Synthesis, Characterization And Ion-conducting Mechanism Study Of Nano Composite Polymer Electrolytes For Rechargeable Lithium Batteries

The rechargeable lithium polymer batteries with high energy density, reliability and good safety, are very important for the research and development of electric vehicles (EV) and hybrid electric vehicles (HEV). In this thesis, the solvent-free and self-standing PEO-based polymer electrolytes (PE) and composite polymer electrolytes (CPE) were prepared with nano-sized SiO2 as fillers. The electrochemical properties such as ionic conductivity, electrochemical stability windows, and the compatibility with Li electrode were investigated with ac impedance, linear sweep voltammetry, and cyclic voltammetry techniques. The lithium transference number (Tli+) was also measured by the ac impedance/dc polarization combined method. The phase compositions and thermal stability of PE and CPE were examined by DSC and TGA. FTIR was used to track the continuous crystallization process at room temperature. The role of nano SiO2 in CPE conductance was proposed. Based on these experimental results and the conclusion, the surface of original fumed SiO2 was modified through silylation reaction. The self-modified SiO2 was added into PEO-LiClO4 PE system and enhanced the conductivity. The highest conductivity of the composite polymer electrolytes reaches 10-5S ?cm-1 at 27.IVThe commercial SiO2 covered with different functional groups were first introduced into PEO-based PE system to investigate the relationship between surfacee groups and the conducting performance of CPE. When SiO2 covered with -OH was added, the ionic conductivity of CPE were not enhanced. And more -OH groups even decreased the conductivity. When the surface -OH was replaced by -CH3, SiO2 could effectively increase the conductivity of the CPE. The more surface -CH3 are, the greater enhancement degree is. When SiO2 covered by long chains of polysilicone was selected as fillers, the CPE showed much lower conductivity.The linear sweep voltammetry results demonstrated that the electrochemical stability windows are affected by the amounts of salt anions on the electrode surface. The more anions are helpful for widening the electrochemical stability window. The windows of CPE are not less than 5.0V(vs. Li+/La), which can provide a higher working potential than liquid electrolytes (4.6V vs. Li+/Li) does. The results of cyclic voltammetry showed the addition of SiO2 decreased Li/electrolyte interface resistance, and the coin cell assembled with CPE containing Si02(TS530) can discharge 161 mAh/g at 60癈. The ac impedance and direct current polarization combined technique shows that the enhancement degree of TU+ of CPE depends on the surface groups of SiO2 used. The TLi of CPE containing Si02 covered with -OH is greater than that of CPE containing SiO2 with -CH3, however, lower than that of CPE containing SiO2 with polyilicone.The phase composition was investigated by DSC technique. The addition of nano SiO2 decreased the crystallinity of the CPE more or less, but it is hard to draw a conclusion how the Si02 surface groups affect phase composition at thisstage.Dependent upon the results of ionic conductivity, electrochemical stability window, lithium transference number, cyclic voltammetry, DSC, FTIR, it is believed that the hypothesis that there two factors influencing the conducting performance of the composite polymer electrolytes: one is the phase composition of the system which mainly works above the melting temperature, and the other is the existence of the interphase between SiO2 and PEO which may permit the ions transfer with lower active energy. The surface groups on SiO2 affect the interphase enhancement degree distinctively. -OH groups on SiO2 will lead to aggregation among particles that may decrease the interphase area, and more importantly -OH will form hydrogen bond with anions that may hinder their transportation speed. Polysilicone on SiO2 may impede the anions cross the interphase due to the inherent long chains. The impedance effects to the anions enhance the lithium transference number indirectly. Hydrophobic SiO2 surface covered with -CH3 ma...