The slope nwhich is obtained from Equation 3.1 can give information about the mech- anisms of the reactions. According to Hancock and Sharp (1972), for n = 0.54 - 0.62 a diffusion-limited rate is inferred, where the rate of diffusion of the reactive species to the nucleation sites is the rate-determining step. Forn= 1.0 - 1.24, a zero-order, first-order, or a phase boundary mechanism between the product and the reagent mixture is rate deter- mining, and forn = 2.0 - 3.0, the formation of nucleation sites is the process that controls the rate. In the present study, the slopen= 2 in the beginning of the reactions (Figure 3.7) suggests that the nucleation of Li2TiO3 sites is the rate-determining step. Then, the slope becomes less steep at higher fractions andn≈1.2 suggests that once enough Li2TiO3nuclei
Figure 3.7: Sharp-Hancock plot for the transformation of P25 titania powder in lithium hydroxide solution to α-Li2TiO3 at 155 ◦C (squares), 168 ◦C (circles) and 169 ◦C (trian- gles).
are formed, the reaction mechanism changes, and then the reaction at the phase boundary is the rate determining mechanism at higher fractions. The rather high activation energy of 66 kJ/mol also suggests a phase boundary or nucleation controlled mechanism. EA < 32kJ/mol would be an indicator for diffusion controlled processes (Lasaga, 1998).
The present study focused on the reaction of the titania powder P25 with a 0.5 M
lithium hydroxide solution. It is expected that by using other titania reactants, or a higher or less concentrated lithium hydroxide solution, the kinetics, and even the reaction mechanisms, might vary. For the formation e.g. of BaTiO3, the activation energies were found to depend mainly on the titania reactant (Walton et al., 2001), thus for BaTiO3 various activation energies between 21kJ/mol (Ovramenko et al., 1978) and 105.5kJ/mol
have been reported (Hertl, 1988). For the hydrothermal formation of the lithium titanate it was reported that coarser particles (≈ 100 nm) cause problems to obtain a complete reaction at 160 ◦C (Laumann et al., 2010), whereas by using a 5 nm anatase, lithium intercalation can be performed in a 2.5M lithium hydroxide solution at 60◦C(Jiang et al., 2008). Obviously, a faster reaction at the same temperature can be expected for a higher concentration of the lithium hydroxide solution. Here, especially at low temperatures e.g. in EXP2, the reactions slowed down before getting to the end of the complete formation of α-Li2TiO3, indicating that low concentrations of remaining lithium ions in the solution decelerate the kinetics (see Figure 3.1).
3.5 Conclusions 35
Figure 3.8: Arrhenius plot used to determine the activation energy of the intercalation of lithium in titania.
3.5
Conclusions
For the first time, in-situ synchrotron radiation has been used to study the interaction of titania powder in a lithium hydroxide solution under hydrothermal conditions. Lithium ions were found to intercalate in the titania structure without a prior dissolution of the titania compounds and the particle growth of the formed metastable cubic α-Li2TiO3 was recorded. Upon heating of the suspension of α-Li2TiO3 particles in a lithium hydroxide solution above the critical point of water, α-Li2TiO3 was found to transform to the mono- clinicβ-Li2TiO3 in a similar way as observed for dryα-Li2TiO3 powder heated in air or in high vacuum. By applying the Avrami-Erofe’ef equation the rate controlling mechanism could be determined for the reactions at lower temperatures. The process was found to be non-isokinetic due to a change from a nucleation controlled to a phase boundary controlled reaction, which might depend on the used titania reactant and the concentration of the lithium hydroxide solution. The calculated activation energy of 66 kJ/mol also will be sensitive to variations of the reaction conditions.
Chapter 4
Two-step synthesis of Li
4
Ti
5
O
12
This chapter describes a new process to synthesize Li4Ti5O12spinel in a two-step man- ner, patent pending, S¨ud-Chemie (Appendix D). This novel method is a combination of a hydrothermal step with a subsequent solid-state reaction. The synthesized products are electrochemically tested and its structural properties are investigated upon heating by impedance spectroscopy (Appendix E) and neutron powder diffraction (Appendix F).
4.1
Introduction
An introduction in synthesis methods for Li4Ti5O12 is given in Chapter 1. Standard solid-state syntheses require, in general, 800 ◦C or higher temperatures to obtain a pure product. These high temperatures cause particle growth and thus lower the rate capa- bility of active materials. As already introduced, Li4Ti5O12 is of spinel-type with a unit cell containing eight formula units of (Li)8a[Li
1/3Ti5/3]16dO324 e, in which lithium (Li1) fully
occupies the tetrahedral 8a sites. The octahedral 16d sites are occupied randomly in a ratio of 1 : 5 by lithium atoms (Li2) and titanium atoms (Kataoka et al., 2008; Julien and Zaghib, 2004), the tetrahedral 32e sites are occupied by oxygen atoms (Ohzuko et al., 1995; Kataoka et al., 2008).
Upon heat treatment of Li4Ti5O12 samples, variations in its activation energy were determined by impedance spectroscopy, which are inferred to relate to structural changes of the order-disorder type (Leonidov et al., 2003, 2004; V¯ıt¸ni˘s et al., 2002). Similar conclusions were reported based on data obtained by infrared spectroscopy (Pecharroman and Amarilla, 2000), Raman spectroscopy (Leonidov et al., 2004), and NMR spectroscopy (Vijayakumar et al., 2009). Based upon impedance data, Li4Ti5O12 is presumed to undergo two order- disorder phase transitions upon heating. First, due to migration of lithium from 8a to 16c
sites, a transition to an ordered F d¯3m structure (Li16c[Li1/3Ti5/3]16dO324 e), and second, due
to the migration of lithium from 16d to 16c sites, a transformation into a disordered NaCl structure F m¯3m (Li16c
4/3Ti165/d3O324 e) (Leonidov et al., 2003, 2004). All proposed structural
transformations are well below 1000 ◦C, the temperature at which the decomposition of Li4Ti5O12 into the ramsdellite-type Li2Ti3O7 and the cubic γ-Li2TiO3 is expected
(Gicquel et al., 1972; Izquierdo and West, 1980; Kleykamp, 2002).