Article pubs.acs.org/cm
Atomic Layer Deposition of the Solid Electrolyte Garnet Li7La3Zr2O12 Eric Kazyak,† Kuan-Hung Chen,† Kevin N. Wood,† Andrew L. Davis,† Travis Thompson,† Ashley R. Bielinski,† Adrian J. Sanchez,† Xiang Wang,‡ Chongmin Wang,§ Jeff Sakamoto,† and Neil P. Dasgupta*,† †
Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States § Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡
S Supporting Information *
ABSTRACT: Lithium solid electrolytes are a promising platform for achieving high energy density, long-lasting, and safe rechargeable batteries, which could have widespread societal impact. In particular, the ceramic oxide garnet Li7La3Zr2O12 (LLZO) has been shown to be a promising electrolyte due to its stability and high ionic conductivity. Two major challenges for commercialization are the manufacture of thin layers and the creation of stable, low-impedance interfaces with both anode and cathode materials. Atomic layer deposition (ALD) has recently been shown to be a powerful method for depositing both solid electrolytes and interfacial layers to improve the stability and performance at electrode− electrolyte interfaces in battery systems. Herein, we present a thermal ALD process for LLZO, demonstrating the ability to tune composition within the amorphous as-deposited film, which is studied using in situ quartz crystal microbalance measurements. Postannealing using a variety of substrates and gas environments was performed, and the formation of the cubic phase was observed at temperatures as low as 555 °C, significantly lower than what is required for bulk processing. Additionally, challenges associated with achieving a dense garnet phase due to substrate reactivity, morphology changes, and Li loss under the necessary high-temperature annealing are quantified via in situ synchrotron X-ray diffraction.
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INTRODUCTION The ability to safely and inexpensively store electrical energy is a key component of future energy systems, enabling the use of intermittent renewable sources, long-range electric vehicles, long-lasting electronics, and more. While current Li-ion technology has enabled a range of technological advances, many applications would greatly benefit from next-generation battery technologies that can store energy at a higher density and lower cost. To be viable for widespread application, these technologies must have a long cycle life, improved safety, and superior energy density and specific energy.1 Several next-generation battery architectures utilize Li metal anodes, but they suffer from poor cycle life and safety concerns due to interfacial instability (including dendrite formation) and undesirable side reactions resulting from the reactivity of Li metal.2 This is true in both Li metal-intercalation cathode cells and in next-generation Li-sulfur cells, which boast of having a dramatically improved theoretical gravimetric energy density (2567 W h/g vs 387 for Li-ion).3 The use of liquid electrolytes in these systems often leads to Li consumption, dendrite formation, and the potential for catastrophic failure and fires.4 There have been many recent efforts to address these concerns by replacing the liquid electrolyte with solid electrolytes.5−8 © 2017 American Chemical Society
Solid-state electrolytes (SSEs) can improve safety by removing flammable liquids, improving chemical and electrochemical stabilities against electrodes and providing a physical barrier to dendrite propagation. The improved stability can dramatically improve cycle life while enabling the use of high-voltage cathodes and high-temperature operation.9 The increased temperature range reduces the need for large, heavy, and expensive thermal management systems, which represent a large part of the overall cost, size, and weight of battery packs for electric vehicle applications. The current state-of-the-art SSE is sputtered lithium phosphorus oxynitride (LiPON). This material is currently used in thin film batteries for a range of applications and has been shown to have excellent cycle life.10,11 The roomtemperature ionic conductivity is moderate (10−6 S/cm), and although LiPON is hypothesized to form an interfacial passivation layer in situ, it is reduced by Li metal.12 Scaling for larger scale applications and deposition on 3D structures are additional challenges.13 Therefore, other materials and Received: March 7, 2017 Revised: April 9, 2017 Published: April 14, 2017 3785
DOI: 10.1021/acs.chemmater.7b00944 Chem. Mater. 2017, 29, 3785−3792
Article
Chemistry of Materials
experiments to obtain the cubic garnet crystal structure. A highly phase pure cubic garnet film is obtained through precise control of the annealing environment, showing the potential of this process to be used for interfacial engineering and electrolyte deposition in 3D battery architectures.
manufacturing methods need to be explored to enable their use in high-capacity and high-power applications such as electric vehicles.14 One of the most promising SSE material systems is the lithium-rich garnet Li7La3Zr2O12 (LLZO), primarily due to its high ionic conductivity at room temperature (∼10−3 S/cm) and improved stability against Li metal.9,15−18 Through aliovalent doping, the superionic conducting cubic phase can be stabilized at room temperature, and this material has shown promise for integration in practical battery systems. Two of the primary challenges remaining for commercialization are the formation of stable low-impedance interfaces with both anode and cathode materials and the manufacture of thin dense films required for high overall energy density. Previous studies have developed processes for deposition of LLZO thin films with techniques including radio frequency sputtering,19 pulsed laser deposition,20 and chemical vapor deposition.21 These techniques are limited in their ability to conformally coat high aspect ratio 3D structures, require high temperatures, and are limited in scalability while maintaining precision and quality control during manufacturing of nanoscale architectures. Li-ion batteries rely on the ability of liquid electrolytes to penetrate into porous electrodes to facilitate ionic transfer and conduction. With SSEs, this is a major challenge, as a planar electrolyte in contact with a porous electrode can suffer from increased impedance due to a lack of ion conduction pathways and low interfacial area.22,23 Moving to 3D and porous electrode architectures could enhance rate capability and enable high areal capacity loading, increasing the energy density.24 This presents a significant challenge for solid electrolyte manufacturing, as it requires intimate contact between a complex 3D geometry and a solid electrolyte film.7,25 In recent years, atomic layer deposition (ALD) has been demonstrated to be a powerful technique for interfacial modification of Li-ion and Li-metal electrodes and for the deposition of solid electrolytes in complex 3D architectures.14,26−31 Furthermore, fluidized-bed and roll-to-roll ALD technologies are being commercialized for large scale Li-ion cathode coating.29,30,32,33 The self-limiting vapor-phase half reactions utilized in ALD enable programmable deposition of ultraprecise thin films on high aspect ratio structures with unparalleled control. For this reason, ALD is of great interest for both interfacial engineering and electrolyte film deposition in solid-state batteries (SSBs). Herein, we present a thermal ALD process for Al-doped LLZO solid electrolytes. The pentenary ALD process was developed by careful combination of constituent binary oxide ALD processes. In ALD of multielement compounds, the precursors must be chosen carefully to avoid unwanted reactions, incompatible temperature requirements, and impurities.34 Moreover, there are many critical aspects of ALD chemistry that can affect film composition, crystallinity, and morphology, including gas-phase ion-exchange reactions,35 chemical etching,36 thermal decomposition,37 and precursorspecific reaction pathways. Thus, a detailed understanding of ALD chemistry on the atomic scale is critical to enable the rational design of materials, which was quantitatively analyzed in this study using in situ QCM measurements of ALD growth. Additionally, challenges associated with lithium loss during annealing ultrathin Li-containing films arise due to the high ionic mobility and reactivity of Li.38 To address these challenges, in this study we perform in situ synchrotron XRD
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RESULTS AND DISCUSSION ALD of Li-containing films has proven to be a challenge due to the lack of sufficiently volatile precursors with a reasonable growth rate and low amount of impurities. In this study, lithium tert-butoxide (LiOtBu) was selected due to previous reports of its reasonable success in ternary films.39−42 Tris(N,N′-diisopropylformamidinato)lanthanum (LaFAMD) was chosen for its improved volatility and growth rate compared to that of other La precursors.43,44 Tetrakis(dimethylamido)zirconium (TDMAZ) and trimethylaluminum (TMA) were chosen because they are well-understood precursors capable of yielding high-quality films under a wide range of conditions.45 Ozone (O3) was used as the oxidizer in all cases to avoid potential complications resulting from the hygroscopic nature of Li2O, La2O3, and the resulting LLZO films.46The constituent binary processes for Li2O, La2O3, ZrO2, and Al2O3 were first optimized on their own and were then combined as shown in Figure 1 to yield dense LLZO films. Each of the processes was characterized to verify saturated pulse and purge times, growth rate, and that the amount of impurities was low (
