Li2-XFeTiO4 As a High Capacity Cathode Material for Lithium Ion Battery

1. Introduction

   Research focusing on iron-based cathode materials for lithium ion batteries tends to favor materials that operate on the (de) intercalation of one Li per Fe atom (based on Fe²⁺/Fe³⁺redox couple); thereby imposing an intrinsic limitation on the energy density of the cathode material. To drastically enhance the energy density, intercalation materials with more than one electron per Fe atom are needed. In commonly used cathode materials, however, more than one electron reaction has not been utilized for charge-discharge reaction. For the development of cathode materials which achieve more than one electron reaction, it is essential to investigate the reaction mechanism.

   This study investigates reaction mechanism of more than one electron reaction in cathode materials. Li_2-xFeTiO₄ is selected as an appropriate model compound, as both the Fe²⁺/Fe³⁺ as well as Fe³⁺/Fe⁴⁺ redox couples are expected to be utilized to achieve high capacity. The possibility of de (intercalation) of two lithiums from the spinel-type Li_2-xFeTiO₄ is examined and the mechanism underlying the Li⁺ (de) insertion mechanism in Li_2-xFeTiO₄is discussed.

    

2. Experimental

   Spinel-type LiFeTiO₄ was synthesized via the conventional solid state ceramics route. Electrodes prepared from this material were examined in two-electrode coin-type cells, using lithium metal as a counter electrode. Electrodes were prepared from LiFeTiO₄ to which carbon black was added and ball-milled at 400 rpm for 6 hours. Polytetrafluoroethylene (PTFE) binder was thereafter added. The weight ratio of LiFeTiO₄, carbon black and PTFE was 6:3:1. The electrolyte used was a 1 M solution of LiClO₄ in ethylene carbonate / diethyl carbonate. Initial lithium extraction from LiFeTiO₄ was carried out based on the capacity of one lithium per Fe atom. Cells were then cycled between 1.6V and 4.2V at various current densities. The electrochemical measurements were conducted at 55.0^oC. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements were conducted at SPring-8 and at SR Center, Ritsumeikan University.

    

3. Results and Discussion

   The pristine LiFeTiO₄ was characterized by Rietveld analysis. As shown in Fig. 1, all the reflections of the XRD patterns of the pristine LiFeTiO₄ are fully indexed to a cubic structure (space group Fd3m). The rietveld analysis shows that all cations are randomly distributed on the tetrahedral (8a) and octahedral (16d) sites. The refined lattice parameters are consistent with the previously reported values [1-3].

   The spinel-type LiFeTiO₄ can be electrochemically delithiated below the voltage of 4.2 V, to obtain FeTiO₄. The subsequent discharge and charge measurements show that a reversible capacity of about 250 mAhg^-1can be obtained under C/20 rate at an average voltage of 2.5 V with good capacity retention.

   For the discharged and charged Li_2-xFeTiO₄, XRD and XAS measurements were conducted to elucidate the crystal and electronic structural changes underlying the high capacity in spinel-type LiFeTiO₄. Fig. 2 shows XRD pattern of charged and discharged Li_2-xFeTiO₄. The spinel  structure of LiFeTiO₄ is maintained during both charge and discharge reaction. Additionally, changes in the lattice constants obey Vegard’s law, indicating a solid-solution behavior during the charge and discharge reaction in Li_2-xFeTiO₄.

    

4. References

1. M. A. Arillo, M. L. Lopez, E. Perez-Cappe, C. Pico, M. L. Veiga, Solid State Ion., 107, 307-312 (1998).

2. M. A. Arillo, M. L. Lopez, C. Pico, M. L. Veiga, A. Jimenez-Lopez, E. Rodriguez-Castellon, J. Alloys Comp., 160-163, 317-318 (2001).

3. M. A. Arillo, M. L. Lopez, C. Pico, M. L. Veiga, A. Jimenez-Lopez, M. L. Vega, Chem. Mater., 17, 4162-4167 (2005).