Synthesis and Crystal Structure of Lithium Iron Titanium Oxides Li1/2-+1/2xFe5/2-3/2xTixO4 with 0 ≤ x ≤ 1.666

Abstract

Li[Li1/3Ti5/3]O4 (LTO) is an ideal negative electrode material for long-cycle life lithium-ion battery (LIB), because of its “zero-strain” character [1]. In the LTO crystal lattice with Fd-3m space group, Li+ ions occupy both tetrahedral 8a and octahedral 16d sites, while Ti4+ ions sit the 16d site. We, thus, expect a solid solution compound Li1/2+1/2x Fe5/2–3/2x Ti x O4 (LFTO) between LTO and Fe[Li1/2Fe3/2]O4 via an intermediate spinel compound Li1/2Fe1/2[Li1/2Fe1/2Ti]O4. Fe[Li1/2Fe3/2]O4 is known to be one of the ferrites, which exhibit a soft ferromagnetic behavior. Here we report the results of powder X-ray diffraction (XRD) and electron diffraction (ED) measurements, and Raman spectroscopy analyses on the LFTO samples with 0 ≤ x ≤ 1.666. According to XRD measurements, all the LFTO samples were assigned as a spinel-framework structure. For the x = 0 sample, the extra diffraction lines were clearly observed in addition to the normal diffraction lines in the Fd-3m. The extra diffraction lines are attributed to the superlattice diffraction lines due to the ordering between Li+ and Fe3+ ions at the octahedral site. As x increases from 0.25, such superlattice diffraction lines disappear, but, surprisingly, superlattice diffraction lines appear again at x > 1.2. The crystal structure at x £ 0.25 and 1.2 < x < 1.6 are assigned as the P4332 space group. Combining with the result of x dependence of cubic lattice parameter (a c), the crystal structure of LFTO is classified into the four regions; (I) regular spinel structure of Fd3-m with x > 1.55, (II) superlattice spinel structure of P4332 with 1.2 ≤ x ≤ 1.55, (III) regular spinel structure of Fd3-m with 0.25 < x < 1.2, and (IV) superlattice spinel structure of P4332 with x ≤ 0.25.AcknowledgementThis work was supported by Grant-in-Aid for Scientific Research (C), 25410207, MEXT, Japan.