Sol-gel synthesis and structural study of a lithium titanate phase Li 3xLa 2/3.x □1/3-2x TiO3 as solid electrolyte

Perovskite lithium lanthanum titanate (LLTO) was prepared as an inorganic solid electrolyte material for lithium ion batteries applications using sol-gel method. Three different compositions of : (a) 3x=0.3 (Li 0.3La 0.57TiO3), (b) 3x=0.5 (Li 0.5La 0.5TiO3) were prepared and calcined at 1000°C for 6h, and (c) 3x=0.1 (Li 0.1La 0.63TiO3) prepared and calcined at 1100°C for 6h. The effects of lithium content and calcination temperature on the structure of the perovskite powders were studied. The synthesized powders were characterized by X-ray diffraction analysis. Results indicate that the Li 0.3La 0.57TiO3 was crystallized in tetragonal structure of space group P4/mmm. On the other hand, Rietveld refinement analysis for the LLTO samples confirmed the formation of the Li 0.3La 0.57TiO3 crystalline phase in the tetragonal P4/mmm space group, the Li 0.5La 0.5TiO3 phase in the tetragonal P4/mmm space group, and Li 0.1La 0.63TiO3 phase in orthorhombic Pmmm space group.


Introduction
All-solid-state batteries lithium are of considerable interest for future technological applications, such as portable electronics, micro-devices and even large-scale applications [1].
Lithium-ion batteries currently represent the main source of power in electronic devices and are widely regarded as one of the most remarkable technologies in transportation electrification. However, lithium-ion batteries still have some limitations, particularly in terms of security. These security problems are often associated with the use of flammable organic electrolytes [2][3].
One of the solutions to these problems is precisely the introduction of solid electrolytes. The advantages of using solid electrolytes instead of organic liquid electrolytes, besides security, are their good thermal stability, higher energy capacity due to the possibility of more compact structures without the need for separators as in the case of conventional batteries, and their wide range of operating temperatures [4].
One of the conditions for the application of the solid electrolyte is that it must have a reasonably higher ionic conductivity. The one potential solid electrolyte is ceramic perovskite ABO 3 , such as lithium lanthanum titanate, of the formula Li 3x La 2/3-x 1/3-2x TiO 3 called LLTO, ( represents vacancy) [5]. The highest bulk ionic conductivity reported for LLTO is about 10 -3 S.cm -1 at room temperature IOP Publishing doi:10.1088/1757-899X/1160/1/012005 2 when 3x= 0.3 [6], but, the total ionic conductivity of LLTO is low (10 -5 S.cm -1 ) due to the low ionic conductivity at the grain boundary [7][8]. Different publications suggest that sintering temperatures, composition (Li to La ratio) and preparation method affect the structure and morphology of LLTO, modifying the resulting conductivity [4][5][6][7][8][9]. That way, although many reports have been made on the properties of LLTO, different preparation steps and sintering conditions may result in different or improved properties.
LLTO is generally synthesized by the sol-gel method or by solid-state reaction, the latter being easier and more economical. In general, the sintering temperature is above 1250°C, which leads to the evaporation of lithium [10]. In this study, the LLTO samples Li 3x La 2/3-x 1/3-2x TiO 3 (3x=0.1; 3x=0.3 and 3x=0.5) were prepared by the sol-gel method, and characterized by XRD diffraction and their structures were refined using the Rietveld method. Figure 1. shows the diagram of the procedure for the synthesis of lithium lanthanum titanate (LLTO) powders. The sol-gel method was used to prepare three different compositions of LLTO for 3x=0.1; 3x=0.3 and 3x=0.5 .
Lanthanum acetate, lithium acetate and titanium sol are mixed in stoichiometric proportions, according to the chemical formulation Li 3x La 2/3-x 1/3-2x TiO 3 (3x=0.1; 3x=0.3 and 3x=0.5) under stirring for 2 hours, to ensure the homogeneity of the final solution. The destabilization of this solution is ensured by evaporation of the solvent in an oven at a temperature of (60°C ~ 70°C) for 96 hours. The xerogel obtained is ground in an agate mortar to break up the agglomerates of the powder and increase its reactivity. The mixed powders were calcined at 1000°C (6h) for (3x=0.3 and 3x=0.5) and at 1100°C (6h) for (3x=0.1) was calcined.
The obtained samples were analyzed by X-ray diffraction using a computer-controlled XPERT-PRO diffractometer with a copper anode K-Alpha1 (λ1=1.54060Å) and K-Alpha2 (λ2=1.54443Å). A scan was adopted with a 0.0170° step. The chosen measuring range is from 10° to 90°. The formation of the LLTO phase starts at 900°C with impurities. The main impurity is the lanthanum titanium oxide La 2 Ti 2 O 7 which was also found by Bohnke et al [11] during the preparation of Li 0.33 La 0.56 TiO 3 by the sol-gel route and the lithium titanate Li 2 TiO 3 which was found by Romero et al [12] during the preparation of Li 0.3 La 0.57 TiO 3 . When the temperature of the treatment increases, the proportion of impurities decreases, and the calcination temperature at 1000°C promotes the removal of these impurities. La 2 Ti 2 O 7 appears at 700°C as indicated by Kitakoa et al [13].  The structure of Li 0.3 La 0.57 TiO 3 heat treated at 1000°C for 6 hours was further refined by means of Rietveld method (Figure 3). In a first step, a refinement was performed using the structural model proposed in the literature by Fourquet et al. [14], which allowed us to verify that the compound is pure LLTO (3x=0. 3   For LLTO samples (3x=0.5), the X-ray diffractogram registered on the heat-treated compound at 1000°C is shown in figure 4. The LLTO compound has indeed been formed, with the presence of impurity containing lithium titanate Li 2 TiO 3 .  Refinement on the compound synthesized at 1000°C ( Figure 5) permitted to obtain the cell parameters a=b=3.8703Å and c=7.7529Å using the P4/mmm space group and α=β=γ=90°. Reliability factors are: R p =39.00%, R wp =26.90%, R exp =17.15% and  2 =2.46. In table 3, the crystallographic characteristics of Li 0.5 La 0.5 TiO 3 are presented, and in table 4, the interatomic distances for refined LLTO (3x=0.5) are reported, these results are confirmed, for example the distances found in reference [15] for Ti-O are in the range of those of this study.   TiO 3 For samples (3x=0.1), the formation of the LLTO phase is produced at 900°C, but with impurities. We have therefore tried to increase the heat treatment temperature to 1000°C in order to obtain pures phases, but unfortunately, we stiff also observe the presence of lanthanum and titanium oxide La 2 Ti 2 O 7 in greater or lesser quantities. The addition of 10% excess of lithium acetate allows us to reduce the quantity of these impurities without eliminating them entirely, an excess of 10% of lithium was added to the stoichiometric mixtures to compensate for lithium loss during processing [16]. (Figure 6).  (Figure 7). Using the structural model proposed by Ibarra et al [15], we have verified that the LLTO compound (3x=0.1), crystallizes well in an Orthorhombic cell (Pmmm; a=3.8743Å; b=3.8632Å and c=7.7816Å , and α=β=γ=90°), and the factors of reliability are: R p =39.40%, R wp =30.40%, R exp =23.93% and  2 =1.61. In table 5, the crystallographic characteristics of Li 0.1 La 0.63 TiO 3 are presented, and in table 6, the interatomic distances for refined LLTO (3x=0.1) are reported, these results are in very good agreement with the literature [15].