Elsevier

Journal of Power Sources

Volume 361, 1 September 2017, Pages 61-69
Journal of Power Sources

Thick-films of garnet-type lithium ion conductor prepared by the Aerosol Deposition Method: The role of morphology and annealing treatment on the ionic conductivity

https://doi.org/10.1016/j.jpowsour.2017.06.061Get rights and content

Highlights

  • Thick films of Li7La3Zr2O12 laced with Al and Ta prepared at room temperature.

  • Aerosol Deposition following the room temperature impact consolidation mechanism.

  • Nano-crystallinity and micro strain in as-deposited films reduce ionic conductivity.

  • Decreases of strain and increases of crystallite size through annealing.

  • Recovery of ionic conductivity and reduction of activation energy through annealing.

Abstract

We fabricated thick films of cubic garnet solid electrolyte AlyLi7-3y-zLa3Zr2-zTazO12 (ALLZTO) by the Aerosol Deposition Method (ADM). Due to the room temperature impact consolidation (RTIC) mechanism, the films become dense. A thermal post-treatment of the film revealed the morphological and process-related impact on the ionic conductivity. As-deposited films show a reduced conductivity around 2·10−7 S/cm. Using electrochemical impedance spectroscopy and high-temperature X-ray diffraction, we found the lattice distortion and nano-crystallinity of the films to be the decisive effect for the conductivity reduction. In our case, post-deposition annealing at 400 °C lead to an increase of the ionic conductivity to 2·10−5 S/cm. With a beginning of sintering at 600 °C, the conductivity successively increased further, reaching values of 7·10−5 S/cm.

Introduction

Lithium ion batteries play a decisive role in mobile electronic devices and hybrid electric vehicles. While current battery cell systems suffer from unsatisfying specific capacity, all-solid-state rechargeable lithium ion batteries with higher specific capacity could pave the way for electric vehicles in mass markets. This encourages research in the field of solid-state lithium ion conductors. Besides material-related issues like chemical stability and ionic conductivity of the electrolyte component, processing is a main issue. For electrolytes, there is an application-born requirement of zero-defects. Another safety related issue is good mechanical stability, which increases when the films become thicker, but leads to a higher electrical resistance. With respect to processing technology, it is a quite advanced challenge to overcome these issues. The Aerosol Deposition Method (ADM), which is based on the room temperature impact consolidation (RTIC) of ceramic powders, yields dense films of adequate thickness directly at room temperature from a ceramic powder. A high velocity aerosol jet with μm to sub-μm ceramic particles is focused on a substrate material where the particles fracture upon impact to nano-crystallites and consolidate due to the hammering of subsequently impacting particles [1], [2]. Due to the room temperature conditions during deposition, there is a high degree of freedom in the substrate choice for this process. From this point of view, even temperature sensitive substrates like polymers could be used [3].

A quite large variety of materials has been already deposited to thick-films by the ADM and their film properties have been studied [1], [4]. Recently it has been shown that aerosol-deposited materials can potentially be applied in the field of gas sensing [5], [6], [7] or energy conversion such as solid oxide electrolyte [8], thermoelectric materials [9], as well as optically active materials for solar energy conversion [10]. Most investigations, especially of more fundamental nature, i.e., dedicated to the deposition and consolidation mechanism, have been conducted on alumina [1], [2], [4], [11], [12]. Besides, there are already some investigations that deal with AD of materials for battery applications like cathode materials LiFePO4 [13], [14], [15], LiNi1/3Co1/3Mn1/3O2 (NMC) [16], as well as anode materials such as Li4Ti5O12 (LTO) [17], silicon [18], [19], Fe2O3 [20] and solid electrolytes like Li1.3Al0.3Ti1.7(PO4)3 (LATP) [21] and Li1.5Al0.5Ge1.5(PO4)3 (LAGP) [22], as well as composites [23]. They assign this effect to the increase in the crystal grain size and to the higher degree in crystallinity caused by sintering.

Further studies deal with the lithium ion conducting oxide ceramic Li7La3Zr2O12 (LLZO). Ahn et al. demonstrated the possibility to fabricate electrolyte films of cubic LLZO [24]. It is a promising material for all-solid-state battery applications, since LLZO ceramics show a good stability against lithium and provide a high Li+-ion conductivity of approx. 2–6·10−4 S/cm [25]. LLZO crystallizes in the garnet-type structure. At room temperature, the highly lithium filled lattice is thermodynamically stable in the tetragonal modification [26], which shows lower bulk ionic conductivity (σbulk ≈ 2·10−6 S/cm) [27]. In order to achieve the cubic modification with higher conductivity, doping with Al3+ leads to a reduction of the number of Li+-ions in the crystal structure (ALLZO, AlyLi7-3yLa3Zr2O12) and to a stabilization of the cubic LLZO [28]. A significant reduction of the activation energy Ea can be observed for the cubic garnet [29]. The exchange of Zr4+ by higher valent elements, e.g. Nb5+ or Ta5+ leads, due to charge neutrality (LLZTO, Li7-zLa3Zr2-zTazO12), also to a lower Li+ content in the crystal structure and to a stabilization of the cubic phase at room temperature [30]. At the same time, the synthesis temperature for mixed oxide preparation can be reduced to 1000 °C.

First studies by Ahn et al. demonstrated that fully cycleable cells of cubic LLZO could be achieved by aerosol deposition, but with reduced ionic conductivity [24]. While LLZO itself appears to be stable against elemental lithium, it shows certain degradation in air [15]. Some studies show instabilities in the presence of humidity or CO2 [31], [32]. Both can be serious issues, since they can drastically increase the transfer resistance in processed films. Therefore, a trade-off exists between the requirements of mechanical stability (thicker membrane) and of low resistance (thinner membrane). Considering the reduced ionic conductivity of the ceramic-electrolyte system, the thickness should be in the range between 10 and 100 μm to achieve an optimum.

In this study, we pick up first results on AD of LLZO and investigate in detail the conductivity of AD-processed films and how it is influenced by film morphology. We process Al and Ta-doped cubic lithium garnet (ALLZTO) and analyze resulting films with respect to conductivity and morphology. Furthermore, we shed light on the effect of a thermal post-processing on the film properties. Usually conductivity measurements are conducted on pellets that were sintered at higher temperatures, e.g. 1100 °C. Since we just use a single temperatures step of 900 °C for powder calcination, we dedicate in a first step to the crystallographic properties of such powders with two different amounts of Al- and Ta- doping.

Section snippets

Powder synthesis

The LLZO garnet-type powder were synthesized by a mixed oxide route. The initial oxides were mixed stoichiometrically with an excess of Li2CO3 of 10 w.-%. Al (Al2O3) and Ta (Ta2O5) was added to achieve the favored cubic modification with enhanced ionic conductivity [28], [30], [33]. In this study, we used two lots of garnet powders with two different amounts of Al and Ta as dopants. Prior to deposition, the powders were examined by XRD to verify the effect of the Al and Ta on the content of the

ALLZTO powders for aerosol deposition

As explained before, two different powders were prepared. Exemplarily, the higher doped powder (y = 0.4, and z = 0.5) is shown in Fig. 2. The SEM image shows the fully treated powder that was used for AD. A broad spectrum of particle sizes reaching from smaller than 100 nm up to several μm is found, with a D50 of 1.9 μm.

After synthesis and milling, the phase content of the ALLZTO powders was analyzed by XRD (see Fig. 3). The transformation to the garnet-type phase is completed for both powders.

Conclusion

This study shows how aerosol-deposited films of Al and Ta-doped Li7La3Zr2O12 behave in respect to morphology and ionic conductivity. The Aerosol Deposition Method (ADM) was used to prepare several micron thick films of cubic garnet-type ALLZTO. In addition to the characterization of as-deposited and annealed samples by EIS and SEM, also HT-XRD analysis were conducted to explain the findings.

EIS measurements of as-deposited films show a reduced ionic conductivity of approximately 2·10−7 S/cm,

Acknowledgement

The authors are grateful to Ms. Laura Schwinger for conducting annealing experiments as well as to Mrs. Mergner and Mrs. Haider for taking SEM images. Furthermore, we thank the Department of Metal and Alloys (Prof. U. Glatzel) for XRD measurements. We are also indebted to Dr. Christine Engel, Michael Butzin and Dr. Ulrich Eisele for intense scientific discussions and the Robert Bosch GmbH for funding this project.

References (44)

  • J. Exner et al.

    Sens. Actuators, B

    (2016)
  • K. Sahner et al.

    Sens. Actuators, B

    (2009)
  • J. Exner et al.

    Thin Solid Films

    (2014)
  • J. Exner et al.

    Adv. Powder Technol.

    (2015)
  • C.-W. Ahn et al.

    Carbon

    (2015)
  • I. Kim et al.

    J. Power Sources

    (2013)
  • C.-W. Ahn et al.

    J. Power Sources

    (2014)
  • S. Iwasaki et al.

    J. Power Sources

    (2014)
  • R. Inada et al.

    J. Power Sources

    (2014)
  • H. Usui et al.

    J. Power Sources

    (2011)
  • H. Usui et al.

    Thin Solid Films

    (2012)
  • R. Inada et al.

    Ceram. Int.

    (2015)
  • T. Kato et al.

    J. Power Sources

    (2016)
  • A. Logéat et al.

    Solid State Ionics

    (2012)
  • J. Awaka et al.

    J. Solid State Chem.

    (2009)
  • H. Buschmann et al.

    J. Power Sources

    (2012)
  • Y. Jin et al.

    J. Power Sources

    (2013)
  • R. Inada et al.

    Solid State Ionics

    (2014)
  • S. Wiegärtner et al.

    Sensor. Actuator. B: Chem.

    (2015)
  • J.H. Shu et al.

    Sens. Actuators, B

    (2010)
  • E. Rangasamy et al.

    Solid State Ionics

    (2012)
  • Y. Liu et al.

    Ceram. Int.

    (2016)
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