리튬-황 전지 양극 고성능화를 위한 촉매 특성을 지닌 다공성 황 담지 소재 개발

Abstract

With a rapid growth of world-wide market of electric vehicles (EVs) and energy storage systems (ESSs), development of rechargeable batteries with high energy densities and low production cost becomes important issues. Even though lithium ion batteries (LIBs) have been commercially used in industrial field, their high production cost and low energy-to-weight ratios (140-250 W h kg-1) inhibit further promotion on the utilization of EVs and ESSs. Among the next-generation batteries, lithium-sulfur (Li-S) batteries have been attracted much attention as the most promising substitute for LIBs. Capacities of sulfur and lithium metal are much higher than conventional electrode materials of LIBs, resulting in high overall energy density (2600 W h kg-1). Moreover, annual production amount of sulfur is tens of millions of tons, leading to the great decrease of battery production cost. However, intrinsic problems including low electrical conductivity of active materials, dissolution of reaction intermediates into electrolyte, and sluggish reaction kinetics cause the poor reversible capacity and unstable cycle life. To tackle these issues, we developed conductive sulfur host materials with high catalytic activity. Especially, we investigated the porous architecture and surface electronic structure of catalytic sulfur host materials for the optimal Li-S cell performance. In Chapter 2, we synthesized ultrahigh pore volume mesoporous carbon microspheres with the incorporation of Fe-N-C molecular catalyst as a functional sulfur host material. The uniform control of particle morphology to sphere results in high electrode density and ultrahigh pore volume enables the stable accommodation of large amount of sulfur into the porous structure of host materials. Most importantly, Fe-N-C site can improve the redox kinetics of the sulfur conversion reaction during the charge and discharge. This synergistic effect of electrocatalysis on the Fe-N-C site and beneficial porous architecture of carbon substrate results in great improvement of Li-S electrochemical performance. In Chapter 3, hierarchically mesoporous and macroporous titanium nitride was synthesized by multiscale phase separation strategy and used as catalytic host materials for sulfur. The multiscale porous structures are highly beneficial for overcoming intrinsic demerits and synergistically combining the merits of single-sized porous structures. In particular, macropore enables the stable accommodation of abundant sulfur and facile electrolyte penetration, whereas mesopore inhibits the dissolution of soluble intermediate products. Besides a porous architecture, titanium nitride with a high electrical conductivity showed an outstanding catalytic activity to enhance the kinetics of sulfur conversion reaction. As a result, Li-S cell with hierarchical porous titanium nitride exhibited stable cycle life and high rate capability. In Chapter 4, we unveiled the fundamental relationship between the binding energy of sulfur species on the catalytic site and kinetics of sulfur conversion reaction. As a model system, the composite of Pt3M (M=second transition metal) nanoparticle catalysts in N-doped mesoporous carbon was used and binding energy of sulfur species on Pt3M surface was systematically controlled by the modulation of d-band structure with different M species. We elucidated that sulfur conversion reaction exhibited the highest kinetics with moderate binding energy of sulfur species on catalytic surface, neither too strong nor too weak. It suggests that moderate control of surface chemical property of catalytic sulfur host material is required to rationally design the cathode of Li-S battery. These studies demonstrated that the control of porous architecture and surface chemical properties of sulfur host materials is a key point to improve the redox kinetics of sulfur conversion reaction and to inhibit the dissolution of active materials. These strategies will create a promising avenue to enhance the electrochemical performance of Li-S battery.