Lithium Garnet Development and Advances for Use in Next-Generation Solid-State Batteries

Abstract

Batteries power our modern society, but their development is outpaced by technology that requires increasing power output and time between charges. Lithium metal is the most energy dense anode choice available and is the most attractive anode material due to its low density, high specific capacity, and low electrochemical potential. Lithium anodes form dendrites which grow from the anode and, if they contact the cathode, lead to short circuit which could result in thermal runaway and ignition of the organic liquid electrolyte that is the most used electrolyte. Instead, solid-state electrolytes (SSEs) have the potential to omit liquid electrolytes, making them a safer alternative with Li₇La₃Zr₂O₁₂ (LLZO), one of the most commonly used SSEs. Fabrication is typically in the form of ceramic pellets or tapes that require high-temperature processing exceeding 1000 ºC. In contrast, the fabrication method used herein relies on a direct liquid precursor to solid film spray method, Sequential Decomposition Synthesis (SDS), that requires a maximum processing temperature of only 500-750 ºC and yields a low thickness of several microns comparable to standard polymer separators in the battery field. Collectively, this contributes to increasing energy density to more than 30% for next generation batteries, by utilizing a solid-state electrolyte with a low-potential lithium metal anode.

In this thesis, two major areas were focused on: in the first part, SDS-synthesized LLZO has been integrated into batteries for the first time and its cycling stability determined in operating conditions. A distinct advantage of SDS is its low synthesis cost combined with a naturally resulting film thickness in a range difficult to achieve by other methods (1-20 µm), making it easier to incorporate these films into existing battery architectures. It is demonstrated that SDS LLZO electrolyte layers can successfully be deposited on glass fiber support structures and integrated to hybrid semi liquid/solid demonstration cell architecture, suitable for next generation battery prototypes. Further, an amorphous LLZO phase was used to mitigate substrate temperature requirements and grain boundary lithium dendrite propagation. Successful cell cycling of 150 cycles at >80% capacity retention has been demonstrated. The successful demonstration of such low-processing temperature films is significant in the field of energy storage where the majority of projected costs from solid-state electrolyte fabrication is born by processing, rather than raw materials cost.

The second thesis part examines the ability of direct-liquid precursor-to-solid-state electrolyte synthesis via SDS for high throughput and Bayesian optimization-guided synthesis of a high number LLZO chemistries and ceramic nanostructures. In general, solid-state electrolyte development takes many years of labor and resource-intensive brute-force methods driven by humans to optimize conductivity, phase, or processing parameters (e.g. temperature). Instead, a data-driven approach using Bayesian optimization can better traverse a large sample space while requiring fewer samples than a simple grid search which could take months or years to reach the same optimization. We demonstrate that this approach can optimize a surrogate objective function to maximize conductivity by applying it to the well-studied LLZO material. Raman spectroscopy is non-destructive, quick, and information rich and fast which aids in rapid advances. In the first for the field, an SDS system was constructed to mix precursor solutions on demand to span an experimentally plausible composition range. In total, fewer than 100 samples were required to achieve a phase-pure and conductive film of LLZO. The computer-guided campaign optimized the cubic-LLZO phase and elucidated limits to its composition and doping that could be applied to guide future studies. This breakthrough platform enables rapid development of solid-state electrolytes for next-generation batteries. This approach can be adapted to novel solid-state materials that haven’t been synthesized before, enabling faster development of next-generation electrolytes and their batteries.