Synthesis and Investigation of Performance of Poly(Vinyl Acetate)-Based Gel Polymer Electrolyte on Electric Double Layer Capacitor via In-Situ Approach

Yap, Austin Yuhang (2023) Synthesis and Investigation of Performance of Poly(Vinyl Acetate)-Based Gel Polymer Electrolyte on Electric Double Layer Capacitor via In-Situ Approach. Masters thesis, Tunku Abdul Rahman University of Management and Technology.

Text 10 Austin Yap Yuhang (MPS, FT).pdf Restricted to Registered users only Download (8MB)

Abstract

Gel polymer electrolytes (GPEs) based on poly(vinyl acetate) (PVAc) has been synthesized via free-radical polymerization of a precursor liquid mixture consisting of vinyl acetate (VAc) monomer, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIMTFSI) ionic liquid, and lithium bis(trifluoromethylsulfonyl)imide salt (LiTFSI). The volume ratio of BMIMTFSI and concentration of LiTFSI are optimized in two separate phases in this research work, and the effects of increasing amounts of the ionic liquid and the salt are studied thoroughly. GPEs are prepared by increasing the percentage of volume (vol.%) of BMIMTFSI at a fixed LiTFSI concentration of 0.1 M in phase 1. Ionic conductivity increases abruptly from (1.31±0.03)×10-6 S cm-1 to (1.90±0.04)×10-4 S cm-1 at room temperature when 35 vol.% of BMIMTFSI is added. Further increment of BMIMTFSI content causes the GPE to behave like a viscous fluid. On the contrary, GPEs are prepared by increasing the mass loadings of LiTFSi in phase 2. The concentration of LiTFSI is increased gradually and a maximum ionic conductivity of (2.89±0.12)×10-4 S cm-1 is obtained at room temperature for 0.4 M LiTFSI. Higher LiTFSI concentration leads to decrease in ionic conductivity due to formation of ion pairs and aggregations. Temperature-dependent ionic conductivity study reveals that the most conductive GPEs in both phases obey the Vogel-Tammann-Fulcher theory of ionic transport, which is based on free-volume model. Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) analysis confirms the complexation between PVAc, BMIMTFSI, and LiTFSI. X-ray diffraction (XRD) studies prove plasticizing effects of BMIMTFSI and LiTFSI. Thermogravimetric analysis (TGA) confirms that the most conductive GPE has sufficient thermal stability for applications in electrochemical cell. Differential scanning calorimetry (DSC) shows that incorporation of BMIMTFSI lowers the glass transition temperature (Tg) of the polymer matrix effectively, whereas LiTFSI suppresses Tg at low concentration but it increases the Tg at higher concentrations due to formation of polymer-salt complexation which enhances the rigidity of polymer backbone. Addition of BMIMTFSI also widens the potential window of the GPE as shown in linear sweep voltammetry (LSV) analysis. Chronoamperometric measurement prove that the GPEs are ionic conductor, while ionic transport parameters suggest that cations are the main contributor to the conductivity while rheological studies show that the GPEs exhibit shear-thinning properties. Subsequently, electric double layer capacitors (EDLCs) are fabricated using ex-situ and in-situ approaches. Specific capacitance (Csp) of the EDLC made from the most conductive GPE is improved by 401% from 2.073 F g-1 to 10.387 F g-1 (or equivalent to 146.67% from 0.0165 F cm-2 to 0.0407 F cm-1) upon implementation of in-situ EDLC fabrication technique. The enhancement of Csp is attributed to reduced equivalent series resistance (ESR) as proven in electrochemical impedance spectroscopy (EIS). Moreover, Galvanostatic charge-discharge (GCD) study confirms that cycling stability and capacitance retention are improved greatly in the EDLCs fabricated with in-situ approach.