3-1 Chemicals
The chemicals employed in the experiments are listed in Table 3-1. The listed reagents were all laboratory-grade and were used without further purification. For the solvents, the ethyl alcohol was used as arrived, and the water was purified through reverse-osmosis (Purelab Maxima/ELGA) being its resistivity higher than 18 MΩ-cm. As a source of TiO2, anatase with a particle size around 10 nm (ST-01, Ishihara-Sangyo Co., Ltd.), rutile with a particle size between 100 to 300 nm (PT-501, Ishihara-Sangyo Co., Ltd.), and P25 (Degussa), that is, a mixture of anatase and rutile, with a particle size around 30 nm were employed as arrived; the particles size was estimated from the SEM micrographs for rutile and P25, but for anatase was estimated from the XRD pattern (approximated to the crystal size). Figure 3-1 shows the SEM micrographs for the different TiO2 precursors. Amorphous hydrous titanium oxide (AHTO) was used (mesoporous titania nanoparticles MTN [38] was provided and synthesized from titanium ethoxide) having a spherical shape of around 300 nm secondary particle, according the SEM micrographs.
21 Table 3-1 List of reagents and solvents.
Chemical Reagent Formula Assay Supplier
Sodium hydroxide NaOH 97 % nacalai tesque Inc.
Japan
Potassium hydroxide KOH 95 % nacalai tesque Inc.
Japan
22
Figure 3-1 SEM micrographs of the TiO2 (a) P25 (b) Rutile PT-501, and (c) Anatase ST-01
3-2 Synthesis of LTO by Hydrothermal Treatment
Three methods have been adopted to synthesize LTO powder in batches as white powder, all of them follow by hydrothermal process, however, the principle mechanism of LTO formation is different among them because of the different ions employed in the treated solution, and hence the morphology is also different. In this thesis the stoichiometric ratio Li/Ti is defined according to the compound Li4Ti5O12 will be used rather than the molar ratio, for example, a molar ratio Li/Ti equal 4/5 will be presented as stoichiometric ratio Li/Ti = 1.
23
In the First Method, illustrated in Figure 3-2, NaOH and LiOH are mixed together in 30 mL of water or ethyl alcohol; the concentration of the solution is 1.0 M and 3.5 M, respectively, and then 1.8 grams TiO2 of is added (P25 or rutile;
the stoichiometric ratio of Li/Ti is 5.8). All the mixture is placed in Teflon-lined stainless steel autoclave and heats at 200 oC for 20 hours. The precipitate powder was washed several times until the conductivity starts to drop below 2.5 mS, dried at around 50 oC, and finally calcined at 500 oC for 2 h.
Figure 3-2 Schematic diagram of the hydrothermal synthesis: First Method 3.5 M LiOH
1 M NaOH
TiO2 (P25 or rutile)
Stirring the mixture: Stoichiometric ratio Li/Ti 5.8
Measurement Hydrothermal treatment at 200 oC for 20 h
24
In the Second Method, illustrated in Figure 3-3, to 20 mL of 0.4 M LiOH solution was added 420 mg AHTO, with this amount of AHTO the stoichiometric ratio Li/Ti is roughly 2.5 (The AHTO was obtained from the hydrolysis of TTIP). Then the mixture is sealed in a Teflon-lined stainless steel autoclave and heats at 185 oC for 8 hours, the white precipitated powder is filtered, washed several times till the conductivity starts to drop below 2.5 mS, dried at around 50 oC, and then calcined at 550 oC for 4 h.
Figure 3-3 Schematic diagram of the hydrothermal process: Second Method 20 mL of 0.4 M LiOH 412 mg AHTO
Stirring the mixture: stoichiometric ratio Li/Ti ~2.5
Measurement conductivity
Filtration of the precipitate
Dilution of the remaining filtrated
< 2.5 mS
> 2.5 mS
Drying the precipitate
Calcination at 550 oC for 4 hours Hydrothermal treatment at 185 oC for 8 h
25
In the third method, shown Figure 3-4 in a general schematic diagram, a basic solution of lithium and H2O2 is prepared. The source of lithium was LiOH H2O, and to increase the pH it was employed NaOH and NH3. The range of concentration of lithium used was from 0.1 M to 0.8 M, and the concentration of H2O2 was from 0.32 M to 1.6 M. Then, titanium dioxide powder (anatase, rutile or a mixture of them) were added and stirred for one hour at room temperature.
The concentration of titanium employed was between 0.1 M - 0.6 M, and the stoichiometric molar ratio Li/Ti employed was in the range of 1 to 5. Upon the time of mixing, the solution remains in the basic pH, and for the nano particles of anatase a clear solution was obtained. The volume used was 30 mL unless it is stated the contrary. The prepared mixture was sealed in a Teflon-lined stainless steel autoclave and heated at 130 – 200 °C for 12 h. After the autoclave cooled down to the room temperature, the obtained precipitate was separated from the mother solution, diluted with deionized water, and then filtrated to recover the precipitate. To the filtrated water, the pH and the conductivity were measured.
This procedure was repeated several times, controlling the pH and the conductivity of the filtrated water until a suddenly decline of either the pH or conductivity was detected, and hence, in this fashion, the excess of lithium ions in solution was controlled. After drying the powder at 50 °C for at least 6 h, the powder was calcined at 550 °C under air atmosphere for 4 h with a heating rate of 250 °C per hour in a muffle furnace.
26
Figure 3-4 General schematic diagram of the hydrothermal process, the concentration and conditions vary for the various experiments: Third Method
3-3 Analysis and Characterization
3-3.1 Phase Identification
To determine the phase of the synthesized powder was performed the X-ray diffraction (XRD) analysis; technique commonly used which gives
Basic solution of
Calcination at 550 C for 4 hours
27
information of the crystallinity, purity, size, and shape of unit cell. An incident X-ray beam is generated and applied to the sample which according to its crystal arrangement diffracts the X-ray obeying the Bragg’s law.
(3-1)
Where n is a positive integer called order of diffraction; λ is the wavelength of the beam; d is the distance between the corresponding crystal plane; and θ is the incident angle between the beam and the atomic layers in the crystal, also called Bragg’s angle.
10 20 30 40 50 60 70 80
10 20 30 40 50 60 70 80
10 20 30 40 50 60 70 80
Li2TiO
3 ; JCPDS No 033-0831
2 Theta (degree) LTO; JCPDS No 049-0207 Intensity (I/I o)
LLT; JCPDS No 047-0123
Figure 3-5 XRD patterns for the involved material in this study
28
In this research, the X-ray powder diffraction (XRD) patterns were obtained on a multi-porpose X-ray diffractometer (Ultima IV, Rigaku Co.) using Cu k alpha radiation and operating at 40 kV, 40 mA, and from 6o to 80o with a constant scan rate of 17 o/minute. In Figure 3-5 is plotted the peaks for the LTO, Li2TiO3, and the layered lithium titanium which are the material involved in this study. These data was extracted from the JCPDS data base.
3-3.2 Morphology Observation
The morphology was examined by field-emission scanning electron microscope (SEM) and transmission electron microscopy (TEM). The scanning electron microscope uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample. Data are collected over a selected area of the surface of the sample, and a 2-dimensional image is generated that displays spatial variations in these properties.
The TEM operates on the same basic principles as the light microscope but uses electrons instead of light. What it is seen with a light microscope is limited by the wavelength of light. TEMs use electrons as "light source" and their much lower wavelength makes it possible to get a resolution a thousand times better than with a light microscope. It is possible to focus objects to the order of a few angstrom (10-10 m).
29
The SEM used in this study was (FESEM; FESEM: FEI/Nova NanoSEM 230). Also, the powder sample was dispersed in ethyl alcohol 95% and a droplet of this suspension was transferred onto a Cu microgrid. After evaporation of the ethanol, the sample was used for characterization by transmission electron microscopy (TEM; Hitachi: H-7100; 75 kV) and high resolution transmission electron microscopy (HRTEM: FEI, TENAI G2: 200 kV).
3-3.3 Surface Area Analysis
In this study, the BET (Brunauer, Emmett, and Teller) surface area of materials was conducted with a surface area analyzer (ASAP-2010/Micrometrics).
The 5-points measurement was utilized in which the relative pressure was ranging from 0.06 to 0.20.
3-4 Electrochemical Test
3.4-2 Electrode Preparation and Assembling of the Coin Cell
The electrochemical performance was tested with coin cells CR 2032 type.
First was prepared the electrodes using mixtures comprising 75 wt% of active material (LTO), 15 wt% of carbon black (XC72), and 10 wt% of polyvinylidene fluoride (PVDF) binder, the recipe is presented in Table 3-3. These components were mixed by mortar and pestle, adding NMP as solvent then was continued by mixing until obtaining well-dispersed slurry. The slurry was pasted over etched aluminum foil as a current collector, and around 40 μm film was obtained. The
30
electrodes were roll-pressed getting around 20 μm thickness of the film, and finally the electrode was dried at 120 °C under vacuum overnight. The resulting loading layers bases on the active material were over 1.5 mg/cm2 in weight of the active material. Coin-type cells CR2032 were assembled with the LTO electrode, lithium metal as counter electrode, and electrolyte 1 M solution of LiPF6 in ethylene carbonate/ethyl methyl carbonate (EMC; Mitsubishi Chemical). The assembling was done in a dry room with atmosphere dew point between -40 and -45 °C. The Figure 3-6 illustrate the assembling of the coin-type cells CR2032.
Figure 3-6 Illustration of parts for the coin cells CR2032.
Upper cap Conic Spring
Stainless steel slice Li foil
Membrane separator Active material layer Current collector
Bottom cap
31 Table 3-2 Electrode composition
Material Composition % w/w
Active material 75
Carbon black (XC72) 15
Polyvinylidene fluoride (PVDF) binder 10
3.4-2 Charge and Discharge Strategies
The electrochemical performances were conducted by constant current charge/discharge (C/D) test, with selected current rates between 3 and 1 V on a battery tester (Arbin, model: MCN6410). The charge and discharge phases of a cycle were conducted at the same selected current rate.
32