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

Li[Li_1/3Ti_5/3]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 Li_1/2+1/2xFe_5/2–3/2xTi_xO₄ (LFTO) between LTO and Fe[Li_1/2Fe_3/2]O₄ via an intermediate spinel compound Li_1/2Fe_1/2[Li_1/2Fe_1/2Ti]O₄. Fe[Li_1/2Fe_3/2]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.

Acknowledgement

This work was supported by Grant-in-Aid for Scientific Research (C), 25410207, MEXT, Japan.

References

[1] T. Ohzuku, A. Ueda, and N. Yamamoto, J. Electrochem. Soc., 142 (1995) 1431.