Correlating Structural and Interfacial Effects of Ceramic Solid Electrolytes with Cycling Stability of Li metal in Solid-State Batteries
Garcia Mendez, Maria
2020
Abstract
Transportation-related emissions comprise the highest contribution of greenhouse gases, thus creating the great need for higher-efficiency, zero-emissions electric vehicles (EVs). However, to meet energy/power density demands, advanced batteries superior to Li-ion are required. Li metal-solid-state batteries (LMSSBs) have the potential to enable about 50 percent higher energy densities, but also improve safety by replacing the flammable liquid electrolyte with a non-flammable solid. Despite the discovery of a variety of promising solid-state electrolytes (SSEs), stable cycling of Li metal remains a challenge. Li metal has been observed to penetrate all ceramic bulk electrolytes upon charging. Currently, the metric used to quantify the resistance of a SSE to Li metal propagation, or the current density at which a Li filament propagates across a SSE is known as the critical current density (CCD). The magnitude of the CCD of a SSE determines the maximum current density at which a LMSSB can be charged without Li metal propagation that results in short-circuit. This thesis aims to gain a better understanding of the role that structural and interfacial effects play in elucidating the poorly understood Li metalpenetration phenomenon. Two inorganic solid electrolyte systems were used as platforms; garnet Li6.25Al0.25La3Zr2O12 (LLZO) as a model system for crystalline materials and 75Li2S-25P2S5 mol% (LPS) as a model system for glass-ceramic and amorphous materials. First, LPS was used to gain a deeper understanding of the interplay between microstructure-processing and electrochemical behavior when paired with a Li metal electrode. The effect of pressure and temperature on structural changes during densification were studied. Consequently, the differences observed in electrochemical performance were correlated to their respective atomic and micro structures. It was demonstrated that the CCD of LPS can be increased by controlling the crystalline structure that precipitates from the mother glass, not only enhancing its ionic transport, but also by improving the particle-particle adhesion during processing. In addition, a dense microstructure with low porosity (< a few percent) is desirable for higher rate capabilities. Thus, higher pressures during densification that can result in dense (~98% relative density) microstructures motivated the subsequent study on LPS. To achieve a dense glassy microstructure, the role that pressure has on densification at the glass transition temperature was investigated. Moreover, correlations between macro and atomic structure with elastic properties and ionic transport was pursued. A five-fold increase in ionic conductivity and a two-fold increase in elastic constants were measured compared to conventional room temperature molding conditions. We believe these results were attained due to favorable coordination environments for Li ion transport and molecular rearrangements. Lastly, the DC electrochemical stability of the optimized microstructure was characterized achieving an increase in CCD compared to the conventional cold-pressed sample. Finally, a mechanistic understanding of the coupled interfacial and quantum mechanical effects have on the Li-LLZO interface stability at low potentials was studied. Integration of a Li reference electrode in a solid-state cell and precise experimental control allowed to confirm empirically, for the first time, that LLZO is stable at Li cathodic potentials below 0 V vs Li/Li+. A new atomistic perspective by which Li penetrates LLZO SSE at the interface is proposed and validated via first-principles theoretical calculations. The findings in this work provide fundamental understanding towards overcoming the challenge of Li penetration in SSEs, which can translate into higher-rate-capability LMSSBs for high-energy demand applications.Subjects
Solid electrolytes Solid-state batteries Li metal Solid-state electrochemistry
Types
Thesis
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Showing items related by title, author, creator and subject.
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Garcia‐mendez, Regina; Smith, Jeffrey G.; Neuefeind, Joerg C.; Siegel, Donald J.; Sakamoto, Jeff (Wiley Periodicals, Inc., 2020-05)
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Yeh, Gregory S. Y. (John Wiley & Sons, Inc., 1977-03)
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