Novel Catalytic Cathode Nanomaterials and Mechanistic Studies for Advanced Lithium–Oxygen Batteries

XIAHUI ZHANG

Since 1996, lithium–oxygen (Li–O2) batteries have been intensively studied as one of the most promising energy storage systems for high-energy applications such as electric vehicles. However, like many emerging technologies, Li–O2 batteries still suffer from several challenges, including high overpotential, poor rate capability, and short cycle life. These shortcomings are mainly caused by the (ⅰ) transport limitations of insoluble discharge products and (ⅱ) parasitic reactions between cathodes/electrolytes and reactive oxygen species. To address these issues, many research efforts have been devoted to promoting the solubility of reaction intermediates in electrolytes via additives and finding alternative chemistries that are less reactive toward cell components than conventional lithium peroxide chemistry. This dissertation describes high-performance Li–O2 batteries that can operate in humid environments via advanced lithium hydroxide (LiOH) chemistry. Benefiting from the tunable structure of metal–organic frameworks (MOFs), we designed and prepared two isoreticular MOF-74 nanomaterials for oxygen cathodes, one with and the other without redox-active metal centers (Mn-MOF-74 and Zn-MOF-74, respectively). We demonstrated that LiOH formed in the presence of moisture via chemical catalysis at coordinatively unsaturated metal centers of Mn-MOF-74.\n To further elucidate the reversibility and reaction mechanisms of LiOH chemistry, we performed a combined experimental and theoretical study. Although LiOH chemistry exhibited excellent cycling and rate performance, systematic characterization studies revealed that LiOH chemistry is irreversible in conventional liquid organic electrolytes using operando cell pressure measurements, isotope labeled mass spectrometry, Fourier-transform infrared (FTIR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. This irreversibility is mainly due to the formation of highly reactive oxygen species that attack organic electrolytes.\n Finally, an outlook for achieving reversible LiOH chemistry is provided, such as using solid-state reactions and redox mediators. Considering that moisture is inevitably present in air, the deep understanding of LiOH chemistry in this dissertation provides insights into practical Li–O2 batteries and other metal–O2 batteries, such as sodium–O2 and potassium–O2 batteries, which can potentially operate in ambient air. Besides, the MOF-based nanomaterials developed in this dissertation might be applied as electrode materials in other energy-related fields, including Li–sulfur batteries and supercapacitors.

Novel Catalytic Cathode Nanomaterials and Mechanistic Studies for Advanced Lithium–Oxygen Batteries

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