Synthesis of Lithium Lanthanum Titanate: History
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Subjects: Chemistry, Physical
Contributor:
Alexandru Okos
,
Cristina Florentina Ciobota
,
Adrian Mihail Motoc
,
Radu-Robert Piticescu

Solid state batteries could potentially improve the characteristics of the conventional Li-ion batteries (capacity, charge/discharge rate, safety and sustainability) by replacing the organic electrolyte of the standard battery with a solid (crystalline, but also polymer and hybrid) electrolyte. One of the most promising solid electrolytes is Li3xLa2/3−xTiO3 (LLTO). A number of synthesis techniques have been employed for the preparation of the LLTO compounds. These can be divided on two subcategories: bulk material synthesis and nanostructured material synthesis. The first category contains primarily two methods: sol-gel method and the solid-state reaction method. Nanostructured materials are obtained by thin film deposition techniques and by electrospinning.

  • solid state electrolyte
  • lithium batteries
  • structure–properties correlation
  • lithium lanthanum titanates

1. Sol-Gel Synthesis

The sol-gel method consists in dissolving organic or inorganic compounds of the metallic ions that form the final product, (for LLTO Li, La and Ti) and the dissociation of the metallic ions in a liquid solution. An organic compound (chelation agent) which forms chemical bonds with the metallic ions is added to the solution. The metallic ions then become bound to the chelation agent in the sought stoichiometry. The solution thus obtained is slowly evaporated and transformed into a gel. The gel is dried and undergoes calcination and sintering processes.
A good example of this approach is recently (2022) found in the works of Diktanaitė et al. [1]. They successfully synthesized Li0.35La0.55TiO3 using for starting materials LiNO3, La2O3, metallic Ti powder and HCl. The chelation agent they used was the tartaric acid (C4H6O6). The Ti4+ ion was obtained by the reaction between the metallic Ti and HCl and the formation of the [Ti(OH2)6]3+ ionic species. A similar approach was conducted earlier (published 2019) by Kežionis [2] when the precursors to the synthesis of Li0.35La0.55TiO3 were again LiNO3, La2O3 and metallic Ti. HCl was used as a solvent for Ti and La2O3 and tartaric acid was used as the chelation agent. Other research groups used different reagents. Tetrabutyl titanate, the chemical composition of the substance is Ti[OCH(CH3)2]4, is reported as a source of Ti ions [3][4][5][6]. The sources for Li and La are usually LiNO3 and La(NO3)8 [5][6][7]. The chelating agent is very often citric acid (C6H8O7 in either anhydrous or hydrated form) or tartaric acid (C4H6O6) [1][2][4][8][9]. Sometimes the starting compounds are heated to remove any traces of water that might influence the weighing errors. Moreover, the Li source compounds (LiNO3) is added with approximately 7 to 10% weight excess to the solution in order to compensate for the evaporation of the compound [1][2][10][11].
The reaction conditions are discussed below. Diktanaitė et al. [1] produced the gel by evaporating the liquid solution at 90 °C. The gel was subsequently dried at 120 °C. The drying stage was followed by a set of calcination treatments at temperatures ranging from 800 to 1100 °C to form the final product (LLTO). The LLTO powder is then reground, pressed into pellets and sintered at 1250 °C. Kezionis [2] produced the gel, then dried it at 120 °C and calcinated the powder at 1000 °C for 5 h. The sintering temperature was approximately 1250 °C. Further examples of synthesis conditions are provided in Table 1. Generally, the temperature ranges employed are the following: evaporation 55–95 °C, drying 100–150 °C, calcination at 350–1000 °C (most authors report calcination temperatures in a narrower range 400–800 °C [1][3][4][5][7][8][12]), sintering between 900 and 1350 °C (most publications between 1000 and 1250 °C [1][2][3][4][7][10]).
Table 1. Typical reaction conditions for the synthesis of LLTO by the sol-gel method.
Final Product Reagents Gel Formation Calcination Sintering Ref.
pristine LLTO and Sr doped LLTO Li0.35La0.55TiO3 Li0.35La0.35Sr0.03TiO3 Sr(NO3)2, La2O3, LiNO3, C6H8O7×H2O 80 °C for 2 h combustion at 250 °C 650 °C for 6 h 1250 °C for 4 h [9]
Li0.35La0.55TiO3 Ti, HCl, C4H6O6 (tartaric acid), LiNO3, La2O3 90 to 120 °C 800, 900, 1000 and 1100 °C 1250 °C [1]
Eu doped Li0.5La0.5TiO3 Ti[OCH(CH3)2]4, LiNO3, La(NO3)3×6H2O,
Eu2O3, C6H8O7 (citric acid), (CH2OH)2 (ethylene glycol)		 120 °C for one hour 350 °C for 2 h 800 °C for 3 h [3]
Al doped LLTO
(Li0.33La0.56)1.005Ti0.99Al0.01O3		 LiNO3,
La2(NO3)3×6H2O,
Ti[OCH(CH3)2]4,
Al(NO3)3×9H2O,
(CH2)2(OH)2,
C6H8O7,
Li2O		 70 °C for 12 h to form a gel then heating to 100 °C to form a resin 350 °C for 6 h
+
750 °C for 3 h		 1350 °C for 6 h [4]
Li0.35La0.55TiO3 LiNO3,
La(NO3)3×6H2O,
Ti(OC4H9)4		 80 °C for gel formation and drying at 150 °C combustion at 350 °C for 4 h + calcination at 900 °C for 2 h no sintering [5]
Li0.33La0.56TiO3 La(NO3)3×4H2O,
LiNO3,
Ti(OC3H7)4		 95 °C for 2 h and
100 °C for 12 h		 combustion at 450 °C for 30 min
+
calcination at 800–1200 °C for 12 h		 1150 °C for 10 h [7]

2. Solid State Reaction

The solid-state reaction method consists in mixing stoichiometric quantities of oxides and/or carbonates of the metallic ions that are required in the final product, pressing the powder into pellets and calcinating the mixture at temperatures in the order of 850–1200 °C. Reaction constants are generally low for solid state reactions therefore this method requires elevated temperatures (and possibly high pressure) and long reaction times. The reaction mechanism is slow for the following reason: in order to obtain a material with the chemical composition AB from two reagents A, respectively, B, it is necessary to remove atoms from the crystal lattice of A and to transfer them into the lattice of material B. The situation is symmetrical from the perspective of B. The reaction occurs initially at the interface between the crystallites of the two reagents. The AB compound (formed at the interface) therefore behaves as a barrier to the continuation of the reaction. In order for the reaction to continue A ions not only have to be removed from the lattice of material A, but also have to be transferred through the crystal structure of the AB material before they can be accommodated into the crystal lattice of material B. Sometimes it is required to stop the heat treatment and regrind the powder, before the reaction can be continued [13].
For the synthesis of the LLTO material, through the solid-state reaction process, the following reagents are commonly used: Li2CO3, La2O3 and, respectively, TiO2. Lanthanum oxide, La2O3, is hygroscopic and, when exposed to air, decomposes reversibly to lanthanum hydroxide, La(OH)3. Many authors employ a heat treatment at approximately 1000 °C for up to 12 h to the lanthanum oxide before weighing [14][15]. Moreover, similarly to the situation of the sol-gel method, excess Li2CO3 is used to compensate the material losses through evaporation. Li evaporation is a major concern for the synthesis of LLTO. One technique to also limit the evaporation, and the potential reactions of the powder with the crucible, is to shield the LLTO pellet with sacrificial powder of the same composition [9].
Kazumasa et al. [16] obtained materials from the LLTO class, namely Li0.16La0.62TiO3 and Li0.33La0.56TiO3 (x ≈ 0.05, respectively, x = 0.11) starting from two intermediary compounds, La2Ti2O7 and Li4Ti5O12, according to the following reaction (2):
 
Li4Ti5O12 is acquired as a precursor. La2Ti2O7 is prepared by calcination of a mixture of La2(CO3)3, TiO2 and KCl at 1200 °C for 8 h.
Generally [11][14][15][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] reaction temperatures are set in the following ranges: calcination at 650–1300 °C (mostly at 800 °C for anywhere between 2 to 8 h dwell time) and sintering at 950–1450 °C with dwell times ranging from 2 to 16 h. More often the sintering conditions are found in a narrower range, between 1200 and 1350 °C for 6 to 10 h. Further, specific examples of solid-state reaction conditions are presented in Table 2.
Table 2. Typical reaction conditions for the synthesis of LLTO by solid state reaction.
Final Product Reagents Calcination Sintering Ref.
Li0.34La0.51TiO2.94 La2O3, Li2CO3, TiO2 800 °C for 4 h +
two heat treatments with intermediary grinding at 1150 °C for 12 h		 1350 °C for 6 h [18]
Li0.33La0.56TiO3 Li2CO3, La2O3, TiO2 800 °C for 8 h 1250 °C +
1350 °C for 12 h followed by quenching		 [11]
Li0.33La0.56−yTiO3−3yF3y
(y = 0.017, 0.05)
Li0.33+3yLa0.56−yTiO3
(y = 0, 0.01, 0.02, 0.04)		 Li2CO3, La2O3, TiO2, LiF 650 °C for 12 h 1350 °C for 1.5 h followed by quenching [17]
Li0.5−xLa0.5NaxTiO3 Li2CO3, Na2CO3, La2O3, TiO2 1100 °C for 4 h 1300–1330 °C for 6 h [26]
LLTO doped with rare earths La2O3, Li2CO3, Na2CO3, TiO2, SrCO3, BaCO3, MgO 800 °C for 4 h,
1150–1200 °C for 6 to 12 h with intermediary grinding		 1350–1400 °C for 3 to 10 h [30]
Li0.33La0.56TiO3−yFy
0 ≤ y ≤ 0.183		 La2O3, TiO2, Li2CO3, LiF 800 °C for 2 h 1200 °C for 10 h [31]

3. Thin Films

Thin film deposition techniques employed for LLTO synthesis include, e-beam evaporation, spin coating, dip coating, pulsed laser deposition (PLD) and magnetron sputtering [24][32][33][34][35][36][37]. The research will only remind the basic working principle of PLD and magnetron sputtering. An excellent and extensive body of literature exists on these topics [13][38]. The two techniques are typically using a target with a chemical composition close (sometimes identical) to the composition of the deposited film. Atoms are removed from the target and transported onto a substrate where they re-arrange and grow into the film. Both techniques require high vacuum. With PLD the target ions are removed by laser ablation. Magnetron sputtering uses an argon plasma to achieve the removal of the target ions. The Ar ions are accelerated towards the target and used for bombarding the target surface, which removes target ions. The plasma can be created by ionizing the Ar gas in either DC (if the target is conductive) or, as it is more often the case, AC (this can be applied to either conductive or insulating targets). Usually, the AC case is referred as radio-frequency magnetron sputtering, the typical frequency is 13.56 MHz [39]. The deposition is achieved by careful control of the many deposition parameters such as RF power, plasma composition (argon or argon/oxygen mixtures), vacuum pressure, distance between target and substrate, substrate temperature, target and substrate composition. A quick overview of deposition parameters used for the growth of LLTO thin films is provided in Table 3.
Table 3. Thin film deposition parameters.
Method Parameters Observations Ref.
e-beam evaporation LLTO target
chamber pressure 7 × 10−2 Pa
O2/Ar ratio 1:2
target to substrate distance 30 cm
beam power 300–600 W		   [24]
PLD
KrF laser 248 nm wavelength, aimed at 45° on rotating target		 LixLa2/3−xTiO3 (x = 0.1, 0.2, 0.3, 0.4, 0.5) target
chamber pressure: 1 × 10−6 to 5 × 10−6 torr
target substrate distance 59 mm
substrate temperature: RT
pulse frequency: 10 Hz
laser power: 180 mJ/pulse		 amorphous film [21]
RF magnetron sputtering chamber pressure: 1 Pa
O2/Ar ratio 30% O2, 70% Ar
magnetron power 80 W		   [36]

4. Electrospinning

Electrospinning is a technique used for generating nanowires [10]. It uses the property of a high strength electric field to cause the formation of a thin liquid jet. Without insisting on the details, the process is as follows: LLTO precursors are mixed/dissolved within some liquid polymer. The obtained mixture is then loaded onto a syringe with the needle connected to some high potential, typically in the range of 7 to 20 kV [10][40]. Since the electric filed intensity is inversely proportional to the distance, on sharp points (such as the tip of the needle, where the tip radius is small), the strength of the electric field is sufficiently high to cause by electrostatic repulsion the deformation of the liquid droplet (the formation of a Taylor cone) and the extrusion of the polymer mix as a narrow stream. This stream is then collected onto a surface where it forms a thin foil. The membrane thus obtained is then dried, calcinated and sintered to form LLTO nanowires.

This entry is adapted from the peer-reviewed paper 10.3390/ma16227088

References

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