Structural Modification on Lithium Host Site in Li2(Sr1-x Na x )Ti6-XNb x O14 (0≤x≤1) Anode Material for Lithium-Ion Batteries (2018)

Introduction For improving operational life and safety of lithium-ion battery, 1.5 V vs. Li/Li + materials such as spinel Li 4 Ti 5 O 12 (LTO) have been extensively studied as good alternatives to graphite as the anode material. Several kinds of isostructural materials such as Li 2 AB 6 O 14 (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 BO 6 octahedra and lithium can be inserted to the tetrahedral vacancy-site. A reversible capacity of approximately 150 mAh/g was obtained in Li 2+ x SrTi 6 O 14 [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 Li 2 (Sr 1 - x Na x )Ti 6 - x Nb x O 14 (0≤ x ≤1) was prepared, and the crystal structure at different state of charge and the electrochemical performance were investigated. Experimental Li 2 (Sr 1 - x Na x )Ti 6 - x Nb x O 14 (0≤ x ≤1) were synthesized by a conventional solid-state reaction method. A mixture of stoichiometric quantities of raw materials such as Li 2 CO 3 , Na 2 CO 3 , SrCO 3 , Nb 2 O 5 and TiO 2 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 Li 2 (Sr 1 - x Na x )Ti 6 - x Nb x O 14 ( 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 Li 2 (Sr 1 - x Na x )Ti 6 - x Nb x O 14 and bond valence sum (BVS) obtained by the Rietveld analysis using neutron diffraction patterns for x =0 and 0.75. The BVS values of 4 a and 8 c were close to 1.0 for x =0, implying that 4 a and 8 c sites are mainly used as a lithium host site, and 4 b 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 Li 2 (Sr 1 - x Na x )Ti 6 - x Nb x O 14 . The detail of electrochemical performance and structural analysis will be discussed in our presentation. References I. Koseva, J.-P. Chaminade, P. Gravereau, S. Pechev, P. Peshev, J. Etourneau, J. Alloys and Compounds, 389, 47 (2005). I. Beharouak, K. Amine, Electrochemistry Communications, 5, 435 (2003). Figure 1