The increasing demand for higher energy capacity and power rate batteries, driven by portable devices and electric vehicles, necessitates the development of advanced energy storage solutions. Solid-state batteries employing lithium metal anodes are promising candidates due to their significantly higher capacity (tenfold) compared to traditional graphite anodes. Solid lithium-ion conductive electrolytes are crucial for realizing this potential. Lithium garnet Li₇La₃Zr₂O₁₂ (LLZO) stands out among solid electrolytes because of its high ionic conductivity (up to 10⁻³ S cm⁻¹) and wide electrochemical stability window, showing stability against both metallic lithium and high-potential cathodes. However, the polycrystalline nature of LLZO presents a significant challenge: its susceptibility to lithium dendrite growth, which can lead to short circuits and battery failure. While the exact mechanisms are still debated, surface defects, bulk defects, and electronic conductivity along grain boundaries are generally considered responsible for lithium metal nucleation and dendrite propagation through the solid electrolyte. Recent research has demonstrated that laser annealing of bulk LLZO can create an amorphized surface layer, improving critical current density (CCD) and lifetime by blocking electron injection and dendrite formation. However, this method lacks control over the amorphous layer's properties. In contrast, ex situ coating methods like physical vapor deposition offer better control over composition and homogeneity. Previous studies have explored amorphous Li-La-Zr-O (aLLZO) thin films prepared by sputtering and pulsed-laser deposition, highlighting the potential of the amorphous phase but without demonstrating dendrite mitigation in battery configurations or as bulk electrolyte coatings. This research addresses the question of whether the amorphous phase of LLZO can effectively prevent lithium dendrite formation, similar to observations with glassy electrolytes like Li₃PO₄ and LiPON. The study presents a comprehensive investigation into the chemical structure and electrochemical properties of amorphous Ga-doped Li-La-Zr-O (aLLZO) films created via magnetron sputtering. By adjusting lithium excess, the ionic conductivity was significantly improved while maintaining negligible electronic conductivity. The resulting ultrathin, conformal, and grain-boundary-free films act as electron injection barriers while facilitating lithium-ion transport. The study evaluates the stability of these films against metallic lithium, examines their application as a dendrite-blocking coating on bulk crystalline LLZO, and demonstrates their use as ultrathin electrolytes in solid-state microbatteries.

The literature review section extensively covers existing research on solid-state batteries, focusing on the challenges and opportunities associated with lithium metal anodes and solid electrolytes. It highlights the advantages and drawbacks of different solid electrolyte materials, including LLZO's high ionic conductivity and electrochemical stability but also its vulnerability to dendrite formation. The review emphasizes the role of grain boundaries, defects, and electronic conductivity in facilitating dendrite growth, citing studies that explored various strategies to address these issues. Key publications examined include those by Janek and Zeier (2016) on the potential of solid-state batteries; Albertus et al. (2018) on challenges in enabling lithium metal anodes; Samson et al. (2019) on Li-stuffed garnet-type LLZO electrolytes; Wang et al. (2020) on garnet-type solid-state electrolytes; and Krauskopf et al. (2020) on physicochemical concepts of the lithium metal anode. The review also considers previous work on amorphous LLZO thin films, analyzing the findings of Kalita et al. (2012) and Garbayo et al. (2018) regarding ionic conductivity and amorphism. It references studies demonstrating the use of other amorphous electrolytes, such as Li₃PO₄ and LiPON, in preventing dendrite formation. Importantly, the literature review sets the stage for the current research by identifying the gap in knowledge—the lack of demonstration of amorphous LLZO's dendrite-blocking capability in practical battery applications—which the current study aims to fill.

Amorphous Ga-doped Li-La-Zr-O (aLLZO) thin films were deposited at room temperature using radio frequency (RF) magnetron co-sputtering. A stoichiometric Li₆.₂₅Ga₀.₂₅La₃Zr₂O₁₂ target was co-sputtered with a Li₂O target to control the lithium content and optimize the ionic conductivity. The films' morphology, composition, and electrochemical properties were characterized using various techniques. Scanning electron microscopy (SEM) and focused ion beam (FIB) time-of-flight secondary ion mass spectrometry (TOF-SIMS) were employed for morphological and compositional analysis. Grazing incidence X-ray diffraction (GI-XRD), Raman spectroscopy, and Fourier-transform infrared (FTIR) spectroscopy were used to determine the amorphous nature and chemical structure of the films. Impedance spectroscopy, current transient measurements, and various electrochemical techniques (including galvanostatic cycling and potentiostatic cycling) were used to assess the ionic and electronic conductivity, as well as the electrochemical stability of aLLZO films against lithium metal. Symmetric Li/Li cells and Li/Pt half-cells were fabricated to study the resistance to dendrite growth and the stability against lithium plating and stripping. To evaluate the performance of aLLZO as a coating, crystalline Ga-doped LLZO pellets were coated with a thin layer of aLLZO and tested in symmetric cells using through-plane and in-plane lithium plating-stripping methods. In-operando optical microscopy was used to visualize dendrite growth. Finally, thin-film microbatteries were fabricated using sputtered LCO cathodes, evaporated Li anodes, and an aLLZO electrolyte layer with a LiNbO₃ coating on the cathode for stability. The batteries' performance was tested by charge-discharge cycling at various current densities.

The study successfully synthesized and characterized amorphous Ga-doped Li-La-Zr-O (aLLZO) thin films via magnetron co-sputtering. By controlling the Li₂O mass fraction during co-sputtering, the ionic conductivity was dramatically enhanced (by up to three orders of magnitude) to 10⁻⁷ S cm⁻¹ while maintaining extremely low electronic conductivity (10⁻¹⁴ S cm⁻¹). This enhancement was attributed to an increased mobile Li⁺ ion concentration and decreased activation energy for ion migration. Electrochemical testing in symmetric Li/Li cells revealed outstanding stability against lithium dendrite formation, with no signs of short circuits at current densities up to 3.2 mA cm⁻². The aLLZO films proved highly stable against lithium metal plating and stripping, as shown by impedance spectroscopy measurements during cycling, which demonstrated the lack of significant changes in electrolyte resistance. When used as a coating on bulk crystalline LLZO, the aLLZO layer significantly reduced interfacial resistance and dramatically increased the critical current density (CCD) by a factor of four (from 0.32 mA cm⁻² to 1.3 mA cm⁻²). In-operando optical microscopy confirmed that the aLLZO coating effectively inhibited dendrite growth, preventing short circuits at higher current densities. Importantly, the study successfully demonstrated the use of 70 nm thick aLLZO films as the solid electrolyte in microbatteries, achieving excellent cycling performance (over 500 cycles at 10C) with only moderate capacity fading. The performance of the microbatteries indicates the feasibility of the aLLZO films as a reliable solid electrolyte for micro-scale applications.

The findings demonstrate the remarkable potential of amorphous LLZO as a high-performance solid electrolyte for solid-state batteries. The ability to significantly enhance ionic conductivity while maintaining exceptionally low electronic conductivity is crucial for preventing dendrite formation. The success in using ultrathin aLLZO films as a dendrite-blocking layer in both thin-film microbatteries and as a coating on bulk crystalline LLZO highlights the versatility of this material. The improved CCD achieved by the aLLZO coating is a significant advancement, addressing a major limitation of currently available solid-state electrolytes. The microbattery results demonstrate that high-performance, long-lasting solid-state microbatteries can be fabricated using ultrathin aLLZO electrolytes, opening avenues for applications in low-power devices. The overall findings significantly contribute to the ongoing efforts to improve the performance and safety of solid-state batteries, offering a promising pathway towards the widespread adoption of this technology.

This study successfully demonstrated that amorphous Li-La-Zr-O (aLLZO) is an effective dendrite-blocking layer for solid-state batteries. By carefully controlling the lithium stoichiometry during sputtering, the researchers achieved high ionic conductivity with negligible electronic conductivity. The resulting ultrathin films showed superior performance both as solid electrolytes in microbatteries and as coatings on bulk crystalline LLZO. Future research should focus on integrating aLLZO coatings into full-cell solid-state batteries to validate the long-term performance and explore further optimization for even higher energy densities and power rates.

The study primarily focused on laboratory-scale experiments. Further research is needed to evaluate the long-term stability and scalability of the aLLZO coating and electrolyte films for large-scale battery manufacturing. The high-temperature annealing steps used in some aspects of the sample preparation may not be compatible with all battery manufacturing processes. The microbattery results were obtained using a specific cathode material; further studies are required to assess compatibility with other cathode materials and their impact on battery performance. In-plane measurements indicated significant overpotentials potentially caused by poor contact between the Li metal contacts and the LLZO pellet, which might have influenced the apparent dendrite suppression.