DESIGN AND TRANSPORT PHENOMENA OF ORGANIC SINGLE-ION CONDUCTORS FOR LITHIUM-METAL BATTERIES

Abstract

The upcoming smart energy era, which will drag in indispensable utilize of the wearable electronic for user-specific health care, self-driving electric vehicles, micromobilities, and smart-home electronics, has motivated the relentless development of advanced rechargeable power sources. Furthermore, the inconsistent energy supply from renewable resources strongly demands for reliable energy storage systems, which balances between energy demand and supply therefrom. Of various rechargeable energy storage systems reported to date, Li-ion batteries have undoubtedly occupied a predominant position. However, despite the remarkable growth in sales, the energy density of commercial LIBs advances laggardly. As one of the most promising alternatives to achieve high energy density, Li metal anode has garnered great attention due to a host-free Li storage behavior and lowest redox potential. The Li metal batteries, which Li metal anode paired with high capacity cathodes, are considered as the next-generation batteries owing to a high increase in energy density. However, the use of organic liquid electrolytes with Li metal anodes tends to cause unwanted passivation of metallic Li, resulting in performance degradation. This issue has spurred the pursuit of advanced electrolytes. Electrolyte is an electronically insulating but ionically conducting medium between a pair of electrodes. Because of its location being paired with electrodes, their interfaces determine the electrochemical reliability of the devices. Ideal ion transport can be achieved by complete ion dissociation and short tortuosity. The most common approach in battery electrolytes is mixing solvents and polymers with salts. However, the accompanying migration of counter anions and solvents tends to be decomposed on the electrodes, spontaneously, consuming the electrolyte components, leading to low Coulombic efficiency, poor cycle retention, and forming thick passivation layers with dendritic Li formation. Thus, huge research efforts have been made to resolve the interfacial problems, with particular attention to the development of solid-state single Li+ conductors. Recently, immobilized anionic domains such as inorganic lattices and polyanions have been extensively investigated but they are suffered from low conductivity caused by their intrinsically strong ionic association and discontinuous conduction pathway. The objective of this thesis is a demonstration of advanced designs for single Li+ conductors to address the formidable challenges. Chapter 2 introduces facile single Li+ transport based on relaxation dynamics of a Li+-centered G-quadruplex (LiGQ). In sharp contrast to the above-described traditional single-ion conductors with strong ion–ion interactions and long tortuosity, ion transport of the LiGQ is enabled by directional Li+ slippage through its central channels which allow weak ion-dipole interaction and straightforward ion pathways. This unusual Li+ slippage behavior of the LiGQ and its self-assembled crystalline structure were elucidated by in-depth experimental and theoretical investigations. Chapter 3 deals with single Li+ conducting gel-polymer electrolytes. Its interconnected cationic ion channel allowed the electrophoretic anion regulation for single Li+ transport along with substantially reduced anion mobility and its decompositions. Driven by the unique transport phenomena, the single-ion conductors enable significant improvements in electrochemical reliability of Li metal anode and electrode kinetics. Moreover, the novel designs are exploited as an artificial solid-electrolyte interphase for Li metal anode and a solid-state electrolyte for an anode-free Li metal battery as a proof-of-concept to explore its practical applicability.