TiO2(B) powders were synthesized by the heat treatment of the rod or belt-shaped H-titanate powders at 450°C for 6 hrs, which had been prepared by a hydrothermal process. The combined process of the hydrothermal and heat treatment has been used for the synthesis of TiO2(B) powders [11, 12]. The synthesized TiO2(B) powders were ball milled using a planetary type ball mill at a fixed rotation speed of 300 rpm for up to 3 hrs to make them smaller sizes. The changes of morphology and size of the powders with the milling time were observed under SEM as shown in Figure 1. The as-synthesized TiO2(B) powders were observed from Figure 1(a) to be long rod or belt shaped. The powders were broken into smaller sizes after 1 hr ball milling (Figure 1(b)). However, they became severely agglomerated after ball milling for 3 hrs as can be seen in Figure 1(c). The size distribution of the TiO2(B) powders with the milling time was graphed in Figure 2. As-synthesized TiO2(B) powders with an average of 24 μm became broken into smaller sizes of 0.7 and 0.2 μm after ball milling for 1 and 3 hrs, respectively.
Figure 3 shows the XRD patterns and Raman spectra for the samples of as-synthesized and ball milled for up to 3 hrs. Typical peaks for TiO2(B) phase were observed from all the samples as in Figure 3(a). It was also noticed that the intensity of the main peak at 24.979 deg. was gradually decreased but the FWHM was increased with the milling time, indicating the particles were becoming smaller sizes. These results were found to be consistent with the SEM observations in Figure 1 for the size changes of the particles with the milling time.
Raman spectra showed that as-synthesized sample was composed of primarily TiO2(B) with the minor phase of anatase as seen in Figure 3(b). However, intensity of the anatase became stronger with the ball milling time, resulting in a dominant phase after ball milling for 3 hrs.
Figure 4 shows the initial charge–discharge performance for the TiO2(B) samples measured in the range of 1.0 – 3.0 V with a lithium foil as a counter electrode. As-synthesized TiO2(B) sample had the charging capacity of about 217 mAh/g, discharging capacity of 190 mAh/g, and the resultant irreversibility of about 12.4%. Meanwhile, the sample ball milled for 3 hrs was measured with the charging capacity of about 180 mAh/g, discharging capacity of 164 mAh/g.
The reduced charge–discharge capacity of the 3 hrs ball milled sample seemed to be caused by the presence of the strong anatase peaks identified with Raman spectra in Figure 2. Similar to our results, TiO2 anode materials with anatase or rutile phase were reported to have a relatively low charging capacity of about 150 – 200 mAh/g [13, 14]. It is also well known that charge–discharge capacity is much affected by the particle size of the electrode materials; smaller particles show better performance [15, 16]. On the other hand, TiO2(B) sample ball milled for 1 hr was measured to have a high charging capacity of 250 mAh/g and discharging capacity of 232 mAh/g, and irreversibility of 7%. Based on the results with 1 hr and 3 hrs ball milled samples, the performance was found to be affected positively by the smaller scale of particles, but negatively by the presence of anatase phase; the presence of anatase phase with a strong intensity in the 3 hrs ball milled sample even with smallest particles deteriorated the performance.
Cyclic stability of charge–discharge capacity and capacity variation as a function of C-rate was compared among the samples in Figure 5. As can be seen in Figure 5(a), TiO2(B) sample ball milled 1 hr showed a high degree of cyclic stability. About 98% of the initial discharge capacity was maintained even after 50 cycles; initial capacity of 232 mAh/g was reduced only to 227 mAh/g. On the other hand, as-synthesized sample and 3 hrs ball milled sample showed a gradual decrease of the capacity with the cycle suggesting smaller particle size without anatase phase in TiO2(B) was an optimum condition for the high electrochemical performance. Discharging capacity was gradually decreased with the C-rate regardless of the sample types as compared in graph Figure 5(b). But the gaps in the initial capacity among the samples were observed to be kept during the whole measurements in the C-rate range of 0.2 – 10 C.