Summary

The overall objective of this work was to develop a solid-state electrolyte (SSE) technology based on garnet-type Li7La3Zr2O12 (LLZO) to enable the integration of metallic Li anodes into rechargeable batteries. If successful, this work could enable a step increase in energy density and facilitate the transition to electric power trains. The critical steps toward the successful completion of this objective include:

• Determine if LLZO SSE can protect metallic Li anodes

• Develop a fundamental understanding of what governs the critical current density in

polycrystalline LLZO

• Establish approaches to maximize the critical current density in polycrystalline LLZO • Determine if polycrystalline LLZO is relevant to vehicle electrification.

Per our hypothesis, defects will govern the maximum current density in polycrystalline LLZO. To this end, the primary focus of this thesis was to understand the role that defects play in governing the stability and transport through the Li-LLZO interface and quantify the contributions of each defect. This knowledge can be used to develop engineering approaches to tailor the LLZO microstructure and interface for maximum resistance to Li metal propagation.

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7.1.1 Stability and transport through the Li-LLZO interface

Comprehensive knowledge of solid-solid interfaces is needed to understand what governs their feasibility in enabling solid-state batteries. The stability and kinetics of the Li-LLZO interface were characterized as a function of temperature and current density (Chapter 3). To this end, polycrystalline LLZO was densified using a rapid hot-pressing technique achieving 97 ± 1% relative density, and <10% grain boundary resistance; effectively consisting of an ensemble of single LLZO crystals. It was demonstrated that temperature and current density significantly affect the Li-LLZO interface kinetics and stability between 30 and 175°C. It was also determined that by heating to 175°C, the room temperature Li-LLZO interface resistance decreases dramatically from 5822 (as-assembled) to 514 Ω.cm2; a > 10-fold decrease. In characterizing the maximum sustainable current density (or critical current density, CCD) of the Li-LLZO interface, it was shown CCD significantly increases with increasing temperature. For the cells cycled at 30, 70, 100, 130 and 160°C, the CCD was determined to be 0.05, 0.2, 0.8, 3.5, and 20.0 mA.cm-2, respectively. The relationship and phenomena observed in this study is useful to better understand the Li-LLZO interface stability to enable the use of batteries employing metallic Li anodes. However, for a SSB to be viable, the stability and charge transfer kinetics at the Li- LLZO interface should foster facile plating and stripping of Li. To compete with conventional Li-ion batteries, the Li-LLZO interfacial resistance should be lower than 10 Ω.cm2_{.Contrary to} these goals, the initial interfacial resistance in Li-LLZO was much higher (>500 Ω.cm2) than expected. Previous studies reported high Li-LLZO interfacial resistance is attributed to a contamination layer that forms upon exposure of LLZO to air. Hence, Chapter 4 clarifies the mechanisms and consequences associated with air exposure of LLZO; additionally, strategies to minimize these effects were described. In this chapter, first-principles calculations revealed that

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LLZO readily reacts with humid air; the most favorable reaction pathway involves protonation of LLZO and formation of Li2CO3. The reaction pathway describing the formation of a contamination layer on LLZO surface proposed by calculations was supported by materials and surface chemistry analysis. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the surface and subsurface chemistry of LLZO as a function of relative humidity and exposure time. Additionally, electrochemical impedance spectroscopy (EIS) was used to measure the Li-LLZO interfacial resistance as a function of surface contamination. These data indicated that air exposure-induced contamination significantly impacts the interfacial resistance: exposure for 240 h resulted in a resistance increase from 54 Ω.cm2 (no exposure) to ~37,000 Ω.cm2_{. Further calculations indicated the possibility for decomposition of Li}

2CO3 and de-protonation of LLZO at high temperatures. The predicted decomposition temperature for H2O evolution from LLZO was at approximately 250°C, and de-protonation of LLZO with decomposition of Li2CO3 was defined between 400°C and 500°C. Thus, Chapter 5 focused on evaluating a simple procedure for removing these surface layers by heat-treatment between 400°C and 500°C in inert gas. It was shown the interfacial resistance of the LLZO SSE against metallic Li can be reduced to 2 Ω.cm2 – lower than that of liquid electrolytes – by controlling the surface chemistry of LLZO and without the need for interlayer coatings. A combination of Li contact angle measurements, surface characterization, first-principles calculations, and EIS was used to show that the presence of common LLZO surface contaminants, Li2CO3 and LiOH, resulted in poor wettability by Li and high interfacial resistance. Furthermore, the direct current (DC) cycling illustrated the importance of achieving low resistance in solid-state cell designs employing metallic Li anodes. It was shown that decreasing the Li-LLZO interfacial resistance

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from hundreds of Ω.cm2 _{to less than ten Ω.cm}2 _{can increases the CCD from 0.05 mA.cm}-2 _{to 0.3} mA.cm-2, respectively. This demonstrates that controlling surface chemistry and reducing Li- LLZO interfacial resistance is an approach to achieve higher CCD. It was also shown that the low resistance interfacial properties were maintained over one hundred cycles, which suggested a pathway to achieving stable, high energy and power density SSBs.

7.1.2 Stability and transport through LLZO

In the context of SSB technology maturation, the CCD must approach values higher than 1 mA.cm-2_{. Monroe et al. proposed that if a SSE has a shear modulus more than a factor of two} higher than metallic Li (4.8 GPa), Li filament initiation and propagation should not occur. Thus, LLZO with high shear modulus (60 GPa) was believed to be stable against metallic Li and to suppress initiation and propagation of Li filaments during DC cycling. However, our studies (Chapter 3, 5) and previous studies by other researchers showed that the CCD of LLZO is <1 mA.cm-2 _{in symmetric metallic Li anode cells. To understand this phenomenon, more efforts} were focused on understanding the mode of Li filament propagation and engineering the microstructure to suppress Li filament propagation in polycrystalline LLZO. Characterization of short-circuited LLZO in Chapter 3, showed direct evidence of intergranular Li propagation through LLZO grain boundaries. This shows grain boundaries are the defects that primarily control the CCD in polycrystalline LLZO. This finding begged the question “how can the LLZO microstructure be carefully engineered to stabilize the Li-LLZO interface?”. In this regard, more efforts were focused on improving the LLZO stability through microstructural engineering of LLZO (Chapter 6). In this chapter, the amount of grain boundary area per unit area of Li-LLZO interface (reduce the number of failure points per unit area) was controlled by growing grains through ceramic processing. It was shown that the average grain size of polycrystalline LLZO

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can be increased by increasing the hot-pressing temperature and annealing time. It was also shown that increasing the average grain size from 5 µm to 600 µm, improved the CCD from 0.3 to 0.6 mA.cm-2, respectively. However, the presence of grain boundaries, albeit reduced, still acts as initiation points to propagate metallic Li during cycling. To improve the stability of the Li– solid electrolyte interface, future efforts should focus on engineering grain boundaries to suppress Li filament propagation in polycrystalline solid electrolytes.