Electrochemical Properties of TiO2(R) 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.