2-1 Introduction to Li-ion Batteries
The electrochemical systems for the storage of energy can be classified in three main types: electrochemical capacitors, batteries, and fuel cells. Batteries can be divided in primary and secondary batteries, and these have been extensively used in daily life. Primary batteries can be used only once and then have to be disposal. On the other hand, the secondary batteries can be recharged after discharge. In an ideal battery, the discharge has to occur at constant voltage, so that a constant power is delivered during the time; this requirement implies that active materials having two phases during the discharge are preferred. High specific energy is achieved by using electrodes which can deliver a high amount of specific charge at a high voltage. High specific power is granted by fast electrochemical reaction, high specific conductivity of the electrolyte, high electronic conductivity of the electrodes and current collectors. In the Figure 2-1 is clear that the storage systems base in lithium has many advantages compare to the other systems. Batteries based on the lithium technology can be divided in two classes: lithium batteries and lithium- ion batteries. The former has many issues due to safety disadvantage, and the latter one is becoming increasingly more popular.
Since researchers at Sony Energytech developed the first commercial Li-ion batteries in the late 1980s, a variety of efforts have been undertaken to
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improve the battery materials. Because the Li ions in the LIB is extract/intercalate from or into the two opposite electrodes during charge and discharge, this kind of batteries are also called rocking-chair batteries.
Figure 2-1 Ragone chart of the main secondary battery systems [8].
In the Figure 2-2 is illustrated the principle of the charge/discharge of the LIB. For example for LiCoO2/C (Cell), during the charging the Li ion will be extracted out from the layered structure of LiCoO2, then the Li ion will migrate through the electrolyte between the two electrodes to finally intercalate into the carbon graphite (anode), and then diffuse into it. During the discharge the reverse process take place. The following chemical equations represent the process:
6 intercalation process (discharge). The anode is composed by graphite while cathode is a layered oxide [8].
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2-2 Li
4Ti
5O
12as Anode Material for Li-ion Batteries
The negative electrode in LIB substitutes metallic lithium. This is necessary because metallic lithium has not a stable surface during charge discharge and the risk of a shot-circuit can arise. To obtain high specific energy, the active material has to store high amount of charge and to have a more cathodic equilibrium potential for the Li exchange. Some limits on the latter properties are set by the stability window of the electrolyte and the deposition potential of metallic lithium (electroplating) on the electrode, which could result in the formation of dendrites. Mainly two kinds of material for the anode of the LIB can be distinguish, this is, intercalation compounds and lithium alloys.
Graphite is the most popular among the anodes for the LIB having a layered compound. The theoretical maximum specific charge is equal to 372 mAh g-1, the flat potentials of the graphite phases are between 0.22 and 0.1 V (Li/ Li+), at this potential range of Li+ intercalation in graphite, most of the electrolytes are thermodynamically unstable, forming a stable passivation film at the surface of graphite particle is necessary, this is, the so-called Solid Electrolyte Interphase (SEI).
Spinel LTO has a theoretical capacity of 175 mAh/g, this material is not toxic. LTO has been considered as a promising alternative to the carbon because of the following reasons: (i) during the Li-insertion process it shows a potential plateau at 1.5 V (vs. Li+/Li) which is ascribed to the two-phase reaction [9]. This higher potential (vs. Li+/Li) is desirable because at this voltage the reduction of the organic solvents and electrolytes is inhibited hence the formation of SEI is
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avoided; moreover, the formation of dendritic is also avoided [3]. The latter causes a serious distress about safety using carbon as an anode; (ii) the zero-strain insertion property, during the lithiation/delithiation [4], for both end members, Li4Ti5O12 (LTO) and Li7Ti5O12 (LTO-rock-salt), are described by the same space group Fd-3m which have similar lattice parameters, as shown in Figure 2-4(a,b). Although substantial chemical changes occur during conversion between the two phases. This special characteristic makes it a robust material during cycling performance which is highly desirable for EVs which use heavy-duty batteries; (iii) The LTO framework provides a three-dimensional network of channels for facile lithium diffusion [9-11], this property provide this material with fast charge/discharge rates which is desirable. Nonetheless, as a drawback the material inherently has an insulating characteristic with band gab ~ 2 eV [5], which prevents seriously its high current performance. To overcome this issue, researches conduct doping it with a conductive material, modifying the surface with a conductive material like TiN [6], or making nanomaterials to increase the surface area.
Figure 2-3 Voltage of LTO anode and various cathode materials [12].
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Figure 2-3 shows an operating voltage of LTO anode with various cathode materials. Various possible candidates stand out to replace the C/LiCoO2 battery.
For example the system LTO/LiFePO4 is very promising because of low cost of the cathode and possible heavy-duty use. The LTO/LiNi0.5Mn1.5O4 cell, among others, is one of the promising and attractive batteries for next generation LIB because of its high voltage (4.7 V), acceptable stability, and good cycling performance [13].
Figure 2-4 (a) Li4Ti5O12 spinel structure type. Blue tetrahedra represent lithium, and the green octahedral represent disorder lithium and titanium. (b) Li7Ti5O12, rock salt. Blue octahedral represent lithium, and green octahedral represents disorder lithium and titanium [14].
The LTO and LTO-rock-salt, both lithium titanium oxides crystallize in the space group Fd-3m (Figure 4). In the structure of the spinel LTO (Figure 2-4a), the octahedral (16d) sites are randomly occupied by lithium and titanium
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whereas the tetrahedral (8a) positions are occupied by lithium. LTO rock-salt structure is formed (Figure 2-4b), here, the octahedral (16c) positions are occupied by lithium; the occupancy of the octahedral (16d) positions is the same as in LTO, whereas in contrast to LTO, the tetrahedral (8a) positions are not occupied [15].
The zero strain property is of key technological importance since lattice strains upon cycling are among the main causes of capacity decays in lithium battery electrodes. It is known that the lithiation-delithiation proceeds through a two-phase equilibrium, which leads to a very constant potential over a large range of overall Li concentrations. On the other hand, recently it has been proposed that in Li4+xTi5O12 this two-phase separation is unstable above 80 K and most likely kinetically induced by the Li-insertion during charge/discharge [15].
As a consequence, in equilibrium the material forms a solid solution at room temperature with associated small changes in the open circuit potential [15-17].
A solid solution implies intimate mixing of 8a and 16c site occupation in contrast to the phase separated system where domains of 8a and 16c occupation are separated on a micrometer length scale [16]. The disorder resulting from the mixed 8a/16c occupation is most likely beneficial for the Li mobility as compared to the ordered coexistence of 8a and 16c domains. Based on observations Wagemaker illustrates this behavior in a phase diagram, shown in Figure 2.5 [15].
Referring the electronically insulating behavior of the LTO, to address this problem it has been proposed: doping with ion metals, modifying the surface of
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the LTO particle with one electronic conductive material, or synthesizing nano material to increase the surface area. The approach of synthesis plays an important role in the electrochemical behavior of the LTO, including new ways of synthesis.
Figure 2-5 The zero strain Li4+xTi5O12 material as a two-phase system is unstable at room temperature, and it is the fast Li insertion that leads to a kinetically induced effective two-phase reaction. The solid-solution-induced disorder, resulting from the mixed 8a/16c occupation, is most likely responsible for the high rate capabilities in Li4+xTi5O12.[15]
Among the most straightforward synthesis processes of LTO is the solid state reaction. Nevertheless, the product suffers from great disadvantages like the broad size distribution and large particles size. [4, 18] A variant of this approach is the surface modification with one conductive material like carbon [19, 20]
(shown in Figure 2-6) or nitrite [6] which improve the electrochemical behavior.
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Even though, a high energy milling is follow the solid state reaction not much improvement is obtained on the electrochemical performance as other researches show [18]. The spray-drying approach is another straightforward process; for instance, Hsiao et al [21] synthesized D- Li4Ti5O12 and P-Li4Ti5O12 by solid-state calcination (D and P stand for dense and porous particles) improving the rate capability for the letter.
Figure 2-6 (d-1) High-resolution TEM images of 20 wt% pitch coated LTO showing penetration of carbon into the particle boundary and (d-2) high-resolution TEM images of 20 wt% pitch coated LTO showing the carbon coating layer [20].
More elaborated processes which achieve products with great electrochemical properties for the anode material were developed; however, those processes are too complicated adding that the precursors are costly. The sol-gel [22, 23], hydrothermal [24], microwave, modified rheological phase (the last two mentioned in the paper review by Yi et al. [12] ), solution combustion
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[25], spray pyrolysis [26] methods or a combination of these methods with a thermal treatment were being developed. The results of these more fancy approaches are astonishing in some cases, for instance, Prakash et al. [25]
prepared LTO using titanyl nitrate [TiO(NO3)2] and LiNO3 as the oxidant precursors and glycine as the fuel in the solid combustion approach. In the Figure 2-7(a) is shown the TEM micrograph, and in Figure 2.7(b) the electrochemical performance; this synthesized LTO is able to cycle at 100 C-rate, even though the loading of the active material over the current collector is ~4 mg/m2.
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Figure 2-7 LTO by the solution combustion approach. (a) TEM micrograph of the synthesized LTO. (b) The electrochemical performance [25].
2-3 Hydrothermal Synthesis of LTO
M. Yoshimura et al. [7] has introduced the concept called Soft Solution Processing, as a process that is inspired basically by natural processes, being able to operate near above ambient conditions. The hydrothermal synthesis fall into
(b)
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the category, being its greatest advantages the possibility to synthesize advance materials controlling the size and morphology of the product.
Fattakhova et al. [24, 27] in previous studies synthesized LTO via hydrothermal and solvothermal process, in these study, however, TiO2 is used as a precursor, employing strong basic solution with high concentration of lithium hidroxile; moreover, the final precipitate is too small as a consequence the product losses great capacity due to the extremely large surface area as is explained by Borghols et al. [17] and kavan et at. [28]. However, the two latter authors concluded separately through their studies that a large surface area leads a larger capacity than the theoretical one for the first cycles but this declined greatly for the further cycles.
In other approaches, in an attempt to soften the condition of the hydrothermal treatment and improve the morphology of the product, amorphous titanium oxide was prepared previously and then performed the hydrothermal synthesis; for example, Tang et al. [29] prepared LTO hollow microspheres from amorphous hydrous titanium oxide (AHTO) followed by calcination. The XRD pattern of the powder before and after the calcination is presented in the Figure 2-8(a), and the high resolution TEM micrograph is presented in Figure 2-8(b). The electrochemical results indicate the right surface area and mesoporous structure of LTO hollow sphere provide the possibility of faster transport of electrons and lithium ions in high-rate LIB, maintaining 131 mAh/g at 50 C-rate.
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Figure 2-8 LTO synthesized from AHTO by hydrothermal process (a) XRD pattern before and after the calcination of the powder (b) High Resolution TEM micrograph [29].
To prepare the nano size LTO, Chen et al. [30] carried out the hydrothermal process of the mixture of titanium tetra isopropoxide (TTIP) with H2O2 in LiOH solution; this approach leads to microspheres of nano flakes, as shown in Figure 2-9(a-b). During the cycling performance at various C-rates given in this research can be seen that at 57 C-rate the as-prepared LTO can deliver 132 mAh/g for 200 cycles; however, the weight of active material over the current collector is not reported in this study; the later point is important because LTO exhibit excellent activity toward Li insertion, even at charging rate as high as 250 C-rate if a thin film is prepared around 2-4 µm [31].
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Figure 2-9 TEM micrographs of as-prepared LTO [30].
Particularly, this thesis focuses in the hydrothermal synthesis using titanium dioxide (TiO2) in their different phases (anatase, rutile, or a mixture of them) as initial precursor (in one set of experiments is employed AHTO);
moreover, the process leads to different morphologies: nano flakes or quasi-square nano particles, those forming aggregated secondary particles or dispersed alone, by controlling the parameters in the hydrothermal synthesis such as ratio Li/Ti, temperature, concentration of H2O2, and pH. The electrochemical performance of the synthesized products was measured showing a greater rate performance than many materials reported before.
2-4 Aqueous Soluble Peroxo Titanate
An aqueous solution of titanium peroxide is achieved from several methods. The most common way is mixing hydrogen peroxide and any soluble titanium, organo titanium (titanium alcoholxides), TiCl4, titanyl sulphate
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TiOSO4·2H2O, Ti(SO4)2, H2TiO3, or dissolving powder of metallic titanium with H2O2 and NH3. The color of this solution is deep orange at strong acid solution, orange below pH 1, yellow around 3, becomes pale yellow when the pH is raised further, and the color disappears in a strong basic solution. Schwarzenbach et al.
[32] explained that the formation of titanium peroxo complex can followed the next global reaction, in acid media:
Ti4+(aq) + H2O2 ====> Ti(O2)(OH)n-2(4-n) + nH+ (2-4) In very acid media an orange-red color of the mono nuclear peroxo titanic acid, TiO2(OH)+ is observed [32], this nono nuclear complex is detected during the colorimetric measurement having a maximum absorbance at around 410 nm wavelength [33]. At higher pH the presence of the mono nuclear peroxo titanic acid is uncertain, rather an equilibrium of dinuclear titanium compounds is formed as Schwarzenbach et al. explained [32].
On the other hand, soluble titanium can be achieved by the dissolution of metallic powder of titanium in excess of NH3 and H2O2 at pH around 11 [34], the suggested global reaction is:
Ti0 + 3H2O2 + NH3 ===>TiO2(OH)3- + 2H2O + NH4+ (2-5)
Water-soluble complexes of titanium have been proved to be very promising reagents for synthesizing titanium compounds. Specifically, the peroxo titanate and their complexes are used by many researchers to obtain TiO2 in various morphologies and phases. The hydrothermal process was employed to obtain nanorod of TiO2 (brookite) in a basic solution [35], in another research the
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hydrothermal synthesis of titanium peroxide in the present of H2SO4 leads to TiO2 (brookite) [36], also Murakami et al. [37] used the hydrothermal process and controlled the shape of the particle with polyvinyl alcohol obtaining anatase.
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