Electrochemical Properties of TiO₂(R) As a High-Potential Negative-Electrode for Lithium Ion Batteries

Abstract

We have previously reported that a high potential negative electrode (TiO2(B)) is promising as a negative electrode material for large-scale Li-ion batteries (LIB), which require high capacity (> 300 mAh g-1), high efficiency, long life, low cost and high safety [1]. However, its morphology, needle-like structure, is a problem for attaining a high volumetric energy density of the electrode. Ramsdellite type lithium titanium oxide (TiO2 (R)) also has a high theoretical capacity (335 mAh g-1) than spinel type lithium titanium oxide (175 mAh g-1) [2, 3]. It crystalizes without any specific anisotoropy, and hence a high packing density of the electrode and a high energy density will be achieved. In the present study, we prepared TiO2(R) from Li2CO3 and TiO2(anatase) as raw materials, and investigated the charge/discharge properties as a high potential negative electrode for lithium ion batteries. Ramsdellite type LiTi2O4was synthesized according to the following processes using solid-state reactions. Li2CO3 + 4TiO2(anatase)→Li2Ti4O9 [750 oC, 12 h, air] Li2Ti4O9 → 2LiTi2O4 [1000 oC, 18 h, Ar/H2(10%)] TiO2(R) was obtained from LiTi2O4 by ion exchange in 1 M HCl for 3 days. Charge/discharge properties of the TiO2(R) were investigated at C/30-10C rates between 1.2 and 3.0 V using a two-electrode coin-type cell with a composite working electrode(TiO2(R): Ketjenblack: PVDF = 8:1:1), a Li metal counter electrode, and 1 M LiPF6dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate(DMC) (1:2, by vol.). XRD pattern of the resultant TiO2 (R) powder is shown in Fig. 1. The pattern indicated that single-phase TiO2(R) was obtained. The color of LiTi2O4 powder was black, while the TiO2(R) was white. These should result from the changes in formal oxidation number of Ti from +3.5 to +4. The diameter of TiO2(R) powder was estimated to be 2.5-10 mm from the SEM image of the TiO2 (R) powder shown in the inset in Fig. 1. Moreover, the tap density of TiO2 (R) powder was evaluated to be as high as about 1.33 g cm-3. Therefore, high-energy density is expected. Charge and discharge curves of obtained for a TiO2(R) composite electrode at C/6 are shown in Fig. 2. A couple of plateaux due to the insertion/extraction of lithium ion were observed at around 1.3 V and 2.2 V. This fact suggests that two types of insertion/extraction sites exist in TiO2 (R). One is an energetically stable site and the other is relatively unstable. The TiO2 (R) electrode showed an initial discharge capacity as high as 205.8 mAh g-1. In addition, the cycleability was high; 98.4 % of the initial capacity was retained in the 50th cycle. The electrode density was determined to be 1.24 g cm-3, and therefore the gravimetric capacity was converted to a volumetric capacity of 255.2 mAh cm-3. The rate performance for a TiO2(R) composite electrode was shown in Fig. 3, together with that for TiO2(B) as a reference. The initial discharge capacity reached as high as 229.7 mAh g-1 at C/30. The capacity retention was higher than that of TiO2(B), particularly at > 3 C rate. The discharge capacity in the 25th cycle (202.7 mAh g-1), which was obtained after charge/discharge operation at a high rate of 10 C, was almost the same as that in the 7th cycle (204.1 mAh g-1). These results suggest the high durability of TiO2(R) electrodes.