Preparation And Modification Of Microporous Polymer Electrolytes

As critical materials used in polymer lithium-ion battery (PLB), Polymer electrolytes affect the performance of batteries directly but decide the process for PLB manufacturing. Through the two methods of radiation and electrospinning, the microporous polymer electrolytes used in PLB were obtained. This study was concentrated on physical and electrochemical properties of the microporous polymer electrolytes. Moreover, their performances in actual batteries were also studied. As a result, this work is important to instruct the future development of microporous polymer electrolytes.As the initial part of this work, by radiation-induced graft copolymerization of Methylmethacrylate (MMA) monomer onto Polyethylene (PE) separator, PE-g-PMMA membranes with different degrees of grafting (DG) were prepared. The novel dual-phases polymer electrolytes based on grafted membranes (GMs) with various DG were prepared originally through immersion in calefactive liquid electrolyte. The novel dual-phases polymer electrolytes all showed sufficient electrochemical stabilities (up to 4.6 V). The PMMA gel layer in polymer electrolytes could improve the interface quality between electrodes and polymer electrolyte and increase retention of electrolyte solvent in polymer electrolytes. It was found that the polymer electrolyte with 42% DG could reach a magnitude of 10-3 S·cm-1 at room temperature. Furthermore, the coin cells made by GMs demonstrated good cycle life and excellent rate performance. The results of this work suggest that radiation-induced graft polymerization provide a rapid and convenient method to substitute blending or coating in preparation of polymer electrolytes.Subsequently, PVDF-based electrospun membranes were prepared from its solutions in the mixture of N, N-dimethylacetamide (DMAc) and acetone. The structured morphology of the EPM is greatly influenced by the processing parameters, such as applied voltage (V), polymer concentration (W), the ratio of solvents (R) and capillary-screen distance (C-SD) etc. Through changing these processing parameters, various electrospun membranes with different structured morphology, mainly including fibers, beads, net, droplets, can be obtained. Many characterizing methods, including SEM, line voltammogram, electrochemistry impedance spectroscopy, cyclic voltammogram, cyclic performance and rate capabilities of lithium ion coin cells, were used to systemically study the physical and electrochemical characteristics of the EPMs. It was found that electrospun PVDF-based fiber membrane (EPFM) composed of nanofibers showed the excellent performance. The EPFMs can offer above 80 % porosity and high ionic conductivity of above 1×10-3 S·cm-1 at room temperature. The cell with EPFM showed better cycling-ability of Cyclic Voltammograms and charge-discharge performance with little capacity loss after 50 cycles. The study illustrated that higher applied voltage generally resulted in the thinner PVDF fibers and the wider jumbled distribution of fiber diameters when all other variables were held constant. As the average fiber diameter (AFD) is decreased, the EPFMs showed the higher porosity and the better tensile strength. As a result, the EPFM with the thinner AFD exhibited the better performance. The Crystal structure and chain conformation of the EPFMs were also investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). DSC and XRD results indicated that the EPFMs exhibited the weaken crystallinity, but had highly oriented molecular chains after electrospinning. Moreover, the electrospinning process transformed PVDF fromα-form toγ-form.Aiming at improving the poor mechanical properties of EPFMs, three original modifying ways were suggested. Method one: it was achieved by thermal treating EPFMs. After thermal treatment, the EPFMs exhibited 5 ~ 6 % increase in crystallinity. Moreover, soften polymer fibers formed an interconnected network at high temperature, which greatly improve physical properties of the EPFMs. Method two: the electrospun membranes of different mass ratio were prepared by the mixture of PVDF and high crystalling PEO. It was found that the PVDF/PEO membrane with 7 : 3 of mass ratio showed an improved tensile strength. Lastly, a new idea of"intensifying electrolytes"was proposed. This novel polymer electrolyte was prepared by using PMMA mixed liquid electrolyte, then which was used as the absorption object of EPFMs. The tensile strength of the novel polymer electrolyte was enhanced, but the ionic conductivity was found to fall to a magnitude of 10-4 S·cm-1. Moreover, the addition of 8-10 mass% nano-Al2O3 could further intensify the mechanical properties of novel polymer electrolyte. At the same time, the ionic conductivity can reach to 0.92×10-3 S·cm-1.In the last part of this work, the EPFM-based polymer electrolyte is three- phase's system, which is mainly composed of liquid electrolyte, solid phase and gel phase swelled in electrolyte. IR,Raman and XPS were employed to analyse the interaction between the electrolyte molecules and the host polymer matrices. According to free volume theory, the lithium ion transport mechanism in EPFM was clarified. It was found that the ionic conductivity of the EPFM electrolyte was mainly contributed by the lithium ion migration in liquid phase and the behavior of ionic conducticity in EPFM polymer electrolyte obey Arrhenius law. The EPFM-based polymer electrolyte showed the higher fitting apparent activatin energy than the liquid electrolyte and PE systems, which mainly attributed to charge interaction between Li+ ion and F atom besides of resistance of microporous matrix.Moreover, the effect of the EPFM's structure parameters (porosity and tortuosity) on the ionic conductivity of polymer electrolyte was also investigated. It was found that the ionic conductivity increase with the increasing of porosity. The EPMs all had low tortuosity less than 3.8, in which the EPFMs showed lower tortuosity between 2.0 and 2.4.