A. James Clark School of Engineering
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Item INTERFACE AND STRUCTURES IN LITHIUM-GARNET QUASI-SOLID-STATE BATTERIES(2024) Gritton, Jack Evans; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A confluence of adoption of the internet of things, mobile electronics, electric vehicles, and shift towards adoption of intermittent green energy sources has led to a need for rapid improvement in battery technology in metrics ranging from rate capability and energy density to safety. While significant strides have been made through traditional liquid-based lithium-ion batteries, these oft-conflicting demands require fundamental shifts in battery chemistry, especially enabling safe incorporation of lithium metal anodes. Given their high conductivity, non-flammability, wide electrochemical stability window, and stability to lithium metal, lithium-stuffed garnets of the family LLZO provide one of the most promising alternative electrolytes to replace traditional flammable electrolytes. Two of the largest factors holding back these ceramic electrolytes are interfacial compatibilities and the interplay between processing and electrolyte mass. While drastic improvements have been made in the interface between garnet and lithium metal to improve rate capability, similar jumps in full cells have not been observed for rate and capacity. Using a varied cathode loading and a combination of EIS and DRT, we showed that garnet-catholyte interface was the main contributor to resistance in quasi-solid-state batteries of reasonable cathode loadings utilizing Pyr14TFSI based catholyte. Two methods were then used to improve this interface: modification of the garnet structure interfacing with catholyte, and modification of catholyte composition. Through the use of these methods, rate capabilities and capacity were drastically improved from the baseline system, both at elevated and room temperature. In addition to reducing interfacial resistance, cell polarization can be reduced through using thinner electrolytes. Given its higher mass density and lower conductivity in comparison to liquid electrolytes, garnet has historically had to rely on its greater stability to higher energy density electrodes to maintain competitive energy densities or utilize thin-film procedures that reduce mass but result in orders of magnitude lower conductivity than bulk produced garnets. To balance conductivity, ease of processing, and cell mass, a new combination of bulk-derived processing has been developed that allows for thin free-standing cubic garnet and thin, flexible, porous garnet. Cells using these new thin garnets achieved high cycling rates, and significant capacities.Item Title of Dissertation: Structural and Electrochemical Variances in Doped Lithiated Cathodes and Ionically Conducting Solid State Materials: Relationships in Solid State Electrolytes, Cathodes, and the Interfaces(2023) Limpert, Matthew A.; Wachsman, Eric D; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lithium-ion conducting Li7La3Zr2O12 (LLZO) garnets are being explored as a replacement for the flammable organic electrolytes used in batteries. However, LLZO garnets require high temperature sintering to densify the structure, but that microstructure and electrochemical properties can vary with lithium content as the lithium volatizes during sintering. The effects of sintering the LLZO garnet requires a detailed examination and study to determine how lithium content can affect physical properties, phase purity and density, as well as performance through ionic conductivity. Studying these parameters produced ionic conductivities above 10-4 S cm-1 in samples that had increased density by enabling liquid phase sintering through the eutectic between Al2O3 and Li2O. Despite this high conductivity, the movement of Li+ through a solid electrolyte encounters even slower kinetics through the rigid electrolyte-cathode interface to the active cathode material. A cathode for LLZO garnets requires a new design with both ionic conduction and electronic conduction pathways while reducing interfacial resistance when co-sintered. Excess lithium within LLZO garnet reduced formation of nonconductive LaCoO3 when co-sintered with the active material, LiCoO2 (LCO), which enables a new completely solid-state cathode for lithium metal batteries to be designed and interfacial resistance to be minimized. LCO, however, is limited to 4.2 V to ensure long life cycle without lattice deformation. Unlocking the potential 5 V cycling with of LLZO garnet necessitated the development of a higher voltage cathode. Chlorinating the oxygen site of lithium spinel, LiMn2O4, using a citric acid method stabilizes the 2 V plateau, which increases the capacity to 180 mAhr g-1, and triple doping with Co, Fe, and Ni enables customization of the properties while shifting the voltage to 5 V. The high voltage spinel and LLZO garnet enables high voltage cycling with increased safety potential enabling a pathway to a safe 400 Wh kg-1 cell, 150 Wh kg-1 higher than the current state of the art.