1, Univ of Michigan, Ann Arbor, Michigan, United States
3, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
4, Pacific Northwest National Laboratory, Richland, Washington, United States
Lithium-conducting solid-state electrolytes (SSEs) are a promising platform for achieving the high energy density, long-lasting, and safe rechargeable batteries needed for a wide range of applications. By eliminating the need for flammable and unstable liquid electrolytes, SSEs dramatically reduce the risk of fire while enabling high voltage and energy density chemistries including Li metal. In particular, the ceramic oxide material cubic garnet Li7La3Zr2O12 (LLZO) is a promising option due to its stability and high ionic conductivity. Two major challenges to commercialization are manufacturing of thin layers and creating stable, low-impedance, interfaces with both anode and cathode materials. Atomic Layer Deposition (ALD) has recently been demonstrated as a powerful method for depositing both solid electrolytes and interfacial layers to improve stability and performance at electrode-electrolyte interfaces in battery systems. The self-limiting reactions afford the ability to conformally coat arbitrary geometries for 3D batteries, powders, and porous cathodes, an important advantage over the current state-of-the-art LiPON solid electrolyte and previous approaches for thin-film LLZO.
In this study1, we present the first reported ALD of the pentenary oxide Al-doped LLZO. Constituent binary processes are successfully combined in a thermal ALD process at 225°C to deposit high purity, dense amorphous LLZO films. The cycle-by-cycle growth of the multi-component film is quantified by in situ quartz crystal microbalance (QCM) measurements. We demonstrate the ability to tune composition within the amorphous as-deposited film, anneal to achieve the desired cubic garnet phase, and characterize the annealed films via in situ synchrotron XRD during annealing. The ability to conformally coat high aspect ratio structures of arbitrary composition with ultrathin layers of dense, amorphous LLZO is shown, demonstrating the potential for integration into 3D battery architectures, including porous electrode structures. Approaches to overcome Li loss and phase segregation during annealing are demonstrated and discussed.
The film exhibits preferential orientation when annealed on single crystal substrates, and the cubic-tetragonal phase transition was observed at ~500°C, significantly lower temperature than reported for bulk synthesis methods. By tuning the composition and annealing conditions, the resulting film can be tuned from pure-phase La2Zr2O7 pyrochlore, a high temperature thermal barrier material with several commercial applications, to high purity tetragonal LLZO or high phase-purity cubic LLZO, the superionic solid electrolyte material.