Unveiling the Formation Mechanism and Phase Purity Control of Nanostructured Li4Ti5O12 via a Hydrothermal Process

Although numerous efforts have been devoted to the spinal Li4Ti5O12 anode material of lithium-ion batteries (LIBs), controllable synthesis of high-purity Li4Ti5O12 nanoparticles under hydrothermal conditions has not yet been achieved. The current work systematically investigates the relationship between phase compositions of the Li–Ti–O system with the corresponding critical conditions. By determining the phase composition using the Rietveld refinement of XRD patterns, the relationship between key hydrothermal parameters and the resultant purity of Li4Ti5O12 is revealed, and the formation mechanism of Li4Ti5O12 using crystalline TiO2 nanoparticles (Evonik Aeroxide P25, Hombikat 8602, and rutile TiO2) and amorphous hydrous TiO2 spheres (AHTS) as TiO2 precursors in an aqueous LiOH solution is demonstrated accordingly. The hydrothermal process cannot generate Li4Ti5O12 directly, while lithium titanium oxide intermediates (LTOIs), e.g., Li2TiO3 and Li2–xHxTi2O4(OH)2, are obtained upon partial lithiation of crystalline or amorphous TiO2 in the LiOH solution, respectively. Nanostructured TiO2@LTOIs can be formed since the underlying phase transition follows the classic in situ crystallization mechanism, as evidenced by the successful self-template synthesis using AHTS. Li4Ti5O12 is formed by the solid-state reaction between the TiO2 core and the LTOI shell at elevated temperatures (∼700 °C). At optimal conditions, the molar ratio of Li/Ti in all TiO2@LTOIs should be greater than a stoichiometric ratio of 4:5 since the solid-state reaction is found to promote the evaporation of lithium species. The comparative studies for the lithium storage properties of our assembled half- and full-cell LIBs demonstrate that the high phase purity of Li4Ti5O12 enhances the electrochemical performances and that the spherical morphology delivers better cyclic stability.