Soft Chemical Synthesis and Electrochemical Li Insertion Properties of Li2Ti3O7 with NaLiTi3O7-Type Structure

Abstract

Introduction Lithium titanium oxides such as spinel-type Li4Ti5O12 and ramsdellite-type Li2Ti3O7 have been extensively investigated as electrode materials for rechargeable Li-ion batteries. In recent, another metastable phase of Li2Ti3O7 with Na2Ti3O7-type layered structure have been reported with the structural details and electrochemical Li-ion insertion properties [1]. On the other hand, NaLiTi3O7 has been investigated as electrode materials for rechargeable Li-ion batteries [2]. This Ti4+/Ti3+ electrode showed lower operating voltage compared to Li4Ti5O12. In addition, NaLiTi3O7 has a three-dimensional tunnel structure which is thought to be favorable for fast Li-ion transfer. This compound can be prepared by a conventional solid state reaction. Li2Ti3O7 with the NaLiTi3O7-type structure may be prepared by ion exchange of NaLiTi3O7 (Fig. 1). Higher Li-ion insertion capacities similar to that observed in Li2Ti3O7 with Na2Ti3O7-type layered structure can be expected. However, to our knowledge, synthesis, the structural details and electrochemical Li-ion insertion reactions in this compound have not been reported yet. In the present study, we have successfully synthesized single-phase sample of Li2Ti3O7 with the NaLiTi3O7-type structure and determined its crystal structure by Rietveld analysis. Furthermore, the Li-ion insertion properties of this compound have been clarified for the first time. Experimental The precursor NaLiTi3O7 was first prepared by a conventional solid state reaction by using a method similar to that reported previously [3]. A mixture of Na2CO3 (99.9% pure), Li2CO3 (99.99% pure) and TiO2 (99.99% pure) in a molar ratio of 1.01:1.03:3 was heated at 950°C for 24 h in air. Then, the lithiated Li2Ti3O7 samples were prepared from NaLiTi3O7 via ion exchange at a low temperature. Sodium/lithium ion exchange experiments were performed using the molten salt of LiNO3. Ion exchange reaction was performed at 400°C for 6h in air. After heat treatment, the reaction mixture was washed with ethanol and then dried at 80°C for 1 day in air. Further Li-ion exchange treatment was accomplished by heating the as prepared Li2Ti3O7 in molten LiNO3at 380°C for 6 h in air. The phase purity and crystal structure of the obtained samples were characterized by powder X-ray diffraction (XRD) using a Bruker AXS D8 ADVANCE diffractometer with Cu Kα radiation source filtered by a Ni thin plate (Cu Kα radiation, operating conditions: 40 kV, 55 mA). The particle morphology and chemical composition were verified by scanning electron microscopy equipped with energy dispersive X-ray spectrometer (SEM-EDX; Keyence VE-8800). The chemical and structural characteristics were evaluated using ICP, DTA and FTIR measurements. Electrochemical lithium insertion/extraction experiments were performed using lithium coin-type cells (CR2032-type). The working electrode were made of 80wt% active materials, 10wt% carbon black (Super-P) as a conductive agent, and 10wt% poly(vinylidene difluoride) as a binder. Copper foil was used as a current collector, and the area of the electrode was a diameter of 14 mm. The counter electrode was a Li foil having a diameter of 16 mm. The separator was a microporous polypropylene sheet. A solution of 1 M LiPF6 in a 1:2 mixture of ethylene carbonate (EC) and dimethycarbonate (DMC) by volume (Mitsubishi Chemical Co., Ltd.) was used as the electrolyte. Cells were constructed in a dry room, and electrochemical measurements were carried out with a current density of 10 mA g−1between 0.5 and 2.0 V at 25°C after standing 6h under an open circuit condition. Results and discussion Figure 2 presents the XRD patterns of the NaLiTi3O7 and Li2Ti3O7 samples. These data suggested that the products were single-phase samples of NaLiTi3O7 and Li2Ti3O7. Chemical analysis confirmed that the removal of Na was not complete at the present experimental condition. The electrochemical measurements revealed that the initial Li insertion capacities were 152 mAh g−1 and 243 mAh g−1 for NaLiTi3O7 and Li2Ti3O7, respectively, which were equivalent to 1.62 and 2.45 electron transfer per each formula unit. In addition, reversible Li-ion insertion and extraction reactions were observed in these samples, although a capacity loss of approximately 50 mAh g−1 and 75 mAh g−1 for the first cycle was observed in NaLiTi3O7 and Li2Ti3O7, respectively. Acknowledgement This work was supported by the “Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING project)” of the New Energy and Industrial Technology Development Organization (NEDO; Japan).