Single Phase Li 4 Ti 5 O 12 Synthesis for Nanoparticles by Two Steps Sintering

Li4Ti5O12 has been noticed about a negative electrode of a high powered and safe lithium ion secondary battery. These properties require single phase, high crystallization, larger specific surface area and fine nanoparticles. This study carried out the noble synthesis of Li4Ti5O12 using a solid phase synthesis by two steps sintering. These results showed Li4Ti5O12 of 6.1 m2∙g−1 and diameter of 110 nm with the single phase and high crystallization. Li2TiO3 will play an important role in this reaction, obtained by pre-sintering as a precursor.


Introduction
Lithium ion secondary batteries have been noticed for many applications to high performances recently, and the developments have been performed [1].For example, material development of negative electrode has been concerned with safety.The material is currently graphite, which has layer structure and is intercalated by lithium ions into the layers.On the other hand, the graphite is not safe to expand with overcharge to intercalate amount of lithium ions over the stoichiometry into the layer.Therefore, noble materials have been investigated instead of the graphite to prevent ignition by internal short-circuit [2].The noble materials need some properties, whose crystal structure shows no change and high stability with intercalate lithium ions.Furthermore, the synthesis is easy and inexpensive to mass production.Here, we have studied Li 4 Ti 5 O 12 as superior negative electrode materials [3].
Li 4 Ti 5 O 12 has spinel structure and LiMn 2 O 4 does likewise, which is applied to the positive electrode in the lithium ion secondary battery.The properties of Li 4 Ti 5 O 12 are hardly expanded by the overcharge, and high stabil-ity in cycles of discharge and charge.The synthesis methods are known as solid phase synthesis, sol-gel process, hydrothermal synthesis, coprecipitation method and vapor phase deposition method.Especially, the solid phase synthesis is a good method for industrial processes, which is simple and easy to control composition of a chemical compound.However, it has some problems, such as a low reactivity, a control of fine particle size and a production of by-products.The synthesis developments have been performed to solve them [4] [5].
Consequently, the synthesis method is needed to produce Li 4 Ti 5 O 12 of a single phase, fine nano-size particles and a high crystalline in order to show high performance as the battery.A synthesis process has been performed by sintering with low temperatures by a two-step process of a pre-sintering at 400˚C -500˚C and a sintering at 700˚C -750˚C, because we have tried to synthesize a single phase and fine nano-size Li 4 Ti 5 O 12 .

Experimental
Li 4 Ti 5 O 12 synthesis was carried out by using CH 3 COOLi•2H 2 O (Wako Pure Chemical Industries, Ltd.) as a Li source that was melted with low temperature at about 300˚C, and anataseTiO 2 (Toho Titanium Co., Ltd.) as a Ti source.The first synthesis process was mixed with the CH 3 COOLi•2H 2 O and TiO 2 with Li:Ti = 4:5 by ball milling (Fritsch, Pulversitte 7) at 1 h, rotating speed at 320 rpm and orbital speed at 110 rpm in agate mortar and balls.The mixed powder was pre-sintering at 400˚C, 450˚C and 500˚C with 10˚C/min, 1 h in air, and a precursor was formed.Furthermore, the precursor was mixed by the ball milling at 1 h similarly, and was sintered at 700˚C and 750˚C with 10˚C/min at 1 h in air.
The obtained powder was identified by XRD (Rigaku Corp., Rint 2000) at scanning step 0.02 deg and scanning speed 5 deg/min by CuKα, and was also measured by BET specific surface area (Shimadzu Corp., Flow-Sorb III 2305) at 0.1 g sample, gas flow rate 80 cm 3 /min in N 2 and current 50 mA, after degassing the sample with heating at 160˚C at 2 h.Particle size of the obtained sample was observed by FE-SEM (Hitachi, Ltd., S-4200), and crystalline estimation tried with TEM (JEOL Ltd., JEM-2100).

Sintering
XRD peaks of samples ware showed at sintering temperature of 700˚C and 750˚C in Figure 4 and  The value at 700˚C was 80%, 84% and 86% at the pre-sintering temperature of 400˚C, 450˚C and 500˚C, which were advantageous with the high pre-sintering temperature.Furthermore, the value at 750˚C was 92%, 99% and 100% similarly, and we could obtain the single phase Li 4 Ti 5 O 12 at 750˚C.These results will show that the sub-phase Li 2 TiO 3 give effects for the Li 4 Ti 5 O 12 synthesis.
A surface separation of (002) face of Li 2 TiO 3 at 4.80 Å is very close to that of (111) face of Li 4 Ti 5 O 12 at 4.83 Å in the spinel structure, which Li 4 Ti 5 O 12 would be able to form from Li 2 TiO 3 with holding the structure [8].The knowledge will show that Li 4 Ti 5 O 12 was synthesized via Li 2 TiO 3 by the pre-sintering, and the single phase was obtained at 750˚C in this process.
SEM images of the samples showed in Figure 6.The average grain size was observed at about 90 nm in any samples at sintering temperature of 700˚C in Figures 6(a)-(c), and the BET specific surface area was about 10.5 m 2 •g −1 for any samples.Similarly, it was at 110 nm under 750˚C in Figures 6(d)-(f), and was 7.3 m 2 •g −1 , 6.3 m 2 •g −1 and 6.1 m 2 •g −1 in the specific surface area.The grain size of the single phase Li 4 Ti 5 O 12 have reported about 600 nm in the solid phase synthesis under 850˚C at 12 h by boll-milling [9]- [11], and Guerfi has especially reported the grain size of 150 nm by mixing graphite into the ball mill [9].On the other hand, we could synthesize that of grain size of 110 nm via Li 2 TiO 3 at 750˚C by the two steps sintering method.
TEM images of the samples showed in Figure 7, and the shape of the grain indicated uniformity in any samples.Furthermore, morphology was improved at the sintering temperature of 750˚C compared with at 700˚C, and facets in the grain were observed by a crystal growth.The morphology have been reported about the synthesis by using nano-particle or nano-wire TiO 2 , which have described that the nanoparticle or nanowire Li 4 Ti 5 O 12 was obtained on keeping the TiO 2 morphology like a mold [12] [13].This knowledge will provide a synthesis mechanism for this two steps sintering method, which the (002) face of Li 2 TiO 3 plays a role of the mold in the spinel structure.These may show the reason why the surface separation of the (002) Li 2 TiO 3 has very close it in the (111) Li 4 Ti 5 O 12 .

Figure 2 .
Figure 2. SEM images of the samples for the pre-sintering temperature of (a) 400˚C, (b) 450˚C and (c) 500˚C.BET specific surface area presented 27.7 m 2 •g −1 , 26.6 m 2 •g −1 and 26.2 m 2 •g −1 under 400˚C, 450˚C and 500˚C.The images of the grain observed two types as deference of contrast at 400˚C in Figure 2(a), and showed a uniform contrast with the high temperature.These results might indicate a localization of unreacted TiO 2 and subphase Li 2 TiO 3 in keeping with XRD patterns.This tendency may confirm TEM images in Figure 3, which will show a low crystallization with corresponded to XRD not to observe facet areas.

Figure 5 .Figure 4 .
Peaks intensity of Li 2 TiO 3 decreased, while those of Li 4 Ti 5 O 12 increased with pre-sintering temperature at 700˚C sintering in Single phase of Li 4 Ti 5 O 12 was obtained at the pre-sintering temperature of 500˚C in Figure5, however the peaks intensity indicated the similar tendency about pre-sintering temperature with the sintering temperature of 700˚C.Here, we defined a single phase rate to estimate it, which utilized the peaks area of them at shown in following equation.

Figure 3 .
Figure 3.TEM images of the samples for the pre-sintering temperature of (a) 400˚C, (b) 450˚C and (c) 500˚C.

Figure 4 .
Figure 4. XRD patterns of the samples sintered at 700˚C with the pre-sintered temperature of 400˚C (black line), 450˚C (blue line) and 500˚C (red line), which show •; Li 4 Ti 5 O 12 and ▲; Li 2 TiO 3 .

Figure 5 .
Figure 5. XRD patterns of the samples sintered at 750˚C with the pre-sintered temperature of 400˚C (black line), 450˚C (blue line) and 500˚C (red line), which show •; Li 4 Ti 5 O 12 and ▲; Li 2 TiO 3 .

Figure 6 .
Figure 6.SEM images of the samples sintered at 700˚C with the pre-sintered temperature of (a) 400˚C, (b) 450˚C and (c) 500˚C; and at 750˚C with the pre-sintered temperature of (d) 400˚C, (e) 450˚C and (f) 500˚C.

Figure 7 .
Figure 7. TEM images of the samples sintered at 700˚C with the pre-sintered temperature of (a) 400˚C, (b) 450˚C and (c) 500˚C; and at 750˚C with the pre-sintered temperature of (d) 400˚C, (e) 450˚C and (f) 500˚C.