Polymerization and Interface Engineering of Polymer Gels for the Development of All-Solid-State Supercapacitors

Abstract

Energy storage device has become an essential part of the modern society. However, one of the key components of the device, liquid electrolyte, poses a potential hazard. Therefore, it comes as no surprise that research on polymer electrolyte is gaining much interest and attention. Polymer-based electrolyte, while maintaining the advantages of the predecessor also provides stronger interfaces stability, mechanical properties, and charge/discharge stability. Making use of such advantages of the polymer electrolyte, all-solid-state supercapacitor with the component has better mechanical properties and reduced interfacial resistance. Theses physical progresses ultimately bring advanced electrochemical properties. Also, by using polymer electrolyte, packaging is omitted thereby once again fueling the interest at hand. In this study, polymer electrolyte was applied to all-solid-state supercapacitor to promote interfacial stability between the electrode and the electrolyte. The study focused on reducing the interface resistance and upgrading the mechanical properties of the proposed supercapacitor. In chapter 1, an introduction to gel electrolyte based on hydrogel and polymer was detailed. The chapter presented schematic explanations to how such properties of the electrolytes could be applied to all-solid-state supercapacitor. In chapter 2, all-solid-state supercapacitor with hydrogel electrolyte having reversible phase transition characteristic was detailed. The sol-gel reversibility was used to assemble the all-solid-state supercapacitor, thereby all the interfaces between the current collector, electrode and electrolyte was fused. This effort was achieved first by letting the solution state electrolyte to sip through the 3D structure current collector and changing the state from solution to gel. Then, during polymerization the interface between electrodes and current collector was fused. The hydrogel electrode prepared as mentioned was covered with the solution state electrolyte then polymerized to fuse the interface between the electrode and the electrolyte. Through the method explained, an integrated all-solid-state supercapacitor was prepared. By integrating the interfaces of all the components, the supercapacitor’s interfacial resistance was decrease, and the electrochemical property improved by 35%. Also, the mechanical properties were enhanced 10 times, and the interface stability made the supercapacitor more resilient to mechanical deformation from repeated use. In chapter 3, all-solid-state supercapacitor with ionogel which has a better stability towards the outside environment was detailed. The electrode was coated the current collector by using functional aqueous polymer binder in slurry. On the prepared electrode, prepolymer electrolyte was applied, then by polymerization the interface between the electrode and the electrolyte was fused. The polymer binder and the ionogel electrolyte was designed so that the two could fuse through covalent bonds. With the method mentioned an all-solid-state supercapacitor with covalent bonding between the electrode and the electrolyte was prepared. The supercapacitor with functionalized binder showed 1.3 times improvement in mechanical strength and 2.47 times improvement in electrochemical function compared to its counterpart. Also, the supercapacitor with ionogel showed electrochemical stability for 15 days even without any packaging process. In chapter 4, advanced all-solid-state supercapacitors that can be visually verified for charge and discharge states was prepared. Electrochromic electrodes had a short diffusion distance for lithium ions due to their porous structure. This porous structure was regularly aligned and had a structural color. The structural color not only indicated the charge/discharge state of the supercapacitor, but also added an aesthetic element. The reflection intensity changed instantaneously with changes in voltage, providing a visual indication of the real-time charge/discharge state. This thesis described the research on all-solid-state supercapacitors that combined soft matter and the interfacial engineering. By controlling the interfaces formed on the supercapacitors, which had improved interfacial stability and mechanical strength enhancement, and confirmed that the electrochemical performance was stable for a long period of time in ambient atmosphere without any packaging process. The combination of interfacial engineering and soft matters could provide new form-factors for supercapacitors. As a result, polymer gel electrolytes were expected to play a key role in future all-solid-state energy storage devices. *Key-words: all-solid-state supercapacitor, interface engineering, polymer electrolyte