titanatos de lantanio litio review

titanatos de lantanio litio review

(Parte 1 de 6)

Reviews

Lithium Lanthanum Titanates: A Review

S. Stramare,† V. Thangadurai,* and W. Weppner

Chair for Sensors and Solid State Ionics, Faculty of Engineering, Christian-Albrechts-University of Kiel, Kaiserstr. 2, D-24143 Kiel, Germany

Received March 14, 2003. Revised Manuscript Received July 2, 2003

To date, the highest bulk lithium ion-conducting solid electrolyte is the perovskite (ABO3)-

temperature with an activation energy of 0.40 eV. The conductivity is comparable to that of commonly used polymer/liquid electrolytes. The ionic conductivity of LLT mainly depends on the size of the A-site ion cation (e.g., La or rare earth, alkali or alkaline earth), lithium and vacancy concentration, and the nature of the B-O bond. For example, replacement of La by otherrare earthelementswith smallerionicradiithan that of La decreasesthe lithium ion conductivity, while partial substitution of La by Sr (larger ionic radii than that of La) slightly increases the lithium ion conductivity. The high lithium ion conductivity of LLT is considered to be due to the large concentration of A-site vacancies, and the motion of lithium by a vacancy mechanism through the wide square planar bottleneck between the A sites. It is considered that BO6/TiO6 octahedra tilting facilitate the lithium ion mobility in the perovskite structure. The actual mechanism of lithium ion conduction is not yet clearly understood. In this paper, we review the structural properties, electrical conductivity, and electrochemical characterization of LLT and its related materials.

1. Introduction

At present, lithium ion secondary battery develop- mentsaremainlybasedon LiCoO2 as a cathode(positive electrode), lithium ion-conducting organic polymer as an electrolyte (LiPF6 dissolved in poly(ethylene oxide) (PEO)), and Li metal or graphite as an anode (negative

electrode). The formation of a solid electrolyte interface (SEI) at the anode leads to a large irreversible capacity lossduringthedischargecycles.A furthermajorconcern is the safety aspect of liquid and common polymeric electrolytes. The liquid-free batteries show some advantagesover the currentlycommercializedones. These include thermal stability, absence of leakage and pollution, resistance to shocks and vibrations, and large electrochemical windows of application, thanks to the use of high-voltage ( 5.5 V/Li) cathode materials. Furthermore, the development in microelectronic and information technologies requires the search for a new generation of energy sources, among which a considerable interesthas been givento the all-solid-statelithium batteries. The main impediment is finding an appropriate solid electrolyte that has a reasonably high lithium ionic conductivity and a good chemical stability in contact with both electrodes, especially with metallic lithium or LiAl alloy anode.1,2

A wide variety of metal oxides are known to exhibit high bulk lithium ion conductivity. Using temperature as scale, they can be divided mainly into two groups:

(i) high-temperature ionic conductors, for example, Li2-

SO4,3 Li4SiO4,4 and Li14ZnGe4O16 (Lithium Super Ionic Conductor, LISICON)5 and (i) low-temperature ionic

Li0.34La0.5TiO2.94.1 They can also be classified into four groups according to the type of compounds: (i) lithium oxyacidsalts,for example,Li2SO4 and Li4SiO4; (i)ç -Li3- PO4 solid solutions, for example, LISICON and ç-Li3.6- deficient perovskite solid solution (Li-ADPESS). To date, the fastest lithium ion-conducting solid electrolytesknownare the perovskite-type(ABO3) oxide, with A ) Li, La and B ) Ti, lithiumlanthanumtitanate

comprehensivereviewof theavailableinformationabout LLT and its related materials (A site, B site, or both substituted LLT), with special focus on compositionstructure-ionic conductivity property.

2. General Considerations

Historically, perovskite-type alkaline-earth titanates

(ATiO3,A ) Ca, Sr, Ba) aroused much interest due to theirfunctionalproperties,for example,dielectricityand

* To whom correspondence should be addressed. E-mail:

vt@tf.uni.kiel.de.Phone: 0049 431 880 6210. Fax: 0049 431 880 6203.Nanogate Technologies GmbH, Gewerbepark Eschbergerweg, D-66121 Saarbrucken, Germany.

10.1021/cm030516 C: $25.0 © 203 American Chemical Society Published on Web 09/23/2003 ferroelectricity,which arise from the displacement of Ti atoms toward one of the octahedrally coordinated oxygen atoms along the c-axis.27a Brous et al.27b reported for the first time the synthesis of cubic Li0.5La0.5TiO3 by thereplacementof A-sitealkalineearthionsin ATiO3 with a trivalent rare earth La ion and monovalent ions

(Li, Na, K). Some years later, Patil and Chincholkar28,29 confirmed this result and extended their investigation in which the La was replaced by other rare earth ions. After these structural studies, the dielectric properties of these perovskite-type titanates were considered by Varaprasad et al.,30 who found the hysteresis and the dielectric anomaly peak at 67 °C in Li0.5La0.5TiO3.O n the basis of their results, they concluded that Li0.5La0.5-

TiO3 exhibits both ferroelectric and antiferroelectric ordering. They and Kochergina et al.31 assigned the structureof Li0.5La0.5TiO3 to that of tetragonaltungsten bronze-type.31b Increaseof the capacitancewith increas- ing temperature, large dielectric loss, and dielectric relaxation were observed from Inaguma et al.,1 who considered these phenomena due to Li-ion motion.

Indeed, the first studies on the conducting behavior and on the stoichiometricrange of stability of LLT were reported by Belous et al.14 They reported that oxides

< 0.17, recognizing that the lanthanum ions, larger in ionic size, are the main contributors to the stabilization of the perovskite-type structure and that the lithium ions are the charge carriers responsible for their high electrical conductivity.14

However, the work of Inaguma et al.1 is referred to as the first study on the ionic conductivity of LLT since they reporteda bulk lithiumion conductivityof 1 10-3 S/cm at room temperature (RT). LLT has attracted the interest of many research groups around the world because of its possible potential application as solid electrolyte in various electrochemical devices, for example, all-solid-statelithium-ionbatteries,sensors,and electrochromic displays. Figure 1 shows the Arrhenius plots for ionic conductivity of LLT together with wellknown solid lithium ion conductors. Since 1993, the influenceof the composition,partialsubstitutionor total substitution of La and Ti with other metal ions, effect of pressure, and sintering conditions on their crystal structure, conductivity, electrochemical properties, and the mechanism for lithium ion conduction have been widely investigated.12,19-26,32-100

3. Synthesis of LLT and Related Materials

LLTs and related compounds have been synthesized mainly by three methods: (i) solid-state reaction, (i) sol-gel synthesis,49 and (i) floating zone.48,85 Among them, solid-statereactionsare commonlyused, allowing the production of relatively large quantities of bulk material. Sol-gel synthesis coupled with dip coating was introducedto preparethinfilmsof LLT for applying these materials in the solid-state devices.49 A floating zone was introduced as a second synthesis step after the solid-state reaction to produce single crystals of

Li0.27La0.59TiO348 and monocrystallinefibersof Li0.5La0.5-

TiO385 to study anisotropic conductivity and the phase diagram under directional solidification, respectively.

4. Structural Characterizations of LLT

Various solid-state techniques came in aid of the researchers for characterizing this family of materials. The evolution of the structure and its dependence on the composition(Li/Laratio),substitution(A site, B site, or both), synthesis/sintering conditions, and external pressure was studied, mainly employing powder X-ray and neutron diffraction (XRD and ND), and in minor cases electron diffraction(ED). High-resolutionelectron microscopy (HREM) has been used to study the superstructure of LLT. Scanning electron microscopy (SEM), transmissionelectronmicroscopy(TEM),and secondary ion mass spectroscopy (SIMS) were also employed for the characterization of LLT and its related compounds. 4.1.X-rayandNeutronPowderDiffractionStudy. The solid solution with the general perovskite

(CaTiO3)-type98,9 formula Li3xLa(2/3)-x0(1/3)-2xTiO3 has turnedoutto be stableovera widerangeof compositions

( 0.04 < x e 0.16).14,15a,32,3,42 The actual crystal structure is still controversial.In fact, depending on the amount of lattice vacancies present and the synthesis method, in addition to the simple cubic unit cell,1,12,14,16,27,28,3,45,56,57,6,68,73,82 hexagonal,75 tetragonal12,14,15,29-3,35,42,4,51,59,6,68,69,73,83,87,89,100 and orthorhombic perovskite-type distorted cells11,14-16,32,3,35,42,48,52,57,59,73,76,97 were reported. An overview of the chemical composition and crystal structure data reported for LLT in the literature is listed in Table 1. 4.1.1.Cubic Perovskite-TypeStructureLLT.The cubic perovskite-type unit cell with the lattice parameter a ) 3.8 Å (space group Pm3m and Z ) 1) was reported for a specific composition14,27,28,3,45,68 and for samples quenched from high temperature (>1150 °C).12,57,6,73,82 The La3+ ions, Li+ ions, and vacancies are randomly distributedover the A sites. Only one work reported the identification of a double F-centered supercell with a

2ap and superstructure reflections, which have been attributed to the ordering of the La3+ and Li+ and vacancies at the A sites.56 The lattice parameter a was found to decrease with increasing x in LLT (Figure 2).57

Moreover, in the case of LixLa0.57TiO3 the lithium was found to evaporate at elevated temperature and the lattice parameter was found to increase for the x ) 0.35

Figure 1. Arrhenius plots of electrical conductivity of per- ovskite-type lithium lanthanum titatate Li0.34La0.51TiO2.94,16 along with several well-known solid lithium ion conductors.

The sharp increase in LiAlCl4 is due to melting.

Reviews Chem. Mater., Vol. 15, No. 21, 2003 3975

and decrease for the x ) 0.30 member with increasing sintering temperature from 1150 to 1350 °C.82 Lithium titanium oxide and lanthanum oxide impurity phases were found to appear when the sintering time was shorter or longer than the optimum condition (6 h at 1350 °C).16

4.1.2. Hexagonal Perovskite-Type Structure LLT. The hexagonalunitcellhas beenreportedfor Li0.5La0.5TiO3-ä (0e ä e 0.06) in a recent ND study,75 the distortion being attributed to the tilting of the TiO6 octahedra. Unit cell parameters were a ) 5.4711(4) Å and c )

13.404(1)Å, withspacegroupR3hc (Z ) 6),corresponding to one of Glazer’soctahedraltilt schemes for perovskiterelated structures,101 which is adopted when the tilting angleof the octahedraremainssmall(e.g.,as in the case of LaAlO3, LaCuO3, or LaNiO3). La, Ti, and O occupy the Wyckoff position 6a site (0,0,0,25), 6b site (0,0,0), and 18e site (x,0,0.25), respectively. Difference Fourier map calculation allowed location of Li in the 18d (0.5,0,0)position.75 The structureis constitutedof nearly regular TiO6 ochahedra (Ti-O 1.943 Å) and La is 12- fold (La-O 2.559-2.911 Å) and Li 4-fold coordination with oxygen (Figure 3). The Li ions are placed in the middleof windows,formedby fourTiO6 units,in squareplanar configuration with Li-O bond lengths of 1.81-

2.07 Å.

Recent investigation of Li0.5La0.5TiO3 by means of TEM and SIMS resulted in the identification of a rigid frame,constitutedof La-Ti-O, with the orderingof the La3+, resultingin a superstructurewhereunit cell edges are doubled. The diffraction patterns of TEM mainly lattice constant (Å) x ) ab c Z space group/crystal system reference a ap ) cubic perovskite lattice parameter. b 27 °C. c Intermediate temperature polymorph (R), disordered phase (prepared by rapid quenching from high temperature to room temperature).3 d Low-temperature polymorph (â), ordered phase (prepared by normal cooling from the elevated temperature).3

Figure 2. Variation of the cubic perovskite lattice parameter

°C into liquid nitrogen. The line passing through the data points is a guide to the eye.

3976 Chem. Mater., Vol. 15, No. 21, 2003 Reviews

follow the rule for which the structure factor F ) 0i f( h

+ k + l) is even and F ) 0.5fLa if (h + k + l) is odd, differentiating itself from the XRD response. This was attributed to the uneven distribution of La3+ ions at the

A sites. Li2TiO3, as a second phase, was found at the grain boundary junctions.68

(Parte 1 de 6)

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