Materials and Structural Engineering of Separators and Composite Electrolytes for Electrochemical Energy Storage Applications

Abstract

Abstract

Materials and Structural Engineering of Separator and Composite Electrolyte for Electrochemical Energy Storage Applications

Ahn, Yong-keon Program in Nanoscience and Technology The Graudate School Seoul National University

Energy storage technology and its applications are regarded as the key strategic components for the green and sustainable energy system. Due to the intermittency of renewable energy sources such as wind and solar power, the importance of energy storing techniques have been increased to overcome the restriction of unlimited energy sources in use. Furthermore, these storage devices have significant potential to reduce the burdensome of environmental issues. The energy storage industries and market have been rapidly flourishing every day. Representative energy storage devices are well known as batteries and supercapacitors. These two types of applications are distinguished from their charge/discharge mechanisms and electrochemical performances. First, the rechargeable batteries, particularly the lithium secondary batteries, exhibit high energy density so that they are regarded as dominant storage devices for providing continuous power. Another type of application is called as supercapacitor (SC) or electrical double layer capacitor (EDLC), which shows to great power density and superior cycle life. Pulse powering system in tram and hybrid bus is a prime example of where SCs are used. Despite the remarkable advantages and usage, LIBs and SCs have faced to several challenges. To overcome the challenges and bring about the substantial improvement in performance, we have been focusing on separator and electrolyte. In the lithium ion battery system, the poly-olefin (polyethylene, polypropylene, polystyrene, or derivative blends) based separators are being commercially used, due to several attractive advantages such as low cost production, electrochemical stability, and suitable mechanical strength. However, severe drawbacks, which are weak thermal stability, inferior electrolyte wettability, and pore irregularity, lead to serious problems in operation of the LIBs. Consequently, the model study of separator with regard to structural and intrinsic properties is necessary, in order to ensure safety of the LIBs. Thus, we introduced an ideal AAO separator structure for the LIB system. The electrochemical performances of lithium ion batteries tend to depend on the structural properties of the separator. The porous framework and vertically straight channels with extremely low tortuosity allow for improved battery efficiency, better conductivity, higher discharging capacity, and superior rate retention capability. Through electrochemical experiments and computer aid simulation, highly ordered hexagonal pore arrays were found to effectively migrate lithium ions and evenly distribute the current density. The second strategy for enhanced energy density in SCs is the material engineering for electrolytes. From the relationship between the energy density and operating voltage, we suggest a solid-state composite electrolyte with the wide electrochemical stability window, in order to boost up the voltage range. In this project, we introduced the composite electrolyte of cross-linked polymer (c-P4VPh) and ionic liquid (EMITFSI). The composite electrolytes are highly ionic conductive solid states due to the rigid framework of c-P4VPh and high ionic conductivity from large contents of EMITFSI over 60 wt%. The IL-CPs are thermally stable over 300 °C and electrochemically stable over 7 V since there are hydrogen bonds between c-P4VPh and EMITFSI. We also introduced all-solid state SCs which operate at 4 V and have high energy density without sacrificing power density.

Keywords: composite electrolytes, inorganic separator, supercapacitor, lithium-ion battery, electrical double layer capacitor, energy storage device Student number : 2013-30730