Microstructure and ionic conductivity of LLTO thin films: Influence of different substrates and excess lithium in the target

doi:10.1016/j.ssi.2014.12.005

Solid State Ionics

Volume 272, April 2015, Pages 1-8

https://doi.org/10.1016/j.ssi.2014.12.005 Get rights and content

Highlights

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Epitaxial growth of Li_3xLa_2/3 − xTiO₃ thin films on oriented SrTiO₃, LaAlO₃ and MgO substrates.
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Unavoidable growth of La₂Ti₂O₇ secondary phase at the oxygen pressure required to maintain low electronic conductivity.
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Secondary phases act as blocking layer to lithium ion diffusion.
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Up to 10% excess of lithium in the target is necessary to compensate lithium loss.

Abstract

The deposition of single phase Li_3xLa_2/3 − xTiO₃ (LLTO) thin films remains very challenging. The growth of the perovskite phase is in competition with the insulating La₂Ti₂O₇ phase when prepared at high oxygen pressure by PLD. Nevertheless, we have achieved epitaxial growth of LLTO on different (001) oriented substrates such as LaAlO₃, SrTiO₃ and MgO despite a large lattice mismatch of up to + 8.8%. We also determined the percentage of lithium excess in the target necessary to reach a maximum ionic conductivity. However, the presence of the blocking La₂Ti₂O₇ phase strongly hinders the lithium ion migration and reduces the total conductivity compared to bulk properties.

Introduction

Ceramic electrolytes have attracted much attention in the field of Li-ion battery development as they represent a safer alternative to the flammable organic electrolytes. Their development, and in particular as thin films, is fundamental to the development of new power storage units in microelectronic components [1], [2], [3]. To achieve the requirements of battery technology, these electrolytes must present properties equivalent to conventional organic electrolytes, i.e., high Li-ion conductivity to facilitate fast lithium transport between the anode and the cathode, and low electronic conductivity to avoid losses and maintain efficiency [4]. Among existing ceramic materials, the perovskite lithium lanthanum titanate (Li_3xLa_2/3 − x□_1/3−2xTiO₃, LLTO) has attracted much attention since the discovery of its high Li-ion conductivity [5], [6]. The presence of vacancies on the A-site, and the ordering of La-rich and La-poor layers, enhance the mobility of lithium ions that migrate from one dodecahedral A-site cage to the next one through square planar windows formed by four oxygen atoms [7], [8]. The highest measured conductivities are obtained when an adequate balance between the lithium content and the number of vacancies in the A-site position of the perovskite structure is achieved. These results have been achieved in bulk ceramic samples for an LLTO composition with x = 0.11, which leads to a lattice conductivity as high as 1 mS⋅cm⁻¹ at room temperature [9]. However, the grain boundaries of these ceramics present a large series resistance that dramatically decreases the total conductivity to values below 0.01 mS⋅cm⁻¹. The reasons for the large grain boundary resistance to Li-ion migration are still not fully understood and, despite recent improvements in the processing of these ceramics [10], a higher total conductivity is necessary for efficient practical applications.

The development of thin film technology over the last few decades has opened a new area of research with the introduction of nanoscale materials and hetero-structures. The properties of a material can be tuned via the influence of the substrate as thin films often develop large stress during their growth, particularly in epitaxial growth conditions. The stress-induced lattice distortions can significantly affect the physicochemical properties of materials. Reports on this interfacial effect are well documented in the literature, for SOFC electrolyte materials for instance, where variation of the oxygen ion conductivity of thin films has been reported to depend on the epitaxial strain developed at the interface between mismatched structures [11]. Important improvements in thin film deposition technology enable an epitaxial growth of thin films where layer-by-layer growth can be achieved. This type of growth displays a slow and well-controlled deposition of the material on the substrate and can present crystalline properties close to single crystal and, more particularly, the absence of grain boundaries. An example can be found in ref [12] where highly ordered thin films, free of high angle grain boundaries, were used to directly access the bulk proton conductivity of Y-doped barium zirconate, higher by two orders of magnitude than the total conductivity of ceramic samples of this material. This finding opens new research direction in ionic conductors, i.e., to dispose of blocking grain boundaries, which would have tremendous consequences for Li-ion conductors and, more particularly, LLTO ceramic electrolytes.

Physical vapour deposition techniques have been shown to give rise to a high loss of lithium during matter transport from the target to the substrate; however, this particular issue can be addressed by adding extra lithium to the targets [13]. Here we review the growth conditions of LLTO thin films on different substrates necessary to achieve epitaxial layers when prepared by pulsed laser deposition and we report on the influence of the lithium content of the targets on the ionic conductivity and the film microstructure.

Section snippets

Sample preparation

Targets used for the PLD deposition were prepared by citrate route as this route yields crystalline particles at low temperatures. The method consists of dissolving LiNO₃, La(NO₃)₃ and titanium(IV) oxide bis(2,4-pentanedionate) in a solution of citric acid with a low volume percentage of nitric acid. A gel was formed after evaporation of the organic solvents and then calcined at 600 °C to obtain a powder. The crystalline phase was formed at 800 °C, and the targets were sintered at 1350 °C in air

Characterisation of the targets

X-ray diffraction analyses of the targets indicate that they possess a tetragonal crystallographic structure corresponding to the P4/mmm space group (a = 3.8741 Å and c = 7.7459 Å [15]), as it can be observed in Fig. 1 for the target prepared with 10 mol% excess of lithium. This corresponds to the thermodynamically stable phase of LLTO at room temperature which consists of the layered structure of La-rich and La-poor layers alternating along the c-axis, necessary for Li-ion conduction. Despite high

Conclusions

In this work, we grew epitaxial Li_3xLa_2/3 − xTiO₃ thin films by PLD on STO, LAO and MgO substrates using targets presenting different lithium enrichments. We demonstrate that it is necessary to use a lithium-rich target with an excess of at least 10 mol% of lithium to achieve the maximal ionic conductivity. However, microscopy studies reveal that these films present a secondary phase restricting the lithium pathway. This phase identified as La₂Ti₂O₇ increases the resistance of the films as lithium

Acknowledgements

We would like to acknowledge Miss Nuria Goméz for ICP experimental work. F.A. would like to thank the support of the Ministry of Industry of the Basque Country for their financial support through the Etortek program untitled Energigune´13.

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Microstructure and ionic conductivity of LLTO thin films: Influence of different substrates and excess lithium in the target

Highlights

Abstract

Introduction

Section snippets

Sample preparation

Characterisation of the targets

Conclusions

Acknowledgements

Solid State Ionics

J. Power Sources

Solid State Commun.

Solid State Ionics

Solid State Ionics

J. Cryst. Growth

J. Solid State Chem.

J. Magn. Magn. Mater.

J. Cryst. Growth

Solid State Ionics

Solid State Ionics

Solid State Ionics