Elsevier

Solid State Ionics

Volume 354, 15 October 2020, 115397
Solid State Ionics

Fabrication of Li1+xAlxGe2-x(PO4)3 thin films by sputtering for solid electrolytes

https://doi.org/10.1016/j.ssi.2020.115397Get rights and content

Highlights

  • Sputtering technique and post annealing using a powder target can be used to prepare LAGP thin films with high ionic conductivities.

  • The crystallization of LAGP films from amorphous was studied using in-situ c-Synchrotron and high-resolution SEM.

  • The sputtering parameters have been optimised to fabricate the LAGP films with high ionic conductivity.

  • Total ionic conductivity values > 10−4 S cm−1 at 20 °C and activation energies as low as 0.31 eV were achieved.

Abstract

Li1+xAlxGe2-x(PO4)3 (LAGP) thin films have been grown on sapphire substrates by RF magnetron sputtering and post annealing. The effects of varying sputtering parameters such as power and pressure were studied, and the deposited films were characterized to investigate how the ionic conductivity values depend on the microstructure. The composition of as-sputtered films was found to be more strongly influenced by power than the pressure. The films deposited at lower powers, which results in lower deposition rates, have compositions similar to that of the target. Heating the substrates during deposition is found to minimise the formation of pinhole defects in films subsequently annealed at higher temperatures. Post annealing leads to a gradual transformation from the as-sputtered amorphous phase to crystalline LAGP thin films. At high annealing temperatures (above 700 °C) both porosity and the GeO2 impurity phase appear in the films and result in lower ionic conductivities. We have optimised the processing conditions to achieve ionic conductivities in excess of 10−4 Scm−1 and activation energies as low as 0.31 eV in films only 1 μm thick, suggesting that LAGP could offer attractive properties as a thin film battery electrolyte material.

Introduction

All solid-state lithium batteries are promising power sources for achieving high energy densities, improved safety and cycle-ability over a wide temperature range [1,2]. However, one of the main challenges in designing these batteries is the development of solid inorganic lithium-ion conducting materials with a sufficiently high ionic conductivity that are also thin, pinhole-free and dense enough to be used as efficient solid electrolytes [[3], [4], [5]]. A large number of different solid electrolytes have been investigated for this application, including oxide- and phosphate-based ionic conductors such as Li1+xAlxTi2-x(PO4)3 (LATP), Li1+xAlxGe2-x(PO4)3 (LAGP), Li2/3-3xLaxTiO3 (LLTO) and Li7La3Zr2O12 (LLZO), sulphur-based ionic conductors such as the lithium thiophosphates (e.g. Li10GeP2S12, Li6PS5Cl, Li7P3S11) and lithium phosphorus oxynitride (LiPON). Of these materials, the oxide- and phosphate-based electrolytes have the advantage of improved chemical stability and safety over the sulphide compounds [3], but reduced values of ionic conductivity [4]. The NASICON-type glass-ceramic electrolyte Li1+xAlxGe2-x(PO4)3 (LAGP) is a promising candidate due to the relatively high ionic conductivity measured in bulk samples, reasonable chemical stability in air and adequate mechanical properties [[6], [7], [8]]. Bulk LAGP has been reported to have a total room temperature ionic conductivity values in the range 3.3–6.7 × 10−4 S cm−1 [5] [6,7], with Kumar et al. reporting a value as high as 5.08 × 10−3 S cm−1 at room temperature [8].

Due to the relatively low ionic conductivity of oxide- and phosphate-based solid electrolytes compared to liquid electrolytes, thin layers of solid electrolytes will be needed to achieve high overall energy densities in all-solid-state batteries. A major challenge for the future commercialization of solid state batteries will be the large scale manufacture of uniform thin layers. LAGP, for example, is generally fabricated in the form of bulk pellets by conventional ceramic processing, but in addition to its high cost, it is difficult by this method to prepare large area, pinhole-free thin layers [7]. Reliable thin film preparation routes will be required, which may also offer opportunities for introducing additional layers to improve the stability and performance of electrode−electrolyte interfaces in operation [4].

Methods such as aerosol deposition, sol-gel processing, radio frequency (RF) magnetron sputtering and pulsed laser deposition have all been used to fabricate thin films of solid electrolyte materials [9,10]. Of these techniques, magnetron sputtering offers the advantages of relatively simple apparatus, low cost, good thickness control and uniform film deposition over relatively large areas [11,12]. Sputtering has been used to fabricate thin films of several solid electrolyte materials, in particular LLZO [13], LiPON [14] and LLTO [9]. For example, LLZO thin films grown by sputtering have shown a higher ionic conductivity (1.2 × 10−4 S cm−1 at 25 °C) than LLZO films grown by other techniques [15]. The influence of sputtering conditions such as gas flow rates, power density, nitrogen pressure and target density on the microstructure and conductivity of LiPON thin films have all been investigated, and it has been reported that both power density and nitrogen pressure have strong effects on the ionic conductivity of the resulting films [16]. Xiong et al. studied the effects of annealing temperature on the conductivity of LLTO films prepared by cold sputtering and found that both as-prepared LLTO films and those annealed at temperatures up to 300 °C were amorphous, but that La0.56Li0.33TiO3 and other impurity phases appeared in films annealed at 400 °C [9]. Pinholes and other defects have been reported to be influential on the ion conductivity of these thin films, and more importantly will cause irreversible failure of the battery due to the formation of short-circuits between the two electrodes [14]. For instance, Ruzmetov et al. state that LiPON thin films deposited at higher pressures (13.3 Pa) contain large cracks and pinholes that result in electrical shorts [14].

There are only a few reports on the deposition of LAGP thin films. Khan et al. studied the effect of annealing temperature on 10 μm LAGP films prepared by aerosol deposition, and reported that annealing at 750 °C leads to a higher ion conductivity of 1.16 × 10−4 S cm−1 that they claimed was due to the reduction of grain boundary area [10], and that annealing at both 600 and 750 °C led to partial decomposition to form AlPO4 [10]. Another report on 75 μm sheets of LAGP fabricated by tape casting described promising ionic conductivity values of 3.38 × 10−4 S cm−1 at 25 °C [7]. More recently, LAGP thin films have been deposited by sputtering in-situ (onto a heated substrate) [17], and crystallization of textured films was observed above 400 °C. The highest ionic conductivity reported was 1.29 × 10−6 Scm−1 (with a low activation energy of 0.25 eV) in amorphous films deposited at 200 °C, with the more crystalline films deposited at higher temperatures showing lower values.

In this work, LAGP thin films (both amorphous and crystalline) were prepared by sputtering and ex-situ heat treatment. We report on the effects of sputtering parameters on the properties of the deposited films, and on the optimization of the processing conditions to achieve ionic conductivities in excess of 10−4 Scm−1 in films as thin as 1 μm.

Section snippets

Experimental methods

LAGP thin films were grown by RF sputtering from powder targets under different conditions. The powder target was simply prepared from commercial LAGP powder from MTI with nominal composition Li1.5Al0.5Ge1.5(PO4)3. The powder was mounted into a 3-inch. Cu-disk holder and pressed flat with a metal plate. (0001)-Al2O3 substrates were cleaned ultrasonically for 10 mins in acetone before being mounted at a distance of 5 cm from the target. The schematic image of our sputtering set up can be found

Sputtering parameters

The optimization of sputtering parameters, especially power and pressure, is important in designing a process to deposit thin films of compound materials with reproducible compositions. To optimise these parameters, LAGP thin films were grown using a range of sputtering conditions, and in two different sputtering gases, pure Ar and a mixture of Ar-10%O2. Sun et al. [17] report an excess of Al in their in-situ sputtered LAGP films, but here we have concentrated on analysing the P/Ge ratio

Conclusions

We have shown that a simple sputtering and post annealing process using a commercial powder target can be used to prepare thin films of LAGP ~1 μm in thickness with ionic conductivities similar to those reported for bulk ceramic samples. These high conductivity values depend on optimising the sputtering parameters including power and pressure to deposit chemically uniform films with compositions similar to that of the target, and by careful study of the processes occurring in the heat treatment

Declaration of competing interest

The authors declare that there is no conflict of interest.

Acknowledgements

The work reported here was carried out with support from the Faraday Institution [SOLBAT: grant number FIRG007].

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