthesis is necessary for the successful formation of the inverse opal. 3DOM-LTO synthetize samples have been studied by means of thermal analysis, x-ray diffraction (XRD) and solid-state nuclear magnetic resonance (NMR). Morphological and microstructural characterizations are carried out by scanning and transmission electron microscopy (FE-SEM, TEM) and by gas adsorption. The remarkable improvement of the electrochemical performances, specially a high current, is shown by comparing with LTO-commercial samples.

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Acknowledgements

This research has been supported by the projects MAT2011-22969, MAT2013-46452-C4-2-R and MATERYENER3CM  S2013/MIT-2753.