단일-이온 고분자 나노입자 기반 고분자 전해질의 합성 및 전기화학적 특성 연구

Abstract

Advances in lithium-ion battery (Li-ion batteries) technologies necessitate improved energy densities, long-cycle lives, and safe operation. However, lithium-ion batteries are facing crucial safety issues prompted by the flammability of conventional liquid electrolytes. The need for non-flammable and mechanically robust electrolytes to replace commercial liquid variants is thus rapidly emerging. Solid-state polymer electrolytes (SPEs) have been widely studied to improve mechanical and thermal properties. Solid or quasi-solid state polymer electrolytes have still main challenges for practical application due to their relatively poor ionic conductivity at room temperature. In addition, lithium salt-containing polymer electrolytes reveal a characteristic limiting current density, which causes concentration polarization and further capacity fading. To overcome these limitations, I synthesized single-ion conducting polymer nanoparticles using emulsion polymerization and chemical modification, where particle size could be easily controlled. Consequently, single-ion conducting polymer electrolytes with effective Li+ conducting pathways were obtained. In this dissertation, the role of nanoparticle-structured single-ion conducting polymers is investigated, and new strategies for utilizing the single-ion conducting nanoparticles in next-generation lithium-organic and lithium-sulfur batteries are proposed. In Chapter 1, I give a brief overview of polymer electrolytes for next-generation Li-ion batteries and introduce current research trends on single-ion conducting electrolytes prior to introducing functional polymer nanoparticles. Research on inorganic/organic hybrid electrolytes and polymer-tethered particle electrolytes is also included. Furthermore, the research motivation and objectives of this dissertation are presented to develop the current research based on the single-ion conducting system. In Chapter 2, I investigated quasi-solid-state single-ion conducting nanoparticle electrolytes for lithium–organic batteries, providing enhanced electrochemical stability, excellent rate performance, and long cycle life. The single-ion conducting polymer nanoparticles were synthesized by emulsion polymerization and post-modification of polymer shells, with different particle sizes. Hybridization of the single-ion conducting polymer nanoparticles with succinonitrile (SN) provided a high conductivity on the order of 10−3 Scm−1, a high lithium transference number of 0.99, and outstanding mechanical strength of >10 MPa over a wide temperature range. The lithium-organic cell comprising single-ion conducting polymer NP electrolyte demonstrated a stable specific capacity of >100 mAhg−1 at a high current density of 794 mAg−1 for 500 cycles. This chapter reveals the great potential of single-ion conducting nanoparticle electrolytes for the safe operation of lithium metal batteries as the first successful demonstration of quasi-solid-state single-ion conducting polymer electrolytes in environmental-friendly and cost-effective lithium-organic batteries. In Chapter 3, I developed the solid-state single-ion conducting nanoparticle electrolytes, which are composited of Li+-conducting polymer nanoparticles and Li+-donating polymer nanoparticles. With a controlled size and number ratio of nanoparticles, binary nanoparticle electrolytes provided an effective Li+ transport pathway. The efficient packing of two nanoparticles enhanced ionic conductivity and outstanding storage moduli of >0.1 GPa in a wide temperature range (25-90 ℃). Due to the wide electrochemical stability window, high mechanical strength, and efficient Li+ ion transport, the lithium cells with single-ion conducting binary nanoparticle electrolyte and sulfur-containing polymer cathode demonstrated a stable specific capacity of >600 mAh g-1 for 200 cycles. This promising approach of binary nanoparticle self-assembly with single-ion conducting characteristics suggests a potential use of nanoparticles as a new building block for high energy density rechargeable batteries.