Abstract
This dissertation presents a new process to prepare microporous, chemically-crosslinked polymer electrolytes based on poly(vinylidene fluoride- hexafluoropropylene) (PVdF-HFP) copolymer as a polymer matrix, polyethylene glycol (PEG) as a plasticizer, and polyethylene glycol dimethacrylate (PEGDMA) as a chemical crosslinking oligomer. The blend electrolytes are prepared by a combination of solvent controlled evaporation and thermal polymerization of PEGDMA. The characteristics of the blend electrolyte membranes were carried out by differential scanning calorimeter (DSC), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The electrochemical properties of the blend electrolytes including ionic conductivity, electrochemical stability and shutdown stability were characterized by AC impedance analysis, linear sweep voltammetry (LSV) and cyclic voltammetry (CV). In addition, the MCMB/LiCoO2 cells using the so-obtained polymer electrolytes were performed practically by battery performance tests including cyclability and rate capability, and safety tests containing nail penetration and overcharge. The results revealed that the blend electrolytes without porous structure show improved mechanical strength due to reinforced effect by PEGDMA network. However, such chemical crosslinking structure resulted in a dense interpenetrating network (IPN) structure that hindered the liquid electrolyte to penetrate into PVdF-HFP matrix and thus deteriorated the transport of lithium ion. Consequently, the ambient ionic conductivity and high-rate performance were considered to be insufficient for portable electronics applications. In order to improve the ionic conductivity of the foresaid polymer electrolyte, an attempt was made to create microporous structure inside the chemically crosslinked polymer matrix by solvent controlled evaporation. Therefore, the blend polymer electrolyte with microporous structure compensated for the decrease in electrolyte uptake and ionic conductivity due to a dense chemical crosslinking structure. Hence, this blend polymer electrolyte exhibits both good mechanical strength enhanced by PEGDMA network, and high ionic conductivity improved by microporous structure. For example, the PVdF-HFP/ PEG/PEGDMA (5/3/2) blend membrane shows a tensile modulus of 52.5 MPa, elongation of 87.2 %. In the presence of 1M LiPF6/EC-DEC, this blend electrolyte exhibits electrolyte uptake of 98.2 % and ambient ionic conductivity of 1.06 × 10-3 Scm-1. In addition, it also shows stable interfacial resistance with lithium metal and electrochemical stability up to 5.0 V vs. Li/Li+. The MCMB/LiCoO2 coin type cell using the resulted polymer electrolyte can deliver about 91% of its C/2 capacity at a 1C rate, and still deliver about 80 % of its C/2 capacity even at a high 2C rate. The cell also retained about 85 % of the initial capacity after 50 cycles. These results indicate that the resulted polymer electrolyte shows good rate capability and acceptable cycleability when compared with that using a commercial separator, such as Celgard® 2300. Finally, the thermal shutdown behaviors of the PVdF-HFP/PEGDMA blend electrolytes were investigated. The crosslinking degree of the blend electrolyte was carefully controlled by casting temperature and casting time during fime-forming. Consequently, one part of the PEGDMA oligomers, which were crosslinked and formed a network, supported the mechanical strength of the said electrolytes, and the other part of the PEGDMA oligomers, which were un-crosslinked, served as plasticizer for PVdF-HFP copolymer under normal situation. However, when temperature rose above 120 oC, the un-crosslinked PEGDMA oligomers started to react and formed network structure in the said electrolytes. Then, such dense network structure hindered the mobility of lithium ion, resulting in increased impedance of the cell and the cell was protected from self-heating and thermal runaway. The results reveal that the resulted polymer electrolyte shows increased impedance by approximately one order of magnitude, which is lower than that of commercial polyolefin separator; however, it exhibits shutdown temperature at 120 oC earlier than that of polyolefin separator, and maintains the thermal stability until 180 oC. Thus, the cells using the so-obtained blend electrolytes can pass the safety tests including nail penetration and overcharge.