Advancements in Solid-State Batteries: Pulsed Laser Deposition of Thin Film Electrolytes

The global transition towards renewable energy sources and electric vehicles has precipitated an urgent need for advanced energy storage technologies. While lithium-ion batteries have been at the forefront of this revolution, they face inherent limitations in energy density, safety, and performance. Solid-state batteries, particularly those employing lithium metal anodes and solid electrolytes, have emerged as a promising next-generation technology to address these challenges. Among various solid electrolyte materials, lithium garnet Li₇La₃Zr₂O₁₂ (LLZO) has garnered significant attention due to its high ionic conductivity, wide electrochemical stability window, and compatibility with lithium metal anodes. However, the fabrication of thin-film LLZO electrolytes with properties comparable to bulk materials has remained a significant challenge, hindering the development of high-performance solid-state batteries.

This thesis aimed to investigate and overcome the challenges in fabricating high-quality LLZO thin films using pulsed laser deposition (PLD). The primary objectives were to develop methods to compensate for lithium loss during thin film deposition and processing, achieve ionic conductivity in LLZO thin films comparable to bulk materials, explore novel fabrication techniques to enhance the properties and performance of LLZO thin films, and investigate the structural and electrochemical properties of LLZO thin films prepared under various conditions.

The research employed various experimental techniques centred around PLD for thin film fabrication. Key methodological approaches included the development of a post-lithiation technique using gas-phase diffusion to compensate for lithium loss during deposition and annealing, and the introduction of a novel segmented target approach for PLD, combining LLZO and Li₂O to incorporate excess lithium during deposition. The study involved a systematic variation of deposition parameters including temperature, oxygen pressure, and laser energy density, to optimise film properties. The characterisation of films was performed using grazing-incidence X-ray diffraction (GI-XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. The ionic conductivity was evaluated by electrochemical impedance spectroscopy (EIS). Additionally, the investigation extended to Ga-doped LLZO thin films to explore the effects of doping on film properties and performance.

The post-lithiation technique successfully compensated for lithium loss in LLZO thin films deposited by PLD, enabling the fabrication of stoichiometric cubic phase LLZO films from either amorphous LLZO or LZO precursor films. This method achieved ionic conductivity comparable to bulk LLZO (approximately 1 × 10^-4 S cm^-1 at 100 °C). The novel segmented target approach allowed precise control over excess lithium content during deposition, enabling room-temperature deposition of amorphous LLZO films with high lithium content. Notably, this technique produced films with exceptionally high conductivity (2.72 mS cm^-1 at 25 °C), surpassing previous records for Ga-doped cubic LLZO (1.7 mS cm^-1 at 25 °C).

A significant discovery of this research was the identification of a metastable amorphous LLZO phase with record-breaking ionic conductivity at room temperature. The study proposed a mechanism involving excess Li-ions occupying higher-energy, metastable positions that facilitate the collective migration of multiple ions. This challenges the conventional thoughts that crystalline phases are necessary for high ionic conductivity and could lead to the development of novel amorphous electrolytes with superior ionic conduction properties.

The investigation of Ga-doped LLZO thin films, fabricated using the segmented target technique, achieved ionic conductivity comparable to bulk non-doped LLZO but lower than bulk Ga-doped LLZO. This work revealed complex interactions between doping, excess lithium content, and film properties, highlighting the challenges in translating bulk material properties to thin-film form.

The significance of the research lies in several key areas. Firstly, the development of innovative fabrication techniques, including the post-lithiation and segmented target approaches, represents novel methods for addressing lithium loss in thin film deposition, potentially applicable to other lithium-containing materials beyond LLZO. Secondly, the discovery of a metastable highly conductive phase challenges existing paradigms in solid electrolyte research and could lead to new directions in developing of high-performance amorphous electrolytes. Thirdly, the comprehensive study of processing-structure-property relationships provides a valuable roadmap for future research and development of LLZO thin films. Furthermore, this work demonstrates the possibility of achieving bulk-like properties in LLZO thin films, a crucial step towards the realisation of high-performance thin-film solid-state batteries. Lastly, the insights gained into doping effects in thin films highlight the unique challenges and considerations involved in translating doped bulk materials into thin-film form.

In conclusion, this thesis makes significant contributions to the field of solid-state electrolytes, offering new fabrication techniques, uncovering novel material phases, and providing crucial insights into the complex relationships between processing, structure, and properties of LLZO thin films. These advancements pave the way for the development of next-generation solid-state batteries with improved performance, safety, and energy density, potentially revolutionising energy storage technologies for a sustainable future.