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

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).

References

[1] K. Chiba et al., Solid State Ionics, 178, 1725 (2008).

[2] S.Y. Yin et al., Electrochem. Commun., 11, 1251 (2009).

[3] L.M. Torres-Martínez et al., Solid State Sci. 8, 1281 (2006).