For improving operational life and safety of lithium-ion battery, 1.5 V vs. Li/Li⁺ materials such as spinel Li₄Ti₅O₁₂ (LTO) have been extensively studied as good alternatives to graphite as the anode material. Several kinds of isostructural materials such as Li₂AB₆O₁₄ (A=M(II), B=M(IV)) have been investigated as a titanium based anode [1-2]. These compounds have a three-dimensional framework of edge and corner shared BO₆ octahedra and lithium can be inserted to the tetrahedral vacancy-site. A reversible capacity of approximately 150 mAh/g was obtained in Li_2+xSrTi₆O₁₄ [2], which is close to the LTO capacity. Although, the charge-discharge curve of this material shows a step-like plateau, the crystal structural change during the charge-discharge process has not been explained in detail. In this work, a new family of Li₂(Sr_1-xNa_x)Ti₆-_xNb_xO₁₄ (0≤x≤1) was prepared, and the crystal structure at different state of charge and the electrochemical performance were investigated.

Experimental

Li₂(Sr_1-xNa_x)Ti₆-_xNb_xO₁₄ (0≤x≤1) were synthesized by a conventional solid-state reaction method. A mixture of stoichiometric quantities of raw materials such as Li₂CO₃, Na₂CO₃, SrCO₃, Nb₂O₅ and TiO₂ was heated initially at 650℃ for 2 hours, then sintered at 1100℃ for 48 hours with an intermittent grinding. The products were characterized by X-ray powder diffraction (XRD). Electrodes of these compounds were prepared as follows; a mixed slurry of the product powder, conductive agent, binder and solvent, was coated on a sheet of Al foil using a doctor blade and then dried at 80℃ under vacuum condition for 12 hours. Electrochemical performance was investigated using a three-electrode glass beaker cell which was assembled using the prepared electrode and a lithium metal as a counter and a reference electrode. The structural changes of the compound during lithium insertion process were characterized using ex-situ X-ray powder diffraction (XRD) at BL19B2 beam line in the SPring-8 synchrotron radiation research facility and ex-situ neutron powder diffraction (NPD) at iMATERIA in the Japan proton accelerator research complex.

Results and Discussion

The charge-discharge curves of Li₂(Sr_1-xNa_x)Ti₆-_xNb_xO₁₄ (x=0 and 0.75) electrodes are shown in Figure 1. The reversible capacity was improved by the substitution of Na and Nb in the potential range between 1.0 V and 2.0 V vs. Li/Li⁺. Moreover, the step-like plateau was shown in x=0, which was flattened with increasing substitution amount. A maximum capacity of 133.4 mAh/g was obtained at x=0.75. The reversible capacity for x=0 was restricted to 106.8 mAh/g in this potential range, while approximately 150 mAh/g has been reported at a lower potential range such as 0.5 V-2.0 V vs. Li/Li⁺ owing to an existence of the step-like plateau [2]. Therefore, we consider that the improvement of the reversible capacity is mainly attributed to the lifted-up redox potential area below 1.2 V to around 1.4 V vs. Li/Li⁺ as shown in Figure 1, namely this results in an enhancement of effective capacity in this potential range. Then, the crystal structure parameters for x=0 and 0.75 were investigated by XRD and NPD measurements to clarify differences among the lithium host sites in the structure.

Table 1 shows proposed lithium host sites for Li₂(Sr_1-xNa_x)Ti₆-_xNb_xO₁₄ and bond valence sum (BVS) obtained by the Rietveld analysis using neutron diffraction patterns for x=0 and 0.75. The BVS values of 4a and 8c were close to 1.0 for x=0, implying that 4a and 8c sites are mainly used as a lithium host site, and 4b site was getting closer to 1.0 for x=0.75. This result suggests that an effective lithium host site can be added by the substitution. From these findings, we believe that the step-like plateau was flattened due to structural modification on lithium host sites in Li₂(Sr_1-xNa_x)Ti₆-_xNb_xO₁₄. The detail of electrochemical performance and structural analysis will be discussed in our presentation.

References

1. I. Koseva, J.-P. Chaminade, P. Gravereau, S. Pechev, P. Peshev, J. Etourneau, J. Alloys and Compounds, 389, 47 (2005).
2. I. Beharouak, K. Amine, Electrochemistry Communications, 5, 435 (2003).