Exploring Local Chemical and Transport Kinetics at Homo- and Hetero-interfaces: Enabling Safe and Fast-conducting Lithium Solid-State Electrolytes
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chu-namo93-phd-dmse-2025-thesis Hyunwon Chu.pdf
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Author(s)
Chu, Hyunwon
Advisor(s)
Rupp, Jennifer L. M.
Tuller, Harry L.
Date Issued
May 2025
Publisher
Massachusetts Institute of Technology
Abstract
Solid-state batteries and related hybrid energy storage systems using garnet-type lithium lanthanum zirconate (LLZO) have immense potential based on their favorable bulk properties. However, their practical application is hindered by interfacial challenges that limit high-current performance and long-term stability. At the Li/LLZO interface, non-uniform Li nucleation and dissolution cause interfacial delamination and short-circuit failure. At LLZO grain boundaries, conductivity heterogeneities create electronic leakage pathways, inducing unwanted Li nucleation and disrupting safe operation. At the cathode/LLZO interface, interdiffusion and oxidation form resistive secondary phases, progressively deteriorating performance. To tackle the critical interfacial challenges in LLZO-based SSBs, this thesis investigates the underlying mechanisms driving these issues and applies the findings to advance engineering practices for enhanced performance and stability.
First, we investigate how the chemistry of different interlayers affects the stability of the Li/LLZO interface by evaluating multiple Li-alloying metals. While interlayers are widely applied for interface stabilization, their dynamic role in controlling Li plating and stripping remains poorly understood. In the study, lithiophilicity emerges as a key factor governing the cycling behavior of interlayer materials, with a bell-shaped dependence observed between lithiophilicity and failure current. To clarify this relationship, we compare interlayers with varying lithiophilicity and assess its influence on Li nucleation and dissolution. This analysis reveals an optimal lithiophilicity range, guiding the discovery of new promising interlayer materials that promote uniform Li deposition and reduce interfacial delamination. By establishing predictive guidelines for interlayer selection, this framework ensures stable high-current operation and long-term Li/LLZO interface stability.
Next, we explore the influence of grain boundary chemistry on local electrical conduction and short-circuit endurance in LLZO electrolytes. Although conductivity heterogeneity at LLZO grain boundaries is a known factor contributing to isolated Li nucleation, the chemical origin of this conductivity variation has not been clarified. In this study, we observe that LLZO grain boundaries preferentially accumulate excess Li vacancies, creating negative electrical potentials that alter the carrier profiles. The carrier redistribution from excess charges leads to decreased ionic conductivity and increased electronic conductivity at the boundaries, which promote isolated Li nucleation under high current operations. By modifying processing atmospheres and introducing multivalent dopants, we regulate local charge accumulation, homogenize ionic/electronic conductivity, and suppress isolated Li growth at the boundaries. These findings provide actionable insights into mitigating grain-boundary-driven failure in LLZO electrolytes, enabling high-current stability of internal electrolyte interfaces.
Finally, we address the need for an effective buffer layer between the cathode and LLZO to suppress interfacial degradation, focusing on lithium lanthanum titanate (LLTO) as a promising candidate. Despite its high bulk conductivity, LLTO suffers from significant grain boundary resistance, which limits its performance. To understand this limitation, we classify LLTO grain boundaries into three types based on chemical composition and charge distribution. Among them, positively charged grain boundaries induce ion-depleted space charge regions, contributing most to the overall resistance. To mitigate this, we introduce polaron defects in LLTO to neutralize space charge potentials and enhance grain boundary ionic conductivity. While this approach focuses on material optimization, it establishes a foundation for further developing LLTO as an effective cathode buffer layer in LLZO-based SSBs.
Together, we provide a comprehensive understanding of the interfacial challenges in LLZO-based SSBs and present novel engineering strategies to address these limitations. By improving the understanding and control of these critical interfaces, the thesis contributes to realizing the full potential of LLZO-based SSBs as high-performance, long-lasting energy storage systems.
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Massachusetts Institute of Technology. Department of Materials Science and Engineering
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